Problem 11.9: A condenser system shows Inhibitor residual on

From Chapter 11 Problem Set. Level 3: System Integration.

Question

A condenser system shows:

• Inhibitor residual on target
• pH in range
• Conductivity in range
• Biocide schedule maintained
• No visible heavy scale

But corrosion coupons show 6.5 MPY. Explain at least two ways this can happen using the distinction between bulk-water chemistry and surface conditions.

Answer

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Two strong explanations are:

• Under-deposit corrosion: Bulk chemistry may be acceptable, but deposits isolate the metal beneath them from inhibitor protection and create local oxygen differentials.
• MIC or localized biofilm attack: The water column may test well while the surface microenvironment remains biologically active and corrosive.

Other acceptable explanations include:

• flow-related erosion-corrosion
• galvanic effects
• poor coupon location
• coupon exposure that reflects a particularly aggressive part of the system rather than the average system condition

The key idea is that bulk water data does not guarantee surface protection.