CHLORIDE STRESS CORROSION C036
All corrosion resistant materials protect themselves by forming a protective oxide layer on the surface of the metal. Aluminum forms aluminum oxide (Al2O3) and stainless steel forms chrome oxide (Cr2O3). These oxides are better known by their generic name “ceramic”.
All ceramics will pit if exposed to chlorides. If the metal piece is under tensile stress either because of operation or residual stress left during manufacture, the pits formed by chlorides attacking the passivated layer will deepen even more. Since the piece is under tensile stress cracking will occur in the stressed portions. Usually there will be more than one crack present causing the pattern to resemble a spider’s web.
Chloride stress cracking is a common 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. This is the main reason that hastelloy C metal is recommended for both metal bellows and spring material.
Here are some additional thoughts about chloride stress cracking that you’ll want to consider:
- Chlorides are a 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 plenty of chlorides and piping is frequently under tensile stress. The worst condition would be insulated, steam traced stainless steel piping.
- If it is necessary to insulate stainless steel pipe a special chloride free insulation can be purchased or the pipe can be coated with a protective film of sodium silicate (water glass) 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 are sure there are no chlorides present. Bolts can be under 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 we have 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 accounting for the reason we sometimes experience cracks after we weld stainless steel and use the die check to confirm the quality of the weld.
- There is some evidence that operating temperatures less than 5°Ç (41°F) and greater than 140°C (284°F) slow down the chloride stress cracking process.