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Effect of Soil’s Acidity and Saturation on Degradation of Fracture Toughness of Buried Cast Iron Effect of Soil’s Acidity and Saturation on Degradation of Fracture Toughness of Buried Cast Iron Muhammad Wasim1; Chun-Qing Li2; Dilan Robert3; and Mojtaba Mahmoodian4 Abstract: There has been limited research in the past on the effect of varying soil’s acidity measured as pH and saturation measured as moisture content on the fracture toughness of buried cast iron as per comprehensive literature review. This paper presents findings obtained from a long-term test designed to investigate the combined effect of varying levels of soil’s acidity and saturation on the fracture toughness of buried cast iron over time. Relations for the change in candidature fracture toughness of the corroded cast iron over time as a function of soil’s pH, moisture content, and corrosion pit depth are developed. Moreover, a relation correlating the change in microstructure and the fracture toughness of the corroded cast iron is also developed. The significance of the current paper is that it reports that soil’s corrosivity induces a change in the microstructure and composition of cast iron, which consequently affects the fracture toughness of the corroded cast iron. DOI: 10.1061/(ASCE)MT.1943-5533.0003258. © 2020 American Society of Civil Engineers. Author keywords: Soil corrosion; pH; Moisture content; Cast iron; Corrosion pit depth; Chemical composition; Fracture toughness. Introduction Cast iron and other ferrous pipes which are buried in soil have a high frequency of failures in the form of leaks and sudden bursting (Rajeev et al. 2014; Mahmoodian and Li 2016; Wang et al. 2019). Several researchers have reported the corrosive nature of the soil as the main cause of their failures (Cole and Marney 2012; Wasim et al. 2017, 2018; Wang et al. 2019). Considerable research has been carried out in the past 50 years on the factors in soil responsible for corrosion-induced failures of buried cast iron pipes (Petersen et al. 2013; Soltani and Melchers 2017; Wasim et al. 2019b). Among various factors, the pH (acidity) and moisture content (saturation) of the soil are reported to be the most influencing factors in causing external corrosion of cast iron pipes in soil (Cole and Marney 2012; Villanueva-Balsera et al. 2018; Wasim et al. 2019b). These two factors have been extensively researched for deter- mining the external corrosion of buried cast iron pipes due to their high frequency of failure rates (see previous text and references). However, long-term research on the effect of these two factors on microstructural change and subsequent change in mechanical prop- erties such as fracture toughness of corroded cast iron is very lim- ited. The significance of investigating the effect of soil’s corrosive factors on the fracture toughness of corroded cast iron is to accu- rately predict the sudden failure of the cast iron pipes, as the degradation of this mechanical property with time will directly in- fluence their service life. Very recently, Wang et al. (2019) proposed a method for evalu- ating the probability of fractured failure of pressurized cast iron pipes as a function of their corrosion pit depths, along with other parameters, and applied their method on a case study. They found that deeper pit depth and reduction of fracture toughness of the pipe with time are the parameters that increase the risk of fractured fail- ure of cast iron pipes. However, the coupled effect of internal pres- sure and corrosion depth influence the probability of fracture failure of cast iron pipes most, as per their developed model. Similar to this research, Soltani and Melchers (2017, 2018) developed probabilis- tic models for the pitting corrosion of aged cast iron pipes and cor- related them with the corrosion-stimulating factors in soils. They found that soils’ higher moisture content (i.e., saturation) and lower pH (i.e., acidity) were responsible for the depth and pitting incre- ments in buried cast iron pipes. There are few other studies on corroded iron-based pipes correlating the degradation of their mechanical properties with the soil-induced corrosion pit depth (Jesson et al. 2013; Mahmoodian and Li 2017, 2018; Mahmoodian 2018). Although the aforementioned studies on predictions of pit- ting corrosion and degradation of mechanical properties of buried cast iron pipes are comprehensive in their approach, the discussion on how factors in soil and their induced corrosion influence micro- structure and subsequently reduce the fracture toughness of cor- roded cast iron with time is scant. Recently, the effect of clay soil of varying acidity and moisture content of 20% on the mechanical properties of buried cast iron pipe sections was investigated by Wang et al. (2018). However, only one saturation level of the soil was selected, with no element change analysis in their test. Also, the one-year duration of the tests was probably not enough time to observe a notable reduction in mechanical properties such as the fracture toughness of corroded cast iron pipes. On the contrary, in a very recently published paper (Wasim et al. 2019b), the long-term coupled effect of low and high acidities (2.5, 3.5, and 5 pH) and saturation levels (40% and 80%) on the corrosion and corresponding change in the microstructure of the corroded cast iron was quantified by performing phase and element change analyses with time. The duration of the tests was relatively long, i.e., 18 months, to obtain conclusive evidence for 1Research Fellow, School of Engineering, Infrastructure Dept., Univ. of Melbourne, Parkville, Melbourne, VIC 3010, Australia (corresponding author). ORCID: https://orcid.org/0000-0002-3900-7985. Email: mwasim@ unimelb.edu.au 2Professor, School of Engineering, RMIT Univ., Melbourne, VIC 3001, Australia. Email:
[email protected] 3Senior Lecturer, School of Engineering, RMIT Univ., Melbourne, VIC 3001, Australia. ORCID: https://orcid.org/0000-0002-5686-7055. Email:
[email protected] 4Senior Lecturer, School of Engineering, RMIT Univ., Melbourne, VIC 3001, Australia. Email:
[email protected] Note. This manuscript was submitted on September 23, 2019; approved on January 2, 2020; published online on April 27, 2020. Discussion period open until September 27, 2020; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561. © ASCE 04020180-1 J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2020, 32(7): 04020180 D ow nl oa de d fr om a sc el ib ra ry .o rg b y C A SA I ns tit ut io n Id en tit y on 0 2/ 25 /2 1. C op yr ig ht A SC E . F or p er so na l u se o nl y; a ll ri gh ts r es er ve d. https://doi.org/10.1061/(ASCE)MT.1943-5533.0003258 https://orcid.org/0000-0002-3900-7985 mailto:
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[email protected] http://crossmark.crossref.org/dialog/?doi=10.1061%2F%28ASCE%29MT.1943-5533.0003258&domain=pdf&date_stamp=2020-04-27 the change in microstructure due to corrosive soil conditions. How- ever, the subsequent change in key mechanical properties of cast iron such as its candidate fracture toughness was not investigated. The previous review and discussions reveal a gap in the under- standing on how the combined effect of soil’s acidity and saturation affects the fracture toughness of corroded cast iron in relation to their change in element composition and maximum pit depth (Wasim et al. 2019a). The current paper will address this gap. This paper aims to present the findings of comprehensive exper- imentation carried out to examine the combined effect of varying acidities and saturations on the candidate fracture toughness of corroded cast iron buried in soil. Test specimens were buried in corrosive soils inside containers that were prepared in the labora- tory and monitored in an environmental chamber for a duration of 18 months. At three planned time intervals of 180, 365, and 545 days, the corroded cast iron specimens were taken out for element composition and fracture toughness testing and analysis. The other microstructural features such as phase analysis of the specimens were also investigated as published in Wasim et al. (2019b). Relations were derived for change in candidate fracture toughness of cast iron over time as a function of corrosion depth and element change along with influencing factors. Test Methodology Test Specimen Specimens with compositions (Table 1) and wall thickness similar to that of old existing buried cast iron pipes in Melbourne, Australia were manufactured for experimentation. The design of the test specimens was the same as that of the pre- vious research (Wasim et al. 2019a). The dimensions of 100 × 20 × 10 mm were selected for the specimens with the intention of testing their fracture toughness after removal from the corrosive soil con- ditions at three intervals of time. Details about the specimens’ prepa- ration will not be repeated but can be found in Wasim et al. (2019a). Selection of Test Variables Three acidity levels, i.e., pH ¼ 2.5, 3.5, and 5, and two levels of saturation, i.e., 80% and 40% (20% and 10% of moisture content), respectively, were studied. Three specimens were tested for each combination of pH and saturation of the soil on 180, 365, and 545 days for the fracture toughness of the corroded cast iron. The testing plan is shown in Table 2, and further details on the se- lection of variables can be found in Wasim et al. (2019b). Description of Soil Clay soil used in the current research was obtained from a landfill site in Melbourne, Australia. Its pH was 8.17, and the mineral and chemical compositions are presented in Table 3. The obtained soil from the field was in the form of lumps, which was crushed to a size of 2.36 mm using sieves, and then its pH was adjusted to 2.5, 3.5, and 5 using hydrochloric acid (HCl). Further details on how these pH levels in soils were achieved can be found in Wasim et al. (2019b). Test Setup A real underground environment was simulated in the current research. Plastic containers, nonreactive to acids, were used to contain acidic soils of known pH and saturation with specimens buried inside. The soils were filled in layers in the test contain- ers, and uniform compaction was applied to keep the density (1.6 g=cm3) uniform for all the layers. The selected density of soil was as per the soil data in the literature (Petersen et al. 2013) and also in consultation with experienced professionals working in local water service in Melbourne. Specimens were buried at a depth of 300 mm along with a variety of sensors to monitor temper- ature, pH, and moisture. The selected burial depth was also close to most of the buried pipes in Melbourne. The test containers were kept in an environmental chamber at a temperature of 23°C and an ambient humidity of 50% for the duration of the experimental program