Corrosion processes of mild steel and low alloy steels in aerated sodium sulphate solution have been investigated by using rotating cylindrical specimens. A part of the corrosion product deposits on the surface of steel and the rest is transferred into the solution. The proportion of the amount of the deposited film to the total amount of corrosion loss do not change even if the immersion time extends but it depends on the alloying minor element; the proportion decreases in the following order, mild steel>P and Cr steel>Cu steel>Ni steel. This order agrees with the magnitude of the solubility product of corresponding alloying metal hydroxide.
The reaction resistance of steels represented by the reciprocal of corrosion rate increases rapidly in the initial period of time with increase of the thickness of the rust layers and becomes constant after four hours. The final resistance is likely to be controlled by the nature of the inital surface condition of steel, e. g. the enrichment of an alloying element.

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A series of underground corrosion tests has been carried out in order to supply corrosion data as well as basis for estimating life of a pipeline transporting raw material gases to a butadiene plant at Yokkaichi-Combinat. The protection procedure of the line was based on the recommendation of Corrosion Prevention Committee, Japan Society for the Promotion of Science.
Total of 144 test pieces was buried near the warf which was considered to be the most corrosive environment in the pipeline area. The coated steel specimens were connected to the cathodically protected pipeline. Some specimens were not cathodically protected.
The seven-year results show among others that cathodic protection is especially effective in reducing corrosion of bare part of coated steel, which otherwise corrodes 5-10times. The tests will be continued up to 20 years.

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