Effect of MnS Inclusions on the Corrosion Resistance of TiN-Coated Stainless Steels

The corrosion resistance of 18-8 stainless steels coated with TiN formed by a hollow cathode discharge (HCD) type ion plating process has been studied by using mainly electrochemical means in sulfuric acid solutions. The influence of the sulfide (MnS) inclusions on the intercepting effect of the TiN film from its surroundings was also estimated. In specimens having greatly diminished MnS content, a homogeneous. film was formed on the steel regardless of the Mn/S ratio in the steel. In specimens with a large MnS content, on the other hand, a homogeneous film can be formed by decreasing the Mn/S ratio. Thus to improve the corrosion resistance of the TiN coated steels, it is necessary to diminish the MnS content and decrease the Mn/S ratio. The starting points of corrosion of the TiN coated steels were film defects formed on the MnS inclusions. In steels with a small amount of MnS and a low Mn/S ratio, spalling of the TiN films scarcely occurred, and corrosion resistance was excellent. When the Mn/S ratio was low, the surface of the MnS after ion bombardment was smooth because the MnS was practically unaffected by physical attack of argon ions. Consequently TiN films formed on the MnS contained hardly any defects, and are superior in terms of the intercepting effect from the environment.