Question: In our company, steel parts are first copper-plated and then electropolished. First, a thinner layer is applied in a cyanide copper bath, followed by a thicker, acid copper plating. After electropolishing, we notice holes that were previously unnoticeable. These holes - or pores - have a depth of around 5 µm. These holes occur mainly in the form of nests, which we cannot explain. We try to carry out the entire process with a very high degree of standardization in order to minimize handling errors, but now we are at a loss.
We are not familiar with such a defect pattern from electropolishing, except for pitting in the high current density range in certain situations. Pitting and gas paths are mentioned in the literature [1], which can occur. Based on the described defect, however, we do not consider this to be the case, but we cannot completely rule it out. We believe that there is probably a problem with the copper plating process. The fault can originate from both electrolytes, although we consider the cause from the cyanide bath to be more likely. The idea is that pores are created here that are closed in the acidic electrolyte and opened during electropolishing. Of course, the pores can also form during acid copper plating and be closed again during electroplating. A lack of convection can be a problem in both electrolytes. In the cyanide electrolyte, the metal content, cyanide and hydroxide are primarily responsible. In acidic copper electrolytes, the cause is usually a lack of basic additive. Of course, an interaction is also possible if something is deposited on the surface between the cyanide and acidic copper plating. However, we assume that this would have been found when examining the pores, provided that the coating does not dissolve during the acidic copper plating. The formation of nests is not untypical, depending on the part geometry. You would first have to look at the current density ranges in which the pores are located. If this appears arbitrary, the flow and suspension (frame) should be considered. If it is a so-called electropolishing drum, we would check whether the diameter of such a nest roughly matches the diameter of the perforation. It could also be useful to look at the removal rate in relation to the total layer thickness and the pore depth. This would make it possible to determine in which process step the pore is created, provided it does not come from electropolishing. Is it in the area of the cyanide copper plating? Acid copper or right on top of it? If this does not help, there is still the option of attempting nickel plating. In the practice of electroplating, it has often been shown that a variety of negative influences, for example from the base material, can be avoided. In addition, possible hydrogen diffusion is avoided or at least minimized [2].
Literature
[1] Electropolishing, Eugen G. Leuze Verlag GmbH & Co KG 2nd edition, 2017, ISBN 978-3-87480-298-7
[2] Online course "Electroplating nickel"; https://www.galvanotechnik-for-you.de/uebersicht-kurse/die-galvanische-vernickelung/
