Trivalent Chromium Passivation Layer for Steel Sheet Corrosion Resistance
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Solution Overview
Problem
Existing electrolytic coating processes for steel sheets use chromium(VI)-containing electrolytes, which are environmentally and health-hazardous, necessitating a replacement to meet future regulatory bans and achieve high chromium oxide content for corrosion resistance and adhesion in packaging applications.
Innovation Solution
An electrolytic process using a trivalent chromium compound-based electrolyte solution without chromium(VI), employing a conductivity-increasing salt and acid or base to set pH, and an anode that prevents chromium(VI) formation, resulting in a passivation layer composed predominantly of chromium oxide and/or chromium hydroxide for efficient and environmentally friendly surface passivation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If chromium(VI)-containing electrolytes are used for electrolytic coating, then good adhesion and corrosion resistance are achieved, but environmental and health hazards increase
Solution Approach 1:
The patent changes the chemical composition parameters of the electrolyte by replacing chromium(VI) compounds with chromium(III) compounds. This parameter change eliminates the environmental and health hazards associated with chromium(VI) while maintaining the electrolytic deposition process to achieve chromium oxide passivation layers with good corrosion resistance on steel sheets
Solution Approach 2:
The patent converts the traditionally harmful chromium(VI) electrolyte system into a beneficial chromium(III) system. By using chromium(III) sulfate as the electrolyte, the process eliminates toxicity while still producing chromium oxide passivation layers that provide corrosion protection, thus converting a harmful process into a beneficial one
2Reliability
If chromium(VI)-containing electrolytes are used, then passivation layer with good adhesion is obtained, but regulatory compliance deteriorates
Solution Approach 1:
The patent changes the electrolyte composition from chromium(VI) to chromium(III) compounds, which eliminates regulatory compliance issues while maintaining the ability to produce passivation layers with good adhesion for organic coatings through electrolytic deposition
3Quantity of substance
If conventional electrolytic coating processes are used, then chromium-containing passivation layer is deposited, but chromium oxide content remains limited
Solution Approach 1:
The patent optimizes electrolyte parameters including using chromium(III) sulfate at concentrations of 20-100 g/L, maintaining pH 1-3 with sulfuric acid, and controlling temperature at 20-60°C to achieve high chromium oxide content (over 90%) in the passivation layer through electrolytic deposition
Solution Approach 2:
The patent employs a two-stage electrolytic process: first a chromium(III) sulfate electrolyte stage to deposit chromium oxide, followed by a rinse and optional second electrolyte stage. This periodic action allows control over chromium oxide content and layer composition
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 process achieves a passivation layer with over 90% chromium oxide content, providing superior oxidation resistance and adhesion for organic coatings, while being completely free of chromium(VI), ensuring compliance with environmental and health regulations.
Implementation Method 1
the steel sheet is passed through a single electrolyte solution in a strip coating line at high strip speeds of more than 100 m/min, acting as the cathode
Implementation Method 2
electrolytic deposition of a chromium metal-chromium oxide (Cr-CrOx) layer
Implementation Method 3
This electrolyte solution contains a trivalent chromium compound (Cr(III)), a complexing agent, and a conductivity-enhancing salt
Implementation Method 4
a conductivity-enhancing salt
Data Source
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AI summary
The invention relates to a method for passivating the surface of a black sheet or a tinplate by electrolytic deposition of a chromium oxide-containing passivation layer on the surface, wherein the electrolytic deposition of the chromium-containing passivation layer is carried out from an electrolyte solution (E) which contains a trivalent chromium compound as well as at least one salt to increase conductivity and at least one acid or base to adjust a desired pH value, wherein the electrolyte solution (E) contains no further components other than the trivalent chromium compound as well as the at least one salt and the at least one acid or base and is in particular free of organic complexing agents and free of buffering agents.