Deposits can not only reduce heat transfer and cause restricting flow problems, but corrosion, specifically under-deposit corrosion can lead to some significant damage. This corrosion can occur both directly and indirectly.

• When the attack is direct, the deposit itself contains corrosive substances which when concentrated at a localized or generalized site can cause wastage. An example of such a deposit would include chlorides.
• Indirect attack is more commonly associated with under-deposit corrosion. This wastage occurs when the accumulation of deposits shields the covered surface from the bulk water system. Corrosion occurs on a metal surface due to some inherent or environmental difference between one area on that surface and another. These differences will create anodic and cathodic areas, setting up a basic corrosion cell. The anode is the area at which the metal is lost. The electrons given up by the metal flow to the cathode to be consumed in a reduction reaction.

The growth of bacteria and formation of biofilms may also result in under-deposit corrosion. Biofilms are notorious for not only acting like a deposit and instigating corrosion and heat inefficiencies, but also for entrapping suspended debris. Biofilms are also capable of the formation of mineral scales by engulfing ions like calcium and magnesium. The trapped ions are now more readily available for a reaction with carbonate and phosphate anions.

Whether the scale or debris accumulation or the biofilm occurred first each is most likely present in even a moderate accumulation. These accumulations inhibit full penetration of biocides and corrosion inhibitors, thus accelerating anaerobic growth underneath deposits and corrosion. Sulfate-reducing bacteria are especially active in covered, anaerobic areas. These organisms reduce sulfate compounds to sulfuric acid and hydrogen sulfide, both creating an acidic attack on the metal surface.

Scale and debris accumulation should be removed and cleaned regularly to prevent under-deposit corrosion. A moderate to severely scaled system will begin experiencing corrosion as soon as a surface have been covered. A covered area is inaccessible to your regular inhibitor program.

Reference- http://www.wallingwater.com/