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. 73: Corrosion Fatigue of 316L Stainless Steel Pipe

 

Stainless Steel Pipe with PittingThe term "corrosion fatigue" can be a catch-all phrase, kind of like using the phrase "I'm sorry" when one is at a loss for words. However, when was the last time your "significant other" let you off the hook for forgetting an anniversary with the words "I'm sorry." Somehow those words just weren't enough. So too with the words "corrosion fatigue." What exactly IS corrosion fatigue?

I've heard the phrase thrown around in complicated failure analysis investigations enough times that I went on a quest to find that "Holy Grail." One day I was ready to chuck out a broken piece of pipe (pictured left) and chalk it up as a simple material fatigue failure, until I looked at it closer.

Close Up View of Pitting in 316L Stainless Steel There were signs of pitting; this of course indicated a corrosive environment. This pitting was in 316L stainless steel. It wasn't intergranular corrosion. If it were intergranular there would have been tell-tale branch-like lines running along the surface. Above the larger pits, and to the left, can be seen smaller pits just beginning. More tiny pits were uniformly distributed amongst the larger pits. They would have eventually joined together to make larger pits.
Pipe View Showing Inside and Outside Further inspection revealed that there wasn't any corrosion on the pipe inner diameter. Pitting was occurring only on the outer diameter. What was surprising was the relationship between the pitting and the fracture surface.

The fracture surface was located along the toe of a circumferential fillet weld (the fillet weld was on the other failed piece not shown in the picture to the left). The pipe extended an additional two feet from the weld. It was a spray nozzle and experienced a lot of fluid turbulence and vibration.

 

View of Fracture Surface Showing Radial Ratchet Marks Multiple radial ratchet marks along the entire pipe diameter indicated a uniformly applied stress. Pitting was also uniformly distributed along the pipe diameter.
Corrosion Fatigue in 316L Stainless Steel

... This is corrosion fatigue. ...

It is simply two different failure mechanisms working together.

One mechanism is corrosion, while the other is mechanical.

This cross section clearly shows the pitting corrosion occurring along the outer diameter of the pipe and progressively migrating radially inwards. The normal working loads were eventually distributed over an increasingly smaller cross sectional area. Hence, the pipe stress increased proportionally. At some point, the remaining material was not strong enough to handle the normal working stress and the crack then proceeded at a greater rate. The smooth fracture surface indicates a low-stress high-cycle fatigue. Vibration generates this type of topography a lot. This pipe was in service for 12 years.

There is a good and interesting explanation for how the two mechanisms work together, especially with materials that generate their own protective film (such as 316 stainless steel). It can be read in H. H. Uhlig's Corrosion Handbook, John Wiley & Sons, 1948 (still good after all these years). The 2nd edition is out now.

 

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