Proposal of a New Stability Index Taking Account of Corrosion Inhibiting Action of Silica and Derivation of Empirical Equations Containing the New Index for Corrosiveness and Scale Formation of Water

Abstract

A new stability index [an improved stability index (SI′)] has been proposed that introduces corrosion inhibiting ability of silica in a broad range of temperature and pH into the conventional index of the corrosive action and scale formation of water, and new empirical equations [Eqs. 1-3] that characterize water were derived using the new index. The derived SI′ was effective in the range of SI>6. The SI′ was balanced at 0, and then SI′=0 differed from SI=6. In addition, a possibility was found for prediction and inhibition of corrosion of ferrous metals based on the equations by controlling the concentration of silica ([SiO₂]).
y=K₀{1-exp(-3.06×10^-1SI′)} (1)
K₀=(2.09T+1.872×10²)exp{-(-1.08×10^-4T+7.62×10^-3)[SiO₂]} (2)
SI′=x-7.21{1-1.71×10^-1exp(-8.59×10^-3[SiO₂])} (3)
[y: corrosion rate (V) (mdd), K0: constant affecting saturation value of corrosion rate (y) (mdd), T: temperature (°C), [SiO2]: silica concentration (mg SiO2/dm3), x: stability index (SI) (-)]