Tensile stress is a necessary component for stress corrosion cracking (SCC). On the other hand, compressive stress have beneficial effects on the localized corrosion decrease, due to modification of electrochemical reactivity and pitting initiation inhibition. Depending on the type of stress, repassivation potential and passive current density could be increased or decreased
Almost all metals (except noble ones like Au, Pt) under go corrosion even when no stress is present on metal surfaces.But presence of the stress greatly enhances the corrosion by the phenomenon named as SCC[Stress Corrosiob cracking].Without going in details about SCC, please find below the reasons for the enhanced corrosion due to SCC :
[1] Active Path Dissolution: While the bulk material is passive, somewhere the grain boundary corrodes and infinitesimally small crack (invisible to human eye) is formed.The effect of stress is to open up (broaden) these cracks; therby allowing easier diffusion of corroded products away from the crack tip.Then this effect is propagated further at a compartively much faster rate.If there were no SCC, then this diffusion process would have taken years and years as it grows with a speed of only 10^-8 m s^-1 while in the presence of SCC , the rate becomes 10^-2ms^-1.
[II] H2 Embrittlement: H2(smallest molecule), has the property of adsobing on all the metals with different rates of diffusion (depending on the nature of the metal) as there are present empty spaces (holes/ interstial sites) .This gaseous H2 is drawn towards the sites having highest stress as the crystal structure has all the more been dialated at those places. The cummulation of the gas makes this site more brittle, i.e. the crack formation rate is enhanced to a large extent and as the effect of corrosion gets propagated, more and more space gets corroded. Further, it may be noted that it the unit cell structure of the metal which will decide the H2 adsorption.For example, it is more pronounced in [fcc] (face centred ccubic) stucture than [bcc] (body centred cubic) structure. The ordinary iron with [bcc] structue is easily corroded than austenitic steel having [fcc] structure.
[III] Film Induced Cleavage: Many a times, there is a formation of very thin (one mu m). Such thin films are more brittle (while the metal below is ductile).The brittleness makes their grains coarse which can be easily pealed off and enhances corrosion under stress.
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