Etching of ABS for Electroplating with chromic-sulphuric acid: disposal, alternatives, operational issues

A. Hi. Etching solution is indeed a very strong mixture of chromic and sulphuric acid. The chrome must be reduced from Cr+6 to Cr+3 by the addition of sodium metabisulfite or sulfur dioxide at a somewhat acidic pH. Too low a pH and you'll release too much sulfurous gasses; too high and the reaction will be slow or non-existent. You might try adjusting the pH up to about 3.5 to 5.0 and add the metabisulfite.

Chromic acid (Cr+6) has a very strong amber color whereas Cr+3 has a faint greenish color, so you may be able to use color as well as ORP (oxidation reduction potential) readings to know when all of the chromium has been reduced. After that, it's a matter of neutralization with an alkali like magnesium hydroxide or lime to a good pH for precipitation.

It should go without saying that this procedure should be done on a beaker sample under a lab fume hood several times until you are completely confident of the amounts of reagents to add, and whether excessive fumes are given off, before you consider treating a production size batch. Here in the USA only chemically knowledgeable people who have received hands-on haz-mat training would attempt neutralization of etching waste. I'm not sure what the regulations are in Maharashtra. Good luck.

Regards,

A. Some large shops use gaseous etching for ABS, Rahul, but it's a substantial capital investment.

Please consider getting a proprietary electroless nickel process from one of the suppliers who have invested countless R&D hours in the effort. Reliable plating on plastic from a home-brewed electroless nickel formulation is a challenge I wouldn't want to undertake, thanks :-)

These days direct metallization (with graphite or organo-metallic proprietaries) is also gaining popularity over electroless processes for metallizing plastic. Good luck.