Residual Smut?

Q. We’re having a problem with small cylindrical parts and small amounts of residual “smut” left over after our cleaning at an outside cleaning vendor. In this case the parts are cleaned as follows 1) vapor degrease 2) alkaline clean, 3) citric acid passivation, 4) rinse. The residue is found in the ID of these parts when wiping the ID with a cotton swab. The material is type 304L stainless steel. Do you have any advice on how to eliminate or remove this smut? Parts are sensor housings and must be clean for later electrical processing. T.V.

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A. If the problem or product is new, it would be worth the time to go upstream in the manufacturing process to understand what has changed as much as possible. Sometimes slight changes in the preceding manufacturing processes can have a significant effect in the cleaning output.

Additionally, it would be useful to understand what the composition or source of the residual smut is on the surface. This could be done by SEM with EDS for elemental analysis. Other methods for analyzing the swabs could also be attempted, although the EDS would be most likely to work with what is probably a small quantity of sample.

Once you have determined the source and composition of the residue, you will be better prepared to determine if it can be eliminated with the existing process. If the upstream steps aren’t the cause of your cleaning problem, then you will have to modify the process to produce better results. Mechanical means may be necessary to remove the residual smut. Depending on size and quantity, internal brushing may be effective. If the parts are too small and numerous for this to be practical, it may be possible to use a mechanical finishing media that would be effective at scrubbing the surface. This would be used wet, possibly with a detergent for the best effectiveness.

Finally, as mentioned in another question in this column, ultrasonics may be an effective means for removal of smut from a large quantity of parts. The ultrasonics will add a significant amount of energy to your cleaning process that heat and chemicals alone cannot accomplish. Suppliers for this process can be found on www.pfonline.com.