THE EFFECTS OF CHLORIDES, HYDROGEN-CHLORIDE, AND SULFUR-DIOXIDE IN THE OXIDATION OF STEELS BELOW DEPOSITS

Thermogravimetric studies have been conducted on the oxidation of a low alloy steel at 500 degrees C and high alloy steels at 600 and 700 degrees C in which either NaCl or a fly ash (from a waste incineration plant) was deposited on the scale of the steels after 24 h pre-oxidation. The chlorides in the deposits react with the scale under formation of chlorine which enters the scale and causes accelerated oxidation, by the formation of FeCl2(s) at the scale/metal interface, evaporation of FeCl2(g) and its oxidation to Fe2O3 at the scale surface; chlorine partially returning into the scale. This leads to a porous unprotective scale and active oxidation, catalysed by chlorine. By variation of several parameters it was shown that the evaporation of FeCl2(g) and its outward diffusion through the scale is the rate controlling step of the active oxidation. The presence of SO2 in the atmosphere causes a minor increase of active corrosion by the sulfation of chlorides and the generation of chlorine. The presence of HCl in the atmosphere causes a transformation of sulfates in the deposits into chlorides, which induce enhanced active oxidation. In the presence of balanced concentrations of HCl and SO2 in the atmosphere, however, the corrosion is limited.