Phelps, Andrew Wells (Kettering, OH, US)
Sturgill, Jeffrey Allen (Fairborn, OH, US)
Swartzbaugh, Joseph Thomas (Clayton, OH, US)

Plaque It!

10/625886

09/09/2008

07/23/2003

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University of Dayton (Dayton, OH, US)

428/444

252/389.4, 106/14.11, 106/14.41, 252/387, 428/457, 148/285, 148/247, 428/469, 148/248, 148/253, 252/389.2, 106/480, 428/432, 148/257, 106/14.05

C23F11/00; C09K3/10

252/389.4, 106/14.11, 106/14.41, 252/387, 428/457, 148/285, 106/14.12, 428/469, 148/247-248, 148/274-275, 148/253, 148/243, 427/385.5, 252/389.2, 106/480, 428/432, 148/257, 106/14.05

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King, Roy

Zheng, Lois L.

Dinsmore & Shohl LLP

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of commonly assigned U.S. application Ser. No. 10/038,150, filed Jan. 4, 2002, now U.S. Pat. No. 7,235,142, and entitled “NON-TOXIC CORROSION-PROTECTION RINSES AND SEALS BASED ON COBALT.” This application is also related to U.S. application Ser. No. 10/625,885, filed Jul. 23, 2003, now U.S. Pat. No. 7,291,217 B2, and entitled “NON-TOXIC CORROSION-PROTECTION PIGMENTS BASED ON RARE EARTH ELEMENTS”, which is a continuation-in-part of U.S. application Ser. No. 10/037,576, filed Jan. 4, 2002, now abandoned, and entitled “NON-TOXIC CORROSION-PROTECTION PIGMENTS BASED ON COBALT”, and U.S. application Ser. No. 10/625,915, filed Jul. 23, 2003 and entitled “NON-TOXIC CORROSION-PROTECTION CONVERSION COATS BASED ON RARE EARTH ELEMENTS”, which is a continuation-in-part of U.S. application Ser. No. 10/038,274, filed Jan. 4, 2002, now U.S. Pat. No. 7,294,211 and entitled “NON-TOXIC CORROSION-PROTECTION CONVERSION COATS BASED ON COBALT”, the disclosures of which are incorporated herein by reference.

We claim:

1. A solid corrosion-inhibiting seal formed on a coating selected from anodic coatings, phosphating coatings, or black oxide coatings, the solid corrosion-inhibiting seal comprising a rare earth element and an inorganic valence stabilizer combined to form a rare earth/valence stabilizer complex within the solid corrosion-inhibiting seal, wherein the rare earth element is selected from cerium, praseodymium, terbium, or combinations thereof and at least one rare earth element is in the tetravalent oxidation state in the rare earth/valence stabilizer complex in the solid corrosion-inhibiting seal.

2. The corrosion-inhibiting seal of claim 1 wherein the rare earth/valence stabilizer complex has a solubility in water of between about 5×10⁻¹ and about 1×10⁻⁵ moles per liter of cerium, praseodymium, or terbium at about 25° C. and about 760 Torr.

3. The corrosion-inhibiting seal of claim 2 wherein the solubility in water of the rare earth/valence stabilizer complex is between about 5×10⁻² and about 5×10⁻⁵ moles per liter of cerium, praseodymium, or terbium at about 25° C. and about 760 Torr.

4. The corrosion-inhibiting seal of claim 1 wherein there is an electrostatic barrier layer around the rare earth/valence stabilizer complex in aqueous solution.

5. The corrosion-inhibiting seal of claim 1 wherein the rare earth/valence stabilizer complex acts as an ion exchange agent towards corrosive ions.

6. The corrosion-inhibiting seal of claim 1 wherein the anodic coatings, phosphating coatings, or black oxide coatings comprise a compound selected from oxides, hydroxides, phosphates, carbonates, oxalates, silicates, aluminates, borates, polymers, or combinations thereof.

7. The corrosion-inhibiting seal of claim 1 wherein the rare earth/valence stabilizer complex has a central cavity containing a cerium, praseodymium, or terbium ion and an additional ion.

8. The corrosion-inhibiting seal of claim 7 wherein the additional ion is B⁺³, Al⁺³, Si⁺⁴, P⁺⁵, Ti⁺⁴, V⁺⁵, V⁺⁴, Cr⁺⁶, Cr⁺³, Mn⁺⁴, Mn⁺³, Mn⁺², Fe⁺³, Fe⁺², Co⁺², Co⁺³, Ni⁺², Ni⁺³, Ni⁺⁴, Cu⁺², Cu⁺³, Zn⁺², Ga⁺³, Ge⁺⁴, As⁺⁵, As⁺³, or Zr⁺⁴.

9. The corrosion-inhibiting seal of claim 1 wherein the inorganic valence stabilizer is selected from molybdates, tungstates, vanadates, niobates, tantalates, tellurates, periodates, iodates, carbonates, antimonates, stannates, phosphates, nitrates, bromates, sulfates, titanates, zirconates, bismuthates, germanates, arsenates, selenates, borates, aluminates, silicates, or combinations thereof.

10. The corrosion-inhibiting seal of claim 9 wherein the valence stabilizer is the inorganic valence stabilizer selected from molybdates, tungstates, vanadates, niobates, tantalates, tellurates, periodates, iodates, carbonates, antimonates, stannates, phosphates, nitrates, bromates, sulfates, or combinations thereof.

11. The corrosion-inhibiting seal of claim 1 further comprising a solubility control agent.

12. The corrosion-inhibiting seal of claim 11 wherein the solubility control agent is a cationic solubility control agent or an anionic solubility control agent.

13. The corrosion-inhibiting seal of claim 12 wherein the solubility control agent is the cationic solubility control agent selected from H⁺; Li⁺; Na⁺; K⁺; Rb⁺; Cs⁺; NH₄⁺; Mg⁺²; Ca⁺²; Sr⁺²; Be⁺²; Ba⁺²; Y⁺³; La⁺³; Ce⁺³; Ce⁺⁴; Nd⁺³; Pr⁺³; Sc⁺³; Sm⁺³; Eu⁺³; Eu₊₂; Gd⁺³; Tb⁺³; Dy⁺³; Ho⁺³; Er⁺³; Tm⁺³; Yb⁺³; Lu⁺³; Ti⁺⁴; Zr⁺⁴; Ti⁺³; Hf⁺⁴; Nb⁺⁵; Ta⁺⁵; Nb⁺⁴; Ta⁺⁴; V⁺⁵; V⁺⁴, V⁺³, Mo⁺⁶; W⁺⁶; Mo⁺⁵; W⁺⁵; Mo⁺⁴; W⁺⁴, Cr⁺³; Mn⁺²; Mn⁺³; Mn⁺⁴; Fe⁺²; Fe⁺³; Co⁺²; Co⁺³; Ni⁺²; Ni⁺³; Ni⁺⁴; Ru⁺²; Ru⁺³; R⁺⁴; Rh⁺³; Ir⁺³; Rh⁺²; Ir⁺²; Pd⁺⁴; Pt⁺⁴; Pd⁺²; Pt⁺²; Os⁺⁴; Cu⁺; Cu⁺²; Cu⁺³; Ag⁺; Ag⁺²; Ag⁺³; Au⁺; Au⁺²; Au⁺³; Zn⁺²; Cd⁺²; Hg⁺; Hg⁺²; Al⁺³; Ga⁺³; Ga⁺; In⁺³; In⁺; Tl⁺³; Tl⁺; Ge⁺⁴; Ge⁺²; Sn⁺⁴; Sn⁺²; Pb⁺⁴; Pb⁺²; Sb⁺³; Sb⁺⁵; As⁺³; As⁺⁵; Bi⁺³; Bi⁺⁵; organic compounds containing at least one N⁺ site; organic compounds containing at least one phosphonium site; organic compounds containing at least one arsonium site; organic compounds containing at least one stibonium site; organic compounds containing at least one oxonium site; organic compounds containing at least one sulfonium site; organic compounds containing at least one selenonium site; organic compounds containing at least one iodonium site; quaternary ammonium compounds having a formula NR₄⁺, where R is an alkyl, aromatic, or acyclic organic constituent; or combinations thereof.

14. The corrosion-inhibiting seal of claim 13 wherein the cationic solubility control agent is selected from H⁺; Li⁺; Na⁺; K⁺; Rb⁺; Cs⁺; NH₄⁺; Mg⁺²; Ca⁺²; Sr⁺²; Y⁺³; La⁺³; Ce⁺³; Ce⁺⁴; Nd⁺³; Pr⁺³; Sc⁺³; Sm⁺³; Eu⁺³; Eu₊₂; Gd⁺³; Tb⁺³; Dy⁺³; Ho⁺³; Er⁺³; Tm⁺³; Yb⁺³; Lu⁺³; Ti⁺⁴; Zr⁺⁴; Ti⁺³; Hf⁺⁴; Nb⁺⁵; Ta⁺⁵; Nb⁺⁴; Ta⁺⁴; Mo⁺⁶; W⁺⁶; Mo⁺⁵; W⁺⁵; Mo⁺⁴; W⁺⁴, Mn⁺²; Mn⁺³; Mn⁺⁴; Fe⁺²; Fe⁺³; Co⁺²; Co⁺³; Ru⁺²; Ru⁺³; R⁺⁴; Rh⁺³; Ir⁺³; Rh⁺²; Ir⁺²; Pd⁺⁴; Pt⁺⁴; Pd⁺²; Pt⁺²; Cu⁺; Cu⁺²; Cu⁺³; Ag⁺; Ag⁺²; Ag⁺³; Au⁺; Au⁺²; Au⁺³; Zn⁺²; Al⁺³; Ga⁺³; Ga⁺; In⁺³; In⁺; Ge⁺⁴; Ge⁺²; Sn⁺⁴; Sn⁺²; Sb⁺³; Sb⁺⁵; Bi⁺³; Bi⁺⁵; organic compounds containing at least one N⁺ site; organic compounds containing at least one phosphonium site; organic compounds containing at least one stibonium site; organic compounds containing at least one oxonium site; organic compounds containing at least one sulfonium site; organic compounds containing at least one iodonium site; quaternary ammonium compounds having a formula NR₄⁺, where R is an alkyl, aromatic, or acyclic organic constituent; or combinations thereof.

15. The corrosion-inhibiting seal of claim 12 wherein the solubility control agent is the anionic solubility control agent selected from fluorotitanates, chlorotitanates, fluorozirconates, chlorozirconates, fluoroniobates, chloroniobates, fluorotantalates, chlorotantalates, molybdates, tungstates, permanganates, fluoromanganates, chloromanganates, fluoroferrates, chloroferrates, fluorocobaltates, chlorocobaltates, fluorozincates, chlorozincates, borates, fluoroborates, fluoroaluminates, chloroaluminates, carbonates, silicates, fluorosilicates, fluorostannates, nitrates, nitrites, azides, cyanamides, phosphates, phosphites, phosphonates, phosphinites, thiophosphates, thiophosphites, thiophosphonates, thiophosphinites, fluorophosphates, fluoroantimonates, chloroantimonates, sulfates, sulfites, sulfonates, thiosulfates, dithionites, dithionates, fluorosulfates, tellurates, fluorides, chlorides, chlorates, perchlorates, bromides, bromates, iodides, iodates, periodates, heteropolyanions, ferricyanides, ferrocyanides, cyanocobaltates, cyanocuprates, cyanomanganates, cyanates, cyanatoferrates, cyanatocobaltates, cyanatocuprates, cyanatomanganates, thiocyanates, thiocyanatoferrates, thiocyanatocobaltates, thiocyanatocuprates, thiocyanatomanganates, cyanamides, cyanamidoferrates, cyanamidocobaltates, cyanamidocuprates, cyanamidomanganates, nitritoferrates, nitritocobaltates, azides, (thio)carboxylates, di(thio)carboxylates, tri(thio)carboxylates, tetra(thio)carboxylates, (thio)phenolates, di(thio)phenolates, tri(thio)phenolates, tetra(thio)phenolates, (thio)phosphonates, di(thio)phosphonates, tri(thio)phosphonates, (thio)phosphonamides, di(thio)phosphonamides, tri(thio)phosphonamides, amino(thio)phosphonates, diamino(thio)phosphonates, triamino(thio)phosphonates, imino(thio)phosphonates, diimino(thio)phosphonates, (thio)sulfonates, di(thio)sulfonates, tri(thio)sulfonates, (thio)sulfonamides, di(thio)sulfonamides, tri(thio)sulfonamides, amino(thio)sulfonates, diamino(thio)sulfonates, triamino(thio)sulfonates, imino(thio)sulfonates, diimino(thio)sulfonates, (thio)borates, di(thio)borates, (thio)boronates, organic silicates, stibonates, cyanides, cyanochromates, cyanonickelates, cyanatochromates, cyanatonickelates, thiocyanatochromates, thiocyanatonickelates, cyanamidochromates, cyanamidonickelates, nitritonickelates, arsonates, diarsonates, triarsonates, organic selenates, diselenates, triselenates, arsenates, arsenites, fluoroarsenates, chloroarsenates, selenates, selenites, fluorothallates, chlorothallates, iodomercury anions, chloromercurates, bromomercurates, osmates, fluoronickelates, chromates, Reinecke's salt, vanadates, or combinations thereof. perchiorates, bromides, bromates, jodides, iodates, periodates, heteropolyanions, ferricyanides, ferrocyanides, cyanocobaltates, cyanocuprates, cyanomanganates, cyanates, cyanatoferrates, cyanatocobaltates, cyanatocuprates, cyanatomanganates, thiocyanates, thiocyanatoferrates, thiocyanatocobaltates, thiocyanatocuprates, thiocyanatomanganates, cyanamides, cyanamidoferrates, cyanamidocobaltates, cyanamidocuprates, cyanamidomanganates, nitritoferrates, nitritocobaltates, azides, (thio)carboxylates, di(thio)carboxylates, tri(thio)carboxylates, tetra(thio)carboxylates, (thio)phenolates, di(thio)phenoLates, tri(thio)phenolates, tetra(thio)phenolates, (thio)phosphonates, di(thio)phosphonates, tri(thio)phosphonates, (thio)phosphonamides, di(thio)phosphonamides, tri(thio)phosphonamides, amino(thio)phosphonates, diamino(thio)phosphonates, triamino(thio)phosphonates, imino(thio)phosphonates, diimino(thio)phosphonates, (thio)sulfonates, di(thio)sulfonates, tri(thio)sulfonates, (thio)sulfonamides, di(thio)sulfonamides, tri(thio)sulfonamides, amino(thio)sulfonates, diamino(thio)sulfonates, triamino(thio)sulfonates, imino(thio)sulfonates, diimino(thio)sulfonates, (thio)borates, di(thio)borates, (thio)boronates, organic silicates, stibonates, cyanides, cyanochromates, cyanonickelates, cyanatochromates, cyanatonickelates, thiocyanatochromates, thiocyanatonickelates, cyanamidochromates, cyanamidonickelates, nitritonickelates, arsonates, diarsonates, triarsonates, organic selenates, diselenates, triselenates, arsenates, arsenites, fluoroarsenates, chioroarsenates, selenates, selenites, fluorothallates, chiorothallates, iodomercury anions, chioromercurates, bromomercurates, osmates, fluoronickelates, chromates, Reinecke's salt, vanadates, or combinations thereof.

16. The corrosion-inhibiting seal of claim 15 wherein the anionic solubility control agent is selected from fluorotitanates, chlorotitanates, fluorozirconates, chlorozirconates, fluoroniobates, chloroniobates, fluorotantalates, chlorotantalates, molybdates, tungstates, permanganates, fluoromanganates, chioromanganates, fluoroferrates, chloroferrates, fluorocobaltates, chiorocobaltates, fluorozincates, chiorozincates, borates, fluoroborates, fluoroaluminates, chloroaluminates, carbonates, silicates, fluorosilicates, fluorostannates, nitrates, nitrites, azides, cyanamides, phosphates, phosphites, phosphonates, phosphinites, thiophosphates, thiophosphites, thiophosphonates, thiophosphinites, fluorophosphates, fluoroantimonates, chioroantimonates, sulfates, sulfites, sulfonates, thiosulfates, dithionites, dithionates, fluorosulfates, tellurates, fluorides, chlorides, chlorates, perchiorates, bromides, bromates, jodides, iodates, periodates, heteropolyanions, ferricyanides, ferrocyanides, cyanocobaltates, cyanocuprates, cyanomanganates, cyanates, cyanatoferrates, cyanatocobaltates, cyanatocuprates, cyanatomanganates, thiocyanates, thiocyanatoferrates, thiocyanatocobaltates, thiocyanatocuprates, thiocyanatomanganates, cyanamides, cyanamidoferrates, cyanamidocobaltates, cyanamidocuprates, cyanamidomanganates, nitritoferrates, nitritocobaltates, azides, (thio)carboxylates, di(thio)carboxylates, tri(thio)carboxylates, tetra(thio)carboxylates, (thio)phenolates, di(thio)phenolates, tri(thio)phenolates, tetra(thio)phenolates, (thio)phosphonates, di(thio)phosphonates, tri(thio)phosphonates, (thio)phosphonamides, di(thio)phosphonamides, tri(thio)phosphonamides, amino(thio)phosphonates, diamino(thio)phosphonates, triamino(thio)phosphonates, imino(thio)phosphonates, diimino(thio)phosphonates, (thio)sulfonates, di(thio)sulfonates, tri(thio)sulfonates, (thio)sulfonamides, di(thio)sulfonamides, tri(thio)sulfonamides, amino(thio)sulfonates, diamino(thio)sulfonates, triamino(thio)sulfonates, imino(thio)sulfonates, diimino(thio)sulfonates, (thio)borates, di(thio)borates, (thio)boronates, organic silicates, stibonates, or combinations thereof.

17. The corrosion-inhibiting seal of claim 1 further comprising a lubricity agent.

18. The corrosion-inhibiting seal of claim 17 wherein the lubricity agent is selected from molybdenum disulfide, fluorinated hydrocarbons, perfluorinated hydrocarbons, graphite, soft metals, polymers, or combinations thereof.

19. The corrosion-inhibiting seal of claim 18 wherein the lubricity agent is the soft metal selected from tin, indium, silver, or combinations thereof.

20. The corrosion-inhibiting seal of claim 1 wherein the corrosion-inhibiting seal is colored.

21. The corrosion-inhibiting seal of claim 20 further comprising an agent which improves color-fastness of the corrosion-inhibiting seal.

22. The corrosion-inhibiting seal of claim 21 wherein the agent which improves color-fastness is selected from an active UV blocker, a passive UV blocker, a brightener, or a combination thereof.

23. The corrosion-inhibiting seal of claim 22 wherein the agent which improves color-fastness is the active UV blocker selected from carbon black, graphite, phthalocyanines, or combinations thereof.

24. The corrosion-inhibiting seal of claim 22 wherein the agent which improves color-fastness is the passive UV blocker selected from titanium oxide, tin oxide, lead oxide, silicon oxide, silicates, aluminosilicates, or combinations thereof.

25. The corrosion-inhibiting seal of claim 22 wherein the agent which improves color-fastness is the brightener selected from sulfonic acids, sulfonates, sulfonamides, sulfonic acids, sulfinates, sulfones, cyanides, nonionic surfactants, or combinations thereof.

26. The corrosion-inhibiting seal of claim 20 wherein the color is formed by a dye selected from vat dyes, mordant dyes, lake dyes, disperse dyes, azo dyes, triazine dyes, triphenylmethane dyes, azine dyes, formazan dyes, phthalocyanine dyes, Schiff Base dyes, naturally-occurring dyes, inorganic pigments, or combinations thereof.

27. The corrosion-inhibiting seal of claim 21 wherein the agent which improves color-fastness is an agent which prevents smudging.

28. The corrosion-inhibiting seal of claim 27 wherein the agent which prevents smudging is selected from phosphoric acid, metaphosphates, orthophosphates, pyrophosphates, polyphosphates, or combinations thereof.

29. The corrosion-inhibiting seal of claim 21 wherein the agent which improves color-fastness is a wetting agent.

30. The corrosion-inhibiting seal of claim 29 further comprising less than about 5 g/L of the wetting agent.

31. The corrosion-inhibiting seal of claim 29 wherein the wetting agent is a nonionic surfactant.

32. A solid corrosion-inhibiting seal formed on a coating selected from anodic coatings, phosphating coatings, or black oxide coatings, the solid corrosion-inhibiting seal comprising a rare earth element and an inorganic valence stabilizer combined to form a rare earth/valence stabilizer complex within the solid corrosion-inhibiting seal, wherein the rare earth element is selected from cerium, praseodymium, terbium, or combinations thereof at least one rare earth element is in the tetravalent oxidation state in the rare earth/valance stablizer complex, and the rare earth/valence stabilizer complex is sparingly soluble in water at about 25° C. and about 760 Torr in the rare earth/valence stabilizer complex in the solid corrosion-inhibiting seal.

33. A solid corrosion-inhibiting seal formed on a coating selected from anodic coatings, phosphating coatings, or black oxide coatings, the solid corrosion-inhibiting seal comprising a rare earth element and a valence stabilizer combined to form a rare earth/valence stabilizer complex within the solid corrosion-inhibiting seal, wherein the rare earth element is selected from cerium, praseodymium, terbium, or combinations thereof, and at least one rare earth element is in the tetravalent oxidation state, wherein the rare earth/valence stabilizer complex has a central cavity containing a cerium, praseodymium, or terbium ion and an additional ion, and wherein the additional ion is B⁺³, Al⁺³, Si⁺⁴, P⁺⁵ , Ti⁺⁴, V⁺⁵, V⁺⁴, Cr⁺⁶, Cr⁺³, Mn⁺⁴, Mn⁺³, Mn⁺², Fe⁺³, Fe⁺², Co⁺², Co⁺³, Ni⁺², Ni⁺³, Ni⁺⁴, Cu⁺², Cu⁺³, Zn⁺², Ga⁺³, Ge⁺⁴, As⁺⁵, As⁺³, or Zr⁺⁴.

34. The corrosion-inhibiting seal of claim 33 wherein the rare earth/valence stabilizer complex has a solubility in water of between about 5×10⁻¹ and about 1×10⁻⁵ moles per liter of cerium, praseodymium, or terbium at about 25° C. and about 760 Torr.

35. The corrosion-inhibiting seal of claim 34 wherein the solubility in water of the rare earth/valence stabilizer complex is between about 5×10⁻² and about 5×10−5 moles per liter of cerium, praseodymium, or terbium at about 25° C. and about 760 Torr.

36. The corrosion-inhibiting seal of claim 33 wherein there is an electrostatic barrier layer around the rare earth/valence stabilizer complex in aqueous solution.

37. The corrosion-inhibiting seal of claim 33 wherein the rare earth/valence stabilizer complex acts as an ion exchange agent towards corrosive ions.

38. The corrosion-inhibiting seal of claim 33 wherein the anodic coatings, phosphating coatings, or black oxide coatings comprise a compound selected from oxides, hydroxides, phosphates, carbonates, oxalates, silicates, aluminates, borates, polymers, or combinations thereof.

39. The corrosion-inhibiting seal of claim 33 wherein the valence stabilizer is the inorganic valence stabilizer selected from molybdates, tungstates, vanadates, niobates, tantalates, tellurates, periodates, iodates, carbonates, antimonates, stannates, phosphates, nitrates, bromates, sulfates, titanates, zirconates, bismuthates, germanates, arsenates, selenates, borates, aluminates, silicates, or combinations thereof.

40. The corrosion-inhibiting seal of claim 39 wherein the valence stabilizer is the inorganic valence stabilizer selected from molybdates, tungstates, vanadates, niobates, tantalates, tellurates, periodates, iodates, carbonates, antimonates, stannates, phosphates, nitrates, bromates, sulfates, or combinations thereof.

41. The corrosion-inhibiting seal of claim 33 further comprising a solubility control agent.

42. The corrosion-inhibiting seal of claim 41 wherein the solubility control agent is a cationic solubility control agent or an anionic solubility control agent.

43. The corrosion-inhibiting seal of claim 42 wherein the solubility control agent is the cationic solubility control agent selected from H⁺; Li⁺; Na⁺; K⁺; Rb⁺; Cs⁺; NH₄⁺; Mg⁺²; Ca⁺²; Sr⁺²; Be⁺²; Ba⁺²; Y⁺³; La⁺³; Ce⁺³; Ce⁺⁴; Nd⁺³; Pr⁺³; Sc⁺³; Sm⁺³; Eu⁺³; Eu⁺²; Gd⁺³; Tb⁺³; Dy⁺³; Ho⁺³; Er⁺³; Tm⁺³; Yb⁺³; Lu⁺³; Ti⁺⁴; Zr⁺⁴; Ti⁺³; Hf⁺⁴; Nb⁺⁵; Ta⁺⁵; Nb⁺⁴; Ta⁺⁴; V⁺⁵; V⁺⁴; V⁺³; Mo⁺⁶; W⁺⁶; Mo⁺⁵; W⁺⁵; Mo⁺⁴; W⁺⁴; Cr⁺³; Mn⁺²; Mn⁺³; Mn⁺⁴; Fe⁺²; Fe⁺³; Co⁺²; Co⁺³; Ni⁺²; Ni⁺³; Ni⁺⁴; Ru⁺²; Ru⁺³; Ru⁺⁴; Rh⁺³; Ir⁺³; Rh⁺²; Ir⁺²; Pd⁺⁴; Pt⁺²; Pd⁺²; Pt⁺²; Os⁺⁴; Cu⁺; Cu⁺²; Cu⁺³; Ag⁺; Ag⁺²; Ag⁺³; Au⁺; Au⁺²; Au⁺³; Zn⁺²; Cd⁺² ; Hg⁺; Hg⁺²; Al⁺³; Ga⁺³; Ga⁺; In⁺³; In⁺; Tl⁺³; Tl⁺; Ge⁺⁴; Ge⁺²; Sn⁺⁴; Sn⁺²; Pb⁺⁴; Pb⁺²; Sb⁺³; Sb⁺⁵; As⁺³; As⁺⁵; Bi⁺³; Bi⁺⁵; organic compounds containing at least one N⁺ site; organic compounds containing at least one phosphonium site; organic compounds containing at least one arsonium site; organic compounds containing at least one stibonium site; organic compounds containing at least one oxonium site; organic compounds containing at least one sulfonium site; organic compounds containing at least one selenonium site; organic compounds containing at least one jodonium site; quaternary ammonium compounds having a formula NR₄⁺, where R is an alkyl, aromatic, or acyclic organic constituent; or combinations thereof.

44. The corrosion-inhibiting seal of claim 43 wherein the cationic solubility control agent is selected from H⁺; Li⁺; Na⁺; K⁺; Rb⁺; Cs⁺; NH₄⁺; Mg⁺²; Ca⁺²; Sr⁺²; Y⁺³; La⁺³; Ce⁺³; Ce⁺⁴; Nd⁺³; Pr⁺³; Sc⁺³; Sm⁺³; Eu⁺³; Eu⁺²; Gd⁺³; Tb⁺³; Dy⁺³; Ho⁺³; Er⁺³; Tm⁺³; Yb⁺³; Lu⁺³; Ti⁺⁴; Zr⁺⁴; Ti⁺³; Hf⁺⁴; Nb⁺⁵; Ta⁺⁵; Nb⁺⁴; Ta⁺⁴; Mo⁺⁶; W⁺⁶; Mo⁺⁵; W⁺⁵; Mo⁺⁴; W⁺⁴; Mn⁺²; Mn⁺³; Mn⁺⁴; Fe⁺²; Fe⁺³; Co⁺²; Co⁺³; Ru⁺²; Ru⁺³; Ru⁺⁴; Rh⁺³; Ir⁺³; Rh⁺²; Ir⁺²; Pd⁺⁴; Pt⁺⁴; Pd⁺²; Pt⁺²; Cu⁺; Cu⁺²; Cu⁺³; Ag⁺; Ag⁺²; Ag⁺³; Au⁺; Au⁺²; Au⁺³; Zn⁺²; Al⁺³; Ga⁺³; Ga⁺; In⁺³; In⁺; Ge⁺⁴; Ge⁺²; Sn⁺⁴; Sn⁺²; Sb⁺³; Sb⁺⁵; Bi⁺³; Bi⁺⁵ organic compounds containing at least one N⁺ site; organic compounds containing at least one phosphonium site; organic compounds containing at least one stibonium site; organic compounds containing at least one oxonium site; organic compounds containing at least one sulfonium site; organic compounds containing at least one iodonium site; quaternary ammonium compounds having a formula NR₄⁺, where R is an alkyl, aromatic, or acyclic organic constituent; or combinations thereof.

45. The corrosion-inhibiting seal of claim 33 further comprising a lubricity agent.

46. The corrosion-inhibiting seal of claim 45 wherein the lubricity agent is selected from molybdenum disulfide, fluorinated hydrocarbons, perfluorinated hydrocarbons, graphite, soft metals, polymers, or combinations thereof.

47. The corrosion-inhibiting seal of claim 46 wherein the lubricity agent is the soft metal selected from tin, indium, silver, or combinations thereof

48. The corrosion-inhibiting seal of claim 33 wherein the corrosion-inhibiting seal is colored.

49. The corrosion-inhibiting seal of claim 48 further comprising an agent which improves color-fastness of the corrosion-inhibiting seal.

50. The corrosion-inhibiting seal of claim 49 wherein the agent which improves color-fastness is selected from an active UV blocker, a passive UV blocker, a brightener, or a combination thereof.

51. The corrosion-inhibiting seal of claim 50 wherein the agent which improves color-fastness is the active UV blocker selected from carbon black, graphite, phthalocyanines, or combinations thereof.

52. The corrosion-inhibiting seal of claim 49 wherein the agent which improves color-fastness is an agent which prevents smudging.

53. The corrosion-inhibiting seal of claim 52 wherein the agent which prevents smudging is selected from phosphoric acid, metaphosphates, orthophosphates, pyrophosphates, polyphosphates, or combinations thereof.

54. The corrosion-inhibiting seal of claim 49 wherein the agent which improves color-fastness is a wetting agent.

55. The corrosion-inhibiting seal of claim 54 further comprising less than about 5 g/L of the wetting agent.

56. The corrosion-inhibiting seal of claim 54 wherein the wetting agent is a nonionic surfactant.

BACKGROUND OF THE INVENTION

This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting rinses and seals for use to impart additional corrosion resistance to structural materials without the use of chromium in the hexavalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-protective rinses and seals for metal phosphating, anodizing, and “black oxiding” processes based on tetravalent cerium, praseodymium, or terbium and methods of making and using the same.

Metals like aluminum, zinc, titanium, iron, cadmium, tin, indium, lithium, beryllium, magnesium, niobium, tantalum, zirconium, lead, rare earths, copper, and silver, their alloys, or items plated with these metals, require protection from corrosion due to their low oxidation-reduction (redox) potentials or ease of oxide formation. These metal alloys have many uses that range from architectural adornments, to protective coatings themselves, to automotive, structural aerospace, and electronic components, to name a few. The unalloyed metals typically form an outer layer of natural oxide: a “passive film” that serves to protect them and reduce their overall rate of corrosion. However, the corrosion protection offered by the naturally formed oxide layer on certain alloys of these metals is not complete and corrosion will eventually occur unless some form of additional corrosion protection is used. Thus, for example, steels are typically “phosphated” to provide an impermeable coating that not only resists corrosive attack, but also provides a paint base. Additionally, architectural and structural aluminum are frequently “anodized” to form an impermeable oxide film for the same reasons.

Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion.

One method of enhancing the corrosion resistance of these alloys includes the use of a chemically- or electrolytically-generated coating such as an anodized coating (typically on aluminum), a phosphate coating (typically on electrogalvanized or bare steel), or a black oxide coating (for high strength bearing and tool steels). The metal is exposed to a compound that chemically alters the surface (in phosphating and black oxiding) or an electric current (in anodizing) and forms a coating that provides some corrosion resistance by forming a barrier film. The morphology and possibly the chemistry of the anodic coating or phosphate coating can allow for the formation of a strong bond with subsequently-applied paint systems. An anodic coating is usually applied via immersion in an electrolytic cell. A phosphating or black oxide solution may be applied by immersion, by spray, or by manual means.

These coatings frequently exhibit “flaws” such as pores, pinholes, or thin portions in the coating after formation and do not contain any inherent means to “repair” these coating breaches. The application of a second solution is necessary to fill the pores in the coating and deposit compounds that will act as long-term corrosion protective species. These “second solutions” are termed “rinses” or “seals” in the corrosion literature. The term “rinse” is typically used for the second solution applied to phosphating and black oxide coatings, whereas the term “seal” usually refers to the second solution applied to anodic coatings. These rinses and seals are typically applied via spray techniques, but immersion, fogging, and wiping are also accepted practices.

Hexavalent chromium has traditionally been the active corrosion-inhibiting agent used in rinses and seals for the formation of protective coatings for iron, electrogalvanized iron, aluminum, zinc, magnesium, titanium, cadmium, tin, indium, lithium, and their alloys. Niobium, tantalum, zirconium, beryllium, lead, rare earths, copper, and silver may also be treated with hexavalent chromium rinses and seals for special applications. The three main coating processes that use these rinses and seals are 1) the phosphating process for steel and galvanized steel products, 2) the anodization process for a host of structural metals, and 3) the black oxide process for high-strength steel and iron used for bearing materials. Table 1 illustrates the processes that typically utilize a final chrome “rinse” or “seal” to impart additional corrosion protection to a given substrate material.

TABLE 1
Current Rinse and Seal Processes Using Hexavalent Chromium
	Comments/		Government/
Process	Examples	Substrate Metals	ASTM/Mil Specs
Rinses for zinc	Used as a paint base	Zinc-coated steel,	MIL-P-50002
phosphating on	on all automotive	zinc, or bare steel are	DoD-P-16232
steel, steel products,	bodies, also for some	usual substrates.	MIL-HDBK-205
and nonferrous	coil and sheet stock.	Also for aluminum,	SAE-AMS2481
alloys	Used as a lubricating	magnesium, copper,	QQ-P-416
	layer on tooling dies.	titanium, cadmium,
		and silver in less
		common applications.
Seals for anodized	Used extensively for	Aluminum and	MIL-A-8625
aluminum including	architectural and	aluminum alloys	SAE-AMS2470
sulfuric, chromic,	decorative		ASTM B580
oxalic, boric,	applications, adhesive		ASTM D1730
sulfonated organic	bonding, siding, etc.		AA46-78
acids, citric, and	Also used as a paint
phosphoric acid	base.
anodizing
Rinses for iron	Used as a paint base	Steel and iron alloys	TT-C-490
phosphating on bare	on coil coatings for		MIL-HDBK-205
steels	general appliance and		SAE-AMS2481
	siding applications.		QQ-P-416
	Different from Zn and
	Mn phosphating.
Rinses for	Used solely as a solid	Mostly bare steel.	MIL-P-50002
manganese	lubricant, not as a	Can also be used on	DoD-P-16232
phosphating on	paint base. Used	high-strength copper	MIL-HDBK-205
steel and steel	extensively on bearing	alloys.	SAE-AMS2481
alloys, also on	materials.
nonferrous alloys
Rinses for “black	Used solely as a solid	Mostly bare steel.	MIL-C-13924
oxide” and other	lubricant, not as a	Can also be used on	MIL-C-46110
oxide lubricating	paint base. Used	high-strength copper	SAE-AMS2485
layers	extensively on bearing	alloys.
	materials.
Seals for anodized	Used as a paint and	Magnesium and	MIL-M-45202
magnesium	adhesive base.	magnesium alloys	ASTM D1732
including sulfuric,			SAE-AMS2475
chromic, oxalic,			MIL-C-13335
boric, sulfonated
organic acids, citric,
and phosphoric acid
anodizing
Seals for anodized	Used as a paint and	Titanium and	SAE-AS4194
titanium including	adhesive base.	titanium alloys	SAE AMS-2488
sulfuric, chromic,
oxalic, boric, citric,
hydrofluoric, and
phosphoric acid
anodizing
Seals for anodized	Used as a paint and	Zinc and zinc alloys	MIL-A-81801
zinc including	adhesive base.
sulfuric, chromic,
oxalic, boric,
sulfonated organic
acids, citric, and
phosphoric acid
anodizing
Seals for anodized	Used as a paint and	Iron, steel, and steel	QQ-P-35
steel including	adhesive base.	alloys
sulfuric, chromic,
oxalic, boric, and
phosphoric acid
anodizing
Seals for anodized	Used for a number of	Copper, cadmium,	QQ-P-416
copper, cadmium,	applications,	silver, tantalum,
silver, tantalum,	principally as a paint	niobium, zirconium,
lead, cobalt,	and adhesive base.	tin, indium,
niobium, zirconium,	For example, niobium	manganese and their
tin, indium, and	and tantalum	alloys
manganese	capacitors, cadmium
including sulfuric,	plate, silver solder,
chromic, oxalic,	and zirconium for
boric, sulfonated	nuclear applications.
organic acids, citric,
and phosphoric acid
anodizing

As shown in Table 1 above, there are three “generic” phosphating processes for steel and steel alloys—zinc, manganese, and iron phosphating. Differences in the coating solutions result in different chemistries and physical attributes in the formed coatings. For example, zinc phosphating is used primarily on galvanized steel sheet, and results in an ideal surface morphology for paint adhesion if the crystals are small in size, and as a solid lubricant for larger size crystals. Manganese phosphating, however, results in a hard, lubricious coating that has no use as a paint base, but exhibits excellent characteristics as a solid lubricant. Manganese phosphating coatings are rarely subjected to a post-chrome rinse, because the corrosion resistance of these coatings is of lesser concern. Iron phosphating is also used as a paint and adhesive base, and always receives post-treatments for corrosion protection.

Similar differences are also noted in anodizing processes. Anodizing processes involve the application of an electric potential under a variety of acidic conditions to the substrate to be coated. Sulfuric acid is the conventional anodizing acid used to form hard oxide films on aluminum, although other anodization solutions have specialized applications. For example, phosphoric acid may be used for adhesive bonding applications on aluminum. Oxalic acid anodization results in a harder, denser coating with higher corrosion resistance than sulfuric acid anodization and is used more often in Europe. Boric acid anodization is used frequently for electronic capacitors although citric and tartaric acid anodization can be used for the same application. Anodization with sulfonated organic acids (such as sulfosalicylic or sulfophthalic acids) is used to impart color during the anodization process. Chromic acid anodization is used on parts with complex shapes where final sealing or rinsing is not possible. Other acids, including hydrofluoric acid, have been used for special applications or in proprietary formulations. Those skilled in the anodization art know that a wide variety of anodizing processes exist due to the multitude of substrate metals, anodizing acids, applied voltages, and final applications.

Finally, “black oxide” coatings are applied to high strength steels and copper-containing alloys to impart a lubricious coating. The difference between “black oxide” coatings and other lubricious coating processes (such as manganese phosphating) is that “black oxide” coatings are applied under caustic, elevated temperature conditions. For example, a concentrated sodium hydroxide solution is raised to its boiling point and the substrate metal is then immersed in this solution. This results in the formation of a lubricious coating of magnetite/ferrite on the surface of steel alloys.

Other coating processes that result in coatings with no inherent self-healing characteristics have also been enhanced through the use of hexavalent chromium rinses and seals. Carbonate coatings on metals such as zinc, iron, magnesium, and especially copper have been described in the early literature as providing some degree of corrosion protection. These coatings can be further enhanced through the use of hexavalent chromium rinses to deposit inhibiting compounds to self-heal coating breaches. Other oxide, phosphate, oxalate, silicate, aluminate, borate or polymeric coatings, or combinations thereof, can also be enhanced via hexavalent chromium rinses and seals.

For each of these three generic coating processes (phosphating, anodizing, and black oxiding), a second, subsequent chemical treatment is often applied. The nature of this second treatment is dependent upon the desired final characteristics of the metal piece. For phosphating and black oxiding processes, this second treatment is usually a rinse of hexavalent chromium, to impart additional corrosion protection to the coating. For anodizing processes, the second treatment can impart a number of useful attributes to the work piece. This second “sealing” process for anodized coatings can include: 1) pure boiling water (to plug the pores with a hydrated alumina composition); 2) silicates (to plug the pores with a silicate composition); 3) dyes or metal-dye complexes (to impart color to the anodic coating); 4) metal salts followed by cathodic reduction (to color the coating via the formation of metals or metal sulfides in the pores); 5) lubricating additives such as molybdenum disulfide or dispersions of polytetrafluoroethylene (to fill the pores with a lubricious additive); and 6) hexavalent chromium seals to fill the pores with chromate species. It is noteworthy that the only one of these six generic sealing processes that results in a coating with self-healing characteristics is the hexavalent chromium seal. The other sealing processes for anodic coatings may temporarily increase the corrosion resistance of the coating by plugging the pores in the oxide coating (e.g., with hydrated alumina or silicate), but the coating does not retain any corrosion-inhibitive species.

The various coating processes to which the art described in this invention is applicable are shown in Table 1 above. The frequent use of hexavalent chrome to “rinse” or “seal” the coating (phosphate, anodic, or black oxide) formed in the first unit operation of the process to impart additional corrosion resistance connects them. These solutions are usually simple formulations consisting of nothing more than dissolved chromium trioxide, chromate, or dichromate. These formulations are usually applied by spraying, although immersion, fogging, or even wiping may also be used.

Sometimes these hexavalent chromium rinse or sealing formulations will contain other constituents. Some formulations include minor concentrations of fluorides. These fluorides act to “etch back” the coating formed in the first unit operation (e.g., phosphate, anodic, or black oxide), thus further facilitating the deposition of corrosion-inhibiting species. Rinsing solutions for phosphate solutions are frequently observed to include phosphoric acid in addition to hexavalent chromium in order to reduce staining of the phosphate coating by the hexavalent chromium. These hexavalent chromium rinse or sealing solutions can also contain other constituents, such as ferricyanides or molybdates. The presence of these other constituents is significant in light of the chemistry developed and presented herein.

Significant efforts have been made to replace chromium with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory concerns. Cerium is one non-toxic, non-regulated metal that has been considered as a chromium replacement. Cerium (like chromium) exhibits more than one oxidation state (Ce ⁺³ and Ce ⁺⁴ ). In addition, the oxidation-reduction potential of the Ce ⁺⁴ —Ce ⁺³ couple is comparable to the Cr ⁺⁶ —Cr ⁺³ couple. For example, in acid solution:
Ce ⁺⁴ +e Ce ⁺³ +1.72 V
Cr ⁺⁶ +3e− Cr ⁺³ +1.36 V
Praseodymium and terbium also exhibit more than one oxidation state (Pr ⁺³ and Pr ⁺⁴ , Tb ⁺³ and Tb ⁺⁴ ). Tetravalent praseodymium and terbium are even stronger oxidizing agents than cerium (with calculated redox potentials of +3.2 V in acidic solution—Nugent, L. J., et al., J. Inorg. Nucl. Chem . 33:2503-30, 1971):
Pr ⁺⁴ +e ^{− Pr +3} +3.2 V
Tb ⁺⁴ +e ^{− Tb +3} +3.2 V
Cr ⁺⁶ +3e ^{− Cr +3} +1.36 V
Accordingly, several processes have been reported in the literature, which make use of cerium in rinsing or sealing bath solutions. However, the coatings formed by these processes provide only limited corrosion protection and do not approach the benefit derived from the use of hexavalent chromium. None of the prior art recognizes the need to “valence stabilize” tetravalent cerium to ensure its long-term stability, nor the need to form tetravalent cerium compounds of optimum solubility characteristics.

The use of film-forming substances, such as polymers, silicates, sol-gel, etc., which have no inherent oxidizing character in sealing or rinsing coating solutions, has been described in the literature. The film formers may enhance short-term corrosion resistance by functioning as a barrier layer. Barrier layers lacking an active corrosion inhibitor have been demonstrated to be capable of inhibiting corrosion as long as the barrier is not breached, as by a scratch or other flaw. Film formers can actually enhance corrosion on a surface after failure due to the well known effects of crevice corrosion.

1) Rinses for Phosphate Coatings

U.S. Pat. No. 2,790,740 to Ayres et al. describes the use of a tetravalent cerium compound (i.e., ceric sulfate) as an accelerator for phosphate coatings on aluminum and zinc. The cerium is added simultaneously with the phosphate treatment. No provisions for post-treatment of the formed phosphate coating through an additional rinse are described. Pores formed during the phosphating step are therefore not sealed. This patent also describes the need to incorporate zinc or manganese compounds with the cerium, as cerium appears to be effective only when used in the presence of substantial proportions of zinc or manganese.

U.S. Pat. No. 2,698,266 to Thirsk decribes the use of hexavalent chromium/tetravalent cerium rinses to seal phosphate and arsenate coatings on aluminum. The use of hexavalent chromium in conjunction with tetravalent cerium represents no appreciable reduction in bath toxicity.

German Patent No. DE 40 41 091 A1 to Metallgesellschaft AG describes the use of trivalent cerium along with tetravalent cerium in a 2:1 to 9:1 ratio for the passivating of phosphated coatings on steel and aluminum. These coating solutions also incorporate fluoride, carboxylate, hydroxycarboxylate, aminocarboxylate, molybdate, and/or tungstate ions in the solution. However, the importance of tetravalent cerium within the formed coating, the “valence stabilization” of this ion, and the solubility ranges for formed tetravalent cerium compounds are not described.

2) Seals for Anodic Coatings

U.S. Pat. No. 5,192,374 to Kindler describes the formation of an aluminum oxide (boehmite) coating on structural aluminum, followed by treatment with a soluble cerium salt and a metal nitrate at 70° C. to 100° C. to form cerium oxides and hydroxides for increased corrosion resistance. The formed oxides and hydroxides are described as filling the pores in the boehmite coating. Also, Stoffer et al. in U.S. Pat. No. 5,932,083 describe the use of a solution containing cerium and an oxidizing agent for treatment of aluminum alloys. The aluminum-containing substrate is electrolyzed in this solution, forming a mixed aluminum oxide/cerium oxide (or hydrated cerium oxide) coating on the aluminum as a barrier film. The formation of tetravalent or hydrated tetravalent cerium oxide is described. However, neither Kindler nor Stoffer et al. teach the use of “valence stabilizers”, which are important for use of tetravalent cerium compounds having aqueous solubilities that are sufficiently high to ensure long-term self healing of the coating. The cerium oxides and hydrated oxides described in these patents function merely as pore-filling barrier layers, and not as active self-healing inhibitors within the coating. Further, the use of tetravalent cerium oxides and hydroxides as corrosion inhibitors results in lower corrosion performance, as is described herein, due to the fact the electrostatic double layers around these species are much smaller than those exhibited by tetravalent cerium species containing 50% or less oxide or hydroxide as attached ligands.

U.S. Pat. Nos. 5,635,084; 5,582,654; and 5,194,138, all to Mansfield et al., describe methods for treating the surface of an aluminum alloy having a relatively high copper content, so as to make the surface resistant to corrosion. The method comprises: a) removing substantially all of the copper from the surface of the alloy, b) contacting the surface with a first solution containing cerium, c) electrically charging the surface while contacting with an aqueous molybdate solution, and d) contacting the surface with a second solution containing cerium. U.S. Pat. No. 5,756,218 to Buchheit et al. describes a process for the corrosion protection of metallic materials that includes sealing a coating with an aqueous solution consisting essentially of at least one soluble metal salt (i.e., Ce). However, the '084, '654 and '218 patents make use of h exavalent chromium in the coating process, and so no advantage in toxicity reduction is achieved. Moreover, electrolysis will only oxidize cerium to the tetravalent state in the outer regions of the already-formed cerium-containing coating. The importance of tetravalent cerium and the functional parameters for tetravalent cerium-containing complexes are not described in any of these prior art references.

U.S. Pat. No. 6,022,425 to Nelson et al. describes the application of a corrosion-resistant coating for aluminum based on cerium, which cerium is oxidized to the tetravalent oxidation state, resulting in the formation of tetravalent or hydrated cerium oxides. However, these references teach tetravalent cerium compounds having aqueous solubilities that are so low they function as barrier films or sealants, rather than active corrosion inhibitors. Moreover, the use of valence stabilizers for forming complexes with tetravalent cerium is not disclosed.

European Application No. EP 0 902 103 A1 by Nippon Steel Corporation describes the application of a trivalent cerium solution with organic oxoacids to aluminum or galvanized steel. U.S. Pat. No. 6,200,672 B1 to Tadokoro et al. describes the use of rare earth and/or Group IVA solutions with selected organic molecules for treatment of metal surfaces. U.S. Pat. No. 5,964,928 to Tomlinson describes the use of a Group IVA compound (i.e., zirconium, titanium, or hafnium) in combination with a rare earth element and optionally a fluoride. European Application No. EP 0 839 931 A2 by Nihon et al. describes an aqueous, metallic surface treating solution comprising a metal element including Ce, an oxidizing source, and an oxyacid or oxyacid salt of phosphorus or an anhydride thereof. However, none of these references teach the presence of a valence stabilized, oxidized rare earth element such as cerium, praseodymium, or terbium in the formed seal, whose availability to the corroding system is controlled via the solubility of the oxidized rare earth compounds. In order to function as a true replacement for hexavalent chromium, which is itself a highly oxidized species, the rare earth compound must be oxidized in the formed seal.

U.S. Pat. No. 6,206,982 B1 to Hughes et al. describes the use of a four component system to provide corrosion protection of aluminum. One of these components includes a rare earth compound, especially cerium.

The use of colloidal suspensions of tetravalent cerium oxide (CeO ₂ ) in anticorrosive coatings is described in U.S. Pat. Nos. 5,922,330 and 5,733,361 to Chane-Ching et al.; PCT International Publication No. WO 96/26255 by Rhone Poulenc Chimie; and PCT International Publication Nos. WO 01/36331 A1 and WO 01/38225 A1 by Rhodia Terres Rares. The CeO ₂ exhibits a solubility that is too low for effective release of corrosion-inhibiting tetravalent cerium ions.

An aqueous dispersion of a cerium compound with other rare earths, transition metals, aluminum, gallium, or zirconium is described for anticorrosive agents in PCT International Publication No. WO 01/55029 A1 by Rhodia Terres Rares. Similarly, an aqueous dispersion of cerium oxide in combination with additives such as beta-diketones, alpha-hydroxycarboxylic acids, beta-hydroxycarboxylic acids, or diols is described for anticorrosive agents in U.S. Pat. No. 6,033,677 to Cabane et al. Neither of these references define the need for cerium to be in the tetravalent oxidation state to achieve anticorrosive effects.

The following U.S. patents and published applications provide further examples of corrosion-inhibiting seals from metallic surfaces: U.S. Pat. No. 6,248,184 B1 to Dull et al.; U.S. Application Publication No. 2002/0003093 A1 by Dull et al.; U.S. Application Publication No. 2003/0019391 A1 by Kendig; U.S. Application Publication No. 2003/0024432 A1 by Chung et al.; U.S. Application Publication No. 2002/0033208 A1 to Krishnaswamy, Jr.; U.S. Pat. No. 6,451,443 B1 to Daech; and U.S. Pat. No. 6,299,983 B1 to Van Alsten. However, none of these references teach the need for at least one rare earth element to be in the tetravalent oxidation state.

Accordingly, the need remains for improved rinses and seals which have an effectiveness, ease of application, and performance comparable to coatings formed with hexavalent chromium and which do so without the use of toxic or currently regulated materials.

SUMMARY OF THE INVENTION

This need is met by the present invention which represents a significant improvement in the formulation of non-toxic rinses and seals through the use of tetravalent cerium, praseodymium, or terbium. Although the present invention is not limited to specific advantages or functionality, it is noted that the rinses and seals of the present invention inhibit corrosion to a higher degree than any other known cerium-based coating. Moreover, the rinses and seals of the present invention inhibit corrosion to a degree comparable to commercial formulations based on hexavalent chromium. As used herein, the term “sealing bath” includes both sealing baths and rinsing baths and the term “seal” includes both seals and rinses.

The present invention utilizes “valency stabilization” of the tetravalent cerium, praseodymium, or terbium ion in the as-formed coating to achieve corrosion resistance that is comparable to hexavalent chromium. More specifically, in order to achieve a high degree of corrosion resistance, a rinse or seal can result in a coating that exhibits the following characteristics:

• 1) The coating can contain an oxidizing species. The coatings that are subjected to rinsing and sealing (e.g., phosphate, anodic, or black oxide) do not contain oxidizing species. Therefore, the sealing or rinsing solution must supply these oxidizing species. Oxidizing species serve two important functions within the coating: a) they act to impede the flow of charged species through the coating, therefore helping reduce the transport of corrosion reactants, and b) if a scratch is formed in the coating, these oxidizing species act to “repair” the breach by oxidizing the underlying metal and quickly reforming an oxide barrier. The effectiveness of the oxidizing species is a function of its individual oxidation-reduction potential and the more highly oxidized species exhibit greater corrosion protection. An oxidation-reduction potential of approximately +0.80 V (at a pH of 0) appears to be the dividing line between inhibitors that offer some corrosion protection and those that do not. The tetravalent cerium ion, with an oxidation-reduction potential of +1.72 V (at a pH of 0), is an exceptionally good oxidizing species. Tetravalent praseodymium and terbium are even stronger oxidizing agents, with reported oxidation-reduction potentials of +3.2 V at a pH of 0. The hydroxyl and oxygen liberated from water when tetravalent cerium, praseodymium, or terbium is reduced will oxidize (“passivate”) nearby bare metal.
• 2) A “valence stabilizer” for the tetravalent cerium, praseodymium, or terbium can be employed to ensure that the ion will not be reduced quickly to the trivalent state in solution or in the coating. The importance of stabilizing the cerium, praseodymium, or terbium ion in its tetravalent state in a solid precipitate is important to the composition of rinsing and sealing formulations.
• 3) The tetravalent cerium, praseodymium, or terbium species formed in the coating (e.g., in the pores) must be present as a “sparingly soluble” material. If the formed tetravalent cerium, praseodymium, or terbium species is too soluble, then it will be washed away. If it is too insoluble, then insufficient tetravalent cerium, praseodymium, or terbium is available to inhibit corrosion. A tetravalent cerium, praseodymium, or terbium species that exhibits low solubility will not only fail to inhibit corrosion, but can also promote localized crevice corrosion and result in enhanced corrosion rates. In order to form an effective rinse or seal, the tetravalent cerium, praseodymium, or terbium compounds formed in the coating pores must be in a “sparingly soluble” form. It is difficult to place specific solubility values to these optimum “sparingly soluble” coating materials because there appear to be several variables associated with what makes an optimum coating material. If the tetravalent cerium, praseodymium, or terbium is incorporated in the coating in the form of a tetravalent cerium, praseodymium, or terbium/valence stabilizer complex which exhibits a solubility in water of between about 5×10 ⁻⁵ and about 5×10 ⁻² moles per liter of tetravalent cerium, praseodymium, or terbium, then appreciable corrosion inhibition will be observed. Coatings that incorporate stabilized tetravalent cerium, praseodymium, or terbium compounds that fall outside of this particular solubility range may also exhibit some corrosion inhibition. For example, compositions with solubilities as high as 5×10 ⁻¹ moles per liter or as low as 1×10 ⁻⁵ moles per liter of tetravalent cerium, praseodymium, or terbium exhibit some corrosion resistance, although not as great as those compounds which fall within the optimum solubility range. The degree of effectiveness will depend on the particular compound itself. The solubility characteristics of the tetravalent cerium, praseodymium, or terbium in the pores of the coating can be controlled through the use of stabilizer materials, which form compounds that fall within a desired solubility range. In this way, a “controlled release” of tetravalent cerium, praseodymium, or terbium can be achieved, much as a “timed release” of hexavalent chromium is achieved in the state-of-the-art systems.
• 4) The “valence stabilization” helps to establish an electrostatic barrier layer around the cation-stabilizer complex in aqueous solution. The nature and character of the electrostatic double-layer surrounding the cation-stabilizer complex may be controlled and modified by careful selection of stabilizer species. Characteristics such as the electrical dipole moment and the shape/conformation (for steric effects) of the stabilizer will influence the performance of the formed inhibitor species. In general, the electrostatic double layer formed acts to protect the cation from premature reaction with hydronium, hydroxide, and other ions in solution. The formation of electrostatic barrier layers also helps to impede the passage of corrosive ions through the coating to which the rinse or seal composition was applied, to the metallic surface.

This phenomenon is exhibited in some of the hexavalent chromium systems. For example, in rinses for phosphate coatings wherein some ferricyanide is added to the hexavalent chromium, the highly charged hexavalent chromium ion is surrounded by very polar ferricyanide ions in the as-formed complexes within the pores. The orientation of the dipoles of the ferricyanide ions with respect to the highly charged chromate ion serves to attract additional layers of ions in the aqueous solution. These ions form a protective shell around the chromium ion/ferricyanide complex.

• 5) The as-formed tetravalent cerium, praseodymium, or terbium/valence stabilizer complex may also exhibit ion exchange behavior towards alkali species. This optional consideration is important because alkali ions (especially sodium) are notoriously corrosive towards alloys which contain metals such as aluminum, zinc, or magnesium. The hexavalent chromium-ferricyanide complex formed in some rinse formulations also exhibits this ion exchange phenomenon. The corrosion resistance of a number of phosphated steel and anodized aluminum alloys as tested using both ASTM B-117 and ASTM G-85 has been enhanced through the use of tetravalent cerium, praseodymium, or terbium species. Their corrosion resistance is comparable to that of hexavalent chromium systems.

The valence stabilizers can be inorganic or organic. A multitude of organic and inorganic stabilizer materials have been used.

In one aspect, the invention comprises a mechanistic and chemical approach to the production of corrosion-resistant rinses and seals using tetravalent cerium, praseodymium, or terbium. This approach uses stabilizer materials which form compounds with tetravalent cerium, praseodymium, or terbium within the as-formed coating that are sparingly soluble in aqueous solution, typically around approximately 5×10 ⁻² to 5×10 ⁻⁵ moles/liter of tetravalent cerium, praseodymium, or terbium. This solubility range provides a release of tetravalent cerium, praseodymium, or terbium from the coating at a rate sufficiently slow enough that protection will be provided for an extended period of time and fast enough to inhibit corrosion during conventional accelerated corrosion testing methods such as ASTM B-117 and G-85.

Compounds that fall slightly outside of this solubility range (as high as 5×10 ⁻¹ to as low as 1×10 ⁻⁵ moles/liter of tetravalent cerium, praseodymium, or terbium) may also prove to be effective under certain conditions. However, formed compounds that exhibit aqueous solubilities far outside of the target range are unlikely to be effective corrosion inhibitors. The solubility of the formed tetravalent cerium, praseodymium, or terbium compounds within the pores therefore plays a significant role in the effectiveness of the formed coating. Solubility control may be achieved using organic or inorganic stabilizer materials.

In another aspect, the invention is the achievement of corrosion-resistant coatings derived from rinses and seals using tetravalent cerium, praseodymium, or terbium. This approach also utilizes stabilizer materials, which form compounds that exhibit dipoles so as to form electrostatic barrier layers composed of ions, such as hydronium (H ₃ O ⁺ ) or hydroxide (OH ⁻ ). The formation of these barrier layers through the use of stabilizer materials can be achieved using organic or inorganic materials.

In an optional aspect, the invention is the achievement of corrosion-resistant coatings derived from rinses and seals based on tetravalent cerium, praseodymium, or terbium by the use of stabilizer materials which form compounds that exhibit ion exchange behavior towards alkali ions. The formation of this ion exchange behavior can be achieved through the use of inorganic or organic stabilizer materials.

In another aspect, the invention is the achievement of corrosion-resistant coatings based on rinses or seals containing tetravalent cerium, praseodymium, or terbium that also uses optional preparative agents in conjunction with the cerium, praseodymium, or terbium to strip off some of the already-formed barrier film in the vicinity of the pores. The typical preparative agents for use with tetravalent cerium, praseodymium, or terbium are fluorides and fluorine-containing chemicals. Acidic species or other halides such as chlorides, bromides, and iodides can be used, but are less effective than fluorides as preparative agents.

Accordingly, it is an object of the present invention to provide non-toxic rinses and seals based on tetravalent cerium, praseodymium, or terbium and methods of making and using the same. These and other objects and advantages of the present invention will be more fully understood from the following detailed description of the invention. It is noted that the scope of the claims is defined by the recitations therein and not by the specific discussion of features and advantages set forth in the present description.

DETAILED DESCRIPTION OF THE INVENTION

A) Starting Materials

Five general starting materials are used for the rinse and sealing baths of the present invention. These include: a cerium, praseodymium, or terbium source; a valence stabilizer source; an oxidation source (optional if tetravalent cerium, praseodymium, or terbium is already present in the rinse or sealing bath); a preparative agent source (optional); and additional solubility control agents (optional). These materials may be included as neat compounds in the rinse and sealing baths, or may be added to the baths as already-prepared solutions. Likewise, all of the described constituents do not necessarily have to be included within one solution, and in some instances (e.g., additional solubility control agents) it is typical that these constituents are used separately. Further enhancements to the formed coating may be imparted through the use of additional starting materials. Foremost among these are agents to improve the lubricity or color-fastness of the coating.

1) Cerium, Praseodymium, or Terbium Source

a) Cerium Source

The cerium precursor compounds can be almost any cerium compound in which the cerium is in either the trivalent or tetravalent oxidation state. Water-soluble precursors are typically used. Examples of inorganic trivalent cerium (“cerous”) precursor compounds include, but are not restricted to: cerous nitrate, cerous sulfate, cerous perchlorate, cerous chloride, cerous fluoride, cerous bromide, cerous iodide, cerous bromate, and complex fluorides such as cerous fluosilicate, cerous fluotitanate, cerous fluozirconate, cerous fluoborate, and cerous fluoaluminate. Organometallic trivalent cerium precursor compounds include, but are not limited to, cerous formate, cerous acetate, cerous propionate, cerous butyrate, cerous glycolate, cerous lactate, cerous sulfonate, cerous alkylsulfonate, cerous alkoxysulfonate, cerous aromatic sulfonate, cerous aromatoxy sulfonate, cerous sulfamate, cerous alkyl phosphates, and cerous acetylacetonate. Complex trivalent cerium precursor compounds include, but are not limited to, ammonium cerous sulfate, ammonium cerous nitrate, ammonium cerous oxalate, magnesium cerous nitrate, magnesium cerous sulfate, alkali cerous nitrate, and alkali cerous sulfate.

The cerium precursor may also be a compound in which the cerium is already in the tetravalent (“ceric”) oxidation state. Examples of these compounds include, but are not restricted to: ceric chloride, ceric fluoride, ceric perchlorate, ceric sulfate, ceric nitrate, ceric acetate, ceric propionate, ceric butyrate, ammonium ceric nitrate, ammonium ceric sulfate, magnesium ceric nitrate, magnesium ceric sulfate, alkali ceric nitrate, and alkali ceric sulfate.

Insoluble trivalent or tetravalent cerium compounds may be acceptable in some coating solutions, particularly if acids are used as the preparative agent. Examples of insoluble trivalent cerium compounds include cerous carbonate, cerous phosphate, cerous sulfide, cerous fluorocarbonate, cerous benzoate, cerous oxalate, cerous malonate, cerous tartrate, cerous malate, cerous citrate, cerous thiocyanate, cerous salicylate, cerous oxide, and cerous hydroxide. Examples of insoluble tetravalent cerium precursors are ceric hydroxide species (i.e., ceric hydroxysulfate, ceric hydroxychloride, ceric hydroxynitrate, ceric hydroxyphosphate, ceric hydroxyperchlorate, and ceric hydroxyacetate) with a hydroxide content of 50% or less.

It may not be necessary to add a separate cerium source for these conversion coating solutions if a cerium-containing alloy is to be treated. The preparative agent contained within these conversion coating formulations can dissolve some of the cerium in the substrate. This will result in trivalent cerium ions being present in the coating solution. A suitable oxidizer can then oxidize the trivalent cerium to the tetravalent oxidation state during or after coating deposition.

b) Praseodymium Source

The tetravalent praseodymium ion (Pr ⁺⁴ ) is an even better oxidizing species than Ce ⁺⁴ . It has a radius of 0.085 nanometers, carries a charge of ⁺⁴ , and has a redox potential of approximately +3.2 V. However, it has a correspondingly lower stability both in and out of solution. Therefore, valence stabilization of this ion is needed in order to use it effectively in a conversion coating. The very large redox potential of Pr ⁺⁴ makes it prone to rapid reduction, and few materials will effectively valence stabilize Pr ⁺⁴ in a sparingly soluble complex, which make its routine application problematic. Tetravalent praseodymium can be made using chemical or electrolytic oxidation, as can trivalent praseodymium.

Praseodymium precursors can be nearly any water soluble praseodymium compound in which the praseodymium has a trivalent or tetravalent oxidation state. Water-soluble precursors are typically used. Inorganic praseodymium precursor compounds include, but are not limited to, praseodymium nitrate, praseodymium sulfate, praseodymium perchlorate, praseodymium chloride, praseodymium fluoride, praseodymium bromide, praseodymium iodide, praseodymium bromate, and complex fluorides such as praseodymium fluosilicate, praseodymium fluotitanate, praseodymium fluozirconate, praseodymium fluoborate, and praseodymium fluoaluminate. Organometallic praseodymium precursor compounds include, but are not limited to, praseodymium formate, praseodymium acetate, praseodymium propionate, praseodymium lactate, praseodymium benzenesulfonate, and praseodymium acetylacetonate. Complex praseodymium precursor compounds include, but are not limited to, ammonium praseodymium sulfate, ammonium praseodymium nitrate, magnesium praseodymium nitrate, magnesium praseodymium sulfate, alkali praseodymium nitrate, and alkali praseodymium sulfate.

c) Terbium Source

The tetravalent terbium ion (Tb ⁺⁴ ) is an even better oxidizing species than Ce ⁺⁴ . It has a radius of 0.076 nanometers, carries a charge of ⁺⁴ , and has a redox potential of approximately +3.2 V. However, it has a correspondingly lower stability both in and out of solution. Therefore, valence stabilization of this ion is needed in order to use it effectively in a conversion coating. The very large redox potential of Tb ⁺⁴ makes it prone to rapid reduction, and few materials will effectively valence stabilize Tb ⁺⁴ in a sparingly soluble complex, which make its routine application problematic. Tetravalent terbium can be made using chemical or electrolytic oxidation, as can trivalent terbium.

Terbium precursors can be nearly any water soluble terbium compound in which the terbium has a trivalent or tetravalent oxidation state. Water-soluble precursors are typically used. Inorganic terbium precursor compounds include, but are not limited to, terbium nitrate, terbium sulfate, terbium perchlorate, terbium chloride, terbium fluoride, terbium bromide, terbium iodide, terbium bromate, and complex fluorides such as terbium fluosilicate, terbium fluotitanate, terbium fluozirconate, terbium fluoborate, and terbium fluoaluminate. Organometallic terbium precursor compounds include, but are not limited to, terbium formate, terbium acetate, terbium propionate, terbium lactate, terbium benzenesulfonate, and terbium acetylacetonate. Complex terbium precursor compounds include, but are not limited to, ammonium terbium sulfate, ammonium terbium nitrate, magnesium terbium nitrate, magnesium terbium sulfate, alkali terbium nitrate, and alkali terbium sulfate.

d) Mixed Cerium, Praseodymium, and Terbium Sources

It is also possible to use mixtures of cerium, praseodymium, and/or terbium sources as feedstock for material preparation. Inclusion of other rare earths (such as yttrium, lanthanum, or neodymium) that cannot be oxidized to the tetravalent state is also permissible. Additionally, minerals that serve as ores for rare earths are ideal source materials for this application. For example, sulfuric acid is often applied to rare earth ores to separate the rare earth mixtures (REM) from native rock. If these sulfuric acid extracts were in turn to be supplied with oxidizers and valence stabilizers, source material for this application is achieved. Examples of rare earth-containing minerals suitable for this application are bastnaesite [(REM)CO ₃ F], monazite [(REM)PO ₄ ], xenotime [(REM)PO ₄ ], loparite [(REM,Na,Ca)(Ti,Nb)O ₃ ], lanthanite [(REM) ₂ (CO ₃ ) ₃ ], rhabdophane [(REM)PO ₄ ], fergusonite [(REM)NbO ₄ ], cebaite [Ba ₃ (REM) ₂ (CO ₃ ) ₅ F ₂ ], aeschynite [(Ca,REM)(Ti,Nb)(O,OH) ₆ ], lucasite [(REM)Ti ₂ (O,OH) ₆ ], stillwellite [(REM,Ca)BSiO ₅ ], samarskite [(REM,Fe) ₃ (Nb,Ta,Ti) ₅ O ₁₆ ], parisite [Ca(REM) ₂ (CO ₃ ) ₃ F ₂ ], gadolinite [Be ₂ Fe(REM) ₂ Si ₂ O ₁₀ ], fluocerite [(REM)F ₃ ], cerianite [(REM)O _2-3 ], churchite [(REM)PO ₄ ], or combinations thereof.

2) Valence Stabilizers

Corrosion resistance comparable to that of hexavalent chromium can be achieved by the use of valence stabilized tetravalent cerium, praseodymium, or terbium ions in the rinse or sealing baths. Valence stabilization has not been recognized previously as an important consideration in the development of effective corrosion-inhibiting rinses and seals. A variety of inorganic and organic stabilizers are available that can control such properties as solubility, mobility, ion exchange, and binder compatibility. The stabilizer complex can also act as an ion-exchange host and/or trap for alkali or halide ions in solution.

Cerium, praseodymium, and/or terbium are effective as oxidative corrosion inhibitors if they can be supplied in sufficient quantities in the tetravalent charge state when brought into contact with unprotected bare metal. The Ce ⁺⁴ ionic radius of 87 picometers is larger than the 44 picometers of the hexavalent chromium ion, and so it will have a correspondingly lower charge density (electrostatic field) per ion. The Pr ⁺⁴ and Tb ⁺⁴ ionic radii of 85 and 76 picometers, respectively, are comparable in size. As noted in the Summary of the Invention, the valence stabilizer may serve one or more important functions in the establishment of a successful rinse or sealing solution. First, the valence stabilizer, when used with tetravalent cerium, praseodymium, or terbium, results in a “sparingly soluble” Ce ⁺⁴ -, Pr ⁺⁴ -, or Tb ⁺⁴ -valence stabilizer complex. Although the exact solubility of this complex can be slightly modified through the incorporation of different cations or anions (either through the dissolution of the coated metal, or by additional solubility control agents), appreciable corrosion inhibition will be observed if the tetravalent cerium, praseodymium, or terbium is incorporated in the coating enhanced via rinsing or sealing as a Ce ⁺⁴ -, Pr ⁺⁴ -, or Tb ⁺⁴ -stabilizer complex that exhibits a solubility in water of between about 5×10 ⁻⁵ moles per liter and about 5×10 ⁻² moles per liter of available Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ . Therefore, any material (inorganic or organic) in the coating bath that complexes with tetravalent cerium, praseodymium, or terbium and results in the formation of a Ce ⁺⁴ -, Pr ⁺⁴ -, or Tb ⁺⁴ -containing complex, which exhibits solubilities within or near this solubility range, can serve as a valence stabilizer for tetravalent cerium, praseodymium, or terbium.

Rinse or sealing solutions that contain valence stabilizers that result in the formation of stabilized cerium, praseodymium, or terbium compounds that fall outside of this particular solubility range may exhibit some degree of corrosion inhibition and may be effective under certain circumstances. Although not as effective as those compounds within the optimum solubility range, compositions with solubilities as high as about 5×10 ⁻¹ moles per liter or as low as about 1×10 ⁻⁵ moles per liter of tetravalent cerium, praseodymium, or terbium at standard temperature and pressure (about 25° C. and about 760 Torr) exhibited some corrosion resistance. For example, in situations where the substrate metal pieces are exposed to environments which require much more immediate corrosion exposure (e.g., sudden immersion in seawater), adequate corrosion protection can be achieved through the formation of a tetravalent cerium, praseodymium, or terbium compound which exhibits a higher solubility in water (e.g., 5×10 ⁻¹ to 5×10 ⁻³ moles/liter tetravalent cerium, praseodymium, or terbium). In this way, a more “immediate” release of protective cerium, praseodymium, or terbium ions can be achieved, although the tetravalent cerium, praseodymium, or terbium will be depleted faster from the coating. Tetravalent cerium, praseodymium, or terbium solubilities that are lower than this optimum range (e.g., 1×10 ⁻⁵ to 1×10 ⁻³ moles/liter of tetravalent cerium, praseodymium, or terbium) may be desirable for some situations (e.g., in nearly pure water with low aeration rates). However, compounds that exhibit solubilities far outside the target range are unlikely to be effective corrosion inhibitors.

The solubility characteristics of the tetravalent cerium, praseodymium, or terbium in the rinsed or sealed coatings are controlled with stabilizer materials that form compounds within the desired solubility range. The exact solubility will be strongly dependent on the application of the rinse or sealing solutions, the nature of the barrier film being treated, and the net aqueous solubility of the overlying paints and coatings.

The formation of coatings with the proper release rate of Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ ions is problematic because of the instability of Ce ⁺⁴ and especially Pr ⁺⁴ or Tb ⁺⁴ out of solution. Tetravalent cerium compounds such as acetate, sulfate, ammonium ceric nitrate, and ammonium ceric sulfate are generally too soluble to serve as effective corrosion inhibitors if formed from a rinse or seal solution. Oxides and hydroxides of Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ are much too insoluble in water to serve effectively as corrosion inhibitors in a coating. For example, ceric oxide (CeO ₂ ) is so insoluble that its solubility has never been accurately determined. The more soluble “hydrated” ceric oxide (ceric hydroxide—Ce(OH) ₄ ) is reported to exhibit a solubility product in water between 4.2×10 ⁻⁵¹ and 1.5×10 ⁻⁵¹ , resulting in a cerium solubility of approximately 5×10 ⁻¹² moles/liter Ce ⁺⁴ (see Tarayan, V. M. and Eliazyan, L. A., Izvest. Akad. Nauk Armyan. S. S. R., Ser. Khim. Nauk 10: 189-93, (1957) in General and Physical Chemistry , vol. 2, col. 9722 (1958) (Abstract)). Similarly, tetravalent praseodymium oxide (Pr ₆ O ₁₁ ) is reported to exhibit solubility in water of 6.5×10 ⁻⁷ moles/liter Pr ⁺⁴ (see Busch, W., Z. anorg. allgem. Chem . 161: 161-79 (1927) in Chemical Abstracts , vol. 21, p. 2412 (Abstract)). For these low solubility compounds, the release rates of Ce ⁺⁴ or Pr ⁺⁴ are too low to compare adequately to Cr ⁺⁶ from the state-of-the-art coatings.

One method of providing a useful source of tetravalent cerium, praseodymium, or terbium at a metal surface is the creation of a sparingly soluble compound in which the Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ is shielded from premature reduction during and after compound formation during the rinsing or sealing treatments. The assembly of a protective shell around the highly charged Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ and its associated oxygen and hydroxyl species can help control the rate at which the cerium, praseodymium, or terbium is reduced and its oxygen is released. Proper selection of materials for forming the protective shell will allow solubility tailoring of the entire assembly to its intended application environment. Valence stabilizers are materials that, when assembled, modify the rate of reduction and solubility of the Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ ion.

The electrostatic character of the complex should also be considered in order to create a Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ stabilizer complex with optimal corrosion resistance. Valence stabilizers may also contribute to the development of a substantial electrostatic double layer. An electrostatic double layer of polar or charged species such as hydronium (H ₃ O ⁺ ) or hydroxide (OH ⁻ ) ions surrounding the stabilized cerium, praseodymium, or terbium complex will help control cerium, praseodymium, or terbium reduction and solubility and enhance the barrier properties of the treated coating. Valence stabilizers which form sparingly soluble cerium, praseodymium, or terbium complexes with enhanced electrostatic double layers will maximize the corrosion-inhibiting character of the rinsed or sealed coating.

The tetravalent cerium, praseodymium, or terbium ions are larger than the hexavalent chromium ion, with less charge density over the surface of the ions. Therefore, the valence stabilizers for Ce ⁺⁴ , Pr ⁺⁴ , or Tb ⁺⁴ must be more efficient in the establishment of dipole moments than the valence stabilizers typically used for hexavalent chromium so that comparable corrosion resistance can be achieved in relation to the state-of-the-art Cr ⁺⁶ compositions. Valence stabilizers which have a comparable dipole moment to the Cr ⁺⁶ stabilizers, or which exhibit even less of a dipole moment than the Cr ⁺⁶ stabilizers can also function as valence stabilizers, but the resultant corrosion resistance of the treated coatings will, in all probability, be less than for the current commercial hexavalent chromium-based rinses and seals.

Large spheres of hydration around corrosion inhibitors can act as electrostatic and physical barriers to the passage of large corrosive ions such as Cl ⁻ and SO ₄ ²⁻ through the coating to the metal surface. The size of the electrostatic double layer is a function of the electrostatic potential at the complex surface and is inversely proportional to the ionic strength of the surrounding solution. Compounds that can carry a charge, have a natural electrostatic dipole, or can have a dipole induced, will likely form an electrostatic double layer in aqueous solution. However, these compounds do not normally act as corrosion inhibitors because they have not been optimized for that purpose.

These facts are relevant when tetravalent cerium's propensity for attracting hydroxide species such as OH ⁻ in solution is considered. While a tetravalent cerium ion surrounded solely by OH ⁻ (i.e., Ce(OH) ₄ ) may have a slight degree of aqueous solubility, the much lower charge density (electrostatic field) that is exhibited by Ce ⁺⁴ (coupled with the muting effect of the surrounding OH ⁻ ions) implies that the electrostatic double layer formed around this assemblage will be small. If fewer hydroxide species surround the tetravalent cerium ion (i.e., Ce(OH) ₂ ²⁺ or Ce(OH) ³⁺ ), the electrostatic double layer around these ionic assemblages is increased, which will result in increased corrosion protection. Tetravalent cerium surrounded by no hydroxide species offers the highest degree of corrosion protection.

A simple laboratory experiment confirms this effect. If tetravalent cerium hydroxide (Ce(OH) ₄ ) is placed into deionized water of pH 7, only a minor pH change will be observed, implying that the ionic attraction of this species for hydronium or hydroxide species is minimal. However, if icosahedral Ce(NO ₃ ) ₆ ²⁻ (note that this ion contains no hydroxide) is placed into deionized water of pH 7, a quite remarkable pH drop to −1 can be observed. The released tetravalent cerium ions will scavenge virtually all of the available OH ⁻ ions in solution (possibly even degrading H ₂ O itself to obtain OH ³¹ ), resulting in this dramatic pH drop.

These factors account for the lower corrosion performance of the hydrous oxides and hydroxides formed in many of the prior art references. Because the electrostatic double layers of hydrated cerium oxides and hydroxides are so small, their ability to impede the progress of corroding species is very low, even in the event that a minor concentration of these complexes become soluble. Unlike other known corrosion-resistant compounds described in the art, which extol the formation of hydrous cerium oxides and hydroxides, this invention recognizes that these species result in lower corrosion performance in side-by-side tests. In fact, any oxo- or hydroxo-coordination greater than 50% on the tetravalent cerium ions (i.e., greater than Ce(OH) ₂ ²⁺ or CeO ²⁺ ) is objectionable. It is also for this reason that this invention does not promote the use of hydroxide or oxide precursors as cerium sources.

Optionally, the incorporation of the valence stabilizer (inorganic or organic) may result in the formation of a Ce ⁺⁴ -, Pr ⁺⁴ -, or Tb ⁺⁴ -valence stabilizer compound that also exhibits ion exchange behavior towards alkali ions. As noted in the Summary of the Invention, this is not a requirement of the Ce ⁺⁴ -, Pr ⁺⁴ -, or Tb ⁺⁴ -valence stabilizer complex, but it is a desirable characteristic for enhanced corrosion resistance. Some existing state-of-the-art chromium systems exhibit this phenomenon, but complexes derived from rinse or sealing solutions that do not exhibit this phenomenon have been successfully demonstrated to inhibit corrosive attack.

Rare earth coordination chemistry, which has been the subject of numerous scientific studies f