CORROSION RESISTANT CHROMIUM FREE CONVERSION COATINGS

Abstract

Process for treating metal such as aluminum and its alloys to prevent corrosion which comprises coating the metal with a chromium-free conversion composition containing effective amounts of metal fluorozirconates and carboxylic metal salts.

Claims

1. A process for treating metal surfaces to protect the metal from corrosion which comprises coating the metal with a chromium-free conversion coating containing from about 4 to 8 grams of a metal fluorozirconate per liter of the chromium-free conversion coating and from about 0.5 to 5.0 grams of at least one carboxylic metal salt per liter of the chromium-free conversion coating.

2. The process of claim 1, wherein at least one of the carboxylic metal salts has an anion or cation different from the other carboxylic metal salts.

3. The process of claim 2, wherein the cation of the carboxylic metal salt is a metal selected from Group II of the Periodic Table.

4. The process of claim 2, wherein the anion of the carboxylic metal salt is selected from the group consisting of oxalate, tartrate, succinate, adipate, phthalate, mellitate and trimellilate.

5. The process of claim 4, wherein one of the carboxylic metal salts is a metal succinate.

6. The process of claim 4, wherein one of the carboxylic metal salts is a metal oxalate.

7. The process of claim 4, wherein one of the carboxylic metal salts is a metal phthalate.

8. The process of claim 4, wherein the metal is selected from the Periodic Table.

9. The process of claim 4, wherein the metal is selected from Group II of the Period Table.

10. The process of claim 1, wherein the carboxylic metal salt is a polycarboxylic metal salt.

11. The process of claim 10, wherein at least one polycarboxylic metal salt has a cation different from the other carboxylic metal salts.

Description

DRAWINGS

[0012] FIGS. 1a to 1e shows 2024-T3 aluminum as treated by Bonderite 5200 (3%) and of PPG X-bond (3% by volume) (1a) commercially available chromium-free conversion coating, a potassium hexafluorozirconate solution (1b) at a concentration of 6 g/L, CFP1 (1c), CFP2 (1d), and CFP3 (1e);

[0013] FIG. 2a shows the commercially available non-chromium conversion coating products, Bonderite 5200 (left and middle) and PPG X Bond (right panel, after 48 hours of salt fog (ASTM B117) exposure;

[0014] FIGS. 2b-e show the potassium hexafluorozirconate solution (2b), CFP1 (2c), CFP2 (2d), and CFP3 (2e) after 48 hours of salt fog (ASTM B117) exposure;

[0015] FIGS. 3a-c show 2024-T3 aluminum as treated by Bonderite 5200 at 3% (3a), CFP1 (3b) a chromium-free conversion coating containing potassium hexafluorozirconate (6 g/L) and zinc oxalate (1.5 g/L) and CFP4 (3c);

[0016] FIG. 4a shows the Bonderite 5200 panels after 24-hour exposure to ASTM B117; and,

[0017] FIGS. 4b and 4c show the CFP1 and CFP4 panels, respectively, after 240 hour (10 days) exposure to ASTM B117.

DESCRIPTION

[0018] This invention is the incorporation of carboxylate metal salts in chromium-free conversion coatings for the purposes of increasing corrosion resistance. The compositions of the inhibitor salts are described as follows. Anions include the polycarboxylates chosen from linear and branched aliphatic molecules like oxalate, citrate, tartrate, succinate, malonate and adipate and the like. Cations include zinc, magnesium, manganese, calcium, strontium, zirconium, scandium, yttrium, lanthanum, and other lanthanides like cerium, praseodymium, neodymium, samarium, europium and gadolinium. The choice of anion and cation will influence water solubility as well as the reactivity with the other chemistries involved in the conversion coating solution and/or the metallic substrate.

[0019] The carboxylate metal salts are added to the chromium-free conversion-free conversion coating in amounts ranging from about 0.5 to 5.0 grams per liter and may be added individually or in combination with other inhibitors. Carboxylate inhibitors may be blended with other carboxylate inhibitors using the same cation. For example, but without limitation, zinc oxalate and zinc malonate may be blended, or they may be blended with different cations with the same or different anions. Another example, but without limitation, cerium oxalate and zinc oxalate or cerium oxalate and zinc malonate may be blended. Carboxylate inhibitors may also be combined with soluble inorganic salts with the same cation, such as zinc oxalate and zinc sulfate or they may be blended with different cations with different anions such as zinc oxalate and lithium phosphate.

[0020] Inhibitors may also be blended with different molar ratios to obtain the maximum synergistic performance. This may range, but without limitation, from relatively low concentrations of a few milligrams per liter to beyond the super saturation point for the carboxylate inhibitor where no more can dissolve in solution.

[0021] The present invention more specifically relates to synergistic metal polycarboxylate combinations and to methods of treating metal to improve the metal's corrosion resistance. The method includes applying to the surface of metal, a chromium-free conversion coating which comprises an effective amount of a synergistic mixture of metal carboxylates. More specifically, but without limitation, a synergistic blend of corrosion inhibitors, consisting of at least two different metal carboxylates, such as polycarboxylics chosen from linear and branched aliphatic molecules like oxalate, succinate, and adipate, and aromatic molecules like phthalate, mellitate and trimellitate and the like. These are specific examples of some molecules. There are many other polycarboxylic acids which can be used for preparing the synergistic combination.

[0022] The cations of the metal carboxylates are identified in the Periodic Table and include, for example, but without limitation, elements chosen from: Group Ia—Lithium, potassium and sodium; Group IIa—Magnesium, calcium, strontium, and barium; Group IIIb—Scandium, yttium, lanthanum and the other lanthanides; Group IVb—Titanium and zirconium; Group Vb—Vanadium and niobium; Group VIb—Chromium and molybdenum; Group VIIb—Manganese; Group VIII—Iron, cobalt and nickel; Group Ib—Copper; Group IIb—Zinc; Group IIIa—Aluminum, and Group Va—Bismuth.

[0023] FIGS. 1a-e show 2024-T3 aluminum as treated by Bonderite 5200 (3%) and of PPG X-bond (3% by volume) and with various additional coatings. FIG. 1a shows the additional coating being a commercially available chromium-free conversion coating, while in FIG. 1b it is a potassium hexafluorozirconate solution at a concentration of 6 g/L., which is used as the base for the following conversion coating with inhibitors. In FIG. 1c, CFP1 is used, which is a subject chromium-free conversion coating consisting of potassium hexafluorozirconate (6 g/L) and a zinc oxalate at a concentration of 1.5 g/L., while in FIG. 1d CFP2 is used. This is another chromium-free conversion coating, which contains potassium hexafluorozirconate (6 g/L) and zinc citrate (1.5 g/L) (the same concentration and cation as CFP1 but with a different carboxylate anion), and CFP3. In FIG. 1e, another chromium-free conversion coating I is used, also containing potassium hexafluorozirconate (6 g/L) and calcium oxalate (1.5 g/L) (the same concentration and carboxylate anion as CFP1 but a different cation).

[0024] For FIG. 1a, the left and middle panels were immersed for 2 minutes in the Bonderite 5200, the left panel had a final rinse after the coating, the middle was dry-in-place. The right panel was immersed in PPG X-Bond for 5 minutes with a final rinse. For FIGS. 1b-1e, panels were immersed for, from left to right, 5, 10, and 15 minutes in their respective conversion coatings. All panels were first cleaned for 10 minutes using an alkaline cleaner (Bonderite C-AK 6849 Aero at 18%) at 140° F., and then chemically deoxidized at room temperature for 1 min. using Bonderite C-1C SmutGo NC at 20%. All conversion coating solutions were at room temperature, approximately 75° Fahrenheit.

[0025] According to this set of neutral salt fog data, it is evident that both the carboxylate anion and cation of the inhibitor contribute to the corrosion protection of the substrate. Further salt fog testing was done using zinc oxalate, FIGS. 3a-c below show 2024-T3 aluminum as treated by Bonderite 5200 at 3% (FIG. 3a), CFP1 (FIG. 3b) a chromium-free conversion coating containing potassium hexafluorozirconate (6 g/L) and zinc oxalate (1.5 g/L), and CFP4 (FIG. 3c), the same as CFP1 but with a different doubled concentration of zinc oxalate (3 g/L). For FIG. 3a, from left to right, panels were immersed for 2, 5 and 10 minutes in the Bonderite 5200. For FIGS. 3b and 3c, the left panels were immersed for 5 minutes in the CFP and the middle and right panels for 10 minutes. All panels were first cleaned for 10 minutes using an alkaline cleaner (Bonderite C-AK 6849 Aero at 18%) at 140° F. and then chemically deoxidized at room temperature for 1 min. using Bonderite C-1C SmutGo NC at 20%. All conversion coating solutions were at room temperature, approximately 75° Fahrenheit.

[0026] This invention is directed to a method of providing chromate-free, conversion coatings when coated onto a metal substrate, such as aluminum or aluminum alloy substrates, said coatings are able to withstand hours of salt spray test without detectable corrosion on the metal. Moreover, the corrosion-inhibiting methods do not cause the health hazards associated with hexavalent chromates coatings.

[0027] While this invention has been described by a number of specific examples, it is obvious that there are other variations and modifications which can be made without departing from the spirit and scope of the invention as particularly set forth in the appended claims.