Case study: Long-term solution for stopping and preventing stress corrosion cracking in two-vessel digester system, TAPPICon25

Severe stress corrosion cracking (SCC) was found in all zones of an impregnation vessel in a South American kraft pulp mill in late 1990’s. The mill assessed different solution options for the problem, and finally an Anodic Protection (AP) installation was chosen for the cost-effectiveness and fastest application time. The AP system was installed during the 2001 annual outage and metal samples were collected for detailed laboratory analysis. When the samples taken from the vessel wall were investigated, over 2.0 mm deep corrosion cracks were detected.

The AP system was commissioned in November 2001 and it has continued to operate up to this day. During the 2024 annual outage, a thorough inspection was conducted by a third-party company and the results showed that the cracking had not progressed in 23 years. Furthermore, the initiation of new cracks had been prevented by the system.

This paper also displays operational data of the anodic protection system over the years, with the elevated electrochemical potential of the vessel wall being the key factor in stopping the stress corrosion cracking.

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