Photo Gallery B4: Under Deposit Corrosion
Under deposit corrosion, often termed “cell corrosion,” is an aggressive pitting condition commonly associated with an internal settlement and buildup of rust deposits within a piping system. It can be initiated by rust migration and settlement from other areas of the system, and is also related to the production of heavy rust tuberculation on the pipe’s surface. Over time and through the progression of various chemical transformation, very localized and severe wall loss takes place.
As we have repeatedly stated to our clients – it is not the rate of corrosion in MPY which defines the remaining service life of a piping system, but the volume of rust created and its deposition in dead-end and low-flow areas which dramatically accelerates localized pitting activity. Even a moderate to high 5 MPY corrosion rate at a condenser water system will still provide extended service life in terms of the pipe wall lost, with such loss at standard 0.375 in. pipe still providing 70 years of service before reaching failure status. Yet, oxidizing that volume of steel into potentially 50 cubic feet or more of rust product may destroy specific areas of the same system within 20 years.
When rust and other particulates settle, they prevent or insulate contact with the underlying bare steel – thereby reducing the efficiency of the chemical water treatment program. Contrary to the claims of most chemical treatment providers, no benefit is realized from the treatment program, leading to dramatically higher localized pitting levels. As a result, severe pitting develops – often resulting in an advanced and entirely unexpected failure. Evidence of such is commonly found by ultrasonically testing low-flow, dead-end, and by-pass piping configurations by the measurement of significantly lower wall thickness along the bottom where the rust has settled.
Unlike the threat of generalized corrosion that can be reasonably measured, under deposit corrosion is extremely random but somewhat predictable to certain areas. Of the many conditions which cannot be represented by corrosion coupons, under deposit corrosion ranks #1.
