The effects of MnS inclusions and alloying elements, Mo, Cu and N, on the corrosion behavior of 18-8 stainless steels have been investigated metallographically in organic acids as a food additive. The composition of MnS depended on the Mn/S ratio in the steel, and dissolution resistance of MnS was increased by decrease in the ratio. The active regions of anodic polarization curves of the steels in organic acid solutions consist of the potential range for dissolution of the matrix and that of MnS. It is, therefore, considered that increasing in the dissolution resistance of the matrix, decreasing in the amount of MnS or reducing of the Mn/S ratio are effective for the improvement of corrosion resistance of stainless steels. Corrosiveness of lactic acid was stronger than that of other organic acids as a food additive, and the corrosion resistance in the lactic acid solution depended on the amount of MnS and the Mn/S ratio. The amount of MnS and the Mn/S ratio affected the dissolution reaction in the active region of the steel. The effect of Mo on the improvement of corrosion resistance was pronounced, and was redoubled with extremely decreasing in the S contents, whereas Cu decreased corrosion resistance of the steels due to the acceleration of the anodic dissolution reaction.

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In order to evaluate the effect of environmental parameters on atmospheric stress corrosion cracking (SCC) of SUS 304 and 316L stainless steels, SCC tests were carried out in laboratory air controlled temperature and relative humidity (RH) environment by using U-bend specimens with the deposition of 0.5N chloride solution (NaCl, MgCl₂, CaCl₂, ZnCl₂) and synthetic sea water droplets.
SCC susceptibilities of steels strongly depend on RH and the maximum susceptibility was observed at the inherent relative humidity (RH_max) for each chloride. The values of RH_max were about 60, 30, 20 and 10% for NaCl, MgCl₂, CaCl₂ and ZnCl₂ solutions respectively, corresponding to their vapor pressure of saturated solution (P_sat), and hence, where the specimen surface is covered with a thin saturated chloride solution layer. In case of synthetic sea water, RH_max was almost same as that of MgCl₂, suggesting that SCC susceptibility was affected by the salt with lowest P_sat even in minor solute composition.
The critical values of relative humidity (RH_crit) below which no-SCC was observed were some what lower than RH_max, where the solution dried up. Above RH_max, the SCC susceptibilities decreased with increasing RH, due to the dilution of the chloride solutions.

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A scanning vibrating electrode system is developed for the analysis of corrosion fatigue of a high strength steel in synthetic sea water. The distribution of current density on the specimen during corrosion fatigue test is measured and displayed in a 3-dimensional map. It is found that local anodic current is observed at the main fatigue crack. However, most of anodic currents are observed in the area covered by corrosion products, under which many small pits associated with micro cracks are found. It is concluded that not only the main fatigue crack but also micro cracks formed under corrosion products contribute to the anodic current which determines the corrosion potential during corrosion fatigue process.

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An organic compound acting as a soft base is readily chemisorbed on a metal acting as a soft acid, resulting in high inhibition efficiency of the compound for corrosion of the metal in an acid solution. With respect to the corrosion of Ti in 3M HClO₄ and of Mn in 0.3M HClO₄, the inhibition efficiency of the compounds of which polar atoms belong to the 6B or 7B group in the periodic table was confirmed to be closely related to the hard and soft acids and bases principle (HSAB principle). Softnesses of the metals as the acids were also estimated by using the relation-ship between the efficiency and the principle.

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