A bipolar plate for polymer electrolyte membrane fuel cell (PEMFC) composed of two stainless steel pieces, i. e. channel former and diaphragm (bottom plate), has been designed to investigate the corrosion behavior of each piece during single cell operation. No apparent corrosion was recognized for type 310S stainless steel as a channel former irrespectively of the paired materials as a diaphragm. Type 430 stainless steel suffered from corrosion when used as a channel former with a faster cell voltage decay and significant contamination of MEA by the released iron ions. When type 430 stainless steel was adopted as a diaphragm, on the other hand, no inferior results were obtained in terms of cell performance and MEA contamination, although some traces of corrosion were recognized on the surfaces.
A micro-electrochemical cell for detecting permeated hydrogen, which is generated by atmospheric corrosion at small scratched areas on coated steel, was developed. It is possible to measure hydrogen permeation currents in the nano-ampere range. Using this cell, the influence of the scratched area on the hydrogen generated by the corrosion was investigated. The formation of galvanic couples between steel substrate and coating plays an important role in the generation of the hydrogen during atmospheric corrosion, and the amount of permeated hydrogen increases with larger areas of scratches.
Coating mixed with superabsorbent polymer and corrosion inhibitor was applied on a steel plate to investigate the corrosion prevention behavior. After creating an artificial defect on the specimen, polarization resistance of the scratched specimen was monitored in a 0.5 wt% NaCl solution at 35°C using an electrochemical impedance method. The polarization resistance of scratched specimen coated with superabsorbent polymer, calcium nitrate and vinylester polymer increased with increasing time in two stages, resulting in self-healing corrosion protective capability. Particulate film was observed in the scratched part of the specimen. Measuring polarization behavior, the film could be prevented cathodic reaction due to superabsorbent polymer in first stage, and anodic reaction due to calcium nitrate in second stage.
Field test piping of copper tubes has been installed in pitting and pitting-free area in Noboribetsu city to reproduce moundless type pitting corrosion. Inner surfaces of the copper tubes sampled from field test piping after 9 years have been investigated by optical microscopic observation and EPMA analysis. Inner surfaces of the copper tubes in pitting-free area were corroded like pockmark, while many pits occurred on the copper tubes in pitting area and have morphological characteristics similar to the moundless type pits. The pitting corrosion like moundless type pits were observed on the copper tubes in pitting area over 7 years after the installing. Depth of pits occurred on the 7 to 9-year old copper tubes is about 100 μm. It is possible that the slow pace of pitting initiation and propagation in this field test in pitting area is attributed to the cover the inner surface of the copper tube with the scale composed of Si and Al or Si and Fe.
The high temperature and high pressure cell for in situ Micro-Raman spectroscopy has been developed for the investigation into the change in the chemical composition of the corrosion products and the surface morphology of steels during the corrosion at the elevated temperatures and pressures. The developed cell was used for the analysis of the corrosion products on various Cr-bearing steels in a solution containing CO2 at elevated temperatures and pressures, and the chemical composition of the corrosion products were analysed continuously. These results demonstrate the utility of the developed cell for the investigation into the corrosion process of steels at elevated temperatures and pressures containing CO2.
As reported before, conventional group of flat heald, heald C, has used to serve for about two years, while a new group of flat heald, heald X, suffered significant localized corrosion earlier in one or two months. In this study, steel C’ of chemical composition similar to heald C is heat-treated to reproduce the heald C in terms of metallurgical characteristics (hardness Hv ; amount of insoluble carbide [MC]; prior austenite grain size dγ). The characteristics of heald C (484 Hv ; 1.2% ; 12 μm) is found to be reproduced by a solution-treatment at 1040°C for 180 s, WQ followed by tempering at 450°C for 20 s, AC. Those of heald X (474 Hv ; 2.0% ; 12 μm), lower in hardness and more in insoluble carbide than C, could be reproduced by solution-treatment at 1040°C for around 100 s, shorter time than heald C. The heat treatment condition determined as above is confirmed to corrode the steel C’ in 6.8 vol% HNO3 solution similarly in surface appearance to healds C and X. Their corrosion rates in the same solution vs. soluble Cr content, [Cr]relationship are nearly equal to the corrosion rates vs. [Cr] relationship for relevant solution-treated steels. The repassivation potential for crevice corrosion, ER.CREV, in 0.85, 2.8 or 28 mM NaCl solution depends also on [Cr], nearly constant between [Cr] range of 11.5 and 12.8%, and above which becomes more noble with increasing [Cr] up to 13.4%. Then, [Cr] values of 11.5% (heald X) and 12.5% (heald C) make no difference in ER.CREV. Real crevice corrosion for heald X was attributed to more noble spontaneous electrode potential, ESP, of heald X than ER.CREV under increased residual chlorine, as reported before.
Corrosion behavior of Al-Zn alloy coating on aluminum alloy substrate by flame splay methods in seawater was investigated using a jet-in-slit testing apparatus. Light brown corrosion product was observed on the surface of the specimen tested in static seawater, however, little corrosion occurred. On the other hand, white corrosion oxide product was observed on the surface of the specimen tested in flowing seawater near freezing point. The difference in the corrosion morphology was caused by the crack of the protective film generated near freezing point and the increase of oxygen supply onto the defected surface due to flowing of solution.