The active base metal remained therefore unprotected and a standard galvanic corrosion occurred between the passive layer (nobler metal or cathode) and the active metal (less noble metal, anode). This was due to chloride attack of the SS passive layer leading to a pitting corrosion. The problem is in fact the mixing of to unfavorable conditions: seawater and chlorine injection against biological fouling.Īfter 3 months only crevice corrosion appeared on the main cooling water pumps impellers made of 1.4408 SS grade (a kind of 316). At Tocopilla combined cycle power station located in the north of Chile, we face a high corrosion rate of our seawater submerged stainless. First of all, let me confirm that the green products you found are probably not issued from the stainless steel grade 316. Look for small spots of red-brown stain on the surface, then probe these spots for a deeper pit under the surface.Ī. The pits are often undercut with only a microscopic opening at the surface. Inspection of the stainless steel process vessels for pitting corrosion can be very tricky. I would recommend a commercial product with technical support from the supplier, rather than a home-brew passivation treatment. In the pharmaceutical industry, this process is often called "derouging", that is to remove the buildup of iron oxides on the stainless steel process surfaces. There are many commercial products for this purpose. Passivation can be used to maintain a good corrosion resistant surface of stainless steel process vessel. Pitting corrosion of stainless steel due to chlorides would certainly produce a rust-colored product. If you are not finding Cr or Ni in the green material, this is further evidence that the color is not from the SS. Green corrosion will occur on stainless steel only under very specific conditions, and the green is due to segregation of chromium. You may or may not have corrosion of the stainless steel from the chloride exposure, but I doubt that the green color that you are observing is the result of such corrosion. Good luck Dan BrewerĬhemical process supplier - Gurnee, IllinoisĪ. I hope this provides a starting point for your investigation. It happens frequently in cooling applications where biofouling has occurred and the operator throws in a bunch of bleach to kill bacteria and algae in the recirculating tank. Generally, this causes a "white rust" and is very damaging. Rene - I have often seen stainless steel have corrosion because of high chlorine content of the water. There are some quite good citric acid proprietary products available now that are a lot safer to use. Hydrofluoric acid is NOT a recommended step. Nitric acid or nitric acid/chromate will reduce this problem. The probable thing that allowed the corrosion to form was trace smearing of ferric iron in the manufacturing step. A final warm water tank and blow dry step will significantly reduce the corrosion. After your NaCl step, increase your rinsing via sprays (mist) or added tanks. Chloride is an excellent rust cause with a little damp air moisture. 316 SS has excellent corrosion resistance, but it is not impervious to rust.
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