Pulp and Paper
Chloride Stress Corrosion Cracking
Specimen
1.25-in. OD tubing.
Material
Type 316L stainless steel, bright solution annealed.
Environment
The tubes were from a liquor heat exchanger in a pulp and paper mill.
Background
The tubes were from an upper liquor heat exchanger that operated at 200 psi and approximately 375°F (190°C) with steam on the outside and liquor on the inside of the tubes. Periodically, the tubing in the heat exchanger was acid cleaned by recirculating inhibited acid – usually hydrochloric acid (HCl).
Service Life
Approximately three months
Findings
Examination of the exterior surface of the tuning revealed the presence of approximately 10 – 12 cracks per linear inch. A metallographic examination of a cross-section transverse to the cracking in the tube wall revealed the presence of branched stress corrosion cracking that had initiated on the exterior of the tubing. The tubing had not been stress-relieved to remove residual stresses from the straightening process. Thus, the residual stresses present in the tubes and the presence of chlorides from the hydrochloric acid cleaning provided optimum conditions for the stress corrosion cracking of the Type 316L stainless steel tubing.