Problem 7.9: A cooling tower shows the following conditions

From Chapter 7 Problem Set. Level 3: System Integration.

Question

A cooling tower shows the following conditions at the same time: elevated chloride, intermittent low pH excursions, biofilm deposits, and 6 MPY carbon steel coupon loss. Using the corrosion-cell model, explain why these conditions do not merely add corrosion risk, they multiply it. In your answer, identify which factors are increasing voltage, which are decreasing resistance, and why the damage is likely to localize rather than remain uniform.

Answer

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Chloride, low pH, and biofilm deposits weaken or remove passive films, which lowers resistance. Biofilms also create oxygen differential cells, which increase voltage by making the covered area anodic and the exposed area cathodic. Corrosion localizes because a small anode is driven by a large cathode, increasing current density and causing pitting rather than uniform metal loss. The 6 MPY is an average weight-loss rate, pitting may be much more severe.