光輝焼鈍したSUS434フェライト系ステンレス鋼の酸化皮膜の結晶構造とその耐食性におよぼす影響

抄録

In the previous paper, ¹⁾ it has been reported that the crystal structures of oxide films formed on the surface of bright-annealed type 430 ferritic stainless steel depend on the bright-annealing conditions and chemical composition, especially Mn and Si contents of the steel, and influence the corrosion resistance of the steel to dilute sulphuric acid.
This paper deals with the influence of oxide films formed on the surface of type 434 steel upon the corrosion resistance to dilute sulphuric acid, when bright-annealed under various treating conditions.
The results obtained are as follows;
Despite that SUS 434 contains Mo as an important element to improve the corrosion resistance, oxidation behaviour of the steel (Si : 0.45%, Mn : 0.52%, Mo : 0.99%) in H₂-H₂O atmospheres with dew points of -10° to -40°C at temperatures of 800° and 900°C is almost same as that of type 430 containing same contents of Mn and Si. The crystal structures of oxide films formed on the surface of the steel, when treated in the atmospheres with dew points of -10° to -30°C at 900°C, are mainly MnCr₂O₄ and SiO₂. With increasing dew point in this range, the formation of inner oxide layer is promoted and SiO₂ precipitates in the substratum near the interface between oxide film and substratum. On the contrary, with decreasing dew point, SiO₂ precipitates densely and forms an external oxide film with MnCr₂O₄.
The corrosion resistance behaviour of bright-annealed SUS 434 can be explained by the variation of the microstructure and thickness of the oxide films formed under the varied treating conditions.