The corrosion of grey cast iron in hot water has been studied under conditions which are similar to those in central heating systems. The rate of hydrogen evolution from a cast iron boiler in a model system at 75°C was measured for 63 days. After an initial increase up to 9 days the rate decreased parabolically (k = 6.5 x 10-2 mg-2 cm-4 h -1 for 3Fe + 4H20 = Fe304 + 4H2). A duplex corrosion scale was found on boiler surfaces which is similar to those reported for corrosion of mild steel in water at 300°C. An increase- in dissolved oxygen stimulated hydrogen evolution, and a mechanism for this process is proposed. The formation of corrosion scales on cast iron at 50°C was studied in vitro. A linear dependence of corrosion rate upon dissolved oxygen in the range 0 to 4.0 ppm was found, the rate, R, being approximately that predicted by R = k[FeOH+] [OH-]2[O2] Scales formed on cast iron in a wide range of solutions contain lepidocrocite, magnetite and a green rust which is similar to, but not identical with, those previously reported. There is evidence for a crystallographic transformation of green rust (G.R) to magnetite CM), the orientation relationship having been partially determined thus: The growth sequences of scales formed at 3.0 ppm and 0.44 ppm O2 are described. The proportions of magnetite and lepidocrocite change continuously with time and with dissolved oxygen level, while there is evidence for electrochemical dissolution of lepidocrocite and precipitation of magnetite as the oxygen level is reduced. A cyclical mechanism for this process is proposed. Scales produced at 3.0 ppm O2 form shell-like "crusts" over a porous subscale; the "crust" exhibits "chimney"-like outgrowths which are orientated in the water-flow direction. Similar morphologies are observed in scales formed on circulating pumps used in central heating systems. These features are attributed to outward diffusion of soluble corrosion products, and possibly hydrogen, from the subscale.
|Date of Award||1976|