## 1 Introduction  
Microbiologically influenced corrosion (MIC) is a serious problem in various industries [1; 2; 3]. MIC can happen not only in oxic environments but also in anoxic environments (e.g., marine sediments, deep seawater, water-logged soil) where anaerobic bacteria occur. Sulphate reducing bacteria (SRB) were found to be one of the predominant types of bacteria associated with MIC [4; 5]. In the absence of oxygen or under low oxygen conditions, these bacteria catalyse sulphate ions to sulphides and corrode metals through a series of oxidation and reduction reactions [1; 6]. Corrosion of stainless steel is primarily due to the presence of sulphide ions [7; 8] which can form metal sulphides and deteriorate the passive film on the metal [4; 6; 9]. Duplex stainless steel, a well-known highly corrosion resistant material, was reported to be more corrosion resistant at higher pH values than at lower pH in abiotic environments [10] though a tenacious passive film of chromium. Alkaline materials (e.g., carbonates) can combine with calcium and magnesium from the environment to form scale at high pH which may help to protect stainless steel from general corrosion [11]. However, it is not clear how duplex stainless steel with its high chromium content and passive film will be affected at high pH and in the presence of SRB. At present, most of the research focusses on studying SRB--induced corrosion at a specific pH, commonly the optimum pH for bacteria to grow [2; 9; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22] which is about 7.4. However, pH of environments can play an important role in microbial corrosion [4; 23]. The bacterial metabolite products such as sulphide ions can accelerate corrosion processes as sulphide is aggressive to passive film on stainless steel [24; 25; 26]. Sulphide enhances the susceptibility of materials to corrosion through different ways such as cathodic depolarisation [27], changes to the local pH upon hydrolysis reactions of dissolved metal ions and initiates pitting [28], supporting active dissolution of steel [29], and increasing electron uptake through metal sulphides [30]. Previous studies showed that at higher pH values, the dissolution rate of H\({}_{2}\)S is faster [31] which increases the concentration of sulphide ions resulting in an increased corrosion rate of carbon steel materials [32]. Environmental pH levels are an important factor for microbial corrosion by SRB but only few details were given [6; 33; 34]. However, microbial corrosion in alkaline environments has received less attention as most microbes are not active in this environment [35]. SRB were found to be inhibited at pH above 9 [36] but can tolerate up to pH 9.5 [37; 38]. SRB have even been found in environments at pH 10 [39; 40; 41; 42] which has implications for equipment operating in alkaline environments such as soda lakes, mine water discharges [41], and paper mills [42]. For instance, nuclear waste duplex stainless steel containers in concrete bunkers can be subjected to MIC from high pH water percolating through the concrete [35]. The presence of SRB in alkaline environments can accelerate the corrosion of equipment operating in such conditions, resulting in equipment failure. An understanding of microbial corrosion processes under alkaline conditions would help to find suitable methods for corrosion control. However, there is not much detailed information in the literature which addresses the influence of environmental alkaline pH on microbial corrosion of materials or associated mechanisms. Many studies do not measure the pH during the corrosion process and the growth of bacteria. Although SRB was reported to survive in alkaline solutions, it is unclear as how they survive these environments and how they interact with metals. This paper tries to answer some of these questions. Even a recent review by [PERSON] et al. [4] has not discussed the effect of high pH on microbes' growth and their effect on microbial corrosion.