Atmospheric corrosion is an electrochemical process which can only proceed in the presence of an electrolyte film. Rain is one of the main factors in initiating the time of wetness and transporting the chemical substance from the atmosphere to metal surface that guide to accelerate the corrosion attack. In this case study, an artificial rainfall equipment was developed with the aim of defining the behaviors of atmospheric corrosion which is affected by rain and chemical species. The Atmospheric Corrosion Monitoring （ACM） sensor which consists of a Fe-Ag couple was used to measure the corrosivity of the rainfall solution by finding the relation between the Corrosion Rate （CR） and the sensor output. The controllable precipitation rate and its distribution facilitate in investigating the effect of rainfall on the corrosion behaviors. The relation between the sensors output and CR enable to predict the corrosivity of the atmosphere. Thus, the developed artificial rainfall equipment is an essential and reliable tool for studying the corrosion behaviors under the effect of rain.

Concentration of SO₄^2－ and HCO₃^－ which affect to corrosion for carbon steels were estimated by analysis using pH, Cl^－ concentration and electrical conductivity κ, in the spent fuel pools （SFP） of Fukushima Daiichi Nuclear Power Station （1F） in which sea water and/or fresh water were injected. Electrical conductivity κ in the SFP of 1F Unit 1 in which fresh water was injected, were presumed accurately by analysis using observed Cl^－ concentration, concentration of SO₄^2－ and HCO₃^－ which were estimated. The pH measurements exceeded the estimate in 1F Units 2 and 3 where sea water was injected. Each of dissolution reaction of iron, dissociation of H₄N₂, and the production of ammonia by the decomposition of H₄N₂ has been evaluated as not the cause of the high pH by itself. It was estimated that the calcium ions from the concrete debris had been continuously supplied to water in SFP of the unit 3 during purification treatment period. It was evaluated that self-passivation conditions of carbon steel were achieved in the water chemistry after the purification process completed for the Fukushima Daiichi Nuclear Power Station of Units 2 to 4. In addition, it was evaluated that self-passivation of carbon steel was achieved in the environment of unit 1.