Problem 10.7: Your corrosion coupon program shows carbon

From Chapter 10 Problem Set. Level 2: Applied Thinking.

Question

Your corrosion coupon program shows carbon steel rates of 1.2 MPY - within the “good” range. But during a shutdown, you find deep pitting beneath silt deposits on horizontal pipe runs. Explain why the coupons missed this, and propose a change to the monitoring program.

Answer

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Standard corrosion coupons are mounted in the flowing water column on clean, smooth holders. They measure uniform corrosion in the bulk water environment. Under-deposit corrosion is localized - it occurs beneath silt deposits on horizontal pipe surfaces where the metal is isolated from the treatment program. The coupon never sees this condition because it has no deposit on it. To detect under-deposit corrosion, the monitoring program should be supplemented with: (a) visual inspection of horizontal pipe runs during shutdowns, looking for deposits and localized pitting beneath them; (b) ultrasonic thickness testing on deposit-prone sections; (c) placing some coupons horizontally with intentional deposits or in spool pieces that mimic pipe conditions rather than in clean, vertical holders.