Comprehensive Analysis of Bitumen-Mastic Coatings on Steel Underground Distribution Gas Pipelines. Part 2: The Mechanism and Key Features of Coating Aging under Operational Conditions

This article is the second part of our study devoted to a comprehensive analysis of bitumen-mastic insulation coatings (BMIC) used on steel underground distribution gas pipelines in the Republic of Belarus. Given the achievement of long service life, the influence of the technological stage (production of bitumen and mastic, pipe insulation works) on the current condition of the coatings has been completely exhausted, and the operational stage is the determining factor. The relationship between soil and subsoil conditions and the key features of operational aging of BMIP has been revealed, i.e. a combination of limited access to molecular oxygen and a moderate temperature regime (low rate of oxygen oxidation), the possibility of biocorrosion of bitumen and polymeric coating components, and the combined effects of soil moisture (hydrolysis, the Rebinder effect, oxygen and catalyst transport) and continuous mechanical impact from soil with the inclusion of moisture. The mechanism of operational destruction of BMIP is revealed, which consists in the accumulation of internal stresses, degradation of the physical-chemical and rheological properties of the mastic material, and its subsequent destruction under the influence of ground loads. The functional failure of the coating consists in the loss of its barrier and dielectric properties, and the formation of physical and electrical contact between the ground electrolyte and the metal surface of the pipe. The impact of plasticizing additives on the durability of bitumen mastics and, consequently, on the durability of bituminous insulation coatings is considered. It has been established that along with the positive effect of expanding the temperature range of bitumen materials used in pipe insulation work, the introduction of petroleum plasticizers in the form of industrial oils further reduces the stability of the bitumen matrix, promotes accelerated aging and the development of defect formation of BMIP (loss of cohesion, cracking, etc.) at the operational stage.

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