engleski

Geometry of pitting corrosion on oxidised stainless steel surfaces

The influence of thermal oxidation on pitting corrosion of stainless steel (SS) was investigated by determination of geometric characteristics of the process. Sheet specimens of SS AISI type 316L were oxidised by heating in air for 20 min, and that isothermally at 200, 400, 600, 800 and 1000 oC respectively or in temperature gradient between 1100 and 100 oC. The specimens were 3 days exposed to 10 % FeCl3&middot ; ; ; ; ; 6H2O solution at 25 oC according to ASTM G48-92. By micrography of cross sections of treated specimens it has been established that the cavities of pits have approximately hemiellipsoidal or hemispherical form. On the observed surface area (S = 90 cm2) of each specimen pits number (np), their depths (hp) and orifice areas (Sp) were determined. The relations between values of np, hp, and Sp are analysed. For comparison, the specimens in the initial state as well as the heated specimens after removal of oxides by pickling in HNO3/HF were tested in the same way. The specimens in the initial state and after heating at 200 oC have no pitting liability under experimental conditions. After isothermal heating at 400 oC and above pits densities (np/S) are high but vary considerably with temperature. The same is true for single zones on specimens heated in temperature gradient. The effect of thermal oxidation on hp and Sp is equivocal. The removal of thermal oxides after heating at 400, 600, and 800 oC as well as after heating in temperature gradient decreases np/S but it does not restore the initial pitting resistance. After removal of oxides from specimens heated at 1000 oC certain increase of np/S is even observed. In the discussion an attempt to explain the mentioned phenomena is made.

stainless steel; thermal oxides; pitting corrosion

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