Investigation of Corrosion Resistance of Duplex, Lean Duplex and Austenitic Stainless Steels in Simulated Offshore Environment
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Some duplex grades are developed to replace the 300 series of austenitic stainless steels in harsh chloride applications due to superior mechanical and corrosive properties. Since the market price for nickel and molybdenum have surged, it has caused an increase in the cost of production of stainless steels and created a competitive market. Some of the duplex and lean duplex grades are now considered to be a cheaper choice in these applications. The purpose of this study is to investigate the corrosion resistance of different types of duplex and stainless steels, namely: 2205 (UNS S32205), 2304 (UNS S32304), 2003 (UNS S32003), 316L (UNS S31603) and 304 (UNS S30400). The grades of lean duplex, 2003 and 2304, are similar to each other in terms of corrosion resistance and are of focus in this thesis, while the other steels serve as reference samples for comparison. Two main experiments were carried out to test the materials resistance in chloride environments, where one was performed as general- and localized corrosion testing in a simulated offshore environment, consisting of three test rounds with different temperatures. The other experiment was performed as an electrochemical test by cyclic polarization according to ASTM G61-86, with two different sodium chloride solutions. The samples were weight tested before and after exposure of the offshore environment and was evaluated after testing based on visual inspection. The interest of the ASTM G61-86 experiment was to test the materials susceptibility to pitting corrosion by finding and comparing the materials pitting-, repassivation- and corrosion potentials. The materials chemical compositions were found by a scanning electron microscope and compared to their corresponding material certificates. The materials pitting resistant equivalent numbers were also assessed and compared to the results of cyclic polarization. None of the samples showed initiation of localized corrosion after being exposed for the simulated offshore environment. General corrosion was observed on many of the samples tested, but there was not registered any deviations in mass after the experiments. However, it was found by visual inspection of the samples after testing that the grades of duplex and lean duplex had less corroded areas on their surfaces. From the ASTM G61-86 experiment it was found that duplex 2205 had the best corrosion resistance against pitting corrosion. Also, lean duplex grade 2003 was found to be superior to 2304 in corrosion resistance against pitting corrosion. The order of the materials resistance to pitting corrosion fits their corresponding pitting resistance equivalent number and are in the following order, from best to worst: 2205, 2003, 2304, 316L and 304. Chemical analysis showed that the materials compositions match their certificates, with a few deviations on elements that were difficult to measure.
Master's thesis in Mechanical engineering