There are many research papers and patents on the decorative chromium plating process of trivalent chromium, but due to various reasons, it has not been successfully put into production and application. The real production application was in 1974.
The International Lead and Zinc Research Institute is mainly committed to adding F- to improve low-temperature plating properties, increasing current efficiency, adding sulfide to increase plating speed, and trivalent chromium plating post-treatment when using hypophosphite as a complexing agent. As well as research on trivalent chromium black chromium plating. Regarding adding F- to improve current efficiency, different researchers seem to have different research results. Edigaryan et al. confirmed that adding F- can improve the current efficiency when using oxalic acid as a complexing agent; however, Hwang et al. reached the opposite conclusion when using hypophosphite as a complexing agent for trivalent chromium plating. Adding F- reduced the current efficiency. NH4+; can improve current efficiency. While trivalent chromium decorative chromium plating has made progress, research on trivalent chromium plating to obtain thick chromium has gradually begun, but there has been little progress. The main reasons are as follows:
(1) The increase in the pH value of the plating solution, especially the pH value of the layer near the cathode surface, leads to the formation of Cr(OH)3 colloid, which hinders the trivalent chromium plating layer. Continue to thicken;
(2) The hydrolysis product of Cr3+ undergoes hydroxyl bridge and polymerization reaction, forming a polymer chain condensate that is adsorbed on the cathode, hindering the reduction of Cr3+;
( 3) The enrichment of Cr2+, the intermediate product of Cr3+ reduction, initiates and promotes the Cr3+ hydroxyl bridge reaction;
(4) The gradual reduction and disappearance of Cr3+ active complexes during the continuous electrolysis process.
In view of the above points, if you want to plate a trivalent chromium coating with a thickness exceeding 50um, you must make adjustments in the plating solution and take corresponding measures, such as lowering the pH value of the plating solution and selecting a better buffer. , speed up solution circulation and stirring, increase the concentration of complexing agent to increase the competitiveness of the complexing reaction and the hydroxyl bridge reaction, maintain a sufficient concentration of active complexes, and add special compounds to reduce the enrichment of Cr2+ to form a dinuclear complex. To reduce its initiating and promoting effect on Cr3+ hydroxyl bridge polymerization reaction. In recent years, reports on thick chromium plating with trivalent chromium are all based on the combined effect of the above measures.
Thick chromium coatings of trivalent chromium generally have cracks, but they do not penetrate the substrate. The coating is often mixed with C and other impurity elements, showing an amorphous or microcrystalline structure. The coating generally has good corrosion resistance, with a hardness between 60O-900HV. If heat treated at an appropriate temperature, the hardness can be increased to 1200~1800HV, and the wear resistance is also greatly enhanced, which can better meet the requirements of hard chromium plating. The chromium salt with tanning properties is trivalent chromium salt, represented by basic chromium sulfate, also known as chromium tanning agent. It has good tanning properties and is suitable for tanning all kinds of leather and fur. The finished leather obtained by tanning has excellent physical and mechanical properties and sensory properties, and is the most important tanning agent.
After aging, trivalent basic chromium salts form tannable chromium complexes, and the strength of their tanning properties can be expressed by alkalinity. The so-called alkalinity is the ratio of the total equivalents of hydroxyl groups (OH) in the chromium complex to the total equivalents of chromium expressed as a percentage. A high alkalinity means that the molecules of the chromium complex are large, that is, the binding ability to the skin proteins is strong; a small alkalinity means that the molecules of the chromium complex are small, and the ability to bind to the skin proteins is weak, but the penetration ability is strong. So alkalinity is an important indicator of chrome tanning agents.
The tanning mechanism of chromium tanning agent is that the trivalent basic chromium salt complex combines and cross-links at multiple points with the carboxyl groups on the side chains of skin collagen, which enhances the structural stability of skin collagen. Therefore, chrome tanned leather has a very high shrinkage temperature (generally over 95°C) and is highly resistant to acids, alkali, enzymes and microorganisms. The leather semi-finished product tanned by chrome tanning agent is blue and is called wet blue leather.