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.

Evaluation of the Protection Ability of a Magnesium Hydroxide Coating against the Bio-Corrosion of Concrete Sewer Pipes, by Using Short and Long Duration Accelerated Acid Spraying Tests

“…The total mass loss at 1000 • C (i.e., the Loss on Ignition, LOI) corresponds to all the water and CO 2 content of the respective powder. This magnesium hydroxide differs mainly in the MgO content, but also in the specific surface area and in the particle size from other examined magnesium hydroxides, previously investigated [28]. The main difference regards the specific surface area, which is very low in this case (7 m 2 /g) to maintain favorable slurry characteristics, such as viscosity and workability and to allow the inclusion of cellulose.…”

“…The main difference regards the specific surface area, which is very low in this case (7 m 2 /g) to maintain favorable slurry characteristics, such as viscosity and workability and to allow the inclusion of cellulose. The results of the previous study [28] indicate that the bigger particle size may lead to slower interactions with the respective (acidic) environment. In that way, this magnesium hydroxide has finer particles, than other examined relevant hydroxides [28].…”

“…The results of the previous study [28] indicate that the bigger particle size may lead to slower interactions with the respective (acidic) environment. In that way, this magnesium hydroxide has finer particles, than other examined relevant hydroxides [28]. Additionally, the purity of raw CCM used to prepare the studied slurry, was higher, and according to previous results [28], this fact may enhance the preservation of alkaline pH values in the protected surface after the application of this coating.…”

“…According to a relevant study [28] two different accelerated sulfuric acid spraying tests were applied, so that coatings can be properly evaluated in terms of the protection they can offer to concrete surfaces in relatively short time period. The same tests and conditions were also used in this study in order to examine the selected magnesium hydroxide, presenting optimized properties.…”

“…This study is mainly focused on a specific magnesium hydroxide (MH) slurry, which after preliminary experiments was found to present the optimal expected properties, e.g., regarding the specific surface area (SSA) and the particle size distribution (PSD) [28]. The magnesium hydroxide is evaluated mainly regarding its action as protective coating on concrete substrates, under conditions that simulate those existing in the sewer pipes.…”