Technical
considered beneficial . Conversely , compounds such as nitrogen and sulphur oxides , as well as ammonia , chlorides and carbon monoxide , react with copper and aluminium to form acids that can cause microscopic indentations . Known as pitting , this can cause severe corrosion damage within just a few months .
As soon as more than one metal is used in components of a cooling system , especially air exposed ones , galvanic corrosion can occur . In conjunction with an electrolyte such as salt water , ions of a less noble metal begin to flow towards a more noble metal . Copper and aluminium components are particularly affected by this , as are metal combinations in environments with high saltwater contamination . In particular , galvanic corrosion can cause significant damage to aluminium microchannel air to water heat exchangers connected via copper tubes . Fracture and blockage of the fine microchannels then leads to partial pressure losses or even refrigerant leakage .
Epoxy resin spray coating of a microchannel heat exchanger .
Modern e-coating dip coating on a microchannel heat exchanger .
PROTECTION MEASURES There are now various options for protecting air to water heat exchangers against corrosion . Thicker aluminium fins protect against premature breakage , however , during plant operation the extra fin width blocks airflow , thereby reducing efficiency . A better option is the use of Cu / Cu coil fins , as they eliminate the effects of galvanic corrosion because both the tubes and fins are made entirely of copper . Cu / Cu coil fins are relatively expensive though and offer only a limited advantage compared to aluminium fins . Since they mainly protect against galvanic corrosion , they are mainly considered for installations in the immediate vicinity of the sea .
In addition to structural measures , modern coating techniques can also help . Spray coating provides protection against all types of corrosion and is relatively inexpensive compared to other types of coating . However , a well-known problem concerning spray coatings is that they can leave gaps in the top coat and while they are recommended for conventional installation conditions , they are less suitable for locations with very high ambient pollution from exhaust gases , acid rain or salty air .
Graphic showing influences on corrosion .
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RACA Journal I February 2022 www . hvacronline . co . za