G.P. Whitelaw Pty Ltd.
ABN 35 005 462 546
Mobile 0438 717 868
October 25, 2003
Chromic acid 180 to 350 g/l
Sulphuric acid 1.8 to 3.0 g/l
Ratio CrO3 : SO4 80 to 120
Temperature 35° to 45° C
The actual best ratio will depend on the operating temperature and any other catalysts and contamination in the solution.
The best way of controlling a chrome bath is by observation of the coverage (and any stains or rainbow effect in the low current density area)
Decide on what concentration of chromic acid you wish to run at.
Higher concentrations 250 –300 g/l give more flexibility and wider operating parameters.
Lower concentrations are cheaper to operate, as there is less drag out and therefore easier waste treatment but more sensitive to dragin etc.
There is no right or best concentration.The balance between chrome and sulphate is all that matters
When you have the concentration at the level you want eg 275 g/l take a specific gravity or baumé’ reading with a hydrometer and then just maintain at this reading with regular small additions of chromic acid.
The analysis of sulphuric acid in a chrome bath is often not accurate so should be used as a guide only.
Observation of the plating is the best method of getting the sulphate to the right level.
When you see “nickel” the sulphate is too high and needs to be lowered with
Barium carbonate reacts with the sulphuric acid and precipitates as insoluble Barium sulphate Add slowly and carefully with good stirring. Keep adding daily until you notice some iridescent rainbow stains in the recesses ie the low current density. Then a small addition of sulphuric acid will stop this and you have your bath at the optimum ratio.
Take a sample of the bath and keep. Give a small sample to your supplier to analyze for reference.
100 g of Barium Carbonate will remove 50 g of sulphuric acid (28 ml)
When adding chromic acid don’t add sulphuric acid as well until it is found that iridescence is showing as usually there is an excess of sulphate there already.
If the ratio is already OK then 75-100 ml would be the amount suggested for each 25Kg added to allow for any sulphate in the chromic acid
If you follow the above you will only need the help from a lab. when something unusual occurs.
Many chrome problems arise from the nickel becoming passive when the problem will usually show up as white blotchy stains, etc.