*2.2.2.1 Removal of hardened surface*

Some materials have the ability to increase hardening. When the erosion process occurs, the abrasive particles continuously hit the surface, and this causes a cold working to be induced on the surface of the material. This cold working induces a hardening effect on the surface and protects the material from further loss by erosion or, in other words, reduces the erosion process. Matsumura and his colleagues [9] investigated the erosive corrosion process in a passive system and found that particles cause the wear of the passive protective film and accelerate the corrosion process. This corrosion flow decomposes the hardened layer on the surface of the material; hence, the erosion resistance decreases.
