Comparison of Different Magnesium Hydroxide Coatings Applied on Concrete Substrates (Sewer Pipes) for Protection against Bio-Corrosion

Abstract

Several coatings and linings have been examined and used for the protection of sewer concrete pipes, against mainly biogenic-provoked corrosion due to the production of bio-sulfuric acid, leading to the degradation of the pipes’ structure and eventually, to their collapse and need for costly replacement. This study aimed to examine the potential differences between five different magnesium hydroxide coatings, prepared from powders presenting different purity, surface area and pore size distribution, when applied as corrosion protection agents. These coatings were tested by using accelerated sulfuric acid spraying tests, both in dry and wet coating conditions. The coating adhesion ability and their microstructure were examined by the application of pull-off measurements and of SEM analysis, respectively and were found to present certain differences, regarding the adhesion ability and the surface morphologies. During the acid spraying procedure, the surface pH and the mass change of coated concrete specimens were recorded daily. The surface pH was reduced towards acidic values and the mass reduction approached almost −20% in comparison with the initial coating mass for certain cases. Additionally, the hardness and roughness of concrete surface under the coating layer (i.e., the interface between the coating and the surface) after four days of acid spraying, exhibited much smaller changes (due to protection) in contrast to the uncoated concrete specimens (used as blank/comparison experiments), which were found to be highly affected/corroded. The formation of concrete corrosion and coating by-products, as noticed after the respective chemical reactions, was recorded by X-ray diffraction (XRD) measurements and the respective quantification of obtained results. In all the coated specimens only very small amounts of the major by-product (gypsum) was observed, indicating that the concrete surface was sufficiently protected from sulfuric acid attack.

Keywords

• concrete corrosion and control
• magnesium hydroxide coatings
• sewerage pipe systems
• acid spraying test