Usualy cathodic protection has been used as protective method for steel structures at harbors, 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, investigation of the effect of cathodic protection applied aloing with painting to steel in sea water and investigation of the effect of corrosion-resistant steel, and so on. So, at first, as one of protective method for steel exposed to splash zone, protective method by means of hardened cement coating was investigated as shown in this report. Untill now, corrosion tendencies of steel embeded in hardened cement coating alone have been investigated in experimental rooms and in field conditions by many engineers, but those of steel in hardened cement coating acting as one zone among many environments around all steel surface have not yet been studied quantitati vely and systematically.
　On the contrary, since steel structures under marine environment contact with many zones, for example, sea water and mud zone saturated with sea water, in applying hardened cement coating to steel above L.W.L., steel structures result in passing through three zones, i.e. hardened cement coating , sea water and mud zone saturated with sea water. And besides, corrosion condition of steel contacting with hardened cement coating with sea water and sea water is equal to one of reinforcing steel in reinforced concrete structures with crack in seawater.
　Therefore, to make corrosion tendencies and protective effect by means of hardened cement coating in practical conditions clear, I investigated corrosion tendencies of steel in alkaline solution i.e. calcium hydroxide solution, in hardened cement coating zone alone, and in hardened cement coating among two or three corrosion zones, and obtained the following results: