Corrosion of Carbon Steel in Tap Water （Ⅱ）

―Growth of Rusted Area on Metal Surface in Relation to Corrosion Potential―

Abstract

This research was focused on the growth of rusted areas on the surface of carbon steel in tap waters, which were collected at 66 locations in Japan.

Rusted area ratio （θ） was defined as θ＝A_rust/A, where A was total surface area and A_rust was the area covered with corrosion products. The changes of θ and corrosion potential （E_corr） with time were investigated.

The followings （1）-（6） were resulted.

（1）As immersion time elapsed, θ increased and E_corr became less noble.

（2）The abrupt changes in θ and E_corr with time appeared simultaneously. Stagnations of both θ and E_corr followed the above. The time of the abrupt change depended on quality of the tap water.

（3）The relationship between log{θ/（1－θ）} and E_corr was similar regardless of water quality.

This meant that the rusted area could be estimated only by measuring E_corr.

（4）The abrupt change was in θ seen from 0.2 to 0.5 and E_corr from －300 to －500 mV vs.SSE, the time at which E_corr reached －400 mV was defined as t_initial. For larger t_initial, the corrosion at small θ rusted longer.

（5）Both average and maximum localized corrosion rates were closely related to t_initial. Insight into those relations surmised the following mechanisms;

（a）When θ＜0.2, the anodes were fixed, inducing localized corrosion.

（b）When θ＞0.5, anodic and cathodic reactions were distributed even on the whole surface, resulting in then uniform corrosion.

（6）The authors found that the maximum localized corrosion rate, v_max（mm y^－1） was a function of t_initial as v_max＝0.32×t_initial^1/3. The unit of t_initial in this equation was hour.