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Understanding the Voltage current relation in hard chrome plating

September 25, 2013

Q. I am trying to understand the voltage v/s current density (CD) relationship in hard chrome plating tank. The purpose of the study is to find the optimum plating density in hard chrome considering the cost of Electricity, plating time, amortised cost of plant per unit output etc. We have an automatic chrome plating line which handles one size of job all year round. If we increase CD, Output rises as well as the cathode efficiency but at a cost of increased electricity and equipment maintenance costs. Reducing CD decreases efficiency, increases fixed costs but overall running costs are lower. I am trying to find out the lowest cost operating condition for this plant. The workable range of CD is 40 - 60 Amp/sqdm. If I plot the voltage v/s CD graphs over this range, and plot a trend line, the equation shows V = 0.33 x CD + 3.05 volts
My question is : 1) What causes this fixed component of 3 volts in the operations? It means, even at zero CD, one would require 3 volts.
2) Is this (3 volts) a characteristic of the chemistry? or density? or trivalent Cr? and how to lower it? We operate a proprietary bath at 260 gpl chromic acid and Cr+3 below 0.2%

We can change the Anode cathode configurations to change the relation but it would require major overhaul of the plant. Besides, This fixed component of 3 volts is a major cost. Just trying to know if playing with the chemistry can change it to our advantage.

Nitin Kibe
Head (Technical) - Pune, India