Corrosion resistance of super duplex stainless steels in chloride ion containing environments: investigations by means of a new microelectrochemical method I. Precipitation-free states

A new mircoelectrochemical method was applied to perform potentiodynamic po larisation experiments on areas in the range of 10 mum For the first time, the individual corrosion behaviour of both single phases in super duplex st ainless steels was determined. The results show a good correlation with the empirical pitting resistance equivalent number (PREN) of the corresponding single phase. The microelectrochemical experiments have revealed two diffe rent kinds of interactions between the ferrite and the austenite phase at t he phase boundary, namely a superposition or a separation of the two polari sation curves of the single phases. Potentiodynamic polarisation of large areas with representative amounts of both phases are performed in hydrochloric acid electrolyte in order to comp are the corrosion behaviour of the single phases with the corrosion behavio ur of the entire alloy. Both. the pitting potentials, evaluated by means of macroelectrochemical experiments in pH-neutral lithium chloride electrolyt e, and the critical crevice corrosion temperatures show a good correlation with the PREN of the weaker phase. Part I of this paper deals primarily wit h solution annealed materials. Solution annealing of the samples was perfor med at temperatures where minimal amounts of precipitates were formed and w here the element partitioning determines the corrosion resistance of the si ngle phases. Part II deals with the influence of precipitations on the corr osion behaviour. (C) 2001 Elsevier Science Ltd. All rights reserved.