Corrosion Investigation of Duplex Stainless Steels in Chlorinated Solutions Mohaddeseh Kahram, Ã Majid Asnavandi, Pramod Koshy, and Charles Christopher Sorrell The corrosion of duplex stainless steel 2205 was investigated both under service conditions and via an electrochemical study in chlorinated media. The steel sample surfaces showed pitting, crevice and selective dissolution, with the ferrite phase being more susceptible to dissolution in this media. The dissolution of MnS was identified to be a contributing factor for pitting corrosion, while electrochemical investigations revealed that the presence of hypochlorite ions caused a switching from a general corrosion phenomenon to a localized one. 1. Introduction Duplex stainless steels (DSSs) are good corrosion-resistant materials for use in aqueous media and have micro- structures consisting of nearly equal proportions of ferrite and austenite phases. [1,2] These steels are increasingly being used in aggressive environments (chemical industry or marine environments such as chlorinated seawater) compared to austenitic stainless steels [2–4] due to the latter experiencing chloride pitting and/or stress corrosion cracking in these media. [5] The high amounts of Cr, Mo, and N in the DSSs are believed to be responsible for the high resistance to both pitting and crevice corrosion in chloride media, with Mo enhancing the stability of the passive film formed on the steel surface. [1] However, external factors such as temperature, chloride concentra- tion, and chlorination levels can have a greater impact on the corrosion resistance compared to the effect of the steel composition. [6] Almost all steels, even DSSs, corrode to varying extents in chlorinated seawater when tested for extended periods. [7] DSS steels are currently used in rotary drum screens, which are industrial facilities that separate solids from seawater and supplies filtered water. In such environ- ments, the biological organisms in the seawater can cause serious issues for the installation and maintenance of the equipment. The resulting fouling (caused by the formation of biofilms) creates friction losses in pipelines and induces erosion of metals; further, low degrees of fouling can lower the thermal efficiency of heat exchangers. To overcome these biological problems, the water is chlorinated [3,8] either by adding hypochlorite solution or by electrolysing the water, continuously or intermittently. [9,10] Therefore, these drum screens are operated in chlorinated seawater, and the strong oxidising properties of chlorine can affect the corrosion behaviour of the steel. This work investigates the corrosion behavior and related phenomena of DSSs in chlorinated solutions under both service conditions and through a laboratory- scale electrochemical study. 2. Experimental Part Table 1 shows the chemical composition of the DSS 2205 samples (bolts, nuts, and washers) from an industrial drum screen that were used for testing. The schematic design of the drum screen is shown in Figure 1. [11] Table 2 provides the compositions of the chlorinated Persian Gulf seawater used for testing (3 ppm chlorine supplied continuously; 5 ppm chlorine intermittently thrice a day for 20 min each time and the service temperature is 238C). X-ray diffraction analysis patterns of corrosion products (removed from the bolts) was done using a X’Pert Multi Purpose Diffractometer (MPD) [CuK a X-ray source (l ¼ 1.5418 A ˚ )]. Scanning electron microscopy (SEM) analysis and energy dispersive spec- troscopy (EDS) analysis of the corroded region were done by a Hitachi TM3000 SEM. For electrochemical experi- ments, samples of 1 cm diameter were cut from the bottom of the bolts and embedded in an epoxy resin (non-conducting), and then polished to 0.05 micron finish. This sample was then connected via a copper wire to an electrochemical cell. Potentiodynamic cyclic polarization studies (at a scanning rate of 10 mV/s) were [ Ã ] M. Kahram, P. Koshy, C. C. Sorrell School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia Email: m.kahram@unsw.edu.au M. Asnavandi School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia DOI: 10.1002/srin.201400293 www.steel-research.de ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 85 (2014) No. 9999 1 FULL PAPER