Erosion and/or corrosion is a conjoint action involving corrosion and erosion in the presence of a moving corrosive substance including a fluid, leading to the accelerated loss of materials. Analytical results of failure accidents by erosion/corrosion are shown. Countermeasures of design and inspection are given for localized metal loss by erosion/corrosion.
After a short introduction for the recent corrosion accident of the Mihama nuclear power plant, characteristics of the erosion/corrosion phenomena including flow accelerated corrosion (FAC) have been reviewed with emphasizing the effect of the flow rate on metal loss. The FAC mechanism of carbon steel in high temperature water was discussed based on the physicochemical model and mass transfer model with hydrodynamics. Several mass transfer models which assume the carbon steel corrosion process to be determined by dissolution of magnetite film were reviewed. It was pointed out that a macro cell corrosion mechanism attributing to difference in oxygen concentration which has been recently proposed is hardly accepted for the flow accelerated carbon steel corrosion in high temperature pure water because of low conductivity of pure water and low driving force for corrosion reaction due to low oxygen concentration.
It is very difficult to interpret the technical term of “erosion-corrosion” which is sometimes encountered in piping systems of power plants, because of complicated mechanisms and several confusing definitions of erosion-corrosion phenomena. “FAC (flow accelerated corrosion)” is recently introduced as wall thinning of materials in power plant systems, as a representative of “erosion-corrosion”. FAC is, however, not necessarily well understood and compared with erosion-corrosion. This paper describes firstly the origin, definition and fundamental understandings of erosion and erosion-corrosion, in order to reconsider and reconfirm the phenomena of erosion, erosion-corrosion and FAC. Next, typical mapping of erosion, corrosion, erosion-corrosion and FAC are introduced in flow velocity and environmental corrosiveness axes. The concept of damage rate in erosion-corrosion is finally discussed, connecting dissolution rate, mass transfer of metal ions in a metal oxide film and film growth.
Effects of chemical compositions of austenitic stainless steel parent metals and weld metals on hydrogen environment embrittlement (HEE) properties were investigated by Slow Strain Rate Testing (SSRT) in gaseous hydrogen environment pressurized at 45MPa. Test results were discussed based on the formation of strain-induced martensite (α′) phase and δ-ferrite phase. Both solution heat treated parent metals and as-welded metals with higher Md30 showed higher HEE susceptibility at lower testing temperatures due to an increase in volume fraction of α′ phase during straining. δ-ferrite phase in weld metals, below 20 volume % in this study, showed no effect on hydrogen embrittlement. The detrimental effect of α′ phase would be due to their successive formation at the crack tip. On the contrary, δ-ferrite phase which was finely dispersed in advance in the weld metals did not affect hydrogen induced crack propagation.
Carbon steel is considered to be the most promising material for an overpack container used for the geological disposal of radioactive wastes. In order to know the effect of alloying elements on corrosion resistance in the disposal environment, corrosion mass loss, polarization curves and electrochemical impedance spectra were measured on carbon steel and low alloy steels in compressed bentonite containing simulated bentonite contact water with chloride at pH 8 to 13. It was found that 0.5%Ni-alloying increases but 0.5%Mo-alloying decreases corrosion resistance in the environments. A relatively large dissolution rate was observed on 0.5%Ni-containing steel in the early stage of corrosion. However, the dissolution rate decreased in a short time by the formation of protective film. The dissolution rates of 0.5%Mo-containing steel and carbon steel were smaller than that of 0.5%Ni-containing steel in the early stage but hardly decreased with time.