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Critical Review of International Standards on Soluble Salts Measurement Methods

Product Number: 51324-20851-SG
Author: Sophia Xiaoxia Zhu; Benjamin T. A. Chang
Publication Date: 2024
It is well known that the residual salts on steel substrates have detrimental effects on the coating performance. There are several international standards on the measurement of residual salts. The most widely recognized ones are ISO 8502-6 and 8502-9. ISO 8502-6 specifies the soluble salts extraction procedures and ISO 8502-9 specifies the calculation of the soluble NaCl salt. The standards use 3 ml deionized (DI) water in 10 minutes dwell time to extract the soluble salts to measure with a potable conductivity meter. The measured conductivity reading in ┬ÁS/cm is multiplied by 1.2 to convert into total (NaCl) salts in mg/m2. The ISO standards assume the extracted salt solution containing only Na+ and Cl- ions and use a simple analytical calculation to convert the conductivity to the total NaCl salt concentration on the contaminated steel surface. In fact, in addition to Na+ and Cl- ions, Fe+2, OH- ions are also in the extracted salt solution contributing to the conductivity reading, but not considered in the total salt calculation. The ISO standards on the total (NaCl) salt measurement are oversimplified that leads to inaccurate estimation of the residual salts. The chloride ions in aqueous condition can cause micro-pits on the carbon steel surface. The pit density and depth are related to the salt levels and corrosion reaction time. During the salt measurement, the DI water can not readily migrate to the micro-pits to extract salts, and the salt extraction efficiency depends on the corrosion severity. In this work we have investigated the salt extraction on doped salted steel panels with five known amounts of NaCl salt. We also used a chloride electrode to measure the chloride ion concentration. The test results showed that the salt extraction efficiency depends on the severity of the pitting corrosion. Based on the chloride level, the extraction efficiency is 90% at low salt content and 50-66% at higher salt content on the grit blasted panels using Bresle patch method. It is recommended to use the residual chloride concentration in the coating specifications as the acceptance criterion, not the total NaCl salt level. More research is to be done to investigate the residual chloride measurements to give the international coating community a more reliable test method in the field.