To prevent the phenomenon that a steel laid in a state where the surface is exposed to the outside comes in contact with chloride ions contained in seawater or the like to be deteriorated, so as to be made thin.

The surface of a steel is coated with an aqueous solution of silver nitrate, so as to cause silver mirror reaction, thus a silver mirror is generated on the surface of the steel. In this way, the surface of the steel is coated with silver, and, when the coated silver comes in contact with chloride ions, it is turned to hardly soluble silver chloride. Thus, the phenomenon that the surface of the steel comes in contact with chloride ions to be deteriorated, so as to be made thin can be prevented.

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