Stains on black anodized components

Question: We are a metalworking company and have recently been experiencing increasing problems with anodized components that we have processed externally. These are components made of the alloy AlMg4.5Mn (cast aluminum). According to the anodizer, the parts CNC-milled by us are coated and dyed using the direct current sulphuric acid process (GS process) in accordance with DIN 17611. The error (photos are available to the editors, but may not be printed) is shown by white spots and light areas, although the surfaces should be a deep black.

As we have been having these and similar items anodized by this supplier for a long time and have always been satisfied, we would like to continue our cooperation. Unfortunately, however, the number of complaints is increasing: Customers are sending us back parts that were coated six months ago.

The coater simply refers to standards and claims that the problem is with the base material - without giving any further explanation - although the alloy has been the same for years. As no changes have been made to the mechanical processing, nor, according to our supplier, has anything been changed in the coating process, we are faced with a conundrum and do not know how we can solve the problem.

Answer: First of all, we notice that there seems to be a communication problem between you and the anodizer. From experience, coaters understand the term "base material" differently from foundries or mechanical companies. Whereas you mean the alloy, the surface coater generally understands the overall condition on delivery or before coating. It includes the alloy, the mechanical processing and everything else that can have an influence on the surface condition.

In many cases, the coater cannot say in advance whether a material is coatable or not, as the effects take place in a microscopic area, if at all. In the course of processing or after coating, he can only examine the results and draw conclusions based on years of experience or only make assumptions if he does not have an appropriate laboratory.

The assertion that it is due to the base material is unsatisfactory for all sides, but will be based on the experience and observations of the coater described above.

Assessment of the defect pattern

In the following, we will attempt to describe and classify the defect pattern in more detail using the photos.

Due to several characteristics, the photos do not show any typical anodizing defects. This can be seen in the following observations:

The defects generally occur at the high AND low current density areas.

• The edges and the center of the surface are conspicuous here, where the defect runs mainly, but not exclusively, in the machining track.
• In the uncovered holes, the color appears to be i. O. in the uncovered holes.
• On the reworked surfaces, such as the countersink in the drill holes, the surface is significantly better than on the "rough" surfaces.

Assumption as to the cause

Typically, such defects can occur if there are fluctuations in the material quality (alloy), even if the fluctuation is still within the norm. These changes can already have an effect on the mechanical processing or the result of the surface. Examples would be indentations, overlaps or smearing of precipitated alloy components. During anodizing, there is a lack of sufficiently thick or sufficiently large pores in such areas for the dye to be absorbed. Simply anodizing for longer or working with higher electrical voltages is not necessarily the solution, as this can lead to other problems. A typical example of this is a higher level of re-dissolution in otherwise flawless areas and problems with dimensional accuracy.

Although the alloy itself can be anodized, it is not particularly suitable for decorative requirements. This means that such alloys can exhibit increased problems even with minor fluctuations in the overall process (alloy, casting, mechanics, anodization).

Black anodized components. The coating ensures a uniform, smooth black appearance as well as durability, corrosion resistance and aesthetics - (Photo: https://at-machining.com)

A pre-treatment problem?

The question of which influences and problems need to be compensated for by the coater and to what extent is as old as coating technology itself. Normally, the general terms and conditions of the coater indicate the condition in which components must be delivered so that they are "ready for coating". The only problem is that there are numerous possible causes of defects that cannot be identified in advance by the coater. For example, smearing caused by certain alloy components, transparent corrosion protection agents that cannot be sufficiently removed during pre-treatment, clogged pores, overlaps, surface compaction or an inadequate microstructure, to name just a few.

In individual cases, changes to the pre-treatment can be helpful. These can be reflected in higher times, concentrations, temperatures or other chemicals. However, in addition to higher costs, there is a risk that other components that were OK before the modification will suffer damage. This can even affect the same parts, but in different places. Or, due to further process fluctuations, a future batch, so that the modifications have to be reversed.

As every coating company has experienced such and similar problems several times, some tend to name a "basic material problem" without in-depth analysis. Although this does not get you any further, it should at least give you a certain understanding of the "other side".

Possible approach

There are three options here:

1. Sampling with different surface processing parameters: To achieve comparability, you should process a few parts as before and vary the milling parameters for others. Parameters that do not exert too much pressure and temperature on the surface tend to be better, i.e. slower processes. It would also be very good to try blasting the parts after normal processing to achieve a different surface condition. The electroplating shop should then coat the sample parts in one batch if possible and keep all parameters as they are. The number of sample parts therefore depends on the batch size of the anodizer.
2. Sampling in another electroplating shop: Sometimes it is helpful to have the parts anodized in another store to get another comparison. If the result is positive, you would have an alternative option to remain able to deliver until the problems have been solved. If it is negative, it will help you to better assess the situation.
3. Investigations in an institute: The general problem with surface problems is that we only look at the surface. We don't know how the material is actually composed, nor do we know what the structure looks like. Such questions can be clarified by carrying out tests in an institute. If possible, three batches should be examined in the original state and three batches in the non-original state. O. should be examined, preferably material before and after anodizing. A material analysis should be carried out on the raw parts. In addition, micrographs should be taken at the critical points before and after anodizing.

In terms of costs, this procedure involves a certain risk, but if nothing comes of point 1 in particular, the procedure is important in order to get to the bottom of the problem.

You mentioned that a complaint can also occur after six months. If there is a theoretical possibility that the process could lead to a recall at some point, for example because the corrosion protection at the points in question is too low, the costs for an examination at an institute would be negligible.

In addition, as mentioned, the alloy is not ideal for decorative anodization. By carrying out more detailed tests, you have a much better case to make to your customer, possibly motivating them to use a different alloy.

Further considerations

It should also be investigated whether there is a correlation between storage times and n. i. O. parts. For example, that the bad parts appear more frequently when they are sent directly for anodizing and not stored for a few days. There are positive and negative examples of both in history. Under favorable storage conditions, a longer dwell time between processing and coating has a more positive effect. In poor storage conditions, on the other hand, corrosion conditions can make coating more difficult or simply impossible. Although this is not common with aluminum due to the natural protective layer, it is not impossible.

Provided there are no problems with the dimensions, anodized parts can also be anodized a second time. The aluminum oxide layer can normally be easily stripped and the surface recoated. However, a second, successful coating does not necessarily mean that the coating was at fault. It is quite possible that disruptive effects have been etched away by a second treatment.