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Type 304 stainless steel was oxidised at various temperatures in air. The pitting susceptibility oj the oxidised and unoxidised specimens was evaluated inJerrie chloride solution. The increased pitting susceptibility oJthe oxidised specimens at about 500°C is attributed to the change oj chemical composition oj the oxide film on the surface. Table 1 Appearance· of oxide films formed at different temperatures DISCUSSIONThere are several factors which affect the pitting susceptibility of the oxide film, such as the thickness,4 the chemical composition,2 the semiconductor nature of the oxide, 1 and chromium depletion in the alloy beneath the oxide. s Isaacs and Kissel 4 found that below 300°C pitting susceptibility increases with increase in oxide film thickness. The greater pitting susceptibility of the thicker oxides was attributed to the oxide acting as a barrier, restricting the flow of chloride anions from the pit. In the same temperature range, Bianchi et al;1 found the oxygen deficient oxide film (n type) to be more susceptible to pitting corrosion than the oxide film with excess oxygen (p type) with respect to complete stoichiometry. The results of this study (Fig. 2) show that there is no significant difference in pitting susceptibility of (ii) beyond 400°C the pitting density increases with increase in oxidation temperature (iii) the pitting density reaches a maximum value of 40 pits cm -2 at 500°C; beyond 500°C it decreases with increase in temperature (iv) after reaching a minimum value of 5 pits cm -2 at 700°C, the pitting density remains almost constant with further increase in temperature up to 1100°C (v) the pitting density of specimens oxidised above 700°C is much lower than that of specimens oxidised below 400°C. The corrosion rates determined by weight loss measurements are shown in Fig. 2b and show the same trend as observed for pitting density at temperatures of up to 700°C (i.e. both the pitting density and the corrosion rate show a maximum at 500°C). The curve of corrosion rate versus oxidation temperature shows another maximum at 800°C. The corrosion rates of specimens oxidised above 700°C are much higher than those of specimens oxidised below 400°C. The specimens oxidised above 800°C show larger diameter pits than those oxidised at lower temperatures.

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