Bathtub ring corrosion in 316L SS tank

[rbrez]

We have a 316L SS mobile tank that we recently found with horizontal lines (relating to product level) of
- Heavy "rust" stains about 1/8" wide by a couple of inches to a couple of feet long
- deep corrosion 1/8" wide, up to 1/8" deep by a 1/4" to a couple of inches long
- heavy "rust" stains bleeding down from the line of corrosions.
However, along the same horizontal line, other areas in the tank are fine.

The product is sodium chlorate solution loaded at about 175 degrees F. (However that does not exclude something else from accidently being loaded in the tank.) We have other tanks in the same service with no indication of the same problem.

There are a total of three different horizontal corrosion / stain lines in the tank. This means the corrosion occurred over 3 different product loads.

The appearance of the material in the corrosion pits was jagged.

Any ideas what could have caused this corrosion?

Thanks

 

[TBP]

If this happened in a shut-down (but with water still at or near the normal operating level) steam boiler, it would be oxygen corrosion. Oxygen from the atmosphere dissolves in the upper layer of water, and this causes pitting of the boiler metal. I've also seen corrosion in a carbon steel supphuric acid storage tank that was due to condensation from humidity in the air. The tank was greatly oversized for the application, and the turnover of acid was very slow - months/years. The diluted acid layer at the surface caused corrosion that was in stepped layers. There was a line of corrosion that formed each time the tank was drawn down a half inch of so, then sat for another period of time.

Is it possible that one section of your tank is not quite the right material for the service, and that atmopheric conditions are contributing to your problem?

 

[aybee]

stainless generally does not like chlorate/chlorides sounds like
wrong stainless
oxygen splash zone type phenomena
temperature related to oxygen concentration..hottest liquor at the top