There are three hexavalent chromic acid bath that is popular for hard chrome.
The conventional or single catalyst bath comprises of chromic acid and sulphuric acid as sulphate source. In this conventional bath the most commonly used strength is 247.17 gm/L (33 oz/gal) by weight of chromic acid and sulphuric acid is added to yield a chrome-to-sulphate ratio of 100 to 1 by weight at 2.47 gm/L (0.33 oz/gal). However, many shops operate around these parameters; some lower and some higher (187.25 gm/L (25 oz/gal) to 374.5 gm/L (50 oz/gal) but maintain 100 to 1 ratio for the sulphate ratio or 80:1 at the low end and 120:1 at the high end).
The mixed-catalyst bath contains chromic acid, sulphuric acid, and hydrofluosilic acid or fluoride salts as a fluoride source. The chromic acid strength by weight is 194.74 gm/L (26 oz./gal) and the ratio for sulphuric acid is 130:1 b.wt. 1.5 gm/L (0.20 oz/gal) with the hydrofluosilic acid b.wt. 0.82 gm/L (0.11 oz/gal) or fluoride salts. This bath has advantages over the conventional bath such as; good coverage and activation, better throwing power, high efficiency of 20% plus, faster deposition rate, better corrosion resistance due to more favourable microcrak structure, more tolerance to current interruptions, inexpensive bath, and easy chemical analysis for in-house labs. However, it has one drawback and that is the fluoride attacks areas of the workpiece or racks, which can be easily overcome by masking.
The third bath is the proprietary, high-speed, non-etch bath, which comprises of chromic acid, inexpensive sulphate source such as sulphuric acid or sodium sulphate, and potassium/sodium salt methane disulfonic acid. The two catalyst salts are; sodium and potassium salts of methane disulfonic acid. At present, Aqua Bond has stock of MDSA - Na Salt for the marketplace.
