External corrosion and its effects on mechanical properties of buried metal pipes

Corrosion is a grave concern for owners and managers of buried ferrous metal pipelines as it can cause structural degradation, which adversely affects the service life of pipes. Leaks or ruptures caused by corrosion are commonly-identified problems in buried metal pipes which can lead to catastrophic failures, causing significant socio-economic losses for consumers and water utilities. Despite the periodic inspection of these buried pipelines and the development of corrosion- protection techniques, corrosion-induced deterioration of pipes remains a major problem globally, and billions of dollars are spent every year on the repair and maintenance of corroded buried pipes. Corrosion may alter the mechanical properties of metals due to chemical and microstructural changes at macro, micro or nano levels which impact their service lives. To date this has not been explored comprehensively for buried pipes by other researchers. Therefore, an understanding of the corrosion mechanism, its contributing factors and its influence on mechanical degradation is necessary for the corrosion protection of newly-installed pipes and the maintenance of existing pipes. In the research reported here, a variety of laboratory-controlled experiments were conducted to observe the variations in the mechanical properties of metal pipes in various corrosive soils and simulated soil environments to investigate the key factors influencing the corrosion of pipes in soils, including anaerobic bacteria. The coupled effects of varying acidity and saturation on the corrosion of gray cast iron specimens was explored by conducting laboratory experiments simulating underground corrosion. A correlation between the corrosion in soil and simulated soil of cast iron was performed for comparison with previous studies. Most experiments were conducted on cast iron and selected ductile iron, and mild steel specimens were also used in simulated soil solutions for the comparison of corrosion severity. Next, the effect of corrosion on their fracture toughness induced by different corrosive environments was investigated. Furthermore, hydrogen embrittlement of steel pipelines in soil environments due to the release of hydrogen during the corrosion process was also investigated. In addition, the variation in mechanical properties due to corrosion was assessed at micro and nano levels. Based on the analysis of various test results, it is concluded that corrosion has a substantial impact on the mechanical properties of metal, thus highlighting a fundamental issue in current approaches to the design of new pipes and the condition assessment of old pipes. The results of the present study have widespread applications, including in corrosion protection and the service life prediction for new and ageing infrastructures.