Stress & Corrosion Cracking, Attack & Mitigation in a Type 316 Reactor Vessel

Several batch reactors constructed of type 316 stainless steel were discovered to have cracking indications after five years of operation. The cracking had not shown up in prior inspections. The cracking was observed both in the reactor shell and internal piping. Other reactors in similar service were found to have a six year life prior to replacement for cracking.

The client contacted Metallurgical Consulting to evaluate the cracking and recommend a replacement alloy. The cracking was determined to be classic branched transgranular stress corrosion cracking (SCC) which is common in stainless steel.

Recent research reports indicate that there can be an incubation period for this type of cracking to occur. This research was consistent with the sudden appearance in this case. Review of the expected process chemistry throughout the course of the reaction revealed severe conditions requiring nickel base alloys. The customers' greatest concern was that the replacement costs were high and vessel lead times were long.

On line corrosion tests using DC potentiodynamic techniques were recommended to the client. These tests can identify conditions leading to SCC as well as producing corrosion rate estimates. Tests of candidate alloys and 316 as a baseline were recommended to be run during the course of one batch cycle. Corrosion probes were installed through blind flanges in the base of the reactor (Figures 3&4). Rapid changes were known to occur during the first hour of startup. One engineer monitored the parameter read outs in the control room and directed the start of rapid potentiodynamic scans to another engineer at the reactor. Corrosion tests revealed the 316 alloy went from active to passive during the first thirty minutes of the batch campaign. This change went through the known region for SCC. The alloy then went passive after this period for the rest of the batch cycle.

It was determined that small changes in the process could eliminate the serious SCC conditions at start up, resulting in increased life of the reactors.

To learn more about this project, or the processes used to complete it, contact Metallurgical Consulting directly.

Images From The Metallurgical Services Provided For This Reactor Vessel Project

(click on thumbnail to enlarge)

Figure 1: SEM picture of extensive SCC in cross section

Figure 2: Extensive cracking on pipe surface (316 stainless)

Figure 3: Corrosion probe installation into the vessel.

Figure 4: This shows the complete corrosion probe assembly