The addition of 500ppm potassium dichromate (PDC, K2Cr2O7) to a silica sand-water slurry decreased the amount of slurry erosion damage of commercially pure iron as well as of aluminium to the value smaller than that of pure erosion. Further, the salt (NaCl) dissolved 3% in the slurry water enhanced the erosion-inhibiting effect of PDC. Thus, it was concluded that PDC adsorbed on the surface of the metals and affected directly the erosion process. It was also found that the erosion-controlling mechanism varried with the particles impact angle: at an impact angle of 30°, it was the reduction of friction coefficient of the metal surface. At 70°, it was the tension applied to the metal surface through the adsorbed ions, which inhibited the plastic deformation of the surface.
Corrosion deterioration of steel structures has been serious problems. It is important not only to detect average corrosion rate but also to observe surface texture of corroded steel, because pitting or localized corrosion gives a large damage to the structure. FFT algorithm was used for characterization of the surface texture. Mild steel plates, 10×10cm2, which were exposed in several places in marine environment for one year were examined. Surface textures of the sample plates were observed by surface roughness meter which was detected electromagnetic induction current. It was found that parameter CFFT which was derived from FFT analysis was related to an amount of corrosion, and was also found that autocorrelation function, which was calculated from FFT result, suggested the distance from anodic area to cathodic area. The corrosion mechanism of mild steel in marine environment was discussed from these results.
Slow strain rate test, SSRT, has been widely used for studies of stress corrosion cracking, SCC. In evaluating SCC susceptibility, it is important to analyze initiation and growth of SCC separately even if SSRT is used. Then we have developed a new SSRT system with a high magnification and high focusing depth CCD camera in order to make in-situ observation of crack behavior on the whole specimen surface during SSRT, and analyzed initiation and growth of crack separately by using fracture mechanics and statistics. As a result, the number of pits, at which crack generated, was found to increase with increase in strain. The Weibull distribution was found for KISCC and its mode was 2MN·m-3×2. While, crack growth rate showed double exponential distribution and its mode was about 10-8m·s-1. Thus, This technique could evaluate SCC susceptibility by separating crack initiation and growth.
Tap water and corroded water pipes have been collected from 28 and 20 places respectively in Japan to elucidate the corrosion phenomena on the interior surface of the galvanized steel pipe for water service. The water quality was characterized by fairly high concentration of dissolved silicate and negative values for Langelier saturation index. The morphology on the cross sections of many of corroded pipes was characterized by the thin uniform corrosion layer on zinc coating and the swollen scale over the localized corrosion part. The principal constituent of corrosion products was hemimorphite (Zn4Si2O7(OH)2⋅H2O) in the uniform corrosion layer, and goethite (α-FeOOH) or magnetite (Fe3O4) in the scale. It is likely that the hemimorphite layer grows by the crystallization of amorphous compounds which are formed from zinc hydroxide and adsorbed silicate on the surface of zinc layer and hemimorphite and its formation prevented anodic oxidation of zinc. These observations suggest that partial formation of hemimorphite layer would obstruct the galvanic protection of galvanized steel pipe and accelerated the localized corrosion on the interior surface of pipes.
Principle, apparatus, and measurement method of Auger electron spectroscopy have been described. Applications of this method to surface analysis of corrosion products on electronic materials, stainless steels, and electroplated Zn-Ni (13%) alloy films are also described. Advantages and limitations for Auger electron spectroscopy are pointed out. Several questions and answers for them are presented.