Pickling or Solubilizing?

Question:

I am in the business of cleaning metal stampings. Recently an automotive customer developed a problem of heat treating a carbon steel part and created a scale on the surface. Engineering decided to use a vibratory media process to remove the scale (expensive). They knew that “pickling” was an option to remove the scale but decided not to because of hydrogen embrittlement. I use a low concentration of inhibited HCl, which removes scale and oxide from parts every day. I’ve tried to convince them that I am not “pickling,” simply solubilizing the surface contaminants (oxides) without going after the steel itself. Am I correct in my thinking? Do you have any research materials on hydrogen embrittlement to help clarify for me and the automotive gurus? Thank you. B.S.

Answer:

The phenomena of hydrogen embrittlement can be found in many material, metals and corrosion books. The concern is that hydrogen generated from the acid cleaning process tends to adsorb to the surface of the part. Elemental hydrogen is so small that it can then penetrate the metal structure and occupy voids in the metal’s crystal structure. The result is a significant loss of strength of the steel.

Based on this, the customers have a right to be concerned, although your cleaning process may not be damaging their parts since you are using the HCl at a low concentration with an inhibitor. These precautions coupled with minimal exposure time may be enough to prevent it. To determine what effect your cleaning process has on the parts, a tensile test could be performed with and without your cleaning process. If there is no hydrogen embrittlement, there should be no loss in strength. Additionally, an extra step to be on the safe side would be to bake the parts after acid cleaning. You would need to do some experimentation to determine the optimum baking cycle, although you could expect it to be around 300F for 2-24 hours. This baking cycle presumably drives off hydrogen that has been picked up from the acid cleaning step.