I am plating decorative chrome on leaded steel. I sporadically have pitting on individual parts on a single rack, both in low- and high-current density areas. I have tried to reduce the pitting by using a number of different cleaning routines to no avail. Adding brightener to my nickel tank reduces my pitting, but I am still not able to eliminate it completely. N. P.
Your problem can have a number of sources. First thing you should do is to try to determine at which step the pits are first appearing. The source of the pits could be in the mechanical steps in the finishing of the leaded steel parts, the cleaning process steps or the plating process itself. The first thing I would do is to inspect the parts as they come to you from the machining area. If the surface areas appear to be pit free, the next step is to inspect the parts after the cleaning process. Both of these inspections steps should be performed using not just your eyeball but also a magnifying lens. If your parts seem to pass these inspections then the culprit is probably the plating process.
You mention in your question that you seem to have this pitting problem on a specific rack. This is a clue that perhaps you have inadequate contact between the rack and the bus bar. Pitting in nickel plating is often caused by poor electrical contact. Other causes of pitting in nickel plating are: high iron concentration, low boric acid concentration, low anti-pitting agent concentration and sometimes, organic impurities.
Keep in mind that leaded steels present a special problem because of the small amount of lead that is present in the alloy to aid machinability. During the machining steps, this lead is smeared over the surface and must be removed prior to the plating steps. Steels of this type must be prepared somewhat differently than typical “run-of-the-mill” steels. You do not mention what your exact cleaning process is but one of the steps used classically has used fluoboric acid or a fluoborate salt for removing the small amount of lead on the surface. An alternate process is to use citric-acid-based solutions for cleaning of leaded steels.