Technical
Unlike spray coating , cathodic dip coating offers high layer uniformity . In this process , an air cooled or air exposed heat exchanger is electrically charged and then completely immersed in a chemical bath . The coating adheres uniformly to all surfaces with no gaps in protection and this also achieves the thinnest protective layer of all coating processes , with little effect on thermal conductivity . Under test conditions spray coating with aluminium pigmented polyurethane ( Blygold ) offers the most effective protection overall , however , the risk of selective protection gaps remains . Ultimately , although it has a lower salt spray value , cathodic dip coating is a more reliable technique .
LIQUID ASSET Most water circuits use a mix of materials that have different chemical properties . Monitoring water parameters in a chilled water circuit is therefore required , in addition to a site specific assessment of individual corrosion risks . Paying attention to the right water quality right from the initial filling stage not only improves system service life , but also ensures higher plant efficiency .
In data centres there are two main water cooled system designs – precision air conditioners with an integrated direct evaporation ( DX ) refrigeration circuit , or chilled water ( CW ) indoor units connected to a central chiller via air to water heat exchangers . Both variants require an oxygen tight water circuit free of foreign particles and sedimenting substances . In the closed circuits found in chilled water systems , corrosion damage can occur within a couple of years . This is a problem not only because of possible pipe damage and leaks , but also because flaked off corrosion particles can clog pumps , filters or capillaries .
BRAVING THE ELEMENTS Oxygen corrosion is caused by dissolved oxygen in water . In closed systems it decomposes over time , but a residual oxygen content can generally be assumed . If oxygen , water and a metal react with each other , oxidation or corrosion of the metal occurs . In the case of acid corrosion , metals are attacked by acid due to a pH value that is too low (< 5pH ) and this begins a decomposition process . The reaction can take place 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 bacteria is sulphur oxide , which causes a strong odour in the surrounding medium .
Formation of a biofilm due to bacterial corrosion on a pump , caused by the use of deionized water without additives .
Another example showing corrosion damage to finned tube heat exchanger .
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 prevent oxygen corrosion it is possible to chemically bind oxygen , yet other factors also contribute to the corrosiveness of circulating water including pH value , water hardness and conductivity .
MADE TO MEASURE A pH value is considered a measure of the strength of an acidic or basic aqueous solution . A too high or too low pH value should be avoided because different materials have different pH ranges in which they can form a protective oxide layer . If this range in an installed material is undercut or exceeded , the risk of corrosion increases significantly – leading to acid corrosion .
After a system has been filled , a large proportion of dissolved carbonic acid is still contained in the circulating water . This proportion gradually escapes as carbon dioxide and , due to the lack of carbonic acid , the pH value increases . An exact adjustment of a pH value is therefore almost impossible . If there is a mix of materials in the pipe network , the range of values in
www . hvacronline . co . za RACA Journal I February 2022 57