A couple thoughts come to mind. First, when using an ethylene glycol based heat transfer fluid, one must always use a heat transfer fluid that contains a corrosion inhibitor. Uninhibited ethylene glycol heat transfer fluids can decompose into various organic acids such as glycolic, glyoxylic, formic, carbonic, and oxalic acids. The decomposition is accelerated by high temperatures and these acids can be quite corrosive to many materials. Second, according to Dow Chemical (a manufacturer of ethylene glycol based heat transfer fluids), galvanized steel is not recommended for use with ethylene glycol heat transfer fluids, even if they are inhibited and especially if the temperature goes above 120 degrees F. The zinc coating of galvanized steel is designed to protect against atmospheric corrosion. At slightly elevated temperatures, zinc becomes cathodic to iron or steel reversing the galvanic series. Thus, the iron or steel will protect the zinc by corroding. In addition, phosphates in the inhibitor will react with the zinc, precipitating an insoluble material that can foul the system and encourage under-deposit corrosion. It would seem that galvanized steel is not a good material of construction choice for your application.