Passive films formed on SUS304 stainless steel that were covered with thin water layer containing various kinds of salt and exposed to atmospheric corrosion environment of 90％ relative humidity and 60℃ were characterized using X-ray photoelectron spectroscopy (XPS). Thickness and Cr content of passive films were affected by a minute amount of salts provided on the surface as a thin water layer. Thickness of the thin water layer on the surface of the steel seemed to be controlled by the equilibrium relative humidity of saturated solution of chlorides supplied on the specimen. Namely, the thickness of the thin water layer containing chloride with low equilibrium relative humidity was larger compared to that containing the chloride with higher equilibrium relative humidity. As a result, the selective dissolution of Fe is stimulated under the thicker water layer to be the more Cr containing passive films. Chlorides enhanced the formation of more Cr enriched passive films, but more concentrated chloride caused localized corrosion. In particular, passive films formed with the thin water layer containing suitable amount of sodium chloride occasionally exhibited high localized corrosion resistance. Sulfates also accelerated the selective dissolution of Fe and increased Cr content in passive films, resulting in stable growth of passive film without localized corrosion.
We investigated a cathodic protection design basis for reinforced concrete structures exposed to atmospheric conditions. If such structures in severely corrosive environments do not satisfy the relevant cathodic protection criteria, the situation can be remedied by supplying more current than the design value. However, as such energization results in rapid consumption of the water surrounding the anode, the voltage DC power supply may rise. Using electrochemical measurement and numerical simulation, we sought to clarify the effects resulting from environmental improvement of rebar surfaces using cathodic protection and devised a new design method. Then, via cathodic protection examination using concrete specimens affected by rebar corrosion, the validity of the new design method was determined. The approach was found to be suitable for economical and stable cathodic protection.