Question: We have steel parts coated externally on the frame with zinc-nickel. The base material is 1.0503 in accordance with EN 10277-2. The coating is to be carried out in accordance with DBL 8451.76. The electroplating company we have commissioned coats the parts in an alkaline bath and repeatedly delivers defective parts, whereby the defect patterns are quite varied*.
Flaking of the coating occurs most frequently. These appear on edges, borders and preferably on mechanically processed areas. Bubbles are not visible. There is often roughness on the same surfaces; these areas usually have a matt, slightly darker surface.
Some parts have an unsightly or non-uniform appearance. This can be seen in black spots and "clouds of dots", with the latter affecting around a quarter of the surface.
There is now the following conflict between the contract electroplating company and us: The electroplating company claims that all of the defects mentioned come from the base material. Our position is that it is an ordinary steel that has been manufactured in accordance with the drawings and can therefore be coated perfectly.
Answer: The case you describe is classic in terms of determining the cause of the defect. The central question is how many deviations from perfect condition an electroplating system should be able to compensate for. Experience shows that an electroplating shop has already done a great deal before blaming the problem on the base material, whereby it often turns out that the electroplating shop and the customer already define the "base material" differently.
What is the base material?
The customer understands it to be what is specified in the standard. If this is fulfilled, everything is fine in their opinion. The point of view is also reinforced if the parts have all the required properties according to the drawing.
Electroplating, on the other hand, usually refers to the initial condition before electroplating. It is the overall surface, i.e. material composition, hardness, dimensions if applicable, but above all the condition of the surface. This often involves properties that are not explicitly defined on the drawing. This will be discussed in more detail below.
Flaking layer
Alkaline ZnNi processes in particular have a problem with the spalling process with some types of steel (whereby not only the alloy, but also the machining and degree of hardness must be taken into account here). This can lead to very different problems. Spalling, blistering or a non-existent coating thickness because the deposition breaks off after just a few nanometers. If these parts are coated several times, subsequent defects occur. Overpickling, general passivity of the surface, high roughness etc. can be the result.
The general challenge lies in the coordination of material quality, processing, pre-treatment and coating. Fluctuating steel quality has a particularly strong effect on surface treatment and the subsequent coating, especially impurities and higher carbon content. Numerous companies have been reporting fluctuating material qualities to us for several months. This affects almost all metals and alloys.
The flaking of the coating on the mechanically processed areas is very noticeable here. As these are not blisters, overlaps can be ruled out. It is much more likely that the surface is highly compacted, which leads to passivities. The rough surface you describe is the same effect, only in the low current density range. The roughness looks like micro flaking.
Spots and clouds
The black spots are in the low current density range and can have two causes:
- There is no layer here
- lack of additives in the electrolyte.
The clouds shown, on the other hand, look like parts that have already been pickled and recoated several times. The surface has become passive at these points and the result is a rather amorphous layer that does not adhere properly.
General recommendation
In relation to your problem, we at the publishing house have the same problem as you and contract electroplating: a lack of information. Our general recommendation would be to have the defective parts examined by an institute first. The coating thickness (and its distribution) and the surface conditions can be examined well in a cross-section. The chemical composition of the surface can also be determined on this occasion. This can be helpful, for example in the case of local accumulations. A microhardness measurement may also help. Without more detailed investigations, you remain in speculative territory and cannot resolve the current stalemate.
It is then necessary to assess whether the initial condition can be improved for electroplating. For example, better material quality and/or other parameters for mechanical processing. If this is not the case, we recommend pre-nickel plating, preferably in two stages:
- Ni-Strike (0.2-0.5 µm)
- Matt nickel (0.5-2.0 µm).
Then, if possible wet-on-wet, coat ZnNi. If this is not possible in one system, the Ni layer must be activated before further coating.
The most important thing is that all information and measures are coordinated between you and the electroplating company.
* = Images are available to the editors, but may not be published
