Underdeposit acid attacks

MIC almost always acts in concert with other corrosion mechanisms and may, at times, appear to be crevice corrosion, underdeposit acid attack, oxygen-concentration cell corrosion, ion-concentration cell corrosion, and CO2... 

Underdeposit acid attack, whereby corrosive attack is accelerated by acidic products of the MIC "community metabolism," principally short-chain fatty acids. 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

Some of the observed wastage was caused by past acid cleanings. However, much of the attack was caused by long-term underdeposit corrosion. 

O. C. Dias and M. C. Bromel. Microbially induced organic acid underdeposit attack in a gas pipeline. Mater Peiformance, 29(4) 53-56, April 1990. 

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