Whereas corrosion is about the breakdown of materials due to a chemical reaction, deposits are where there is a build up of a substance which causes a problem. This is usually in the form of an inefficient operating system. This is a major problem in boilers and cooling towers as a build up of deposits can lead to higher costs due to an inefficient system, or even worse, it can lead to a higher chance of the outbreak of legionnaires disease.
Deposit formation depends largely on certain parameters, such as residence time, system metallurgy, water temperatures and water velocity. The highest level of deposits are found when you have high surface temperatures and low water velocities. There are, broadly speaking, two types of deposits: Scale deposits and Foulant deposits.
Scale Deposits:
These are caused by precipitation at a surface that's in contact with water. This happens when the substance in the water is saturated. Although they may be completely soluble in the lower-temperature bulk water, these compounds (calcium carbonate or calcium phosphate, for example) supersaturate in the higher temperature water adjacent to the heat transfer surface and precipitate on the surface.
Foulant Deposits:
Fouling occurs when deposits are formed on a surface from insoluble particulates that are suspended in the recirculating water. These deposits enter a cooling system through airborne contamination, makeup water, process leaks and corrosion. Most potential foulants enter with makeup water as particulate matter, such as clay, silt and iron oxides.
