To determine corrosion rate of steel structures, we carried out at Suma coast and Yasuki, Moji and Hakata Harbors both the non-destructive corrosion survey with new ultrasonic thickness meters (resonance and pulse echo types for underwater use) and destructive survey using steel pieces cut from steel structures, and we obtained the following results :
(1) The number of possible locations measurable with the pulse echo type thickness meter was 100 per cent of the total determination sites, but with the resonance type thickness meter from 4 to 76 per cent.
(2) The deference between steel thicknesses with ultrasonic thickness meters was within about ±1 mm.
(3) Corrosion rates of steel sheet piles contacting with uniform sea water were 0.02～0.05mm/yr between H.W.L. and mud line.
(4) Thicknesses of steel pipe piles (after or under five years) were nearly equal to the initial thicknesses between sea water level and mud line.
(5) Most potentials of the measured steel structures, which was appled with cathodic protection by power-impressed method were more positive than -770mV (S.C.E).