Relationship between preferential dissolution behavior and the holding potential and its duration of the potentiostatic test of a duplex stainless steel in simulated solution inside corroding crevice was investigated. In the active region (−370, −300 and −260 mV vs. SCE), the current density decreased towards a certain value after the measurement started, then it stayed rather constant value until the end of the measurement, i.e., for 24 h. In the passive region (−200 and −150 mV), it decreased greatly towards certain values after the measurement started, then it stayed lower value until the end of the measurement. In all potential regions, current density values determined by potentiostatic tests showed about one order of magnitude lower than those of the dynamic anodic polarization curves. At the less noble potential (−370 mV) of the active region, ferritic phase preferentially dissolved, while that of noble (−260 mV) of the active region, the austenitic phase dissolved preferentially. In the passive region, both phases dissolved evenly. These dissolution behavior were observed in the potentiostatic tests for 15 min as well as for 24 h. Chemical composition of dissolved metal ions after dissolution tests at certain potentials in the active region was also analyzed. Chemical composition of dissolved metal ions was almost same as chemical composition of a preferential dissolution phase of the specimen at each holding potential. It would appear that chemical composition of dissolved metal into crevice solution reflects to the preferential dissolution phase.
Corrosion resistance to crevice corrosion attack of welds made with various Ni-Cr-Mo(-Ta) alloy fillers connected to Ni-based alloy plates in a hot concentrated NaCl solution was investigated using the technique of measuring repassivation potentials for crevice corrosion (ER,CREV). The ER,CREV of the test electrode with a weld could be regard as it of the weld. Under this condition it was made clear that the dendrite core where was the alloy element depleted region determined the corrosion resistance to crevice corrosion attack of the weld to be degraded it. ER,CREV of weld has linear relationship to Pitting Resistance Equivalent with tantalum (PRE(Ta)=[%Cr]+3.3([%Mo]+0.5 [%W])+7.7 [%Ta]) which is calculated from dendrite core composition. The line consistents with the relationship between ER,CREV of base metal and PRE(Ta) which is calculated from its chemical compotion. Under more severe conditions for crevice corrosion such as higher temperature, the ER,CREV of alloy 22 showed the same value with it of the weld made with the filler having the same composition of the base metal. It was suggested that the depnendance of the corrosion resistance to crevice corrosion attack of the weld changed into its average chemical composition from the dendrite core composition.
In order to monitor the corrosion rate of carbon steel under deep under-ground environment which is depleted with oxygen, the corrosion rate measuring apparatus using the hydrogen permeation current was developed. Inner surface of the carbon steel monitor electrode made of beverage can was coated with Ni or Pd-Ni plating, which was used as the hydrogen detecting electrode. Corrosion rate which was estimated from hydrogen permeation current well consisted with literature data. Corrosion rate in acidic solution at pH < 3.5 was found to be not changed with time, but the corrosion current in neutral and alkaline solution was decreased with time according to the parabolic rate law and approached the value below 0.01 μA/cm2 after long time elapsed.