Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Corrosion - Case Study No. 1: Corrosion Attack of Stainless Steel Weld Metal


Corrosion usually occurs when there is a mismatch between the material and its environment, or when operating conditions change. Continual weld repairs should raise a red flag. In Figure 1, the weld that attached the manway opening to the top of the vessel continually failed. The normal mode of repair was to patch the pinholes (Fig. 2) with an additional layer of weld.

When the manway cover was removed for the first time during an outage, and inspected, the inspection revealed that the weld metal was being selectively corroded away (Fig 3). Each successive repair would eventually corrode away and leak. Note the "ring-worm" pattern where the weld bead use to be located. Although this material was stainless steel, the environment in this particular region of the vessel was unique. In addition, the continual repair welding had effected a change in the parent metal as evidenced by the granular appearance in the heat-affected zone located at the bottom of the manway, Fig. 4. The black line at the bottom of the picture locates where the last repair weld bead was placed on the other side of the outer nozzle wall.

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