Salt Precipitation from QPQ processed parts

[TGLewis]

I have a weldment made of A513 Tube. The part is finished with the QPQ process. The final assembly has this part placed inside another tube of 4140 material and enclosed with plastic caps to keep moisture out. Recently we are finding a noticeable amount of wet salt deposits on the inside of the QPQ Tube. We clean it out and after a few weeks it comes back. Any ideas why our parts "sweat" salt?

 

[TVP]

The outer portion of the white layer (also called compound layer) is porous and can trap various chemicals, liquids, etc. This portion should be removed prior to the oxidation step (final salt bath that turns the color black). Nitromet has some good technical information on their website including cross section images that should show the porosity.