Halogen-Free Trivalent Chromium Conversion Coating Process
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Solution Overview
Problem
Existing trivalent chromium conversion coatings for metal substrates contain high levels of halogens, which can negatively impact the corrosion life of the underlying metal substrate, posing environmental and operational hazards.
Innovation Solution
A process for forming halogen-free trivalent chromium conversion coatings on metal substrates using a solution comprising soluble trivalent chromium compounds and non-halogenated ligand compounds like zirconium or hafnium salts, with controlled pH and temperature, to produce a coating with minimal halogen content (1 atom % maximum).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If trivalent chromium conversion coatings are formed using halogenated bath solutions, then the coating formation process is effective, but the halogen content in the coating reaches 4 to 6 atomic %, which negatively impacts corrosion life
Solution Approach 1:
The patent removes the harmful halogen component from the bath solution composition. By using non-halogenated activators (such as zinc, nickel, or copper salts) instead of halogenated compounds, the coating formation process eliminates the source of halogen contamination, achieving halogen-free coatings with 0 atomic % halogen while maintaining effective conversion coating formation
Solution Approach 2:
The patent changes the chemical composition parameters of the bath solution by replacing halogenated activators with non-halogenated metal salts. This parameter change in the bath chemistry directly results in a corresponding change in the coating composition, reducing halogen content from 4-6 atomic % to 0 atomic %, thereby improving corrosion resistance without sacrificing coating effectiveness
2Reliability
If hexavalent chromium is used in conversion coatings, then corrosion inhibition is achieved, but hazardous working conditions and high waste disposal costs occur
Solution Approach 1:
The patent converts the harmful hexavalent chromium into beneficial trivalent chromium. By using trivalent chromium compounds in the bath solution, the coating maintains effective corrosion inhibition properties while eliminating the environmental hazards associated with hexavalent chromium. The trivalent chromium forms a protective coating that provides corrosion resistance without the toxicity and waste disposal problems of hexavalent chromium
Solution Approach 2:
The patent changes the oxidation state parameter of chromium from +6 (hexavalent) to +3 (trivalent). This parameter change fundamentally alters the properties of the coating, maintaining corrosion protection while eliminating environmental hazards. The trivalent chromium forms a stable, non-toxic coating that provides the same protective function without the harmful environmental effects of hexavalent chromium
3Ease of manufacture
If halogenated activators are used in trivalent chromium processes, then conversion coating formation is effective, but the resulting coating contains 4 to 6 atomic % halogen which affects corrosion life
Solution Approach 1:
The patent extracts and removes the halogenated activator component from the bath solution formulation. By replacing halogenated compounds with non-halogenated metal salts (zinc, nickel, or copper), the process maintains effective conversion coating formation while eliminating the source of halogen contamination that would otherwise be incorporated into the coating at 4-6 atomic %
Solution Approach 2:
The patent introduces non-halogenated metal salts as intermediary activators that mediate the conversion coating formation process. These intermediary substances (zinc, nickel, or copper salts) perform the same catalytic function as halogenated activators but without introducing harmful halogen atoms into the coating, thus maintaining manufacturing effectiveness while improving corrosion resistance
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 resulting halogen-free trivalent chromium coatings exhibit superior corrosion resistance and environmental acceptability compared to previous coatings with higher halogen content.
Implementation Method 1
Conversion coatings are applied through chemical reactions between the metal and the bath solution which converts or modifies the metal surface into a thin film with required functional properties
Implementation Method 2
The resulting halogen-free trivalent chromium coatings exhibit superior corrosion resistance and environmental acceptability compared to previous coatings with higher halogen content
Data Source
AI summary
A process is provided for forming a non-halogen containing trivalent chromium conversion coating on a metal substrate comprising the steps of: (a) preparing a conversion coating solution comprising from 1 to 3 wt% soluble trivalent chromium compound, and from 1 to 3 wt% of a non-halogenated ligand compound of a metal selected from the group consisting of zirconium, titanium, hafnium, and mixtures thereof; (b) adjusting the pH of the conversion coating solution so that it lies in the range of 1.5 to 4.5; (c) controlling the temperature of the conversion coating solution to a temperature in the range of 15 to 95°C; and (d) contacting a metal substrate with the conversion coating solution to form a non-halogen containing trivalent chromium conversion coating on the substrate.