Some alkaline solutions were boiled in boiler tubes under heat transfer at 350°C, and corrosion characteristics of the tubes were observed. Local corrosion developed at the lower part of the tubes, and no corrosion was observed in the upper part, owing to the rinsing action of bubbles from the lower part. Sodium hydroxide was the most aggressive among the three reagents tested for the tube surface under the deposit. Corrosion characteristic changed from pitting corrosion to severe general corrosion with increasing thickness of the deposit. Sodium phosphate seemed to be least corrosive if it does not thickly precipitate on the wall. The mill-scale and the rust had almost no influence on corrosion by themselves. It was, however, important for corrosion that they detached from the wall and deposited at the stagnant spots, where local corrosion occurred under the heat transfer. The surface scratch had no effect on corrosion in sodium phosphate solution and also in potassium phosphate solution. In case of sodium hydroxide solution, local corrosion occurred at the scratches under the heat transfer when their depths were more than 0.3mm. The deposit formed by hideout of sodium phosphate solution was also examined.

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Cathodic protection has been extensively used as protective method for steel structures under marine environments, but there are still some problems to be investigated, for example, establishment of protective method for steel exposed to splash zone, confirmation of the effect of cathodic protection for macro-corrosion and investigation of the effect of cathodic protection applied along with painting to steel in sea water. So, fundamental experiments regarding the above important problems were carried out and the following tendencies were found.
1) When steel exposed to splash zone alone was covered with hardened cement coating, steel in hardened cement coating served always as cathode against bare steel in mud zone, whether corrosion proceeded or not on steel surface in sea water.
2) Cathodic protection was effective for macrocorrosion as well as for micro-corrosion.
3) When cathodic protection was applied along with hardened cement coating, excess protective current did not flow into steel in hardened cement coating. Therefore, it seemed that application of both protective methods to steel structures under marine environments was more useful.
4) By coating the surface of steel in submerged zone with some painting materials, i. e. zinc-rich paint and others, macro-corrosion between sea water and mud zone could be suppressed, and current density required for cathodic protection could be reduced, for example, to about one-third for three or four months.

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