Intensive investigations have been conducted on the failures experienced during the past years due to the corrosion of the furnace wall tubes of high pressure boilers in modern thermal power plants in Japan. The process and development of corrosion and its causes have now almost been determined, and it can be outlined as follows; (1) The iron and copper materials of the feed water system become corroded by the oxygen entering into the system when the boiler is banked. (2) Corrosion products such as the copper and iron oxides accumulate and are deposited on the inner surfaces of the furnace wall tubes of the boilers. (3) The boiler water containing free caustic alkaline penetrates into the very small crevices beneath the deposits; and concentrated by heat, it corrodes the tube surfaces. As a result of these investigations, the writers recommend the“Zero Caustic Method”as the boiler water conditioning and chemical cleaning in order to avoid corrosion. In these cases the phenomenon of the corrosion was decided to be alkaline corrosion, but its mechanism is not yet sufficiently clarified. So we propose to adopt the theory of thermal diffusion potential as one of the clues to make the mechanism clear. The result of our consideration is as follows; (1) To simplify calculation the factors such as mobility are assumed to be constant irrespective of temperature and concentration. Then the relation between the concentration (ratio of activity) and the temperature of NaOH aq. soln, and thermal diffusion potential is calculated with Nernst formula. (2) In case the concentration (ratio of activity) of the concentrated layer is 102-103 times higher than that of the usual boiler water at a boiler, of which the pressure is 100kg/cm2, the thermal diffusion potential reaches 100-200mV provided the average temperature of boiler water is 586°K. (3) Since the galvanic corrosion occurs when the potential between the two kinds of metals in contact with each other is more than 100mV, the mechanism of the corrosion of the tubes can be illustrated by introducing the conception of the thermal diffusion potential.
At present there are two main theories for sulfide corrosion cracking, namely hydrogen embrittlement mechanism and stress corrosion cracking mechanism. We have studied the relation between sulfide corrosion cracking and hydrogen embrittlement, and discussed on this report about the effect of the hydrogen on sulfide corrosion cracking. The results are as follows.: (1) The cracks in the heat affected zone under the welded bead of mild steel were caused by diffused hydrogen only. It is suggested that sulfide corrosion cracking is caused by hydrogen embrittlement. (2) Absorption of hydrogen from ACOH+sat. H2S solution has a similar tendency to that of cathodic electrolysis in the quantity absorbed and absorptivity. It is suggested therefore that the absorption of hydrogen comes from the same mechanism in both of these instances. (3) Hydrogen sulfide raises hydrogen overvoltage, which accelerates the atomic hydrogen diffusion rate. Accordingly, sulfide corrosion cracking is interpreted as hydrogen embrittlement which is caused by diffused atomic hydrogen.
Studies have been made of the influence of stress rate in trapezoidal strain waves on low cycle corrosion fatigue strength. The electrode potential of the specimens has been observed against the saturated calomel electrode during the cycles. The following facts have been made clear; (1) The corrosion fatigue strength is affected by the stress wave forms. The durability of the material gets cut short by the longer strain time in which the strain ranges from zero to maximum, owing to the fact that the corrosion potential of the specimens is retarded and does not follow up to the equilibrium state of stress. (2) The effect similar to the effect made on the strain time is found to be made on the stress rate. In the range of lower stress, however, the stress rate has less effect because the maximum duration of time of stress becomes more effective than the strain time. (3) When lower stresses are applied, the strain time has its effect on the period of pitting formation and of crack propagation, while the effect on the latter is much smaller than on the former under higher stress range.
The effectiveness of anti-corrosive mechanism mostly depends on the condition of painted films. In the other words, the perfect film of the paint is most necessary for corrosion protection. At present, the inspectors usually use the paint film tester (mesurement of electromotive force by DC current) or the magnetic thickness gauge to check the painted films. The authors have studied two items concerning above problems as follows. (1) by using the impedance meter (automatic-calculated), we has been check the imperfect condition of painted film as reported already (2) we could determined the control limit of checking for film performance.