Erosion — corrosion of duplex stainless steel under Kuwait marine condition

Desalinated water produced by a reverse osmosis (RO) filtering method forms about 22% of total production of desalinated water in the world. However, the RO environment is very corrosive due to the presence of various chemicals for water treatment and the flow of sand particles leading to corrosion. Recently, there has been much effort to substitute cheaper and more corrosion resistant stainless steels for copper based alloys as a valve material in RO. Nevertheless, the effects of chemicals and particles on the corrosion of stainless steels have rarely been studied. In this study, we report the effects of flow velocity under conditions with and without sand particles and chemicals such as NaOCl, FeCl3, and Na2S on the corrosion behaviors of various stainless steels. UNS S31603, S31803 and S32750 based cast stainless steels were used for tests. All electrochemical tests were conducted at 35 oC in artificial sea waters containing various chemicals and flow-velocity with/without sand particles using three-electrode system composed of a specimen of interest as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a Pt wire as the counter electrode. Prior to measurements, all the specimens were carefully polished using abrasive papers and diamond paste. Influence of chemicals for water treatment in RO, such as NaOCl, FeCl3, Na2S was investigated using electrochemical impedance analysis as well as cyclic-polarization tests. In addition, current noise of the samples in potentio-static test was monitored in solutions with and without sand particles under the jet impingement condition. After the corrosion tests, morphological change of the specimens was examined by an optical microscope and a scanning electron microscope. Among the chemicals tested, Na2S exhibited a greater effect on corrosion of the stainless steels under investigation compared with other chemicals such as NaOCl and FeCl3. At cyclic polarization tests, the obtained corrosion current density (icorr), as a measure of corrosion rate, of the stainless steels was found to be almost similar to each other. However, the cast austenitic stainless steel clearly exhibited the lower resistance to pitting corrosion compared with duplex stainless steels. In addition, EIS tests confirmed that the charge transfer resistance (Rct) and the resistance of the passive film (Rf) measured in sulfide-containing solution decreased more significantly compared with those in other chemical-containing solutions. Under various flow-velocity, all the current density of stainless steels increased at passive range. In particular, at the flow rate of higher than 6 m s-1 erosion corrosion was detected on the surface. Increasing the flow-velocity was found to increase the physical damage in the passive film. In seawater containing sand particles, the erosion in stainless steels was accelerated further, and passive current density was more increased than that in the seawater without particle.

From the introduction of its first-generation, Duplex (austeno-ferritic) steel has seen a steady increase in popularity [1]. Recently, the production of highstrength, corrosion resistant super-duplex coil has been implemented in several applications. The case study we present in this paper concerns its use in a set of tubes of a heat exchanger affected by corrosion. The tubes material, S31803 Duplex stainless steel, is particularly suited to applications where corrosion is likely due to chlorides and/or sulphides being present. The heat exchanger operates in a chloride environment due to marine water on the inner side while the outer side is in contact with a hydrocarbon mixture rich in sulphur. It also has high resistance to general, pitting, crevice and erosion corrosion and to corrosion fatigue.

Role of microstructure on corrosion of duplex stainless steel in presence of bacterial activity

The effects of nitrogen and high-temperature aging on the uniform corrosion of duplex stainless steel (DSS) immersed in 60°C, 40% sulfuric acid (H2SO4) were studied. Under 800°C, regardless of nitrogen content aging at 0 to 1 h and 1 h to 10 h, the corrosion rates of DSS significantly increase. But, in general, DSS containing high nitrogen content, under the same aging treatment, has a lower corrosion rate. The higher the nitrogen content, the fewer σ phase precipitates for a given aging treatment. After DSS containing different nitrogen contents have undergone aging treatment for more than 1 h at 900°C, their corrosion rates all tend to be saturated.

This paper compares the suitability of 22 Cr duplex stainless steel (UNS S31803), low alloy steel, and Alloy 625 (UNS N06625) for MEG reclamation systems. Oxygen is of particular interest. Corrosion of duplex stainless steel in the presence of high oxygen concentrations under MEG reclamation conditions is manifested as preferential ferrite dissolution and/or localized corrosion at crevices. Oxygen entry must be minimized to prevent corrosion of duplex stainless steel. Corrosion most likely occurs where salts deposit. Low alloy steels pit at low oxygen concentrations. Duplex stainless steel (with oxygen restrictions) and Alloy 625 are suitable for the MEG reclamation service conditions.

Effect of post weld heat treatment and TiAlSiN coating on the tensile strength of autogenous plasma arc welding of duplex/ super austenitic stainless steels

Erosion–corrosion behaviour of lean duplex stainless steels in 3.5% NaCl solution

Corrosion of duplex stainless steels (DSS) usually initiates at a specific phase, causing preferential phase-dissolution in aqueous environments. Although the scanning Kelvin probe force microscopy (SKPFM) can characterise the phase-corrosion at a micro- or even nanoscale, conventional data-processing methods are insufficient to analyse massive data to derive mechanistic information, and sometimes, even cause a misunderstanding of corrosion evolution. Here we utilised root mean square, power spectral density and fractal evaluations to analyse the time-dependent topographic and Volta potential results obtained by SKPFM during corrosion progression of 2205 DSS in a chloride solution. Corrosion of the DSS preferentially occurs in the ferrite phase and at the ferrite/austenite boundaries in the initial stage. The corrosion changes from the initial galvanic mechanism to mass transfer control with increased time. By using proper numerical tools, the Volta potential results are more insightful of analysing corrosion than the topographic profiles.

Since contradictory data can be found in the literature, it is often difficult to assess the susceptibility of crevice corrosion of stainless steels in service conditions for a given marine application. The initiation and propagation of crevice corrosion in natural seawater were evaluated for five different duplex stainless steel grades together with some austenitic grades. A CREVCORR-type assembly was used to simulate crevice configurations involving the use of plastic crevice formers. The standard pressure applied on the crevice assembly was 3 N/mm2. Pressure of about 20 N/mm2 was also applied on some selected specimens in order to assess the effect of crevice geometry on crevice corrosion. The effect of environmental parameters (i.e. temperature, flowing conditions, residual chlorine, and dissolved oxygen content) and of surface roughness on the crevice corrosion initiation and propagation were investigated, allowing the assessment of limits of applications for some tested stainless steel grades. The less alloyed duplex stainless steels were evaluated in stagnant seawater at 5 °C and 20 °C. The duplex stainless steel UNS S32205, with PREN = 37, was also evaluated under the same conditions of exposure. The high alloyed stainless steels with PREN above 40 were evaluated in the expected most severe conditions of exposure, namely in 0.5 ppm-chlorinated seawater at 20 °C, in seawater at 30 °C (not chlorinated and with 0.5 ppm of residual chlorine) and in seawater at 50 °C (not chlorinated and with 0.5 ppm of residual chlorine). As expected the less alloyed duplex stainless steels showed limited crevice corrosion resistance in the tested media while UNS S32205 showed better resistance in the less severe tested condition of exposure. In demanding media it was shown that the limits of application of highly alloyed stainless steels are highly dependent on the crevice geometry (i.e. specimen roughness and applied pressure at gasket location).

The mechanism of hydrogen-induced pitting corrosion in duplex stainless steel studied by SKPFM

Duplex stainless steels (DSSs) were first invented in the early part of the 20th century, but it was not until the 1970s and 80s, with the introduction of argon oxygen decarburization (AOD) melting and the recognition of the benefits of nitrogen additions, that DSSs became attractive for widespread industrial use. DSSs have found applications in most industries, including oil and gas, marine, desalination, power, chemical and process, pulp and paper, and mineral processing. DSSs have been adopted by many industries to varying degrees. They are the workhorse corrosion resistant alloy (CRA) of the oil and gas industry, and are also widely used in the chemical and process industries for their SCC and corrosion resistance. Superduplex stainless steel has become the main alloy for piping, pumps, and valves in seawater reverse osmosis (SWRO) desalination plants, because of its resistance to crevice corrosion in seawater. In this book, Roger Francis reviews various duplex alloy compositions, mechanical properties, and design stresses for vessels and pipes to various codes. He also covers the basics of welding duplex alloys, both to themselves and to other alloys, and their corrosion resistance. Of most importance, the book looks at a variety of types of corrosion that may affect DSSs in service, presenting the available data and, in some cases, how to avoid problems.

This paper reports in situ atomic force microscopy (AFM) and SEM studies of the corrosion of duplex stainless steels. The results showed selective dissolution of the duplex structure, with one rapidly dissolving phase and one slowly dissolving phase. Electron probe microanalysis (EPMA) measurements showed that the weak phase contained more Cr and Mo and the resistance phase contained more Ni, which was used to assign these phases to ferrite and austenite, respectively. Copper deposition experiments showed that the resistant phase was more noble and acted as the cathode. The mode of dissolution was apparently modified after polarization of the steel in the passive range. Copyright © 2002 John Wiley & Sons, Ltd.

Electrochemical noise analysis of localized corrosion of duplex stainless steel aged at 475 °C

Crevice corrosions of duplex stainless steels were investigated under the aggressive condition of sea water pumps in offshore petroleum platforms. The study aims to compare crevice corrosion behavior of three grades of duplex stainless steels, UNS S32101 (2101), UNS S31803 (2205) and UNS S32750, in seawater with 200 ppm of hypochlorite. Duplex steels were compared with a widely used austenitic steel UNS S31603 (316). Specimens were tested under synthetic sea water prepared at pH4, pH6 and pH8 by the cyclic potentiodynamics polarization technique and the potentiostatic technique. The results show that the duplex stainless steels had higher corrosion resistance compared with the 316. The released charges and the corrosion rates of the 2101 and the 2205 were similar, but much lower than those of the 2507especially after 3 months period.

Duplex stainless steels are finding increasing usage in the pulp and paper industry. The resistance to pitting and crevice corrosion of duplex stainless steels depends upon the microstructure and distribution of alloying elements between the austenite and ferrite. In the case of weldments of duplex stainless steels, these are affected by welding parameters. The effect of heat input and welding techniques on the pitting resistance of a duplex stainless steel, FERRALIUM alloy 255, was examined. High heat input, which results in slow cooling, was found to be beneficial for pitting resistance. Pitting corrosion in the weld metal occurred preferentially in the ferrite along the austenite-ferrite boundaries. Qualitative explanations for the beneficial effect of slow cooling on pitting resistance are given.