Calcium sulfate scale deposited on spot-welded stainless steel sulfite digester linings is believed to be the cause of pitting (from concentration cell effect) and stress corrosion cracking of the liner experienced at the British Columbia plant of Columbia Cellulose. The chloride concentration expected in the scale was not found in analysis. A 1955 test showed high chloride concentration in both the cooking acid and blow liquor.

Some theories are outlined about reasons for the corrosion damage observed in the digester, and repair procedure is described. Because of difficulty with Hastelloy welding rods, Ni-O-Nel was adopted for all welding in both the shell and vapor phase areas.

Lack of trouble with a clad digester is attributed to improved plant operations and some improvements in scale removing procedures. Better design of collector rings and pipe nozzles could eliminate many crevices in which scale forms, the author believes. Stainless steel for sulfite service should contain a minimum of 2.75 percent molybdenum and maximum 0.05 percent carbon. All surfaces should be ground to a high luster finish and fully protected during manufacture. 7.6.6

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