Solving Corrosion on Black-Dyed 7000 Series Alloy

What caused corrosion on one shop’s black-dyed parts? Anodizing expert Larry Chesterfield offers troubleshooting advice.


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Q. We anodize several different 7000 Series alloys. Recently, on a load of Type II anodized and black-dyed extrusions, some of the parts had small areas of heavy corrosion. This was only observed after the parts had been dyed black and sealed in hot deionized water. As far as we could tell, the parts did not have corrosion spots when they were racked. What might have caused this?

A. The photos you sent show small areas of heavy corrosion having a significant amount of “bloom.” This does not appear to be electrolytic pitting (electrolysis), nor does it look like “blistering” that is sometimes seen on 7000 Series alloys. 

Troubleshooting any defect that occurs in process is made somewhat easier if the tank where the defect first showed up can be pinpointed. If the anodizing line has no inspection aisles at strategic intervals, finding the location where the defect occurred is very difficult. If this is the case, it is sometimes necessary to bring the rinsed load back to the racking area after each process step to inspect for signs of corrosion or any other condition that might be occurring.

After further correspondence, we were able to surmise that the most probable place of occurrence was your black dye tank. There are three possible reasons for this. One, the black dye compound is relatively high in chlorides. As the bath gets older (as opposed to a newly made-up bath) the additions of dye increase the concentration of chlorides in the bath. Two, the use of titanium racks, or partially titanium racks, increases the probability of galvanic corrosion because of dissimilar metals. Three, the 7000 Series alloys (high zinc) can be particularly sensitive to pitting and galvanic corrosion. Edges of the parts are particularly sensitive to corrosion of this type.

You can be the judge of the condition of the bath regarding chlorides and buildup of other contaminants. The best way to reduce the chance of this type of corrosion would be to make up a new dye bath and then check for the presence of chlorides in the water that is used to make up that new bath. If the water is high quality, it will have no chlorides whatsoever. An easy way to do this is to get a sample of the makeup water in a small beaker and add a few drops of silver nitrate. The presence of any white cloudiness (silver chloride) will indicate that there are chlorides present. If chlorides are present in the bath makeup water, this water should not be used. Check the RO and/or DI water equipment and ensure that the outlet water is chloride-free. 

The best rack for black dye will be made entirely of aluminum. It can be a difficult decision whether to use aluminum or titanium, however, because other factors also often come into play.

Q. Is there is a set standard for measuring and matching colors?

A. There has been confusion in the anodizing industry about the efficacy of using devices for measuring and matching the color of anodized finishes ever since machines were first marketed for that use in the 1970s. I have never compared the readings of different machine brands on the same anodized parts; machines of two different brands might not give the same results. I have found, however, and there are others in the industry who agree, that tri-stimulus colorimeters, in general, do not give reliable color value readings for anodized coatings. They are fine for opaque, applied organic coatings (paint), but the metallic, reflective nature of the substrate throws off the color value readings for anodized coatings. This makes it virtually impossible to obtain reliable color readings. This is also true for painted coatings with metallic, or mica, flakes. Even though these machines were advertised to the anodizing industry, there was nothing in the literature that stated they could be used effectively for color-matching anodized aluminum coatings. Colorimeters should not be used to gauge colors for anodized coatings. 

You are not the first to have this problem or ask this question. I believe the use of color range samples and the good old “mark 1 eyeball” under consistent lighting conditions is the best way to evaluate anodized colors, including clear anodized finishes.

Q. Sometimes we have trouble stripping sealed or even unsealed Type III coatings completely off the substrate before parts have to be rerun. If the parts are left in the caustic etch bath for too long, they are often unusable, either because they are undersize or turn out too matte, or both. Occasionally, we find that some of the coating has been stripped and some still remains in patches. Even if these patches can eventually be removed, it takes so long to do so that too much metal is stripped away from those areas that were initially stripped. This leaves high and low areas on the stripped parts and renders them scrap. This usually is only a problem with coatings more than one mil (25 microns) in thickness. Is there an effective technique that can be used to strip the anodic coating from the parts completely on the first try?

A. There is a way to do this that works most of the time, and it’s pretty simple. The hardcoat coating is usually thick and dense, and can be very difficult to remove. If it is sealed, the task of stripping becomes even more difficult. Soak the parts to be stripped in a desmut tank with a strong acid bath for 30-45 minutes. This will break the seal, soften the coating and make it much easier to strip. After the acid soak, rinse the parts and then strip in the etch bath until the coating is completely removed. It helps to have the caustic etch bath at around 140oF (60oC). Lower than 130oF (55oC) usually won’t work. It usually takes some practice in order to refine this technique and determine an appropriate soak time and strip time for different coating thicknesses. Keep at it until you find the most effective times. Hopefully, you will be pleased with the results. 
An anodizing bath may be used in place of the desmut bath for soaking. No matter which bath is used, it could cause a slight disruption in production while the parts soak.  


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