GEOMETRY OF PITTING CORROSION ON OXIDISED STAINLESSSTEEL SURFACES

摘要

The influence of thermal oxidation on pitting corrosion of stainless steel (SS) was investigatedrnby determination of geometric characteristics of the process. Sheet specimens of SS AISI typern316L were oxidised by heating in air for 20 min, and that isothermally at 200, 400, 600, 800 andrn1000℃ respectively or in temperature gradient between 1100 and 100℃. The specimens were 3rndays exposed to 10 % FeCl3·6H2O solution at 25℃ according to ASTM G48-92. Byrnmicrography of cross sections of treated specimens it has been established that the cavities of pitsrnhave approximately hemiellipsoidal or hemispherical form. On the observed surface area (S =rn90 cm2) of each specimen pits number (np), their depths (hp) and orifice areas (Sp) wererndetermined. The relations between values of np, hp, and Sp are analysed. For comparison, thernspecimens in the initial state as well as the heated specimens after removal of oxides by picklingrnin HNO3/HF were tested in the same way. The specimens in the initial state and after heating atrn200℃ have no pitting liability under experimental conditions. After isothermal heating at 400rn℃ and above pits densities (np/S) are high but vary considerably with temperature. The same isrntrue for single zones on specimens heated in temperature gradient. The effect of thermalrnoxidation on hp and Sp is equivocal. The removal of thermal oxides after heating at 400, 600, andrn800℃ as well as after heating in temperature gradient decreases np/S but it does not restore therninitial pitting resistance. After removal of oxides from specimens heated at 1000℃ certainrnincrease of np/S is even observed. In the discussion an attempt to explain the mentionedrnphenomena is made.