Of all processes used in electroplating Chromium plating is, without a doubt, the one that has changed the least since it entered the industrial practice in 1920. However, there are formulas devised by many researchers in order to improve the operating conditions, or to modify the structure of the chromium deposit. Since a typical chromium bath was too simple to be marketed, everyone wanted to have their own «special bath».
Therefore, to chromic acid were added the most varied products; that led to formulas containing: chromium sulphate, boric acid or borates – acetic acid – hydrofluoric acid, fluoride, chromium sodium fluoride – sodium acetate – sodium bisulphate, various metal salts of iron, copper, cobalt, nickel, aluminium, magnesium, cadmium, zinc, mercury, etc..; not to mention numerous other oxidants, colloidal products, or potassium permanganate, hydrogen peroxide, pepsin, peptone…
Almost all materials, or chemicals, have been tested, whether to improve the penetration power, or to increase the yield or brilliance.
But because we want to stay in the practical field of chromium plating, and because these different additives have not, in fact, produced really tangible results, today the dominant method is the one based on chromic anhydride, or simply «chromic acid» with the addition of 1% in weight of pure sulphuric acid at 66 °B.
The concentration of chromic acid can vary within wide limits, from 250 to about 400 grams per litre and the addition of sulphuric acid will vary respectively from 2.5 to 4 grams per litre. But taking into account the often highly variable qualities of common chromic acid, we recommend reducing the dosage indicated above of sulphuric acid, both at the time of the formation of a new bath, and when adding significant amounts of chromic acid to an already operating bath, in order to take account of the possible presence of an excess of sulphate in the chromic acid used.
When preparing a new Chromium plating bath, some precautions should be taken: chromic acid can be dissolved at the concentration of approximately 30 Kg. every 100 litres of water heated at about 5 °C, both in the same chromium plating tank, and in bare iron containers (well etched) and, when the total amount is dissolved, 1% sulphuric acid is added (preferably pure quality from the lab).
The bath mass must be stirred for a long time to make the chromic-sulphuric acid mixture uniform; this is necessary, in view of the small percentage of sulphuric acid compared to chromic acid.
This operation is too often overlooked. The liquid is simply “cut”, which cannot lead to the smooth and uniform mixing of the two elements; the sulphuric acid may not produce its effect. Hence, the possibility of errors, the most serious of which could be a new addition of sulphuric acid, with the consequent appearance of typical defects due to excess; and this, simply because the bath has not been stirred well at first.
This observation relates to both the first formation of a new chromium plating bath and the subsequent feedings, as regards which, it should be noted that the addition of chromic acid need not necessarily be accompanied by sulphuric acid in a ratio of 1%.
It will also be useful to treat a rather considerable number of objects after the addition of chromic acid, and only when the classics defects due to excess or lack of sulphuric acid appear, an adjustment can be made.
We will resume this topic when dealing with the defects related to chromium plating. For a smooth bath with minimum risk, the ratio of 1% between SO4/CrO3 should be maintained; and if the examination of the deposit or the working conditions of the bath, do not give clear signs proving that this ratio is right, an analysis will be needed to get an exact indication.
In the presence of an excessive quantity of chromate, the normal ratio may be changed by slightly forcing the dose of sulphuric acid; and this, until all the chromates have been eliminated. However, this is a practice to be avoided as far as possible, because, after this elimination, the excess acid will bring consequences, and then it will be necessary either to treat the bath with barite, or to increase the concentration of chromic acid.