Typical austenitic, ferritic and duplex stainless steels with different surface treatments were put to laboratory simulated seaside environment corrosion testing. Tests were measurement of pitting potential in 3% NaCl solution at room temperature, and corrosion under MgCl2 deposit at 40°C and 50% relative humidity, which is an accelerated seaside environment. Kelvin probe was used to study the surface properties of materials. Pitting potential in 3% NaCl, and simulated seashore atmospheric corrosion testing gave adequate data for evaluating corrosion of stainless steels under chloride particles. Kelvin probe was useful in corrosion monitoring on whether stainless steel was attacked or not by chlorides.
The surface degradation of metals that occurs in high temperature corrosive environments has been severe problems of boiler materials in fuel combustion power plant systems. The damage to materials has been often accelerated by mechanical removal of oxide films formed on the material surface. It is therefore very important to investigate mechanical properties and adhesiveness of oxide films at practical high environments, in order to discuss life cycle assessment. Corrosion/oxidation tests were conducted at high temperature corrosive environments of wet air, water steam with hydrogen chloride to look into basic behavior of the corrosion/oxidation. Impact tests were performed on the surface of carbon steel covered with oxide films in the same environments to obtain mechanical properties and adhesiveness of the oxide films. It was found that the corrosive environments increased oxidation of carbon steel a little, and that decreased the fracture strength and other mechanical properties of the oxide films.
Corrosion resistance of duplex stainless steels containing 4%Ni-23%Cr-0.3%Mo-0.2%N and 0 to 2%Cu was investigated in aqueous chloride solutions. The addition of Cu showed no effect on the pitting potential, while it improved the crevice corrosion potential and the repassivation potential. The Cu alloying was effective on suppressing the pitting corrosion in boiling 25%NaCl solution with phosphoric acid, but was not effective in that with dichromate. The Cu addition was effective on improving the resistance to depassivation in acidic environments, while it increased the passive current density and the critical passivating current density.
The electrochemical method to obtain the i-E curve of carbon steel in the fresh water has been developed. The electrochemical impedance spectra were measured during the measurement of potentiodynamic polarization curve. The precise solution resistance could be determined by the 3-D impedance plots, and the iR drop was compensated for the i-E curve. Furthermore, the variation of the solution resistance was monitored to discuss the variation of solution nature during the measurement of i-E curve. The Pd-H electrode was employed as a reference electrode, because the Pd-H electrode is stable in the aqueous solution and yields no contamination of the solution. Beside, the electrochemical cell in which the solution was impinged onto the working electrode was constructed in order to avoid the variation of the solution composition due to the working electrode reaction. The present method was applied to the measurement of i-E curve of carbon steel in the tap water, and the relationship between i-E curve and the corrosion mechanism of the carbon steel was discussed.