Two Type 316L SS rotary valve impellers that were used in cheese curd production were evaluated to determine the cause of pitting corrosion. The pitting was localized, biased towards the keyed drive end of the rotors, reportedly the same location where convey air is introduced into the valve. The convey air was reported to be as high as 170 degrees F, and promoted deposit formation and concentrated salts due to dry-out of the impeller surface adjacent to the convey air outlet during valve operation.
This dry-out and re-wetting cycle resulted in a saturated salt solution on the wet blade surfaces, and when combined with the calcium phosphate deposits, formed a crevice condition at 170 degrees F that exceeded the critical crevice pitting temperature (CCPT) for Type 316L SS impellers. Continued use of Type 316L SS for these impellers with reduced or no pitting would therefore require a significant reduction in convey air temperatures. If convey air temperatures cannot be reduced, use of an alloy exhibiting higher pitting resistance would be required.
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