Effect of ligand field on cation forming complexes with precipitation inhibitors for metallic corrosion in aerated neutral solution are discussed based on the hard and soft acids and bases （HSAB） principle. The energy levels of three 3d orbitals, d_xy, d_xz and d_yz in Fe^3＋, for example, decrease and two orbitals, d_x²_－y² and d_z² increase markedly by approaching an effective ligand, ：CN^－, resulting in the formation of d²sp³ hybrid orbital. Contrary, less effective ligand like ：NH₃ provides sp³d² hybrid orbital. Because chelating agents enhance the ligand field effect markedly, the d²sp³ hybrid orbital of Fe^3＋ can be provided for the formation of chelate compound, resulting in stable coordinate bonds between hard acid and hard base.

This study focused on the differences in initial corrosion behavior of spheroidal graphite cast iron, carbon steel and weathering steel in atmospheric corrosive environment, the atmospheric exposure test and dry/wet cyclic test were conducted to examine the effect of graphite and Cu of spheroidal graphite cast iron on its initial corrosion behavior. After exposure to the atmosphere for two years, the mean corrosion depth of spheroidal graphite cast iron is smaller than that of carbon steel and similar to that of weathering steel, whereas the initial corrosion of spheroidal graphite cast iron progresses earlier than carbon steel and weathering steel, regardless of the temperature, humidity, and the amount of sea salt. From the initial corrosion test results in the wet-dry cyclic test, that Cu precipitation was observed at the corrosion origin of the spheroidal graphite cast iron, which is same as that in weathering steel. Based on the findings obtained in this study, a schematic diagram of the initial corrosion mechanism of spheroidal graphite cast iron was speculated.

Corrosion resistance of carbon steel covered with resin coating containing nickel sulfate has been evaluated under chloride and sulfuric acid mist environment. The structure of corrosion products formed on steel surface was investigated by XRD and XAFS analyses using synchrotron radiation. Nickel sulfate promoted the formation of goethite and akaganeite. It was considered that this akaganeite was not tetragonal β-FeOOH but monoclinic akaganeite containing nickel.