While various initiating conditions exist, under deposit corrosion is primarily due to the settlement of rust deposits at by-pass, dead end, futures, full flow headers, crossover connections, or wherever low flow conditions favor rust and debris deposition.  The issue is very common to open condenser water systems where naturally higher corrosion levels are added to by captured airborne particulates.  The problem can also be the result of microbiologically influenced corrosion activity known as MIC – a closely related subject.

A telltale indication of under deposit corrosion is the identification of tuberculation nodules visually, or the measurement of random low wall thickness ultrasonically.  In both examples, the depth of penetration into the steel pipe is directly proportional to the volume of rust deposits above.

Although most large diameter piping systems can easily handle a 5 MPY wall loss, it is the volume of rust produced and its settlement which then elevate pitting to 35 MPY levels.  Under deposit corrosion is generally random.  Very quickly, and without aggressive actions, it will doom the piping system affected.