Hard chrome plating over Stellite 6 Welding - trivalent contamination, pitting and peel off issues
May 11, 2022
Q. I am SOM from Chennai Tamilnadu. My plating tank is 20,000 liters capacity. It has trivalent contamination of 20 gms/liter and Iron Contamination of 8 gms/liter. The plating solution looks black.
While plating over Stellite 6 welding material and normal alloy steel, I get cluster pinholes. We do plating thickness of 250-300 microns. The chemical looks black.
I have bad adhesion (peel off issues) of hard chrome plating over Stellite 6 welding (cobalt chromium-based material).
I have lots of pinholes, peel off issues.
How to solve this issue and how to reduce trivalent contamination in this 20000-liter tank and recover my chemical?
Due to these issues, I am not able to put any job. Please Help Me.
- Chennai, Tamilnadu, India
May 16, 2022
A. With high Cr3+ and low Cr6+, the electrolyte can become greenish.
I got a black electrolyte when a poor chromium anhydride was purchased, a dark coating was obtained from this electrolyte. Hull cells showed a lack of chromium anhydride, although the result of the analysis showed an excess of the content of chromium anhydride.
Perhaps you can explore your chrome electrolyte using the Hull cell, what does it show?
Good adhesion on alloy is provided by special training. This can be processing on the reverse current > 2 min, with a gradual raising of direct current to the operating values, shot processing, etching in a mixture of acids, preliminary nickeling and other coating).
The use of additives for sulfate electrolyte can increase adhesion and reduce hydrogen implementation, making the coating less fragile and less prone to cracks and exfoliation on large thicknesses.
Using electrolytes with fluoride salts.
Cleaning electrolyte from pollution with trivalent chromium is standard.
It is necessary to conduct the process with the size of the area of the anodes 5-10 times larger than the area of the cathode.
The work must be carried out with periodic mixing of the electrolyte so that ions of trivalent chromium are constantly present in the electrolyte layer near the anodes. The current strength is 10 -30 a/dm2. The duration of work is 24-72 hours (depending on the amount accumulated in the electrolyte of the chrome of trivalent). The effectiveness of the work is controlled according to the testimony of a voltmeter, in the process of work a gradual reduction in the readings of the voltmeter should occur.
Used methods of cleaning electrolyte from iron contained in it.
Dilution of electrolyte by 2 times with water and adding chromium anhydride there (the effectiveness of this method is first checked by the Hull cell).
Adding substances (included in the composition of additives to improve adhesion) that connect iron, copper and others in complexes that are partially deposited to the bottom of the chromium tank, and partially leave them in electrolyte in complexes that do not affect the operation of chrome electrolyte.
The use of special membrane installations that allow you to clean the electrolyte.
- Nizhniy Novgorod, Russia