Localized corrosion of lean duplex stainless steels in H2S-containing wet atmosphere from urban wastewater treatment units

With lower alloying costs and higher mechanical properties, lean duplex stainless steels can be a good alternative to the more commonly used austenitic stainless steels. A study was initiated to define the limits of the use of lean duplex stainless steels for urban wastewater treatment (WWT) units. This paper gives and discusses the corrosion results in an aerated wet atmosphere containing H₂S at different levels. Exposures were performed both at laboratory scale and in the field WWT plant for 1 year. A specific probe was also designed to study the corrosion process below water condensate film contaminated with H₂S. Under such conditions, the properties of stainless steel were strongly modified with an enhanced risk of localized corrosion. The results obtained on lean duplex materials (UNS S32101, S32202, and S32304) are compared with austenitic UNS S30403 and UNS S31603 and with the more standard duplexes UNS S82441 and UNS S32205. The results show that lean duplexes can be used in aerated wet atmospheres in case of moderate contamination of H₂S (<10 ppm) and chloride (<200 ppm). For higher contaminations (e.g., H₂S around 100 ppm/chloride around 1000 ppm) the duplex S32205 should be preferred.     

核电厂水池覆面用不锈钢钢板在硼酸水溶液中的点蚀行为

采用电化学方法研究了温度、Cl~-以及SO_4~(2-)含量对不锈钢板S30403、S32101和S32205在硼酸溶液中点蚀行为的影响。结果表明:3种材料的点蚀电位(Eb)和再钝化电位(Er)均随Cl~-含量的升高而降低,S32205不锈钢的点蚀电位和再钝化电位普遍高于S32101和S30403不锈钢的;存在临界温度(约60℃),当温度高于临界温度,S32205不锈钢的点蚀电位大幅降低,再钝化电位的临界温度介于40~60℃;SO_4~(2-)含量对3种材料点蚀电位和再钝化电位的影响不明显。S32205不锈钢的耐点蚀性能优于S32101和S30403不锈钢的,而S32101和S30403不锈钢的耐点蚀性能相当。     

The effect of CeCl3 inhibitor on the localized corrosion of stainless steel in chloride solutions

Stainless steels have been used extensively in many sectors such as medical and household appliances as well as construction. This has been primarily due to their high resistance to corrosion attacks, reasonable cost, and excellent mechanical properties. However, when placed in corrosive media stainless steel is susceptible to localized corrosion attacks, especially when placed in chloride solutions. The paper explores the issue of corrosion liability of austenitic (AISI 316) and duplex (UNS S32205) stainless steels in et aconditions of seawater environment as well as under the influence of an inhibitor in the same environmental conditions. The behavior of stainless steels was examined via electrochemical testing relying on DC and AC techniques, optical metallographic analysis, scanning electron microscopy, and X‐ray diffraction analysis. Duplex stainless steel showed better resistance to localized corrosion but a higher tendency toward general corrosion in all examined solutions. Cerium chloride in a chloride solution showed inhibiting properties for both the AISI 316 and the UNS S32205.     

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

Laboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic-austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits-crevices were found under the corrosion crust deposits on the duplex stainless steel.     

Effect of laser surface melting on intergranular corrosion behaviour of aged austenitic and duplex stainless steels

In the present study, the effect of laser surface melting (LSM) on intergranular corrosion behaviour of aged austenitic stainless steels (UNS S30400, S31603, S32100 and S34700) and aged duplex stainless steels (UNS S31803 and S32950) were investigated. LSM of the aged stainless steels was carried out using a 2.5 kW CW Nd:YAG laser. The microstructure of the aged stainless steels after LSM depends on their compositions. After LSM, the aged austenitic stainless steels mainly contain austenite (γ) with some ferrite (δ) as the minor phase, but the carbide phases are completely eliminated. For the aged duplex stainless steels after LSM, δ becomes the major phase and the δ/γ phase balance is disturbed, whereas the sigma (σ) phase is eliminated. The degree of sensitization (DOS) and corrosion morphology of the aged stainless steels before and after LSM were determined by the double loop electrochemical potentiokinetic reactivation (DL-EPR) using a potentiostat and SEM observation, respectively. Desensitization of the aged stainless steels has been successfully achieved by LSM and their intergranular corrosion resistance is found to be significantly enhanced as reflected by the decrease in DOS due to dissolution of the carbides or σ phase, which reduced Cr depletion or the possibility of solute segregation at the grain or phase boundaries, despite the presence of δ and disturbance of δ/γ phase balance.     

Corrosion performance of selected stainless steels in brine from reverse osmosis desalination plants

Abstract

The corrosion performances of some commercial stainless steel alloys in the brine reject solution from a reverse osmosis sea water desalination plant was studied in terms of their pitting susceptibilities (investigated under aerated conditions at ambient temperature using a cyclic polarisation technique) and crevice corrosion resistances (evaluated in the plant over a 3 month exposure period using multiple crevice test assemblies). The alloys used were four austenitic steels, UNS S31603, UNSS 31703, UNS N08904, UNS S31254, a ferritic steel UNS S44635, and a duplex steel UNS S32550. Cyclic polarisation studies show that the pitting or breakdown potentials for S31603 and S31703 occurred at more active values than for N08904, S31254, S44635, and S32550 alloys, and indicated a reduced resistance to pitting corrosion. The multiple crevice tests show that the alloys S31603, S31703, and N08904 do suffer crevice corrosion in the brine reject solution at ambient temperature, while the S44635 S32550, and S31254 alloys showed considerably higher crevice corrosion resistance.     

Interactions between stainless steels and sulphur anions in liquor mixtures typical of kraft pulp digesting

Interactions of certain stainless steels (UNS S30400, S31603 S32205, S32304, hard chromium plated S31603 as well as hard chromium plated S32205) and sulphur anions in concentrations typical of kraft liquors were investigated using wave length dispersive X‐ray fluorescence (WDXRF) and contact electric resistance (CER) techniques. Ion chromatoghraphy (IC), ion selective electrodes (ISE) and ABC titration were applied in the determining the behaviour of the sulphur anions. Hydroxide content was found to correlate linearly with the adhesion of sulphur onto steel. Impact of sulphide was evident, but nonlinear. Effect of other anions and temperature was less significant. The adhesion increased in the following order: S32304, S32205, S30400 and S31603. Film forming on S30400 possessed highest resistance properties. Hard chromium plated steels appeared to be chemically inert. For the solution, real/analysed concentrations of sulphide was less than the weighted concentrations. Thiosulphate had a tendency to convert into sulphite.     

Corrosion resistance of low‐nickel duplex stainless steel rebars

The use of stainless steel bars in reinforced concrete structures may be an effective method to prevent corrosion in aggressive environments where high amounts of chlorides may penetrate in the concrete cover. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for these rebars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, etc.) should be assessed. To reduce the cost of stainless steel reinforcement, duplex stainless steels with low nickel content have been recently proposed as an alternative to traditional austenitic steels, even though, few results are available regarding their corrosion performance in chloride contaminated concrete. This paper deals with the corrosion resistance of low‐nickel duplex stainless steel rebars (1.4362 and 1.4162) as a function of the chloride content. Comparison is made with traditional austenitic steels. An attempt to define a chloride threshold for the different stainless steels is made by comparing the results of several test procedures both in concrete and in solution.     

Welding Heat Input Influence on UNS S82441 Lean Duplex Stainless Steel Corrosion Resistance Assessed by Scanning Vibrating Electrode Technique (SVET)

The precipitation of the intermetallic phases can impair mechanical and corrosion properties. In the case of lean duplex stainless steels, they present solidification mode as primary ferrite, with austenite precipitation in the solid state, during cooling. This means the heat-affected zone balanced microstructure can be changed during the welding by the cooling rate, which is associated to the welding heat input, the thickness of the plate and the physical properties of the material. This work intends to present the influence of welding heat input on the corrosion resistance of UNS S82441 lean duplex stainless steel. The gas metal arc welding process was used to make the weld beads, using ER2209 as filler metal, which characterizes a dissimilar welded joint. The corrosion resistance of these specimens was analyzed through scanning vibrating electrode technique. It was observed that welding heat input variation distinctly influences corrosion in different zones of the weld beads over the specimen exposure time. The weld bead under intermediate welding heat input showed the highest corrosion rate and the one under the highest welding heat input, the lowest corrosion rate intensity.     

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Abstract

Anisotropy in stress corrosion cracking of lean grade UNS S32101 and standard grade UNS S32205 hot rolled duplex stainless steels was evaluated in this study. Microstructures were characterised with stereology methods in three orthogonal orientations to quantify anisotropy in phase distribution. Constant extension rate tests were conducted in an autoclave at 2×10^?6 s^?1 in chloride containing alkaline sulphide solution at 170°C. The maximum crack lengths and average crack densities were measured for the rolling longitudinal and transverse longitudinal orientations after stress corrosion cracking tests. Results indicate that stress corrosion crack initiation and growth are a strong function of the microstructure in each orientation. Maximum crack length and average crack density were maximum for the rolling longitudinal orientation. Crack initiation and growth were favoured along the transverse direction. Chlorides enhanced anisotropy in crack growth behaviour by facilitating crack initiation and crack coalescence along phase boundaries, which lead to more severe stress corrosion cracking in the alkaline sulphide environments that were studied.     

22Cr双相不锈钢与304L、316L钢在氯化物溶液中耐应力腐蚀性能的比较

采用恒载荷法、恒应变法研究了22Cr双相不锈钢在不同Cl-浓度、不同温度的氯化物渗液中耐应力腐蚀的性能,并与316L和304L普通奥氏体不锈钢进行了对比.结果表明,22Cr双相不锈钢在氯化物环境中具有更好的应力腐蚀破裂(SCC)抗力.     

Chloride-induced stress corrosion cracking of Duplex stainless steels. Models,test methods and experience

Stress corrosion cracking (SCC) induced by chlorides frequently causes problems in applications where standard austenitic stainless steels are being used. Often this problem can be solved by the use of duplex stainless steels. In this report the mechanisms for SCC have been surveyed, and the cause for the high SCC resistance of duplex stainless steels has been discussed and evaluation of test methods for SCC and how duplex stainless steels respond to them, as well as practical experience of duplex stainless steels. The study shows that no single mechanism can be attributed to the good resistance to SCC of duplex stainless steels. Probably a synergistic effect of electrochemical and/or mechanical effects is responsible for the good performance. Test methods for SCC often give relatively good correspondence with real applications, but ranking is often doubtful, and comparisons of different material types should be made with caution. Numerous cases with SCC on standard austenitic stainless steels have been solved by the use of duplex stainless steels.     

Cl~-浓度对双相钢和镍基合金腐蚀速率的影响

针对H2S、CO2、Cl-同时存在的高温、高压腐蚀环境,选择22Cr、25Cr双相不锈钢和028铁-镍基合金、G3镍基合金进行对比试验,在其它条件相同的情况下(H2S/CO2分压、温度、液体流动等条件一致,仅Cl-浓度变化),分析Cl-含量对各种耐蚀合金管材发生均匀腐蚀与点蚀的腐蚀速率的影响。结果表明,在温度为160℃,CO2分压为4.13MPa,H2S分压为2.66MPa,流速为3m/s的条件下,随着Cl-浓度增加(25g/L、100g/L、120g/L、250g/L),UNS S32205和UNS S32750两种双相不锈钢的均匀腐蚀速率和局部腐蚀速率增加;028和G3两种镍基合金的均匀腐蚀速率变化不大,且无局部腐蚀发生。UNS S32205、UNS S32750双相不锈钢试样表面钝化膜的主要成分为Cr2O3和/或Cr(OH)3;028和G3镍基合金试样表面钝化膜的主要成分为NiO、Cr2O3和/或Cr(OH)3。     

Application limits of stainless steels in the petroleum industry

Stainless steels have a great variety of potential applications in the petroleum industry, mainly as an alternative to carbon steel in corrosive environments. Within a number of media that can cause corrosion problems with these materials, only chloride solutions and hydrogen sulfide are of importance in oilfield service. A reliable tool that permits the proper selection of stainless steels has yet been missing. In order to provide engineering diagrams for this purpose, pitting and stress corrosion cracking (SCC) tests were performed. Specimens were exposed to NaCl solutions containing from 3 to 100,000 ppm Cl^? at temperatures from 40 to 200 °C. This test configuration was chosen to give a better representation of actual service conditions than accelerated standard test procedures do. Tested materials were the austenitic stainless steel grades 321, 316Ti (API LC30‐1812) and 254 SMO, and 22Cr duplex (austenitic‐ferritic) steel (API LC65‐2205). Based on an optical examination of the specimens, no‐risk regions of chloride concentration vs. temperature have been identified. Subsequently, service temperature limits have been deduced for each tested material. Thus, material failures by pitting and SCC can be prevented without overdesigning. The results of the testing series are applicable to all chloride environments without presence of H₂S, as they have to be handled by primary production equipment, as well as transportation and gas processing facilities.     

Corrosion behaviour of corrugated lean duplex stainless steels in simulated concrete pore solutions

This work studies the corrosion behaviour of two corrugated lean duplex stainless steels (SAF 2001 and 2304 grades) in eight alkaline solutions (carbonated and non-carbonated, saturated Ca(OH)₂ solutions with different chloride contents). 2001 stainless steel is a new grade in market because of its composition. 2304 is a grade previously studied under different conditions. However, its use as reinforcement in concrete is new. Studies are carried out by polarization curves following scanning electronic microscopy (SEM) and optical observations. Results are compared to those of carbon steel and austenitic AISI 304 and duplex SAF 2205 under similar conditions. After corrosion tests in alkaline media with chloride, ferrite tends to corrode selectively in 2304 duplex, while austenite corrodes selectively in 2001 under the same conditions. The influence of the duplex microstructure on attack development and morphology is analyzed. The electrochemical parameters obtained from the polarization curves suggest 2001 could replace 304 keeping the structure its corrosion performance (and with clear economical advantages). 2304 shows better corrosion behaviour than the more expensive 304, but somewhat lower than the excellent behaviour shown by 2205.     

Corrosion resistance of stainless steel pipes in soil

To be able to give safe recommendations concerning the choice of suitable stainless steel grades for pipelines to be buried in various soil environments, a large research programme, including field exposures of test specimens buried in soil in Sweden and in France, has been performed. Resistance against external corrosion of austenitic, super austenitic, lean duplex, duplex and super duplex steel grades in soil has been investigated by laboratory tests and field exposures. The grades included have been screened according to their critical pitting‐corrosion temperature and according to their time‐to‐re‐passivation after the passive layer has been destroyed locally by scratching. The field exposures programme, being the core of the investigation, uses large specimens: 2 m pipes and plates, of different grades. The exposure has been performed to reveal effects of aeration cells, deposits or confined areas, welds and burial depth. Additionally, investigations of the tendency of stainless steel to corrode under the influence of alternating current (AC) have been performed, both in the laboratory and in the field. Recommendations for use of stainless steels under different soil conditions are given based on experimental results and on operating experiences of existing stainless steel pipelines in soil.     

Localised effects of macrofouling species on electrochemical corrosion of corrosion resistant alloys

Abstract

Interactions between macrofouling and corrosion have been studied on two duplex stainless steels (UNS S31803 and UNS S32760), an austenitic stainless steel (UNS S31354), a nickel base alloy (UNS N06625), and a cobalt base alloy (UNS R30006) in long term tests conducted in natural sea water off the west coast of Scotland. After an 18 month exposure period, the specimens were heavily fouled primarily with barnacles and mussels and all the materials exhibited crevice corrosion at the specimen/resin interface, although this was less extensive on the nickel base alloy. Localised corrosion was observed under the base of live barnacles on UNS S31603 stainless steel. Direct current electrochemical anodic polarisation tests undertaken after the 18 month exposure period yielded unusually high currents in the range of potentials between the free corrosion value and the breakdown potential. However, long term exposure and associated macrofouling had little effect on the overall breakdown of general surface passivity as indicated by values of the breakdown potential. The higher currents were associated with the appearance, after the anodic polarisation, of black sulphide corrosion products at the specimen/resin crevices, around barnacles, and around mussel byssus threads.     

Study of passive film on stainless steels and high grade nickel base alloy using X-ray photoelectron spectroscopy

Abstract

In the present study an assessment has been made of the air formed passive film and the passive film existing after a short exposure to a synthetic sea water environment on austenitic (UNS S31603), superaustenitic (UNS S31254 and UNS S32654) and superduplex (UNS S32760) stainless steels and a Ni base alloy (UNS N06625) using X-ray photoelectron spectroscopy. Assessment of their corrosion behaviour has been made through use of electrochemical accelerated tests supported by post-test microscopy. The passive film on the Ni base (low Fe) UNS N06625 alloy is remarkably similar to those formed on the high Cr and Mo stainless steels. It has been demonstrated that the Fe/Cr ratio in the passive film is dependent on alloy composition and on the conditions to which the material has been exposed, and a general correlation between passive film constitution and corrosion resistance is evident.     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Corrosion behavior of a superduplex stainless steel in chloride aqueous solution

Super duplex stainless steels (SDSS) have been widely used as structural materials for chemical plants (especially in those engaged in phosphoric acid production), in the hydrometallurgy industries, and as materials for offshore applications due to their excellent corrosion resistance in chloride environments, compared with other commercial types of ferritic stainless steels. These alloys also possess superior weldability and better mechanical properties than austenitic stainless steels. However, due to their two-phase structure, the nature of which is very dependent on their composition and thermal history, the behavior of SDSS regarding localized corrosion appears difficult to predict, especially in chloride environments. To improve their final properties, the effect of the partition of the alloying elements between the two phases, and the composition and microstructure of each phase are the key to understanding the localized corrosion phenomena of SDSS. This paper concerns the effects of the SDSS microstructure and heat treatment on the SDSS corrosion resistance in aqueous solutions, containing different amounts of NaCl at room temperature.