Define Abrasion of steel structures?

Abrasion of steel structures can generally be recognised by a worn, smooth, polished appearance of the surface.  Abrasion is caused by continual rubbing of adjacent moving steel surfaces, or by the exposure of structural components to wave action in areas of sandy bottom.  Steel sheet piling and pile-supported structures are particularly susceptible to sand abrasion in exposed locations.  Abrasion of steel structures is a problem because it removes both protective coatings and protective layers of corrosion products, thus accelerating corrosion.