Scale Protection Criteria in Natural Waters

Abstract The alternating current impedance of a wetted porous scale is shown to provide a quantitative criterion for its effectiveness as a corrosion protector. This is based on a quantitative analysis of the rate determining step in the corrosion process (the diffusion of oxygen through the porous scale) and of the ionic conductivity of the electrolyte in the pores. A method for measuring the scale impedance was developed and verified on three representative types of scale. Correlation between respective impedances of 15 natural scales formed in pipelines and their crystalline phase composition as well as the composition of the water flowing through the lines was studied. The latter led to the evaluation of a new criterion for the tendency of scale deposition from waters. According to this criterion (provided sufficient temporary hardness exists), the presence of chlorides and sulfates improves the protective properties of the scale.