Corrosion of Immersed Ceramic Heat Exchanger Tubes in Aluminium Foundry Baths

Abstract : The corrosion of three non-oxide ceramics by Al-9Si-3Cu baths and by fluxes (mixtures of chlorides and fluorides of sodium and potassium) at about 750 degrees C was studied in a foundry environment. Comparison of results of the metallurgical examination of A, a silicon-nitride-bonded silicon carbide and of B, a reaction-bonded silicon nitride, surface treated to fill all the external porosity provides some insight into the role of the bonding phase and the porosity. Grade C is a graphite bonded silicon carbide with an external protection by a ceramic glazing. The SiC phase in the tubes is inert to the corrosive liquids (attributed to the silicon content in the metal). A and C ceramics react only in the presence of a fur. Sodium and chlorine were identified in the corrosion products as well as AlN (A) and Al4C3 (C), resulting from reaction of the silicon nitride or of the graphite bonding phase with aluminium. This suggests that the fluxes are responsible for the corrosive process, by causing the formation of gaseous aluminium halides which penetrate the porous bonding phase and react with it to form AlN or Al4C3.