Trivalent Chromium Coating Oxidation Suppression
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Solution Overview
Problem
Conventional trivalent chromium chemical conversion coating films on zinc or zinc alloys suffer from oxidation to hexavalent chromium over time, leading to environmental and health concerns, and existing methods fail to provide sufficient corrosion resistance and wastewater treatment efficiency.
Innovation Solution
A trivalent chromium chemical conversion coating film with a specific composition that suppresses oxidation from trivalent chromium to hexavalent chromium, using a treatment liquid with controlled trivalent chromium, cobalt, and hexavalent chromium generation suppressing agents, ensuring high corrosion resistance and minimal hexavalent chromium elution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick trivalent chromium chemical conversion coating film is formed by carrying out the treatment at an elevated temperature in high concentration of chromium and organic acid, then good corrosion resistance is obtained, but the concentration of chromium and organic acid in the treatment bath becomes high, causing difficulty in wastewater treatment
Solution Approach 1:
The invention changes the parameters of the treatment bath by reducing chromium concentration to 0.1-10 g/L and organic acid concentration to 0.1-5 g/L, while adjusting treatment temperature to 20-80°C and treatment time to 1-60 minutes. These parameter changes enable formation of a coating film with sufficient corrosion resistance (96 hours or more in salt spray test) without creating high concentration waste that is difficult to treat
Solution Approach 2:
The invention creates a coating film that copies the corrosion resistance function of conventional thick high-concentration coatings, but achieves this through a different mechanism: by controlling the oxidation suppression function and using low concentrations of chromium and organic acid, the invention produces a coating film with equivalent or superior corrosion resistance (96+ hours in salt spray test) compared to conventional methods
2Object-affected harmful factors
If a trivalent chromium chemical conversion coating film is used as an alternative to hexavalent chromium, then environmental and health safety is improved, but trivalent chromium in the coating film is oxidized to hexavalent chromium when left for a long period in natural environment
Solution Approach 1:
The invention applies preliminary anti-action by incorporating oxidation suppressors (vitamin C, citric acid, tannic acid, or phosphoric acid) into the treatment bath before coating formation. These substances pre-establish a protective chemical environment that prevents oxidation of Cr³⁺ to Cr⁶⁺, achieving hexavalent chromium content of less than 0.01 μg/cm³ in the coating film and elution of less than 0.05 μg/cm³ after 30 days storage at 80°C and 95% humidity
Solution Approach 2:
The invention uses oxidation suppressors as intermediary substances that mediate between the trivalent chromium and the oxidizing environment. These intermediaries (vitamin C, citric acid, tannic acid, phosphoric acid) chemically interfere with the oxidation process, protecting the Cr³⁺ ions from converting to Cr⁶⁺, thus maintaining the environmental safety of the coating film over time
3Reliability
If hexavalent chromium is used for chemical conversion coating, then sufficient corrosion resistance is achieved, but harmful effects on human bodies and environment occur
Solution Approach 1:
The invention converts the potential harm of chromium by using trivalent chromium (Cr³⁺) instead of hexavalent chromium (Cr⁶⁺). Cr³⁺ is environmentally safe and non-toxic, yet through the addition of oxidation suppressors and optimization of treatment parameters (temperature 20-80°C, time 1-60 minutes), the invention achieves corrosion resistance of 96 hours or more in salt spray test, thus converting a potentially harmful substance into a beneficial protective coating
Solution Approach 2:
The invention changes the oxidation state parameter of chromium from +6 to +3, and adjusts other parameters (chromium concentration to 0.1-10 g/L, organic acid concentration to 0.1-5 g/L, treatment temperature to 20-80°C) to achieve both environmental safety and sufficient corrosion resistance, eliminating the harmful effects associated with conventional hexavalent chromium coatings
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 achieves corrosion resistance equal to or exceeding conventional hexavalent chromate coatings, with negligible hexavalent chromium elution and reduced organic acid and nitrogen concentrations, enhancing wastewater treatment and environmental safety.
Implementation Method 1
a trivalent chromium chemical conversion coating film formed on a surface of zinc or zinc alloy plating
Implementation Method 2
having a function of suppressing oxidation from trivalent chromium in the coating film to hexavalent chromium
Data Source
AI summary
A trivalent-chromium chemical conversion coating from which substantially no hexavalent chromium is released. The trivalent-chromium chemical conversion coating is one formed on the surface of a zinc or zinc-alloy deposit. In a brine spray test, the chemical conversion coating has unsusceptibility to corrosion (time required for white-rust formation) of 96 hours or longer. The chemical conversion coating has a hexavalent-chromium concentration less than 0.01 μg/cm2 in terms of metal atom amount. The amount of hexavalent chromium released after 30-day standing in a thermo-hygrostatic chamber at a temperature of 80° C. and a humidity of 95% (amount of hexavalent chromium released when the coating is immersed in 100° C. water for 10 minutes) is smaller than 0.05 μg/cm2.