Proposal of a New Corrosion Index for Domestic Tap Water in Japan Available to Purchase

This paper proposes a new corrosion index that can be effectively applied to tap water having low calcium hardness and high soluble silica (SiO₂) concentration. Tap water at 70 different locations in Japan were sampled, and corrosivity for carbon steel dipping in flowing tap waters were evaluated. The average corrosion rate V_avg was obtained by the weight loss method. The results showed that V_avg was strongly dependent on the concentrations of SO₄²⁻ and SiO₂. The application of conventional corrosion indexes to Japanese tap water was examined with unsuccessful results. In view of these facts, a new corrosion index SS ratio = [SO₄²⁻] / [SiO₂] was proposed. It was quantitatively demonstrated that V_avg was directly proportional to the SS ratio. At the same time, the maximum localized corrosion rate V_max was determined by measuring the corrosion depth with a point micrometer. The results show that V_max could also be well evaluated by the SS ratio, and V_max was inversely proportional to the SS ratio.

Japanese tap water has a low calcium hardness compared with those of countries in Europe and the Americas.¹ There are many steep mountains and volcanos in Japan. These geological factors lead to features such as short rivers with water rich in SiO₂. This leads to a lack of calcium compounds in river water because the flow rate is high and the dissolution rate of CaCO₃ into water is low. That is, Japanese tap water is soft. The relationship between the corrosion rate and components in tap water has often been studied, mostly in laboratory settings. It has been said that Cl⁻ and SO₄²⁻ are corrosive and HCO₃- is inhibitive.^2-6

In general, the corrosivity of tap water is predicted by corrosion indexes. The Langelier Index (LI), Ryzner Stability Index (RSI), and Larson-Skold Index (LSI) are commonly used.^7-11 However, the experience of the authors has been that these indexes often do not reflect actual corrosivity. This discrepancy is notable for Japanese tap water. In this study, corrosion tests were performed for tap waters sampled from 70 locations in Japan. The new corrosion index was proposed on the basis of these results.