Reliability Engineering Snapshot TM

Corrosion - Case Study No. 94: Consequences of a Leaking Corrosive Solid Gas or Liquid

Letting a corrosive medium get past the first line of defense is like letting the cat out of the bag. It's mean, it's ornery, and it wants revenge. If the first line of defense is the best and only line of defense, then it is important to design safety systems around this type of failure in the event of its occurrence. Here are some simple examples of what happens when the "what if" factor isn't considered. In all examples, the best materials were used to maintain a barrier between the corrosive medium and the outside world. However, when the corrosive medium finally got past the first and best line of defense everything in its path was easily destroyed. The picture to the left shows a steam coil that has general pitting attack. The steam coil finish drys sodium carbonate that is on the outside of the tube coil. The sodium carbonate is already low in moisture to begin with and poses no threat to the carbon steel tube coil. However, when water from a leak elsewhere in the system mixes with the sodium carbonate it becomes quite corrosive.
In the upper two pictures phosphorous gas is contained with stainless steel. The stainless steel flanges are held tight by carbon steel screw clamps. Stainless steel is suitable for phosphoric acid service, while carbon steel is not suitable. When the gasket started leaking, the gas mixed with the water to make phosphoric acid. The acid ate through the carbon steel clamp (picture left, center) within a couple of weeks. An exterior carbon steel wall structure was also attacked. The unit was shut down and repaired. The picture to the upper right was taken while the unit was down. The red mark to the left shows how much thickness was attacked (3/16"). The bottom stainless steel weld is not attacked. The red line in the center shows a crack where the material thinned to failure. The whitish blur beneath the red line is water leaking out from behind the wall.
The upper two pictures show a problem in the making. There is no history to this "nasty cat." It still isn't "out of the bag" yet. These are 304 and 316 stainless steel flanges that have been aggressively attacked, within two months, by sodium chlorate. Notice the granular appearance on the wetted portion of both weld-neck flange faces, and on the inner diameter. If this is the best line of defense, there will be hell to pay when the sodium chlorate gets out. Stay tuned for this corrosion article.

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