Aluminium as a stand-alone material, when selecting the proper alloy, is resistant even to aggressive environments, including sea water conditions.
The automotive industry is the best example of an industry which has been able to switch to all aluminium heat exchange solutions due to applying good system designs. Hydro has significant experience in this field and is looking forward to supplying its know-how also to the solar thermal industry.
When designing a solar thermal system, there are two main types of corrosion to consider.
Internal corrosion
To secure the system against internal corrosion, Hydro recommends its HyLife™ Solar alloy which is a very corrosion resistant alloy and therefore suitable in solar thermal systems. Also, the system needs to be a closed system (hermetically sealed) that contains a heat transfer fluid containing an inhibitor. An open aluminium system, or a system with fresh water without an inhibitor, will corrode over time. Most commercially available heat transfer fluids contain inhibitors.
Filler material for brazing should be AlSi based and not contain zinc. We also only recommend the use of aluminium or stainless steel fittings in hot areas, i.e. at the absorber. We do not recommend brass fittings in these areas since brass contains zinc. This is because zinc may introduce corrosion in a solar thermal system at high temperatures.
External corrosion
There are mainly two areas of risk for external corrosion, at joints to copper or brass fittings, or at joints between the absorber tubes and sheet. External galvanic corrosion may occur between aluminium and other metals if the joint is subjected to humidity from rain or condensate.
To avoid corrosion where aluminium is directly joint to copper or brass fittings, it is good practice to protect the joint area and keep it dry with a suitable heat shrink sleeve, or similar.
To avoid corrosion between the absorber tubes and sheet, it is important to select the right alloy combination for the aluminium absorber plate and tube. The tube has to be slightly more noble than the plate in order to guarantee that any potential material sacrifice is of the plate and not tube.
The larger the difference in material nobility, the higher the risk that the less noble material will sacrifice electrons to the more noble material, introducing corrosion. Thanks to lower differences in potentials in an all aluminium absorber, compared to an absorber with copper tubes and an aluminium sheet, the overall corrosion risk is reduced and a longer collector life-time may be obtained.
Read more about Hydro’s HyLife™ Solar alloy.
