Metals gradually corrode when exposed to air, water or even other metals, if not properly treated. This can be a serious problem for your heating and air-conditioning systems. Corrosion can reduce equipment efficiency, increase operating costs and cause equipment failure.
Factors that Affect Corrosion
In order to minimize corrosion, manage each of the following:
Water pH: As the pH in untreated water drops, corrosion may increase. This lower pH can also dissolve various oxides and exposes more of the metal surface to corrosion.
Water velocity: Low water velocity allows solids to settle and deposit on metal surfaces, making it impossible for any corrosion-fighting chemicals to do their job. The protective film in the corrosion inhibitor settles on the deposit rather than on the metal surface.
Galvanic corrosion: Most systems consist of several different metals – when two different metals are connected and exposed to moisture, corrosion can occur.
System deposits: When dirt, corrosion products and scale settle out in your system, they cause deposits, under which corrosion can occur.
Dissolved gases: Oxygen and carbon dioxide cause the primary corrosion problems in HVAC systems. Both will contribute to corrosion if not removed or chemically treated. Oxygen is problematic because it tends to quickly combine with the metals used in the system. Carbon dioxide can affect boiler systems, but is not a problem in cooling systems.
Water temperature: Chemical reactions proceed faster in hot water. As the water heats, more oxygen reacts with electrons and water, forming hydroxides. Every increase in water temperature of 18 degrees Fahrenheit doubles the corrosion rate.
Dissolved solids: Dissolved solids are minerals dissolved in water, such as chlorides and sulfates, which can interfere with the formation of corrosion inhibitor films.
Microbiological matter: Microbiological matter causes pitting corrosion only in cooling systems. They are seldom a problem in heating systems because higher temperatures kill the organisms.
The most effective way to control corrosion is corrosion inhibition, a combination of mechanical and chemical control. It incorporates three steps:
- System cleaning: A clean system is most important for corrosion control. Oils, scale and deposits all contribute to corrosion by developing corrosion concentration cells. The system must be clean to gain maximum benefit from corrosion inhibition.
- Pre-treatment: A newly cleaned piece of equipment is susceptible to corrosive attack. If placed back into service without being chemically treated, the corrosion starts immediately. Pre-treatment chemicals lay down a coating on the metal to protect it during start-up.
- Chemical treatment: When the system has been cleaned and pre-treated, ongoing protection can begin. You must maintain the recommended levels of inhibitors in order to assure this protection. Corrosion inhibitors should be used in favorable water conditions if you want them to work properly. Water’s pH levels are important to fight against corrosion. When the pH becomes too low, even the most powerful inhibitor will be ineffective.
Courtesy of Facility Management Resources