Common Defects: Smut

Smut is often seen on color work because it is easy to spot. Sometimes it presents itself as a powdery residue and other times as a black part with an iridescent green or blue cast. The color of the residue after wiping on a tissue can be a good clue as to the source, however it may be prudent to use is an isolation strategy to pinpoint where a process change needs to be made. The source of smut can usually be tracked back to either of the following process steps.

Seal

This is usually the source. Ensure the pH isn’t too high. Hydrothermal sealing naturally causes smut, so proprietary chemicals are used for control. These chemicals do decompose over time, particularly when exposed to high temperature surfaces, so seals have a naturally life expectancy. The use of filtration combined with replenishment will extend the life, but there will come a time when it is time to dump. This could be days for a hot water seal, weeks for a hot nickel acetate seal, months for a mid-temperature surfactant based seal, or years for a cold seal.

Dye Bath

If the dye is new and the smut is isolated to this step, it may be a sign that the dye is overly active and a reduction in temperature may help to promote adsorption more deeply into the pores. It is more likely however that the dye bath has aged. Try dye bath filtration, buffer, and pH adjustment. A related “dye smut” problem is when isolated spots of dye particle precipitation occur. These particles may be prevalent on edges or machined or knurled surfaces and they can be easily wiped off the surface. This defect is typically related to chloride content in the dye bath. Sometimes buffering can help.

E-Color

Grey smut may be due to over-plating. Cut back on time, voltage, and modulation. Anodize conditions may need adjustment to increase the thickness and/or porosity of the coating. Easy-to-remove yellow or white smut isolated to this tank is likely due to fall-out of tin salts on to the part surface. Better filtration may be the only course of action.

Anodize

Soft coatings are prone to produce smut, particularly on copper containing alloys. Critically evaluate the anodize temperature and current density. The latter may be easier to do in retrospect by asking the line man how long it took to anodize the part, measuring the coating thickness, and applying the 720 rule. It shouldn’t take 50 minutes to get a 0.25 mil coating!

De-Smut

Surprisingly, parts that are not well de-smutted can carry this smut all the way down the line. 6063 T52 is a good example. That tenacious silica rich residue may appear to be gone, but appearances can be deceiving. Check the temperature (> 65°F) and chemistry. Make sure it is well agitated. You can boost the effectiveness with additives like nitrates, persulfates and bifluorides.

To our best knowledge, the information and recommendations contained herein are accurate and reliable. However, this information and our recommendations are furnished without warranty, representation, inducement or license of any kind, including, but not limited to the implied warranties of merchantability and fitness for a particular use or purpose. Customers are encouraged to conduct their own tests and to read the product labels carefully before using them. Furthermore, the customer assumes sole liability for any patent infringement that occurs by reason of their following our recommendations or using the information given.

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