Problems caused by uneven substrate surface in electroplating processing

The protrusions of the coating are uneven, some are visible to the naked eye and can be felt by hand, while others have no obvious hand feel, and require careful observation with a magnifying glass to distinguish. It is generally best to observe carefully with a magnifying glass of more than 20 times, which makes it easier to accurately identify the phenomenon and the degree of unevenness, facilitate the analysis of different causes, and find corresponding solutions.
1. Problems caused by uneven surface of the substrate
For galvanized parts, except for individual requirements, grinding and polishing treatment is generally not performed before plating. However, galvanizing solution generally only requires fine crystallization and good brightness, and its leveling ability is not very good. Therefore, the final roughness after galvanizing depends on the original condition of the workpiece. Cold rolled sheet parts, cold drawn wire materials, and precision machined workpieces without rust have good flatness after plating; Hot rolled plates, profiles, and severely rusted workpieces, after being pickled to remove oxide skin and rust products, will even expose surface pits, spots, etc. Although they can be polished, they cannot achieve excellent leveling effect. Hard chrome plated parts cannot be expected to be brighter and smoother than before after chrome plating. Generally, the roughness will slightly increase, and there is currently no chrome plating brightener available. The function of chromium plating additives is nothing more than to appropriately improve the deep plating ability and cathode current efficiency, or to produce microcracks.
For decorative electroplating, parts need to undergo polishing treatment before plating (excluding generally high gloss cold rolled thin parts), and the final brightness and flatness depend on two factors: first, the effect of polishing the substrate before plating, and second, the brightness and leveling effect of one-step electroplating solution. Before plating, if the substrate is polished with coarse sand, fine sand, or oil sand according to its condition, and then polished with soft cloth or hemp wheels, and the residual sand path is very fine, it is possible to obtain a high gloss and high leveling appearance. The ungrounded part of a partially polished component maintains its original rough appearance. Improper design of the pre-treatment process before plating may cause damage to the appearance of the polished parts. For example, many low-end electroplating factories do not first perform strong corrosion rust removal on the parts, but directly grind and polish them. After oil removal, strong acid is used in the activation tank to simultaneously remove rust from the ungrinded parts. As a result, the already polished parts are corroded and the surface is coarsened, with residual ash hanging. If directly plated after cleaning, not only will the polishing effect be greatly reduced, but there is also a risk of matrix hydrogen permeation.
When using one-step electroplating, if the maintenance of the bright electroplating itself is poor or the requirements for the mirror surface of the workpiece cannot meet the requirements, the workpiece must be coated with thicker bright copper before polishing the copper layer, and then coated with bright nickel after oil removal. High processing unit prices for automotive aluminum wheels, bumpers, large mud guards for motorcycles, and noise reduction tubes require polishing of copper layers (bright nickel polishing is no longer commonly used). It must be pointed out that even if the plating solution is normal, leveling requires a certain thickness of the coating, and it is unrealistic to achieve a high leveling effect by plating a few microns. On the contrary, if the substrate is well polished before plating, but if the maintenance of the bright plating solution is poor, resulting in unevenness faults, the surface smoothness of the plated parts may be worse than before plating. This is the issue to be discussed in this lecture.