Sherardizing: corrosion protection of steels by zinc diffusion coatings

Sherardizing is a zinc diffusion coating process, which uses zinc vapor to form zinc alloys with the base material. This chapter provides an overview of the fundamentals of the process and its applications. For sherardizing it is necessary to have the source (Zn powder) close to the coating surface, because of the low partial pressure of zinc. The coating ‘grows’ out of the base material forming uniform diffusion bonded coatings. The process allows the treatment of complex shapes, so that even hollow parts like pipes can be coated, and threads do not fill. Because of the rather low process temperatures, hardened and tempered steel can be processed without losing their beneficial mechanical properties. Sherardizing is performed in a hydrogen-free atmosphere, thus avoiding any hydrogen embrittlement. In conjunction with special passivation systems or paints, the corrosion resistance is excellent, forming a sacrificial zinc coating and an inert barrier coating. Small rivets, screws, clamps, chains, pipes, press formed and hardened steel can be protected giving a wide range of applications.