A coulostatic method for rapid evaluation of corrosion rates was developed and applied to mild steels in aqueous solutions. In this method, the decay of polarized potential was recorded on open-circuit after applying a small amount of charge. According to the theory developed, polarization resistance, R_p, for SB46 immersed for 2 hours in 1N H₂SO₄ was estimated to be 7.9Ωcm². This agrees fairly well with the value of 5.6Ωcm² obtained by polarization curve measurement. The method is also applied to SS41 in distilled water. The R_p values obtained were 2.4kΩcm², 2.1kΩcm² and 1.1kΩcm² for immersion times of 45, 51 and 74 hours, respectively. These values are similar to the average polarization resistance, 2.2kΩcm², or the corrosion rate of 21mdd obtained from the weight loss measurement at the immersion time of 75 hours. The coulostatic method developed in this study can be applied without ohmic drop correction, even in distilled water and the time needed to complete the measurement is considerably shorter compared to the usual linear polarization method.

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In view of the accelerated atmospheric rusting in Shibukawa area, Gunma, presumably due to the presence of HCl released from chemical plants, a series of atmospheric exposure tests has been carried out for one year starting in Sept. 1972. Data were obtained with steel, zinc and silver specimens at polluted area, Osaki and Nakamura, the others at less polluted area, Toriba and Shinto. Corrosion products on the exposed specimens were analyzed by X-ray and the weight loss of specimens was measured. The result of X-ray analysis showed that amounts of Cl on all specimens and produced AgCl on silver are higher at Nakamura than other areas. Amounts of S on steel and zinc are similarly higher at Nakamura. On the basis of these data, it is estimated that corrosion at Nakamura is mainly due to HCl and Sulphur pollutants.

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In order to enable the use of cheap ferrous materials for heat exchanger tubes in an MSF desalination plant, the application of special cathodic protection to the inside of mild steel tubes was studied by laboratory experiments. At first, the sponge ball coated with metallic zinc powder was passed intermittently at intervals of 1.5 seconds through a mild steel tube with continuous flow of deaerated 6% NaCl solution, and the whole inside of the tube was polarized to about -900mV (SCE) and was sufficiently protected. However, the durability of polarizing capacity of the ball was not appreciably long on account of dropping zinc from the ball, and further studies for ball design were necessary to fasten the zinc on the ball. In the second step of this work, deaerated 6% NaCl solution suspending metallic zinc grains ranging in size from 48 to 200 mesh was passed through the tube with and without the use of sponge ball whose diameter was slightly larger than the inside diameter of the tube, and the current generated from the dispersed anodes, namely zinc grains, protected the whole inside of the tube. The results showed that the potential of the tube was maintained below -900mV without the ball under such conditions that the extremely deaerated test solution suspending zinc grains in concentration of 1 to 2%. It was considered that satisfactory protection might be performed by dispersed anodes, and that the accelerating effect of simultaneous use of the ball was revealed when the zinc grains were very fine or dissolved oxygen content slightly increased. In air-saturated 6% NaCl solution the tube potential did not effectively polarize by the dispersed anodes even in cooperation with the ball. The self-corrosion rate of zinc grains in stagnant deaerated artificial sea water (double salinity) at 95°C and pH 7 was 0.02 to 0.03%/hr, and the life of zinc grains was expected to be about 5, 000 hours. On the other hand, zinc powder revealed severe self-corrosion and the polarizing effect by the powder was not remarkable. Then, in heat recovery section and brine heater, it is expected that satisfactory protection is performed in the inside of the steel tubes through which zinc grains having several tens mesh size are passed with brine in concentration of 1 to 2%. However, further experiments are necessary to establish the application under such severe conditions in MSF desalination plant as of 120°C and 2m/sec, and it may be necessary to test the polarizing power of larger zinc grains in order to protect sufficiently the steel tubes in heat rejection section through which air-saturated sea water is passed.

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Stainless steels with duplex structure of ferrite and austenite are superior in some properties such as corrosion resistance and mechanical strength to austenitic stainless steels. Composition ranges of them are about 18 to 28% Cr and 2 to 10% Ni. New types of these steels have low carbon content and adjusted chemical composition to keep appropriate phase balance of ferrite and austenite. Some of them contains 1 to 3% Mo, 0.1 to 0.2% N to improve the corrosion resistance to pitting. In this paper, the following subjects of the new duplex stainless steels are mainly described:
(1) Resistance to stress corrosion cracking and pitting corrosion.
(2) Effects of heat treatments on microstructure and other properties.
(3) Examples of application.

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This paper describes sulfur dewpoint corrosion of steels in the low-temperature parts of oil-burning boilers such as economizer, air-preheater and chimney.
The contents of this paper are summarized as follows.
1. The outline of sulfur dewpoint corrosion: The phenomena of sulfur dewpoint corrosion is explained as regards the effects of several influential factors on the corrosion.
2. The development of laboratory test method for sulfur dewpoint corrosion which is correlated well with field test in boilers: This laboratory test method has been used to study the effects of alloying elements on the corrosion resistance of steels.
3. The mechanism of sulfur dewpoint corrosion: It has been cleared that sulfur dewpoint corrosion proceeds by three-step mechanism, i, e., first, second and third steps. The third step is the most important, because corrosion resistant low alloys are passivated at this stage whereas other steels still continue active dissolution.
4. Corrosion preventive measures:
i) Use of low-sulfur content heavy oil.
ii) Boiler operation at low excess oxygen ratio.
iii) Use of additive such as dolomite to neutralize flue gas.
iv) Use of corrosion resistant low alloys.
There have been developed corrosion resistant low alloys, i. e., CRIA, S-Ten 1, NAC-1, RIVERTEN-41S, TAICOR-S. It is recommended for them to be used to low-temperature parts of boilers from the viewpoints of economy and utility safety.

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