The corrosion survey method for steel structures at harbors has not been yet standardized and the non-destructive survey using Sakata-type thickness meter has been made in this country.
Results by the non-destructive method, however, were not compared with those by destructive method carried out in situ. It is shown in the previous paper that the corrosion rate of the steel pile under H. W. L. is fairy smaller than the values estimated formerly by the non-destructive survey.
Then, to determine the corrosion rate of steel structures, we carried out at Hirohata and Osaka harbors both the non-destructive survey with a new ultrasonic thickness meter (resonance type for underwater use) and the destructive survey using steel pieces cut from quay walls.
Results obtained are summerized as follows:
(1) The thickness measurement with the ultrasonic thickness meter was nearly satisfactory. However, since the number of locations measurable with the thickness meter was from 60 to 90per cent of the total determination sites, it is desirable to make a survey with both non-destructive and destructive methods.
(2) Corrosion rate of steel piles formerly estimated by the non-destructive survey is at least several times as much as the one obtained in this survey.
(3) Corrosion rates of steel piles were 0.01-0.03mm/yr above H. W. L., 0.10-0.12mm/yr at H. W. L. and 0-0.05mm/yr between H. W. L. and mud line.

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A filing system for corrosion data retrieval has been studied. This system of computer programs accepts as input the original data obtained at laboratories or plants and generates as output the data file by which retrieval for desired corrosion data becomes easier and faster. This system can also generate as output the corrosion charts to show the influence of environmental conditions e. g. the concentration of corrosives and the temperature on materials. These charts are useful to know not only the range of the corrosion resistance of materials but also the influence of environmental factors.

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In order to investigate the corrosion behavior of some alloys in high velocity sea water, experimental tests have been conducted by using disc shaped specimens and tension type welded ones with and without zinc anodes, at the coast of Seto Inland Sea. Gun metal, mild steel and four kinds of stainless steels were immersed in flowing (18m/sec) natural sea water for about nine months.
The weight-loss and the depth of pitting, besides the reduction of tensile strength were measured.
Results obtained are as follows:
(1) Corrosion rate of mild steel was 0.75mm/yr, and it could be decreased to 0.034mm/yr by cathodic protection with zinc anode.
(2) Corrosion rate of gun metal was 0.2mm/yr, and that of type 304 stainless steel was less than 0.003mm/yr.
(3) Reduction rate of tensile strength of welded mild steel was about 29%, and it could be reduced to nearly zero with zinc anode.
(4) It was found that stainless steel could be safely used in high velocity flowing sea water.

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