Water used in building systems must be monitored and treated to control scale, corrosion, microbiological growth, and dirt/silt fouling. These problems can cause inefficient heat transfer, higher energy costs, reduced reliability, damage to equipment, and increased risk to the health of occupants. Proper, ongoing water treatment is required through a combination of mechanical and chemical processes. In this article, we’ll focus on corrosion.
If not properly treated, metals gradually self-destruct when exposed to air, water, or even other metals. This is the process of corrosion, which is a serious problem in heating and air-conditioning systems. Corrosion and its deposited by-products can reduce equipment efficiency, increase operating costs, and cause equipment failure.
Eight Factors that Affect Corrosion
To minimize corrosion in HVAC systems, each of the following factors must be managed:
As the pH in untreated water drops, corrosion tends to increase. The lower pH also dissolves various oxides and exposes more of the metal surface to corrosion.
Low water velocity allows solids to settle and deposit on the metal surfaces. These deposits make it impossible for any corrosion-fighting chemicals to do their job. The protective film provided by the corrosion inhibitor settles on the deposit rather than on the metal surfaces. As a result, localized corrosion can thrive unchecked underneath the deposits.
Most systems consist of several different metals. When two different metals are connected and exposed to moisture or water, corrosion can occur.
When dirt, corrosion products, and scale settle out in your system, they cause deposits. Corrosion can occur underneath these deposits.
Two principal dissolved gases, oxygen and carbon dioxide, cause the primary corrosion problems in heating and cooling systems. Both gases, if not removed or chemically treated, will contribute to corrosion within water systems.
Oxygen is troublesome in both heating and cooling systems because it quickly combines with the metals used in the system and fuels the corrosion process. Carbon dioxide is not a problem in cooling systems because there is no breakdown of natural alkalinities in water to form carbon dioxide. However, carbon dioxide can affect boiler systems.
Chemical reactions proceed faster in hot water. As the water heats up, more oxygen reacts with electrons and water at the cathodes, forming hydroxides. As a general rule, every increase in water temperature of 18°F doubles the corrosion rate.
Dissolved solids are minerals dissolved in water. The effect they have on corrosion depends on their type and amount. These dissolved solids, such as chlorides and sulfates, can increase corrosion by interfering with the formation of corrosion inhibitor films.
Microbiological matter causes pitting corrosion. Microbiological problems only affect cooling systems. They are seldom a problem in steam and heating systems because the higher temperatures kill the organisms.
There are many ways to reduce corrosive effects, if not prevent them totally. Corrosion-resistant materials, such as copper, stainless steel, PVC, plastics, and concrete, are used in the construction or retrofit phase. However, these materials can be expensive.
Another method is to rustproof the system by coating metal surfaces with corrosion-resistant metals or compounds. However, some heating and cooling systems are so large that, while you may be able to coat a section here and there, coating the entire system is impractical.
The use of sacrificial anodes actually relies on corrosion itself to protect the system. These anodes are pieces of metal, such as magnesium or zinc, that have a higher potential to corrode than the base metal itself. In this system, metal bars are strategically installed on tube sheets or baffles. The sacrificial anodes corrode instead of the equipment, and are periodically replaced.
However, sacrificial anodes can only protect selected parts of a heat exchanger. Thus, the tubes will not be protected. Also, corrosion still goes on, releasing corrosive products into the water and causing deposits the system.
Using Mechanical Equipment to Control Corrosion
The use of mechanical equipment to reduce the effects of corrosion is most common in the steam heating systems of large commercial and institutional facilities. There are two primary systems, which have many variations.
Ion exchange/softening is used to condition water prior to its entry into the system. Generally, a polystyrene resin is used to remove and/or exchange various dissolved solids in the water that contribute to the scaling and corrosion process. Another process is deaeration, which involves removing the oxygen and carbon dioxide gases before they enter the boiler system. Any gases that remain after deaeration are chemically removed.
Deaeration is the most important dissolved gas removal process in steam boiler systems. It is not an ultimate cure because both oxygen and carbon dioxide can enter or develop later in the system. Therefore, there is usually need for chemical treatment in addition to deaeration.
The most effective and economical way to control corrosion is corrosion inhibition, a combination of mechanical and chemical control. An effective corrosion inhibitor program will interrupt anode reactions and slow the reactions at the cathodes. Effective corrosion inhibiting incorporates three steps:
- System cleaning
A clean system is most important for any corrosion control program. Oils, scale, and corrosion deposits all contribute to corrosion by developing corrosion concentration cells. The system must be clean to gain the maximum benefit from corrosion inhibition.
A newly cleaned piece of equipment is susceptible to corrosive attack. If placed back into service without chemical treatment, the corrosive attack will start immediately. Pretreatment chemicals lay down a coating on the metal to protect it during system start-up.
- Chemical treatment
When the system has been cleaned and pretreated, the ongoing protection process can begin. The recommended levels of inhibitors must be maintained to assure protection. Corrosion inhibitors must be used in favorable water conditions to perform properly. The acidity or alkalinity (pH) of the water and its conductivity are important in the fight against corrosion. Whenever pH drops below recommended levels, corrosion will increase.
Furthermore, when pH becomes too low, even the most powerful corrosion inhibitors are ineffective because their protective coating is stripped away from the metal. Conversely, very high pH can create scale problems and also prevent inhibitor films. Therefore, it is important to keep pH within the recommended ranges.
This article is adapted from BOMI International’s course The Design, Operation, and Maintenance of Building Systems, Part I. More information regarding this course is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.