Scale on boiler surfaces can lead to serious problems. Even an extremely thin layer of scale is enough to retard heat transfer and reduce boiler efficiency. Scale can cause boiler metal to overheat and tube failures can result. Scale can be controlled by careful control and maintenance of all make-up and feedwater pretreatment systems to limit the amount of impurities entering the boiler. Proper chemical treatment also plays a critical role in scale control and is used to inhibit the deposition of any minerals that do make their way into the boiler.

Corrosion can occur throughout a boiler system. The most common causes of corrosion are the following:

• Oxygen Attack
• Alkalinity Concentration
• Carbon Dioxide Attack/Acid
• Chelate Attack
• Ammonia Attack

Different kinds of corrosion are found in different areas of the boiler. Corrosion in boiler systems is not a simple matter to define because the various chemistries involved are often interrelated and complex. Corrosion of boiler metal is generally considered to be one of two types: general corrosion or localized corrosion. General corrosion is a uniform attack of the total metal surface resulting in a slow, even wastage of the boiler metal. General corrosion does not normally result in enough metal loss to cause tube or wall thinning and subsequent tube failures. Localized corrosion results in a fairly rapid and severe metal loss in a small, localized area. Such corrosion results in a gouge or pitting attack that can cause tube failures in a very short time. The concentrating corrosion mechanism (such as may be found under iron oxide deposits) is an example of severe, localized corrosion.

Metal oxides and organics are the two primary sources of deposits in modern-day boilers. Metal oxide deposits are a major cause of feedwater heater and boiler tube failure in both lower pressure industrial and higher pressure utility systems. Deposits formed from organics can occur in feedwater systems and boiler tubes of both high-pressure and low-pressure units. Like metal oxide deposits, organic deposits can insulate boiler tubes causing an increase in metal temperature that could lead to tube failure.

Steam quality is the percent steam in a mixture of steam and moisture. The percent moisture is thus 100 minus the quality. Steam purity does not refer to moisture content; however, it is the amount of solids entrained or dissolved in the steam.