Cleaning Low Carbon Steel

Question:

I have constant velocity joint components from an automotive application that I believe to be a low carbon steel. In some areas they have a 58-62 HRc surface hardness with a 0.040-0.060-inch case depth up to 50 HRc. I wanted to clean them with a 20-30% phosphoric acid, 70-80% water solution warmed to 180F for 10-15 min in order to remove grease and rust. I have run this experiment and made a dimensional, hardness and surface finish comparison. Only the surface finish changed slightly. Are there other things to be concerned about such as hydrogen embrittlement? D.F.

Answer:

For your application, probably the most significant area of concern would be hydrogen embrittlement. There are a few things you can do to minimize this. First, it is much more efficient to add a separate degreasing step to your operations than try to do both in one tank. This has the obvious disadvantage of requiring two extra steps (wash and rinse if aqueous) although it will minimize the time in the acid cleaner for rust removal, minimizing the chance of hydrogen embrittlement.

Depending on the composition and severity of the oil and grease residue, you can use a moderate to highly alkaline aqueous cleaner for the degreasing step. The cleaner will consist of several components. One category is builders that are alkaline salts, which will buffer the liquid and saponify some lubricant residues. Another category is surfactants. These are ingredients that lower the surface tension of the liquid and help to lift and remove the soils on the metal surface. Other ingredients may include inhibitors, such as silicates to minimize base metal attack, as well as sequestrants and dispersants to help suspend contaminants. Other cleaning methods could also do a good job at oil and grease removal, although I suggested aqueous since you will be doing that with the phosphoric acid.

Most aqueous cleaners will require heating, so if you cannot supply heat to the product, an alternate cleaning method is necessary. Solvent cleaning would be another possibility. Something as simple as mineral spirits may provide enough cleaning. If not, there are more powerful solvents available on the market. For suppliers of either aqueous or solvent cleaning chemistries, you can go to www.pfonline.com and click "Cleaning and Pretreatment," then "Cleaning Chemicals, aqueous" and "Solvents, degreasing" on the Suppliers page. Or, you can check the 2001 Products Finishing Directory And Technology Guide (PFD) under the same categories.

Other things you can do to minimize the possibility of hydrogen embrittlement would be to use a specially formulated phosphoric acid mixture that would contain surfactants and/or inhibitors. The surfactants would lower the surface tension and allow any hydrogen bubbles formed to release from the surface more quickly, lowering the chance of hydrogen migration into the base metal. Formulated acid products will sometimes contain an inhibitor that will minimize base metal attack. The advantage of this would be that the chemical would aid in dissolving the rust but would not attack the base steel. This will minimize the formation of hydrogen bubbles, also minimizing the chance of hydrogen embrittlement. You can look under the same category as above (Cleaning Chemicals, aqueous) or in "Inhibitors, acid pickling" for a supplier of acid cleaners.

There are two ways to eliminate the concern about hydrogen embrittlement. One would be some type of mechanical cleaning following the oil and grease removal. Various blasting and abrasive methods could be effective at the rust removal if these are available to you. The worse the rust, the more aggressive the mechanical method would be, so you would have to check your hardness to make sure it has not changed significantly. Another method to remove concern regarding hydrogen embrittlement would be an oven bake at about 400F for up to a few hours after pickling. This is a generally accepted method to eliminate hydrogen embrittlement concerns.