The influence of quantity of dewing and interval of dew-cycle on the corrosion of mild steel panel exposed in an atmosphere of 20ppm SO₂, 95-100% R. H. was investigated and evaluated from the viewpoint of similarity to field test, acceleration of test and reproducibility of test result.
The grade of dewing was classified into the critical dewing (temperature difference between specimen and atmosphere: TD≅0), the light dewing (TD=2°C) and the heavy dewing (TD=5°C).
As dewing period (min)/drying period (min)15/15, 30/30, 60/60 and 120/120 were selected, besides continuous dewing during the test period of 24hr.
In heavy dewing, the shortest interval of 15min/15min showed the largest amount of corrosion, but the similarity to the corrosion pattern of field test was weak. The reproducibility under the condition of the cyclic heavy and light dewing was very good, while that in the cyclic critical dewing was unstable. Continuous dewing except critical one had the tendency to reduce acceleration and similarity.
To satisfy the similarity, acceleration and reproducibility in the laboratory test, the condition of light dewing and comparatively rapid dew-cycle as 30min/30min was recognized to be optimum.

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Though the high temperature corrosion observed at heavy oil burning boilers is known as a typical vanadium attack, the presence of sulfide under the oxide layer and carburization phenomenon are observed by the investigation on the corroded tubes. The vanadium attack occurs under oxygen rich condition, while the latters are usually observed under oxygen poor condition.
Those phenomena can not be explained thoroughly by the customary accelerated oxidation mechanism.
This paper clarifies the followings on the sulfidation-carburization mechanism by the investigation of the high temperature corrosion on austenitic stainless steel superheater tubes and the change of the ash composition following the corrosion reactions.
1) Deposits (heavy oil ash) adjacent to tube wall principally consist of sodium and sulfur independently of the quantity of vanadium in fuels. Therefore the reactions with sodium sulfate compounds play a principal part in the corrosion reactions following the elevation of tube wall temperature.
2) Sulfides are the corrosion products of the tubes with the sodium sulfate compounds.
3) Free alkali liberated from the sulfide formation reaction reacts on the carbon dioxide in the combustion gas to form sodium carbonate. The sodium carbonate together with carbon monoxide, carbon dioxide and unburned carbon raises the carbon potential near the tube wall. This seems to accelerate the carburizing action upon the tube surface which is poor in inhibitive film by the corrosion reactions.

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Galvanostatic-cathodic reduction behaviour of anodic oxide films on iron has been investigated in neutral boric-borate solutions. The amount of iron cathodically dissolved from the film and the decay of potential during the cathodic reduction are much affected by both the solution pH and the cathodic current density. The current efficiency for the reductive dissolution of the film increases with lowering the solution pH and attains 100% at pH=6.35. In the solution of pH=6.35, however, dissolution of iron substrate begins to occur immediately after completion of reduction of the film. Addition of arsenic trioxide into the solution is proved to inhibit the iron corrosion completely, and the maximum amount of dissolved iron from the film is now equal to the total amount of iron in the film calculated from the anodic film charge assuming the film as Fe₂O₃. This result not only provides evidence for the single hydrous ferric oxide layer model of anodic oxide films on iron, but also gives an electrochemical method for estimating the amount of iron in anodic oxide films on iron.
This method has been applied to determination of iron in the films formed in the oxygen evolution potential region. It is shown that the film in the oxygen evolution region thickens with rise of potential, but becomes thin at a critical potential where transpassive dissolution occurs. Further rise of potential results a constant film thickness independent of the potential.

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