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The BiogaSS project explores stainless steel for cost effective design for the fabrication of biodigesters. Although concrete is extensively used, it is not necessarily the most economic choice despite its low purchase cost. To prove stainless steels’ cost-efficiency, three aspects are investigated in detail, namely corrosion resistance, mechanical properties and innovative construction solutions. First, the corrosion resistance was investigated under increasingly realistic conditions, ranging from electrochemical testing in artificial solutions over exposure in lab scale reactors to field exposure in operational biodigesters. The test results delimited alloying content as a function of surface finish. Furthermore, a series of laboratory tests and numerical analyses was carried out to extend the design rules from Eurocode 3: Part 1-4 (EN 1993-1-4). This was done for stainless steel shells under axial or circumferential compression including the post-buckling strength. By taking advantage of the post-critical strength, reduction factors could be significantly increased. Also adhesive bonding was explored as a part of innovative cost-effective design. It was concluded that adhesive bonding was not a suitable method for fabricating stainless steel biodigesters, although more adhesives and more rigorous surface preparation should be studied before definitive recommendations can be made. Combining the project results with cost and maintenance data gathered from the industry, the costs for equivalent in-situ concrete, carbon steel and stainless steel tanks were assessed. Stainless steel was the cheapest option in terms of whole life costs over a 25 and 40 year lifetime. Finally, the project generated design guidance for a range of suitable stainless steel grades for application in biodigesters.

The focus of the project was the development of a comprehensive way of assessing the risk for localised corrosion of stainless steels. Pitting, crevice corrosion, stress corrosion cracking and crevice-crack interactions were evaluated in terms of the effects of temperature, chloride concentration, potential, pH and cold work. The main emphasis was on the testing of the austenitic 1.4404 and the superduplex 1.4410 in solutions of NaCl and MgCl2. An intercomparison of the electrochemical test methodologies between laboratories was carried out and refined until consistent results were obtained. Pre-exisiting data for the two grades of interest were compiled and the environmental conditions prevailing in different applications were characterised. Measurement of pitting potentials and critical pitting temperatures gave reasonable agreement, while spring-disc crevice formers yielded lower critical temperatures for crevice corrosion than use of a modified flushed-port cell. The presence of crevices was found to have only a marginal influence on the development of stress corrosion cracking, and results from U-bend and slow strain rate testing were largely compatible. Results from field testing gave generally good agreement with laboratory data, but could in cases result in higher limiting conditions for localised corrosion. The results have been compiled into design diagrams defining the regimes in which there is a danger for different types of localised corrosion as a function of the main variables, chloride concentration and temperature. This provides indicative information for materials selection and a framework into which future data from both laboratory testing and field testing can be included.