G.P. Whitelaw Pty Ltd.

ABN 35 005 462 546

Chemical & Metal Finishing       Consultant               

115 Esplanade East

PORT MELBOURNE 3207

                                                                                                Phone 613 9646 4846

                                                                                                Mobile 0438 717 868

                                                                                                October 25, 2003

 

 

Standard Chrome Bath Control

 

 

A standard chrome bath is composed of

 

Chromic acid               180 to 350 g/l

Sulphuric acid             1.8 to 3.0 g/l

Ratio CrO3 : SO4               80 to 120

Temperature               35º to 45º 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)

 

Chromic Acid

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 Baume’ reading with a hydrometer and then just maintain at this reading with regular small additions of chromic acid.

 

Sulphuric 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.

 

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)

 

As sulphate will always increase due to drag in of acid dips and nickel solution regular small additions of barium carbonate will always be required.

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.