Chromium-Free Black Conversion Coating on Zinc
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Solution Overview
Problem
Existing methods for protecting zinc-containing surfaces from corrosion, such as chromating and chromium-free conversion layers, either use toxic metals like chromium and cobalt or fail to achieve a high-quality black color impression due to limitations in light absorption across visible wavelengths.
Innovation Solution
Aqueous, acidic treatment solutions containing sulfur compounds, oxo cations, and oxidizing agents are used to produce black-colored conversion layers on zinc and its alloys without chromium and cobalt, utilizing sulfur-containing compounds like thioglycolic acid and zirconoyl sulfate to achieve enhanced corrosion protection and a deep black color.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If chromium(VI) compounds are used for chromating, then corrosion protection is significantly improved, but acute toxicity and carcinogenic potential increase
Solution Approach 1:
The patent removes chromium(VI) compounds from the treatment solution entirely, replacing them with chromium-free alternatives consisting of oxo cations, halogen complex ions, and sulfur compounds. This extraction of the harmful substance while maintaining the protective function resolves the contradiction between corrosion protection and toxicity.
Solution Approach 2:
The patent changes the chemical composition parameters of the treatment solution by substituting chromium(VI) with specific oxo cations (such as ZrO+, HfO+), halogen complex ions, and sulfur-containing compounds. This parameter change allows achieving corrosion protection without the harmful effects of chromium(VI).
2Object-affected harmful factors
If chromium-free conversion layers are used, then toxicity is reduced, but the ability to generate black color impression is lost
Solution Approach 1:
The patent introduces sulfur-containing compounds (such as thioglycolic acid, thiosulfate, or sulfite) as intermediary substances that enable black color formation in chromium-free conversion layers. These sulfur compounds act as mediators that facilitate the formation of black-colored precipitates or complexes on the zinc surface, bridging the gap between chromium-free composition and black color appearance.
Solution Approach 2:
The patent creates a composite conversion layer system combining oxo cations, halogen complex ions, and sulfur-containing compounds. This composite approach allows the layer to exhibit both low toxicity (due to absence of chromium) and black color impression (through sulfur compound-mediated pigment formation).
3Illumination intensity
If cobalt is added to generate black pigments, then black color impression is improved, but health safety deteriorates
Solution Approach 1:
The patent removes cobalt from the treatment solution, eliminating the health safety concerns associated with cobalt exposure. The black color formation is achieved instead through sulfur-containing compounds that form black pigments without requiring toxic cobalt additives.
Solution Approach 2:
The patent replaces expensive and toxic cobalt with cheaper and safer sulfur-containing compounds (such as thioglycolic acid or thiosulfate) that can be used in higher concentrations without health risks. These sulfur compounds serve as disposable, safe alternatives to toxic metal additives for achieving black coloration.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides effective corrosion protection and a high-quality black color impression while being free from toxic metals, with the black conversion layers demonstrating improved resistance to zinc corrosion products, as evidenced by extended service life in neutral salt spray tests.
Implementation Method 1
These are reaction products of the non-precious coating metal or its alloy with the reaction solution that are insoluble in aqueous media over a wide pH range
Implementation Method 2
it is not possible to generate sufficient absorption of the light reflected from the surface across all wavelengths of visible light, which would be perceived as a black color impression
Implementation Method 3
In order to reduce the corrosion of the coating metal or to prevent it for as long as possible, so-called conversion layers are often used especially on cathodically protective base metals such as zinc and its alloys
Data Source
AI summary
Treatment solution for producing black chromium- and cobalt-free conversion layers, comprises: (a) at least one complex, water-soluble metal cation selected from oxo cations of the formula M' cd +>, and/or complex halogen ions of the formula M''X ab ->; (b) at least one oxidant; and (c) at least one organic sulfur compound. Treatment solution for producing black chromium- and cobalt-free conversion layers, comprises: (a) at least one complex, water-soluble metal cation selected from oxo cations of the formula M' cd +>, and/or complex halogen ions of the formula M''X ab ->; (b) at least one oxidant; and (c) at least one organic sulfur compound selected from compounds of the Formula 1: HS-R 1>-COOR 2>, and Formula 2: R 5>OOC-R 3>-S-S-R 4>-COOR 6>. c : an integer from 1-3; d : an integer from 1-3; X : selected from F, Cl, Br and I; a : an integer from 3-6; b : an integer from 1-4; and M' and M'' : selected from Mn, V, Ti, W, Mo, Zr, B, Si and Al. R 1> : selected from 1-8C-alkyl, linear and branched, and aryl; R 2> : selected from H, NH 4 +>, Li +>, Na +>, K +>and 1-4C-alkyl, linear and branched; R 3>and R 4> : selected independently from 1-8C-alkyl, linear and branched and aryl; and R 5>and R 6> : selected independently from H, NH4 4 +>, Li +>, Na +>, and 1-4C-alkyl, linear and branched.