Stainless Steel UNS Research Articles

Pitting corrosion of welds in UNS S32304 lean duplex stainless steel

The pitting corrosion of welded joints of lean duplex stainless steel (LDSS) UNS S32304 was studied. LDSS S32304 thick plates were welded by different processes commonly employed in the manufacture of equipment and piping: shielded metal arc welding, gas metal arc welding, and flux cored arc welding. The electrochemical behavior of different weldment zones (fusion zone, base metal, and heat affected zone (HAZ)) was characterized both independently, using an electrochemical microcell, and together by testing the whole welded area. The electrochemical testing was performed in acidified glycerin, a process fluid of the biodiesel industry. The HAZ was the most susceptible zone for pitting corrosion for all samples. The results were correlated to the microstructural features of the materials. Finally, the results from the microcell and whole weld approaches were compared. According to the results, the gas metal arc welding process with 2209 as filler metal was found to be the optimum for application of 2304 duplex stainless steels in the acidified glycerin environment.

Influence of Deleterious Phases Precipitation in the Corrosion Resistance Measured by Double Loop Electrochemical Polarization Reactivation Test in Superduplex Stainless Steel

This work investigates the influence of deleterious phases precipitation on the corrosion resistance of two wrought superduplex stainless steels UNS S32750 with similar composition, but different grain sizes. Isothermal treatments were carried out to introduce different amounts of deleterious phases, such as chi (χ), sigma (σ) and secondary austenite (γ2). The specimens were tested by double loop electrochemical potentiodynamic reactivation test (DL-EPR). The kinetics of precipitation in the two steels was different due to the difference of the grain sizes. The results show a correlation between the sensitization degrees, measured by DL-EPR, with the amount of deleterious phases precipitated.

Estudo da Influência da precipitação de fases deletérias na resistência à corrosão por ensaio de polarização anódica cíclica em aços inoxidáveis superduplex

A influência da precipitação de fases deletérias na resistência à corrosão em dois aços inoxidáveis superduplex (AISD) UNS S32750 com diferentes tamanhos de grão foi investigada. Foram realizados tratamentos isotérmicos de modo a introduzir na microestrutura diversas quantidades de fases deletérias, tais como chi, sigma, austenita secundária e nitretos de cromo. Nestas condições foram efetuados ensaios de corrosão por polarização anódica cíclica. Estudos por microscopias ótica (MO) e eletrônica de varredura (MEV) foram realizados para caracterizar as fases precipitadas e a sua cinética de precipitação. Os resultados permitem correlacionar os valores obtidos pelos ensaios de polarização com o percentual de fases deletérias precipitadas para diferentes condições de tratamento em AISD com composições similares, mas com diferentes tamanhos de grão. Observou-se ainda que o tamanho de grão e a temperatura de envelhecimento influem na cinética de precipitação, na morfologia e no tamanho das fases deletérias precipitadas, incidindo na resistência à corrosão por pites dos AISD.

Brazing of UNS S32101 and UNS S32304 lean duplex stainless steels and UNS S32750 super duplex with AWS A5.3 BNi-7 (Ni–Cr–P) filler metal

Due to their corrosion resistance, duplex stainless steels are widely applied in various industries in which corrosion resistance is a required property. Unlike ferritic and austenitic stainless steels, duplex stainless steels have superior mechanical strength and can be used to produce lighter equipment. When thermally cycled, the properties of these steels can change, especially when using joining processes such as brazing, and when the austenite/ferrite volume fraction becomes imbalanced. This study aims to describe the brazing of lean duplex and superduplex stainless steels, verifying the effect of the joint gap and brazing time on the formation of continuous intermetallics inside the brazed joint. In this study, samples of duplex stainless steel UNS S32101, UNS S32304 and UNS S32750 were brazed in a continuous hydrogen atmosphere furnace, using BNi-7 (Ni–Cr–P) filler metal for brazing at a temperature of 1100°C, for 12 and 32 min, using joint gaps of 0.0 and 0.3 mm, followed by in-furnace cooling. The samples were characterized using optical microscopy, scanning electron microscopy with energy dispersion and X-ray diffraction. The results showed that the best condition for the in-furnace brazing used was a joint gap of 0.0 mm. Both of the joint gaps had different microstructures, yet the joint gap of 0.0 mm showed a continuous centred face cubic phase rich in nickel, which is frequently called γ-Ni. This region is composed of intermetallic phases rich in phosphorus. Sigma phase precipitation in the base metal on the duplex stainless steel UNS S35750 (32750?) was observed in the brazing conditions studied.

Repassivation Behaviour of UNS S32101 and UNS S30403 Stainless Steels after Cathodic Stripping of the Native Passive Film in a CO<sub>2</sub>-Saturated Oilfield Brine

Repassivation behaviour of the passive film formed on lean duplex stainless steel UNS S32101 and austenitic stainless steel UNS S30403 in a CO2-saturated oilfield environment has been studied. The native passive film on the alloys was thinned/removed by stepping the potential of the alloy to ﹣850 mV/Ag/AgCl for 30 minutes. Potentiostatic measurements were then taken at potentials of ﹣200, ﹣100, 0, 100 and 200 mV versus Ag/AgCl. Results show that the passive film repassivates at potentials of ﹣200 and ﹣100 mV and 0 mV for both alloys at 50°C. The current density however continues to rise for potentials of 100 and 200 mV. This shows that both alloys are susceptible to pitting at potentials above 100 mV at the test temperature of 50°C.

Characterization of weld strength and toughness in the multi-pass welding of Inconel 625 and Super-duplex stainless steel UNS S32750

The present study investigated the weldability of dissimilar metals involving Inconel 625 and Super-duplex stainless steel obtained from continuous current (CC) and pulsed current (PC) gas tungsten arc welding (GTAW) processes employing ER2553 and ERNiCrMo-4 fillers. A comparative analysis on these dissimilar weldments was carried out to establish the structure-property relationships. Microstructure examination was carried out using optical microscopy (OM) and scanning electron microscopy (SEM) techniques. Grain coarsening was observed at the HAZ of UNS S32750 for all the cases. Mechanical tests ascertained that the PCGTA weldments employing ERNiCrMo-4 offered better weld strength and impact toughness. Elaborative studies on the structure – property relationships of these dissimilar weldments were discussed.

Crevice corrosion performance of high-alloy stainless steels and Ni-based alloy in desalination industry

Several high alloys are candidate materials for the severe corrosive environment of reverse osmosis desalination plants. In the present study, the crevice corrosion performance of duplex, superduplex, superaustenitic stainless steels, and nickel-based alloy was investigated in natural and chlorinated seawater, at different temperatures. Several crevice configurations were evaluated including flanges assembled with gaskets, bolts mounted with nuts to plates, and the standard CREVCORR-type crevice formers. It was thus possible to illustrate the effect of crevice configuration on the corrosion behavior of the different alloys. The crevice geometry was confirmed to be of major importance in terms of risk for initiating crevice corrosion. In natural seawater (i.e. not chlorinated), the most severe temperature was 30°C, due to high biological activity and high corrosion kinetics. Chlorination at 0.5 ppm increased the risk of localized corrosion but decreased the corrosion propagation rates. In chlorinated seawater, crevice corrosion risk significantly increased with temperature for all tested alloys. For each of the tested configurations, the superaustenitic stainless steel UNS S31266 showed better crevice corrosion resistance than superaustenitic UNS S31254 and S34565, superduplex S32750, and also nickel-based alloy N06625. The present results are part of a three-year Joint Industry Program supported by end-users, engineering companies, and material producers.

Effect of Austenite Stability on Pack Aluminizing of Austenitic Stainless Steels

Aluminide coatings were applied to the surfaces of several austenitic stainless steels—UNS S30300, S30400, S30900, S31000, and S31600 (Type 303, 304, 309, 310, and 316)—by the halide activated pack cementation process. The coating compositions, microstructures, and hardness were determined for the different steels coated at 850°C for 25 h. The stability of the austenite phase for each type of steel was calculated by determining the ratio of the nickel to the chromium equivalents based on their nominal compositions. The thickness of the inner diffusion zone in the coating was shown to be inversely related to the austenite stability of the steels. Microhardness measurements were obtained across the coating thickness and into the substrate. The hardness values followed the same trends as the aluminum composition profile into the substrate.

Low-Temperature Stress Corrosion Cracking of Austenitic and Duplex Stainless Steels Under Chloride Deposits

Corrosion performance of austenitic-ferritic (duplex) stainless steels UNS S32101, S32202, S32304, and S32205 and austenitic stainless steels UNS S30403 and S31603 was studied in the presence of ch...

Suitability of UNS S31603 steel for geothermal brines in volcanic areas – Influence of different physicochemical conditions on its corrosion behavior

Geothermal sources are found to be the most aggressive natural environments for construction materials in terms of corrosion. This study was focused on investigating the suitability of stainless steel UNS S31603 in volcanic areas of geothermal sites Sibayak and Lahendong (Indonesia). Influence of different factors (temperatures up to 175°C, chloride content up to 1500mg/L and pH down to 2) was assessed to determine a “safe operating window” of a power plant and assure the system integrity. Investigations were performed by means of electrochemical methods and long-term exposure tests.

Effect of Solubilization Temperature on the Sigma Phase Precipitation of a Superduplex Stainless Steel

Superduplex stainless steel is an important class of stainless steels because it combines the benefits of ferrite and austenite phases, resulting in steels with better mechanical properties and corrosion resistance. However, a significant problem of this steel is the precipitation of deleterious phases during heat treatment. Among these precipitated phases, the most relevant is the sigma phase, because it causes higher loss of properties. The objective of this work therefore is to study the sigma phase precipitation in the superduplex stainless steel UNS S32520 when submitted to heat treatment of solubilization in three different temperatures (1050 C, 1150o C and 1250° C) and subsequently aged in the temperature of 850oC during 10 minutes, 30 minutes, 60 minutes, 3 hours and 10 hours, followed by water quenching. The results showed that as the solubilization temperature increases, there is a significant grain growth and an increase of the ferrite volumetric fraction, which delays the sigma phase precipitation in this superduplex stainless steel. Moreover, it can be verified that the hardness of the material is directly related to volumetric fraction of sigma present in the steel.

슈퍼듀플렉스 스테인리스강 UNS S32750과 탄소강 A516-70의 이종금속 FCA 용접 특성에 대한 연구

The metallurgical and mechanical characteristics, toughness and corrosion resistance of dissimilar welds between super duplex stainless steel UNS S32750 and carbon steel ASTM A516Gr.70 have been evaluated. Three heat inputs of 21.12, 24.00, 26.88kJ/cm were employed to make joints of dissimilar metals with flux cored arc welding(FCAW). Based on microstructural examination, vermicular ferrite was formed in the first layer of weld at low heat input(21.12kJ/cm) and <TEX>$Cr_{eq}/Ni_{eq}$</TEX> of 1.61 while acicular ferrite was formed in last layer of weld at high heat input(26.88kJ/cm) and <TEX>$Cr_{eq}/Ni_{eq}$</TEX> of 1.72. Ferrite percentage in dissimilar welds was lowest in the first layer of weld regardless of heat inputs and it gradually increased in the second and third layers of weld. Heat affected zone showed higher hardness than the weld metal although reheated zone showed lower hardness than weld metal due to the formation of secondary austenite. Tensile strengths of dissimilar welds increased with heat input and there was 100MPa difference. The corrosion test by ferric chloride solution showed that carbon steel had poor corrosion resistance and pitting corrosion occurred in the first layer(root pass) of weld due to the presence of reheated zone where secondary austenite was formed. The salt spray test of carbon steel showed that the surface only corroded but the amount of weight loss was extremely low.

In situ colonization of marine biofilms on UNS S32760 duplex stainless steel coupons in areas with different water qualities: Implications for corrosion potential behavior

In the presence of biofilms, stainless steels (SS) exhibits an increase in corrosion potential, called ennoblement. In the present study, the corrosion potential (E corr) behavior of the duplex SS UNS S32760 was recorded simultaneously with the in situ marine biofilm formation in two areas at Arraial do Cabo, Southeastern Brazil. The biofilm at Forno Harbor (an anthropogenically disturbed area) was characterized by higher relative abundances of Bacteria at day 2, followed by diatoms (especially Navicula sp.) on day 10 and dinoflagellates on day 18, whereas no clear trend was recorded at Cabo Frio Island (an undisturbed area). The ennoblement of E corr values was site-dependent. In a complementary laboratory assay, biofilms were removed and the E corr values registered in sterile conditions for the subsequent 10 days and corroborated in situ results. Understanding biofilms and SS interactions has important implications for materials science and engineering decisions as well as helping to fill in important gaps in this knowledge.

Metallurgical and mechanical characterization of electron beam welded super-duplex stainless steel UNS 32750

This paper addresses the weldability of 6mm thick plates of super-duplex stainless steel by electron beam welding (EBW) process. Microstructure investigations conducted using optical and SE microscopy showed the presence of large ferrite grains with intra- and inter-granular austenite. Ferrite content studies on the weld zone estimated that there is no significant element partitioning between austenite and ferrite phases. Hardness studies portrayed that the weld hardness is found to be greater than the parent metal which is acquainted due to the solid solution strengthening effect. Tensile results corroborated that the joints obtained by EB welding process have better strength than the base metal. A detailed structure–property relationships has also been carried out through point and line mapping EDAX analysis across the weldment to substantiate the discussions.

Corrosion testing in superheated geothermal steam in Iceland

Corrosion testing was conducted in a superheated geothermal steam from the IDDP-1 well in Iceland at 360°C for 113 days. Severe silica scaling occurred in the test. The average corrosion rate was extremely low; less than 0.1mm/year for all the metals tested which can be explained by the combined protective effect of the silica and the high superheat in the steam preventing any condensation. Despite the low average corrosion rate, the microstructural analysis show that even the most corrosion resistant materials tested, such as UNS N06625 and titanium UNS R52400 show localized corrosion damage after the test. Only the low alloy stainless steels UNS S30403, UNS 31603 and UNS 31803 showed evidence of Stress Corrosion Cracking.

Transpassivity characterization of the alloy UNS N08367 in a chloride-containing solution

We investigate the transpassivity of super-austenitic stainless steel UNS N08367 in 2.5 M LiCl solution by using cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). The CPP curve exhibits negative hysteresis, which indicates a transpassive dissolution process instead of pitting corrosion. The transition from the passive region to the transpassive region is characterized by EIS and equivalent circuit analysis. During the transpassive dissolution of the N08367 alloy, two reactions of adsorbed intermediates are dominant, as indicated by the two inductive loops at the transpassive region. The first inductive loop is associated with the faster reaction, i.e., the adsorption of Fe intermediates. This fast reaction is significantly influenced by the preferential dissolution of Fe during the transpassive dissolution. The second inductive loop is correlated with the adsorption of the Cr intermediate. In contrast to Fe, the Cr content on the surface increases in the transpassive region compared with the content in the passive region. The XPS spectra support the time and frequency domain approach for the preferential dissolution, and the dominant species resulted from the interfacial processes at the transpassive region.

Characterization of weld strength and impact toughness in the multi-pass welding of super-duplex stainless steel UNS 32750

This paper investigates the weldability, metallurgical and mechanical properties of the UNS 32750 super-duplex stainless steels joints by Gas Tungsten Arc Welding (GTAW) employing ER2553 and ERNiCrMo-4 filler metals. Impact and tensile studies envisaged that the weldments employing ER2553 exhibited superior mechanical properties compared to ERNiCrMo-4 weldments. Microstructure studies performed using optical and SEM analysis clearly exhibited the different forms of austenite including widmanstatten austenite on the weld zone employing ER2553 filler. Also the presented results clearly reported the effect of filler metals on strength and toughness during the multi-pass welding. This research article addressed the improvement of tensile and impact strength using appropriate filler wire without obtaining any deleterious phases.

Evaluation of Pitting Behavior on Solution Treated Duplex Stainless Steel UNS S31803

The pitting corrosion resistance of duplex stainless steels UNS S31803 annealed at different temperatures ranging from 1050 °C to 1200 °C for 24 h has been investigated by means of potentiostatic critical pitting temperature (CPT). The microstructural evolution and pit morphologies of the specimens were studied through optical microscopy and scanning electron microscopy. The potentiostatic CPT measurements show that the CPT was elevated with the annealing temperature increased from 1050 °C to 1150 °C and decreased as the temperature further increased to 1200 °C. The specimens annealed at 1150 °C exhibited the highest CPT and the best pitting corrosion resistance. The pit morphologies show that the pit initiation sites transfer from austenite phase to ferrite phase as the annealing temperature increases. The results were explained by the variation of pitting resistance equivalent number (PREN) of ferrite and austenite phases as the annealing temperature was varied.

Characterization of Aluminum Coatings Formed on Porous Ferritic Stainless Steels by Pack Cementation

Pack cementation is an attractive method for the formation of aluminum coatings on metal substrates. When applied to porous stainless steel substrates, the aluminum vapour species can diffuse into the pores and coat the interior surfaces. In this study, aluminum coatings were grown on commercial porous UNS 430 stainless steel and freeze cast UNS 440 stainless steel substrates by the pack cementation method. The effects of pack mixture composition, temperature, time, and the morphology of the substrates were characterized by scanning electron microscopy. It was shown that the degree of coverage of the aluminum coatings increased with porosity of the substrate. In addition, increased reaction time and temperature of the pack led to a more homogeneous distribution of aluminum across the substrate.

Brasagem dos aços inoxidáveis duplex 'baixa liga' UNS S32101, UNS S32304 e superduplex UNS S32750 com metal de adição AWS A5.3 tipo BNi-7 (Ni-Cr-P)

Os aços inoxidáveis duplex, devido à sua resistência à corrosão, têm uma grande aplicação em diversos segmentos industriais, onde a resistência à corrosão é uma propriedade requerida. Por terem resistência mecânica superior aos aços inoxidáveis ferríticos e austeníticos convencionais, podem produzir equipamentos mais leves. Quando ciclados termicamente, estes aços podem ter suas propriedades alteradas, principalmente quando se emprega processos de junção, como a brasagem, e quando ocorre o desbalanceamento da fração volumétrica ferrita/austenita. O objetivo deste trabalho é caracterizar a brasagem de aços inoxidáveis duplex 'baixa liga' e superduplex, verificando o efeito da folga e do tempo de brasagem na formação de intermetálicos contínuos no interior da junta brasada. Neste trabalho foram brasados, em forno continuo com atmosfera de hidrogênio, corpos-de-prova de aço inoxidável duplex UNS S32101, UNS S32304 e UNS S32750 com a adição para brasagem BNi-7 (Ni-Cr-P), na temperatura de 1100ºC, por tempos de 12 min e 32 min, utilizando-se folgas das juntas de 0,0 e 0,3 mm, seguidos de resfriamento em forno. Os corpos de prova foram caracterizados utilizando-se microscópio óptico, microscópio eletrônico de varredura com EDS e difração de raios X. Os resultados mostraram que a melhor condição para a brasagem em forno empregado foi a brasagem com a folga de 0,0 mm. Ambas as folgas apresentaram microestruturas variadas, porém a folga de 0,0 mm mostrou a presença de uma fase contínua CFC (cúbica de face centrada), rica em níquel, que é denominada, frequentemente, de γ-Ni. Para folgas de 0,3 mm, forma-se uma região eutética contínua no centro da junta, que pode prejudicar as propriedades mecânicas da mesma. Esta região é composta de fases intermetálicas ricas em fósforo. Foram observadas precipitações de fase sigma no metal de base no AID UNS S35750, nas condições de brasagem estudadas.