Webinar Overview

Pictured is a grade 316 stainless steel rolled groove pipe joint after service in water with chlorides seven times the accepted limit. The rough surface where the rollers formed the groove is fine but the crevice at the edge of the rubber seal has perforated. This webinar will look at the multitude of factors that affect corrosion resistance beyond chloride concentration.

Except with very aggressive chemicals, most attack on stainless steels is localised and occurs in crevices or by pitting or by stress corrosion cracking (SCC). All too frequently, the problem is excessive chlorides for the specific alloy although bromides and iodides can also cause problems. Material selection is often based on the 3-element composition-based PREn and its correlation with the temperature required to initiate pits or, with suitable test coupons, crevices. The data only really applies to oxygenated chloride environments and the prepared metal coupons. There is no consideration of surface finish or cleanliness, impurities in the metal or local variations in the exposure.

 Topics considered and illustrated include:

• The source and breadth of application of the PREn and its correlation with Critical Crevice or Pitting Temperature (CPT/CCT) in standard test solutions
• Using PREn for stainless families and nickel alloys
• The difference between corrosion initiation and propagation
• The differences between a pit and a crevice
• Modeling of crevice corrosion for different alloys
• Accepted chloride limits for waters based on experience
• The dramatic effects of surface finish, chemical treatment, temperature, flow, scale and deposits on corrosion resistance
• Compositional issues in stainless steels, especially carbon and sulphur
• The strengths and weaknesses of corrosion resistance tables
• Precautions in using the aggressive stress corrosion cracking test data