CHLORIDE STRESS CORROSION.17-07

If a 300 series stainless steel component is under tensile stress, either because of operation or residual stresses left during manufacture, chloride induced pitting will deepen even more. Since the piece is under tensile stress cracking will occur in the stressed piece. Usually there will be more than one crack present causing the pattern to resemble a spider's web.

Chloride stress cracking is a serious problem in industry and not often recognized by the people involved. In the seal business it is a serious problem if you use stainless steel springs or stainless steel bellows in your seals. This is the main reason that Hastelloy C is recommended for spring material. Here are some additional thoughts about chloride stress cracking that you'll want to consider:

• Chlorides are the big problem when using the 300 series grades of stainless steel. The 300 series is the one most commonly used in the process industry because of its good corrosion resistant proprieties. Outside of water, chloride is the most common chemical found in nature and remember that the most common water treatment is the addition of chlorine.
• Beware of insulating or painting stainless steel pipe. Most insulation contains chlorides and piping is frequently under tensile stress. The worst condition would be insulated, steam traced, stainless steel piping.

If it's necessary to insulate stainless steel pipe, a special chloride free insulation can be purchased, or the pipe can be coated with a protective film prior to insulating.

• Stress cracking can be minimized by annealing the metal after manufacture, to remove residual manufactured stresses.
• Never replace a carbon steel bolt with a stainless steel one unless you're sure there are no chlorides present. Bolts can be subject to severe tensile stress.
• No one knows the threshold values for stress cracking to occur. We only know that you need tensile stress, chlorides, temperature and the 300 series of stainless steel. We do not know how much chloride, stress or temperature.
• Until I figured out what was happening I had trouble breaking stainless steel fishing hooks in the warm water where I live in Florida.
• Many cleaning solutions and solvents contain chlorinated hydrocarbons. Be careful using them on or near stainless steel. Sodium hypochlorite, chlorethene. methylene chloride and trichlorethane are just a few in common use. The most common cleaner used with dye checking material is trichloroethane, explaining the reason we sometimes experience cracks after we weld stainless steel and dye check it to inspect the quality of the weld.
• There is evidence that the rate of chloride stress corrosion slows down below 40 F (5° C). and above 285 F (140° C) and.

See: Corrosion - problems with the 300 series of stainless steel 4-1

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