SUS304ステンレス鋼のすきま内外における腐食に及ぼす塩種と乾湿条件の影響

Abstract

As the requirement for durability of automobile increase, the application of type 304 stainless steel is expanding because of its high workability and corrosion resistance. On the other hand, one of the main corrosion environments of automobile is chloride condition caused by deicing salt or sea salt particle, and it becomes more severe when added to the wet/dry cyclic condition that is specific to automobile. In this case, the corrosion resistance is not enough even if the stainless steel is applied. In the case of stainless steel, crevice corrosion is mainly concerned in chloride environment. Pitting corrosion, however, can sometimes occur at non-crevice site. The purpose of this study is to clarify how the corrosion factor such as salt composition or wet/dry condition affects to the corrosion behavior of type 304 stainless steel inside and outside of crevice. The corrosion factors described above were evaluated by the corrosion test method developed in our previous work. Furthermore, electrochemical measurements were conducted. As a result, when NaCl is used for chloride addition, there is more severe corrosion at crevice than non-crevice site regardless of wet/dry condition. However, when sea salt particle or CaCl2 that is a major component of deicing salt are used, corrosion sites differ depending on wet/dry condition. That is, corrosion is severe at crevice site under high humidity condition of more than 60%RH, while pitting corrosion is prominent at non-crevice site under low humidity of less than 40%RH. Considering the corrosion behavior under various wet/dry conditions, main corrosion factor of sea salt particle is not NaCl but MgCl2 that is not major component.