Performance of Three Magnesium-based Coatings for Corrosion Protection of Concrete against Sulfuric Acid

The biogenic deterioration of large diameter concrete pipes constitutes a major problem in most sewerage systems. The protection of concrete against sulfuric acid produced in situ biogenically by certain microorganisms is of major interest. This study is focusing on the protection of concrete by applying relatively low-cost, magnesium-based coatings that can react and capture (neutralize) effectively the produced sulfuric acid. Three specific magnesium-based coatings were examined, i.e., a magnesium hydroxide/calcium hydroxide coating (C1), a magnesium hydroxide/calcium hydroxide coating with the supplementary addition of an acrylic additive (C2), and a magnesium phosphate cement coating (C3). The respective magnesium coatings were applied onto appropriate concrete specimens and evaluated by various experimental methods, regarding their anticorrosion capability. More specifically, the coated and the uncoated (used for comparison reasons) concrete specimens were subjected to two accelerated tests, i.e., a sulfuric acid immersion test and a sulfuric acid spraying test, attempting to simulate the real conditions existing in a sewer pipe internal wall surface. Multiple measurements performed to evaluate the respective coating performance, such as mass recordings, compressive strength, and surface pH measurements. The surface pH of the coated specimens was higher, as compared to the uncoated specimens, with the coating C3 presenting 7.8 surface pH value after four days of acid spraying. Overall, the coating C3 seemed to offer the best corrosion protection to concrete with respect to the other two examined coatings.