ボイラ蒸発管腐食について
1964 年 13 巻 130 号 p. 517-526
詳細
1964 年 13 巻 130 号 p. 517-526
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.
