Problem 10.2: A cooling tower has excellent water chemistry

From Chapter 10 Problem Set. Level 1: Conceptual Mastery.

Question

A cooling tower has excellent water chemistry: inhibitor residuals on target, biocide fed on schedule, pH and conductivity within range. Corrosion coupons look acceptable, but during shutdown you find aggressive pitting beneath silt deposits on horizontal pipe runs. The basin has 3 inches of settled silt. Explain how these observations are consistent.

Answer

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The bulk water chemistry is excellent, but it is only measuring the water column. The 3 inches of settled silt have created deposits on horizontal pipe surfaces. Beneath those deposits, oxygen is consumed but cannot be replenished, creating oxygen differential cells that drive under-deposit corrosion. The corrosion inhibitor, pH, and biocide measured in the bulk water are not reaching the metal surface beneath the deposits. The coupons, which are typically placed in the flowing water column on clean holders, are measuring the chemistry the pipes are not actually receiving. Excellent bulk chemistry and aggressive pitting are entirely consistent when deposits are present.