applied fracture mechanics

Early Corrosion Fatigue Damage on Stainless Steels Exposed to Tropical Seawater:A Contribution from Sensitive Electrochemical Techniques 235initiation on passive metal surfaces [26]. Stainless Steels (SS’s) with higher concentrations ofalloying elements were found to have an increased pitting resistance due to the formation ofpassive films containing fewer defects and greater stability.Despite the efforts made to understand this phenomenon, localized corrosion is a nonpredictabledegradation process of metals and alloys in contact with aggressiveenvironments. Given that most of the information related to the stable stage of localizedcorrosion has been obtained throughout studies of pitting and crevice corrosion, this aspectof the localized attack will be presented in the next section dedicated to pitting corrosion.3. Pitting corrosion fatigue3.1. Breakdown of the surface passive filmCorrosion is an electrochemical reaction process between a metal or metal alloy and itsenvironment [27], which involves complex mass and charge transfer taking place at themetal-electrolyte interface. These charge transfer reactions at the interface are the origin ofthe instability of metals. The high corrosion resistance of Austenitic SS’s is primarilyattributed to the adherent metal-oxide film formed on its surface called passive film. Thisnatural coating acts as a barrier that avoids the contact between fresh metal surface and theelectrolyte limiting the corrosion reaction and such alloy is said to be passive. Whenstainless steels are exposed to oxygen containing electrolytes, a chemically stable, nonsoluble film is formed from a mixture of iron and chromium oxides, with hydroxide andwater-containing compounds located in the outermost region of the lm, and chromiumoxide enrichment at the metal-lm interface [28]. However, the resistance of this passivelm is determined by the environmental conditions to which the SS is exposed to, as well asby the alloy composition. Many metals and alloys become covered with oxide films onexposure to aqueous environments. Copper, aluminum or tin and their alloys develop thickfilms, while metals which exhibit passive behaviour like stainless steels have a very thin filmof the order of 9 to 20 nm. The passive films are cathodic to the metal matrix. However, thesusceptibility of passive films to suffer local breakdown depends upon the nature of thefilm, the quantity and nature of non metallic inclusions, the chemical composition of theelectrolyte or solution, and the electrochemical state at the metal/solution interface, but alsoto a possible stress state.For CF test, when the passive metal or alloy is subjected to cyclic loading, the passive film isdamaged mechanically due to the creation of slip steps from beneath and following arepeated process of microdeformation-activation-dissolution-repassivation.During the initial cyclic loading, fine slip lines appear on the specimen surface becausedislocations, at or near the surface, the stress in localized sites can exceed the nominal stress.By using microscopic techniques has been possible to observe localized plasticity atmicroscopic scale in the grains, where fatigue damage is caused by microplasticity producedby large shear stresses.