Erosion repassivation

Film is locally removed by dissolution, surface shear stress or particle/bubble impact but it can repassivate. Erosion corrosion rate is a function of the frequency of film removal, bare metal dissolution rate and subsequent repassivation rate. 

The materials properties most important in relation to erosion corrosion are thermodynamic nobleness, the ability to form mechanically stable and protecting surface films, and the liability to rapid passivation at the start of exposure as well as rapid repassivation after removal of surfaee films. The materials listed in Table 7.5 can roughly be divided into three groups based on their behaviour at the three velocities represented in the table ... 

In the absence of suspended particles, the corrosion rate of passive metals such as stainless steel or titanium in neutral media is not markedly affected by hydrodynamic conditions (Table 10.26). However, when exposed to slurries, these metals are subject to erosion corrosion because the suspended particles that impinge on the surface damage the passive film. As a consequence an anodic partial current flows which serves for film repair and repassivation of damaged areas. In the presence of aggressive anions such as chloride, passive film damage can lead to metal dissolution by pitting [23]. 

Figure 15.3 Current transients in erosion-corrosion due to depassivation/repassivation. Reproduced from Ref [8], Reprinted by permission of the publisher (Taylor Francis Ltd, http //WWW. tandf co. uk/journals).

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