Page 37 - Climate Control News magazine March 2022

Product Focus
without oxygen and proceeds more aggressively in a more acidic environment.
For electrochemical, or galvanic, corrosion to occur, two materials must be in contact. The less noble metal gives up electrons and is oxidised, with steady decomposition of the material. Meanwhile, with bacterial corrosion, bacteria extract electrons from the metal, whereupon the material oxidises. The waste product of the bac- teria is sulfur oxide, which causes a strong odour in the surrounding medium.
Oxygen content also has a massive impact on corrosion in a chilled water circuit. If there is high oxygen content in the water, an enhanced risk of corrosion can also be assumed, but in closed systems this is rapidly degraded. To pre- vent oxygen corrosion it is possible to chemical- ly bind oxygen, yet other factors also contribute to the corrosiveness of circulating water includ- ing pH value, water hardness and conductivity.
A too high or too low pH value should be avoid- ed because different materials have different pH ranges in which they can form a protective oxide layer. If this range in an installed material is un- dercut or exceeded, the risk of corrosion increas- es significantly – leading to acid corrosion.
After a system has been filled, a large propor- tion of dissolved carbonic acid is still contained
in the circulating water. This proportion gradu- ally escapes as carbon dioxide and the pH value increases. An exact adjustment of a pH value is almost impossible. If there is a mix of materials in the pipe network, the range of values in which the pH value does not have a corrosive effect on any components is reduced.
“DAMAGE TO THE MATERIAL STRUCTURE IS IRREVERSIBLE AND REQUIRES EXPENSIVE REPAIRS OR REPLACEMENT.”
Water hardness is defined by the content of alkaline earth metal ions. On the one hand, there is carbonate hardness, which is also called temporary hardness, and on the other there is permanent hardness. Non-permanent hydrogen carbonates, which are dissolved in water when a circuit is filled, precipitate into carbon dioxide and carbonates locate at the warm sections of a circuit and can cause sig- nificant efficiency losses.
The conductivity of water is determined by the amount of anions and cations dis- solved in the water including minerals such as magnesium, calcium and hydrogen car- bonate, as well as salts and dissolved metal ions. The more particles present, the higher the conductivity and thus the susceptibility to electrolytic corrosion. In chilled water systems lime always precipitates at the warmest point with the lowest flow velocity, and in most cases this is the air to water heat exchanger. As such, there is a risk that it will become clogged, which can also lead to stress cracks.
When checking water conditions, special at- tention should be paid to salts. Dissolved salts in particular are critical for the chilled water cir- cuit, as they are very reactive and cause precipi- tation and corrosion. Chloride is the salt of hy- drochloric acid and is the most stable parameter in the loop system. Primarily, it serves as a chemical catalyst that also accelerates corro- sion processes.
In addition to mandatory leak and function tests, a comprehensive cooling system mainte- nance strategy should include preventive corro- sion protection for air cooled or air exposed heat exchangers. ✺
CLIMATE CONTROL NEWS MARCH 2022
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