Water-ethylene glycol mixtures have important cost advantages for use in solar thermal collector systems when compared to other heat transfer fluids such as alkylated aromatic and silicone liquids. However, under certain circumstances, aqueous glycol causes the development of severe pitting in aluminum.
The pit depth (D) of aluminum alloys exposed to water generally follows a cube root law 1 with respect to exposure time (t):
where k is defined as the pit growth constant.
This cube root relation has been established by many laboratory pitting rate tests in fresh waters of various composition, as well as by actual service data obtained from an industrial water pipe (5052 alloy nominally 2.5% Mg, 0.25% Cr, balance Al) over extended periods of time.2 Furthermore, it holds for 6061 alloy (nominally 0.6% Si, 0.28% Cu, 1.0% Mg, 0.2% Cr, balance Al) in sea water. However, there...
