engleski

Analysis of the topographic criteria for pitting evaluation

Pitting corrosion is the most frequent form of the electrochemical deterioration caused by local depassivation of metals which are usually in passive state like stainless steels, Ni-alloy, Al and Al-alloys, Ti-alloys etc. Pitting has often catastrophic consequences for thin wall and mechanically loaded structures (eg leakahes of pipes and vessels, deformation and fracture of high pressure equipment and carryng constructions). Technical damaging effects of pitting depend, first of all, on topography of metal destruction causing local decline of pipes and vessels walls as well as loss in mechanical properties. Topographic characterisation of pitting is brought about by means of some quantitative criteria. According to ASTM G 46-94 they are: - pits density (pits number per unit of the observed area) - average pit size (orifice area) - average pit depth - pitting factor (the ratio between maximum pit depth and average corrosion penetration). The significance of these and many other criteria has been analysed. Among them are eg: - maximum pit depth - pit size and depth variability indices - average corrosion non-uniformity index - pitting penetration density and intensity etc. By combination of criteria from ASTM G 46-94 except pitting factor, the meaning of which is ambiguous, with several other criteria it is possible to get better insight into technical damaging risk of pitting corrosion.

pitting corrosion; topographic criteria; damaging risk

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