Cleaning Q&A: Cleaning Smut from Aluminum Weldments

Q. We are looking for better ways to remove weld-related black deposits (smut, carbon, soot and so on). Spray-on, wash-off chemicals are ineffective. Sand or soda blasting is too abrasive and dirty, and dry ice blasting is not abrasive or fast enough and leaves a white heat-related residual appearance. Our products are typically constructed of welded 6061 aluminum tubing and structural profiles, often restricting access for mechanical brushing, which is slow and results in surface marking. How do other aluminum welders solve this problem when the final product will not be anodized or coated?

A. It sounds like you have tried numerous options to clean the welded aluminum parts. You can expect differing surfaces after welding depending on power, shielding gas, base alloy (and variety within the same alloy), thickness and the person doing the welding. Combine those issues with difficultly accessing all surfaces, and you have a difficult cleaning task. In this case, either a caustic or acid-immersion etch would be optional to remove smut and provide a uniform finish to the part.
A caustic etch can be aggressive in removing smut and base aluminum. Most alloyed aluminum will require a nitric acid step to remove the smut (residual alloying elements) created by the caustic etch with rinsing. This type of treatment will result in a uniform, matte finish on the aluminum. An acid etch of the part will typically consist of a low (about 1 percent) level of hydrofluoric acid in nitric acid (about 20 percent). This can be used at room temperature and will produce a uniform, though shinier, surface than the caustic etch. Because of the pH value, both solutions need to be used with extreme caution and properly disposed.

Originally published in the March 2016 issue.