水道用亜鉛めっき鋼管中における珪酸亜鉛化合物の生成と配管の腐食形態との関係

抄録

Tap water and corroded water pipes have been collected from 28 and 20 places respectively in Japan to elucidate the corrosion phenomena on the interior surface of the galvanized steel pipe for water service. The water quality was characterized by fairly high concentration of dissolved silicate and negative values for Langelier saturation index. The morphology on the cross sections of many of corroded pipes was characterized by the thin uniform corrosion layer on zinc coating and the swollen scale over the localized corrosion part. The principal constituent of corrosion products was hemimorphite (Zn₄Si₂O₇(OH)₂⋅H₂O) in the uniform corrosion layer, and goethite (α-FeOOH) or magnetite (Fe₃O₄) in the scale. It is likely that the hemimorphite layer grows by the crystallization of amorphous compounds which are formed from zinc hydroxide and adsorbed silicate on the surface of zinc layer and hemimorphite and its formation prevented anodic oxidation of zinc. These observations suggest that partial formation of hemimorphite layer would obstruct the galvanic protection of galvanized steel pipe and accelerated the localized corrosion on the interior surface of pipes.