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. 115: Corrosion Testing Using Welded Coupon

Chloride stress corrosion cracking is a very ugly problem when it comes to stainless steels. Under the right conditions it will initiate, especially where high stress is involved. Therefore, it is best to find potential problems ahead of time, before they have a chance to begin. This is where bend-coupon testing is an affective tool in the arsenal of predictive maintenance.

Wherever chlorides can congregate there is always a chance for stress corrosion cracking of stainless steels. Inducing high stress into a stainless component can cause cracks to quickly propagate once they have initiated via chloride attack. Of particular interest in this case was how well 316L stainless steel would do in sodium chlorate service. The environment being tested was a scrubber. Its operating conditions varied between wet and dry, and the temperature varied from ambient to 400 F. The alternating wet-dry environment was perfect for concentrating chloride attack.

316L Stainless Steel Coupon in Sodium Chlorate Service 316L Stainless Steel Coupon - Right Side
To push the test to its limits, a welded sample of 316L stainless steel was placed inside the scrubber. The coupon sample came from a welding procedure qualification test. In that test the weld was tested for strength and ductility. It was also checked for lack of fusion. The ductility of course was tested by bending a sectioned strip of the weld around a mandrel of a specific radius. If there were no tears along the weld face, as is shown in the pictures above, then the procedure would pass one part of the qualification test. It passed the test. What you see in the pictures above is the cross section of the weld. A half inch wide by eight inch long strip containing a portion of the weld in the middle, was bent in the form of a "U" shape. The weld was located at the bottom portion of the "U" shape. The root of the weld is on the bottom in both pictures.

The shape, although it is bent, is still under a very high tensile stress along the outer portion of the bend. If the coupon was going to be attacked by chlorides coming from the sodium chlorate, then it would initiate along the heat affected zone at the weld faces in this coupon. As it turned out, there wasn't any noticeable attack after nine months of service.

So what is the value in all of this? Well, there's about $10 million dollars, or more, worth of equipment downstream of this coupon. A lot of which have the same wet-dry conditions and temperature conditions. If this coupon started corroding then there would be good reason to believe that a lot of high stressed welds downstream could be at risk. An inspection program, the likes of which nobody is use to, would be warranted. Inspection programs always cost a lot of money, so there has always got to be a good reason to start one. If you took a corroded coupon into the boss, he probably wouldn't argue with the request for inspection. In fact, he might even thank you for the good work. Is this the end of the story? No it isn't. This coupon will be periodically checked.

In this business, with processes and products changing all of the time, it pays to be vigilant.

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