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Hard chromium plating rate is way low

Q. I am a plating processes engineer. Our firm begins to run a hard chromium plating line using standard hard chromium plating solution. Bath temperature is 57 °C. The solution analysis is:
Chromic acid - 250 g/L, Sulfate - 2.63 g/L, Chromic acid/sulfate = 95.
Chromium (+3) 2.5 g/L (1%), Chloride 0 ppm, Fe 119 ppm, Ni 3 ppm, Cu 19 ppm, Al 1 ppm, Pb 22 ppm,
Zn 0 ppm, Co 0.6 ppm, Se 1 ppm.
We plate small shafts of 6 mm diameter using cylindrical conforming anode about 2 cm around the shaft.
Our customer permit to plate at current density of 31-47 A/dm². (2-3 A/in²), so we plate it at 45 A/dm².
According to the book of Mr. Guffie (page 35) ⇨
plating rate should be at 30-35 microns per hour at the same conditions. Canning Handbook [on eBay, Amazon, AbeBooks affil links] (page 463) gives almost the same data (25 microns per hour).
We experience plating rates of 10 microns per hour.
Please advise, what could be the reason for so slow plating rate in our solution and how it could be improved.
Thank you

Alex Sirota Inquirer often helps others, so especially deserves our help
- Lod, Israel
September 8, 2024

A. Alex, check the chrome, sulfate ratio should be 100:1, chromic acid
32 oz/gal sulfate should be 0.32 oz/gal, check trivalent chrome concentration

Popatbhai B. Patel
electroplating consultant - Roseville, Michigan

November 23, 2024