CORROSION OF NICKEL-BASED ALLOYS IN MELTS OF MGCL2-KCL-NACL TERNARY CHLORIDE EUTECTICS

Corrosion tests of a series of heat-resistant nickel alloys were perfo rmed in a molten 48.7% MgCl2-38.2% KCl-13.1% NaCl ternary eutectic at 500-900 degrees C and with up to 30-h exposure. Chloride corrosion exh ibits intergranular features and causes weight loss an order of magnit ude higher than common gaseous oxidation does. Time dependence of the weight loss is intermediate between parabolic and linear (1 < n < 2), while the temperature dependence is exponential. The loss is significa ntly decreased only by alloying with aluminum. The corrosion resistanc e of NiAl intermetallide is relatively high (it surpasses nickel 60-70 times and commercial heat-resistant alloys 20-30 times). As a functio n of chromium and titanium contents in binary nickel alloys, corrosion has minima at 10-12% Cr and 5-7% Ti, respectively; the minimum in mul ticomponent alloys is located 2-3% lower. Cobalt, molybdenum, and tung sten affect the chloride corrosion resistance nonadditively, but even the best variants of alloying are characterized by corrosion of up to 20-30 g/(m(2) h). In order to achieve the required corrosion resistanc e, commercial high-temperature alloys should be aluminized or protecte d by a Ni(Co)Al-(beta-phase) intermetallic-based coating.