CHROMIUM(VI)-FREE COATING AGENT FOR METALS

Abstract

The invention relates to a coating agent for coating acid-sensitive metal surfaces, which is an aqueous solution of an acidic phosphate binder, the coating agent comprising a peroxo compound and being free of chromium (VI) salts. The invention further relates to a method of coating a metal surface with such a coating agent, and to the use of such a coating agent for increasing the corrosion resistance of metal components in various industrial fields.

Claims

1. A coating agent for coating acid-sensitive metal surfaces, which is an aqueous solution of an acidic phosphate binder, wherein the coating agent comprises a peroxo compound and is free of chromium (VI) salts.

2. The coating agent according to claim 1, wherein the peroxo compound is a peroxo salt, a peroxo acid or the salt of a peroxo acid, and/or that the concentration of the peroxo compound in the coating agent is at least 0.1 wt. %.

3. The coating agent according to claim 1, wherein the concentration of phosphate salts in the coating agent is greater than 5 wt %.

4. The coating agent according to claim 1, wherein the pH of the coating agent is from pH 1.5 to pH 4.0.

5. The coating agent according to claim 1, wherein the phosphate binder comprises acidic monohydrogen or dihydrogen phosphates of one or more cations selected from the group consisting of aluminum, magnesium, chromium(III) and/or zinc.

6. The coating agent according to claim 1, wherein the coating agent is free of volatile organic solvents.

7. The coating agent according to claim 1, wherein the coating agent is a slurry suspension further comprising a metal powder suspended in the aqueous solution of the acid phosphate binder.

8. The coating agent according to claim 1, wherein the coating agent further comprises an alkyne or cycloalkyne, the concentration of the alkynes or cycloalkynes being at least 0.01 wt. %.

9. The coating agent according to claim 1, wherein the coating agent further comprises a reducing agent.

10. The coating agent according to claim 1, wherein the coating agent further comprises a ceramic pigment and/or dispersed PTFE particles.

11. The coating agent according to claim 1, wherein the coating agent is free of nitro compounds, and of highly oxidized complex anions.

12. A method of coating a metal surface with a coating agent according to claim 1.

13. The method according to claim 12, wherein the coating agent is applied to the metal surface by spraying, brushing or pouring.

14. The method according to claim 13, wherein the coating agent is applied two or more times after intermediate drying in between each application.

15. A method for increasing the corrosion resistance of metallic components for the aviation industry, the energy industry, the automotive industry, the oil industry, the metal processing industry or the maritime industry, said method comprising applying the coating agent of claim 1 to a metal surface.

16. The coating agent according to claim 1, wherein the peroxo compound is a perborate, a perphosphate or a percarbonate, hydrogen peroxide or an organic peroxo compound.

17. The coating agent according to claim 1, wherein the peroxo compound has a concentration in the coating agent of between 0.5 wt. % and 10 wt.

18. The coating agent according to claim 1 wherein the concentration of phosphate salts in the coating agent is greater than 15 wt. %.

19. The coating agent according to claim 1, wherein the pH of the coating agent is from pH 2.0 to pH 2.7.

20. The coating agent according to claim 1, wherein the coating agent further comprises an alkyne or cycloalkyne, the concentration of the alkynes or cycloalkynes being between 0.05 wt. % and 0.3 wt. %.

Description

[0041] Further details and advantages of the invention will become apparent from the examples described in the following, with reference to the figures. The figures show:

[0042] FIGS. 1a-1b: pictures of phosphate-based base coatings with and without peroxo compound after application;

[0043] FIGS. 2a-2b: pictures of the coatings of FIGS. 1a-1b during water exposure;

[0044] FIGS. 3a-3b: pictures of the coatings of FIGS. 2a-2b after water exposure;

[0045] FIGS. 4a-4b: pictures of the coatings of FIGS. 3a-3b after mechanical abrasion;

[0046] FIGS. 5a-5b: pictures of phosphate-based aluminum slurries with and without peroxo compound after application;

[0047] FIGS. 6a-6b: pictures of the coatings of FIGS. 5a-5b after baking at 350 C.; and

[0048] FIGS. 7a-7b: pictures of the coatings of FIGS. 6a-6b after conductive blasting.

EXAMPLE 1

[0049] Chromium(VI)-free phosphate-based base coating agents with identical compositions except for an addition of the peroxo compound were applied to a mild steel surface, directly or by spraying, and dried.

[0050] Both compositions are aqueous solutions of 25 wt. % of chromium(VI)-free acid phosphate salts with a pH of about 2.5. The inventive variant comprises 2.5 wt. % of disodium peroxide in the solution.

[0051] The results are shown in FIGS. 1a (without peroxo compound) and 1b (inhibited with peroxo compound, according to the invention). The appearance of the non-inhibited coating is dull and yellow-green. The surface is relatively rough. The appearance of the inventively inhibited coating is glossy, dark green and smooth.

[0052] FIGS. 2a (without peroxo compound) and 2b (inhibited with peroxo compound, according to the invention) illustrate the behavior of the coatings upon contact with water. While the non-inhibited coating clearly changes appearance and is penetrated by water, there are no visible changes for the inhibited coating.

[0053] FIGS. 3a (without peroxo compound) and 3b (inhibited with peroxo compound, according to the invention) show the coatings after water contact. While the non-inhibited coating shows a water spot, there are no visible changes for the inhibited coating.

[0054] Lastly, FIGS. 4a (without peroxo compound) and 4b (inhibited with peroxo compound, according to the invention) illustrate an abrasion behavior after water contact, in the form of the coatings of FIGS. 3a and 3b after mechanical abrasion treatment. The non-inhibited system was severely damaged, there are no visible changes for the inhibited coating.

EXAMPLE 2

[0055] Chromium(VI)-free phosphate-based aluminum slurries with identical compositions except for an addition of the peroxo compound were applied to a mild steel surface and dried.

[0056] The slurries were obtained by mixing 55 wt % of the non-inhibited or inhibited base coating agents of Example 1 and 45 wt % of metallic aluminum powder.

[0057] The results are shown in FIGS. 5a (without peroxo compound) and 5b (inhibited with peroxo compound, according to the invention). The appearance of the non-inhibited coating is rough, porous and discolored, with visible bubbles and pores on the surface. The appearance of the inventively inhibited coating is compact, flat and homogeneous. The film thickness of the non-inhibited coating is 74 m, as opposed to 45 m for the inhibited coating, indicating the difference in porosity and compactness.

[0058] FIGS. 6a (without peroxo compound) and 6b (inhibited with peroxo compound, according to the invention) illustrate the behavior of the coated surface after baking at 350 C. While the damage to the non-inhibited coating already described in connection with FIG. 5a is emphasized, the inhibited system remains compact, flat and homogeneous.

[0059] FIGS. 7a (without peroxo compound) and 7b (inhibited with peroxo compound, according to the invention) show the coatings shown in FIGS. 6a and 6b, respectively, after conductive blasting with corundum. While the non-inhibited system was severely damaged and ablated, the inhibited system remained intact. The measured electrical resistivity of the coating in the case of FIG. 7a is greater than 12, and in the case of FIG. 7b is only 0.5.