The effect of large heat input on the microstructure and corrosion behaviour of simulated heat affected zone in 2205 duplex stainless steel

In the simulated heat affected zone of 2205 duplex stainless steels, effects of large welding heat inputs on the microstructure and corrosion behaviour were investigated. Reformed austenite content increased with the coarsening of grain boundary austenite (GBA) and the growth of intragranular austenite (IGA) and Widmanstatten austenite (WA), thus improving the low temperature toughness and affecting corrosion state. Reduction of chromium nitrides contributed to better resistance to pitting corrosion. Moreover, the pitting corrosion and intergranular corrosion were improved resulting from the formation of more GBA and WA. The specimen with a Δt_8/5 of 100 s presents better comprehensive performance.     

The influence of solution treatment temperature on microstructure and corrosion behavior of high temperature ageing in 25% Cr duplex stainless steel

In 25% Cr duplex stainless steels, the effect of prior-solution treatment temperature (STT) on the microstructure and corrosion behavior with ageing at 750 °C and 850 °C was investigated. The results revealed that the precipitation rate of σ-phase was fast in the early stage of ageing for 80 min, and then got slower with ageing time up to 330 min. The σ-phase formation was effectively suppressed by raising STT from 1060 °C to 1230 °C especially for ageing at 750 °C. Consequently, the corrosion rate of specimen was dependent on the amount of σ-phase precipitation, and was lowered due to higher STT, and more σ-phase precipitation can lead to the transition from metastable to stable pitting with ageing at 750 °C up to 330 min. Pitting occurred easily around coarse σ precipitates and caused selective dissolution in ferrite. The longer ageing time increased intergranular corrosion (IGC) susceptibility, whereas higher STT contributed to better resistance to IGC.     

Influence of heat treatments at 475 and 400 °C on the pitting corrosion resistance and sensitization of UNS S32750 and UNS S32760 superduplex stainless steels

Some precipitation reactions of superduplex stainless steels (SDSS) can improve hardness, yield, and ultimate strength, but with substantial decrease of corrosion resistance. One of these reactions is the spinodal decomposition of ferrite into Cr‐rich fine precipitates (α′) and Cr‐depleted matrix during low temperature aging. In this work, the effect of the α′ precipitation on the pitting corrosion resistance was investigated. The results showed that short duration heat treatments at 475 °C do not decrease the pitting corrosion resistance, but increase the hardness of UNS S32750 and S32760 SDSS. The limits of heat treatment duration for both steels were determined.     

Effect of high temperature heat treatment on intergranular corrosion of AlMgSi(Cu) model alloy

Copper containing 6000-series aluminium alloys may become susceptible to intergranular corrosion (IGC) as a result of improper thermomechanical processing. Effect of cooling rate after solution heat treatment on the corrosion behaviour of a model AlMgSi(Cu) alloy of nominal composition (wt%) 0.6 Mg, 0.6 Si, 0.2 Fe, 0.2 Mn and 0.1 Cu was investigated. Slow cooling rates were simulated by isothermal treatment for predetermined times in lower temperature baths immediately after solution heat treatment. Treatment for 10-100 s at temperatures below 400 °C introduced susceptibility to IGC. Longer heat treatment at the same temperatures introduced susceptibility to pitting. A corrosion resistant time zone was found between the zones of IGC and pitting at temperatures lower than 350 °C. Quenching in water after solution heat treatment prevented IGC. IGC was related to microgalvanic coupling between the noble Q-phase (Al₄Mg₈Si₇Cu₂) grain boundary precipitates and the adjacent depleted zone. Pitting was attributed to coarse particles in the matrix. Possible mechanisms causing the corrosion resistant intermediate zone are discussed. The results indicate possible methods for obtaining increased corrosion resistance of similar alloys by proper thermal processing.     

热处理对双相不锈钢组织和腐蚀性能的影响

    研究了热处理温度和时效时间对双相不锈钢微观组织及腐蚀性能的影响,结果表明:随着固溶温度提高,双相钢中奥氏体含量增加.固溶温度为1060℃,铁素体含量大约在45%~50%之间,两相比例大约为1∶1,抗点蚀性最好.时效处理时间越长,双相不锈钢中σ相析出越多,其耐腐蚀性能越差.析出的σ相周围形成的贫铬区优先被腐蚀,降低了双相不锈钢抗点蚀性能.     

Effects of solution heat-treatment and nitrogen in shielding gas on the resistance to pitting corrosion of hyper duplex stainless steel welds

The effects of solution heat-treatment and shielding gas on the pitting corrosion of hyper duplex stainless steel (HDSS) welds were investigated in highly concentrated chloride environments. The pitting resistance of a solution heat-treated HDSS after welding with an Ar shielding gas supplemented with N₂ was greatly increased due to the dissolution of Cr₂N in α-phase, which followed the diffusion of N atoms from the α-phase to the γ-phase and an increase of the γ-phase in the weld metal and heat affected zone. It was also attributed to a decrease of the pitting resistance equivalent number difference between the two phases.     

Annealing temperature effect on the pitting corrosion resistance of plasma arc welded joints of duplex stainless steel UNS S32304 in 1.0 M NaCl

Pitting corrosion resistance of 2304 duplex stainless steels after autogenous plasma-arc welding and subsequent short-time post-weld heat treatment at different temperatures, determined by critical pitting temperature in 1.0 M NaCl solution, has been investigated. The results showed that the as-welded joint displayed impaired pitting corrosion resistance and that pitting preferentially occurred at ferrite grain in heat-affected zone near the fusion line. Short-time annealing treatment at 1020–1120 °C has a beneficial effect on the pitting corrosion resistance of welded joint. The most favorable annealing temperature for the analyzed welded joints was found to be 1080 °C, at which the joint restored the pitting corrosion resistance lost during welding entirely.     

Evaluation of sensitization in stainless steel 304 and 304L using nonlinear Rayleigh waves

Austenitic stainless steels have a wide range of applications in the energy industry, but the corrosion resistance of these stainless steels can be reduced by sensitization, particularly in the heat affected zones in welds. Sensitization is the formation of chromium carbide precipitates along the grain boundaries, causing the formation of a zone of chromium depletion around the grain boundary. Since chromium is the primary alloying element that makes stainless steel corrosion resistant, this chromium depleted zone is susceptible to intergranular stress corrosion cracking (IGSCC). Sensitization occurs when a stainless steel is exposed to a high temperature for an extended time period, such as during welding. The objective of this research is to determine the sensitivity of nonlinear ultrasound to the presence of sensitization by using nonlinear Rayleigh waves to quantitatively track the sensitization of 304 and 304L stainless steels as a function of holding time at 675 °C. The effect of the carbon content of the alloys (304 versus 304L) to the sensitization process and the measured nonlinearity parameter, β are investigated. Annealing of these specimens isolates the effect of just sensitization, removing the presence of cold work which can also affect the material nonlinearity. Complementary electrochemical potentiodynamic reactivation (EPR) measurements and microscopy are used to confirm the absence or presence of sensitization. The results show that the acoustic nonlinearity parameter is sensitive to the presence of chromium carbide precipitates in sensitized austenitic stainless steels.     

Corrosion behaviour of sensitized and unsensitized Alloy 900 (UNS 1.4462) in concentrated aqueous lithium bromide solutions at different temperatures

Duplex stainless steels can undergo microstructural changes if they are heated improperly. When that happens, duplex stainless steels are sensitized and intermetallic phases appear. The high Chromium and Molybdenum content promotes the formation of secondary phases as a consequence of the heat treatment. These secondary phases, which are rich in alloying elements, such as Cr and Mo, deplete these elements from the neighbouring phases, leading to a reduction in corrosion resistance. In order to study the influence of the secondary phases on the corrosion parameters, samples of duplex stainless steel, Alloy 900 (UNS 1.4462), have been heated in argon atmosphere at 825 °C for 1 h. The corrosion behaviour of sensitized and unsensitized Alloy 900 has been analyzed in a concentrated aqueous lithium bromide (LiBr) solution of 992 g/L by means of cyclic potentiodynamic curves. Secondary phase presence reduces the pitting potential value of Alloy 900. Besides, the pitting potential decreases with temperature. On the other hand, the corrosion potential and open circuit potential values increase with temperature and sensitization.     

Influence of chemical composition on the pitting corrosion resistance of non-standard low-Ni high-Mn-N duplex stainless steels

The pitting corrosion resistance of a new family of duplex stainless steels has been evaluated. These non-standard duplex stainless steels are characterised by low Ni content and high N and Mn levels. Potentiodynamic polarisation scans in NaCl solution have been carried out to determine pitting potentials. A crevice-free cell has been used to perform the electrochemical tests.An exponential equation is obtained in the regression analysis between the pitting potential and chemical composition which allows an estimate of the pitting resistance of these new duplex stainless steels.     

Pitting and Intergranular Corrosion Resistance of AISI Type 301LN Stainless Steels

The pitting and intergranular corrosion (IGC) resistance of AISI type 301LN stainless steels were evaluated using ASTM methods, anodic polarization, and electrochemical impedance techniques. The IGC results indicated that the microstructure of the samples after sensitization heat treatment at 675 °C for 1 h shows step or dual structure for both imported and indigenous materials indicating insignificant Cr₂₃C₆ precipitation. The results of immersion tests in boiling 6% copper sulfate + 16% sulfuric acid + copper solution for 24 h followed by the bend test (ASTM A262 Practice-E method) indicated no crack formation in any of the tested specimens. Pitting corrosion resistance carried out in 6% FeCl₃ solution at different temperatures of 22 ± 2 and 50 ± 2 °C (ASTM G 48) up to the period of 72 h revealed pitting corrosion attack in all the investigated alloys. The potentiodynamic anodic polarization results in 0.5 M NaCl revealed variation in passive current density and pitting potential depending on the alloy chemistry and metallurgical condition. The passive film properties studied by electrochemical impedance spectroscopy (EIS) correlated well with the polarization results. The x-ray diffraction (XRD) results revealed the presence of austenite (γ) and martensite (α′) phases depending on the material condition. The suitability of three indigenously developed AISI type 301LN stainless steels were compared with imported type 301LN stainless steel and the results are highlighted in this article.     

In vitro electrochemical investigations of advanced stainless steels for applications as orthopaedic implants

Potentiodynamic anodic polarization experiments on advanced stainless steels (SS), such as nitrogenbearing type 316L and 317L SS, were carried out in Hank’s solution (8 g NaCl, 0.14 g CaCl₂, 0.4 g KC1, 0.35 g NaHCO₃, 1 g glucose, 0.1 g NaH₂PO₄, 0.1 g MgCl₂, 0.06 g Na₂HPO₄ 2H₂O, 0.06 g MgSO₄ 7H₂O/1000 mL) in order to assess the pitting and crevice corrosion resistance. The results showed a significant improvement in the pitting and crevice corrosion resistance than the commonly used type 316L stainless steel implant material. The corrosion resistance was higher in austenitic stainless steels containing higher amounts of nitrogen. The pit-protection potential for nitrogen-bearing stainless steels was more noble than the corrosion potential indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed very little tendency for leaching of metal ions, such as iron, chromium, and nickel, at different impressed potentials. This may be due to the enrichment of nitrogen and molybdenum at the passive film and metal interface, which could have impeded the releasing of metal ions through passive film.     

6种不锈钢的化学和电化学腐蚀行为

采用化学和电化学加速腐蚀试验方法对６种不锈钢的耐点蚀和缝隙腐蚀性能进行了评价。结果表明：两种评价方法之间具有良好的相关性;６种不锈钢按照点蚀和缝隙腐蚀抗力由大到小的顺序排列为３＾＃〉１＾＃〉６＾＃〉２＾＃〉４＾＃〉５＾＃,详细描述了６种不锈钢各自的腐蚀行为特征。     

Influence of Step Annealing Temperature on the Microstructure and Pitting Corrosion Resistance of SDSS UNS S32760 Welds

In the present work, the influence of step annealing heat treatment on the microstructure and pitting corrosion resistance of super duplex stainless steel UNS S32760 welds have been investigated. The pitting corrosion resistance in chloride solution was evaluated by potentiostatic measurements. The results showed that step annealing treatments in the temperature ranging from 550 to 1000 °C resulted in a precipitation of sigma phase and Cr₂N along the ferrite/austenite and ferrite/ferrite boundaries. At this temperature range, the metastable pits mainly nucleated around the precipitates formed in the grain boundary and ferrite phase. Above 1050 °C, the microstructure contains only austenite and ferrite phases. At this condition, the critical pitting temperature of samples successfully arrived to the highest value obtained in this study.     

Application of the modified electrochemical potentiodynamic reactivation method to detect susceptibility to intergranular corrosion of a newly developed lean duplex stainless steel LDX2101

The present work aimed at defining optimal conditions using double loop electrochemical potentiokinetic reactivation (DL-EPR) method for evaluating intergranular corrosion (IGC) susceptibility of lean duplex stainless steel (LDX2101) aged at 700 °C between 3 min and 300 h. The results demonstrated that the modified DL-EPR measurement (solution of 33% H₂SO₄ + 0.1% HCl at 20 °C and scan rate of 2.5 mV/s) could successfully characterize the interactions between precipitation, chromium depletion and IGC of LDX2101 with high sensitivity and reproducibility. In addition, there was no indication of healing because the effect of formation of chromium-enriched precipitates was more dominative than that of redistribution of chromium in depleted zones.     

Measuring secondary phases in duplex stainless steels

The use of duplex stainless steels is limited by their susceptibility to the formation of dangerous intermetallic phases resulting in detrimental effects on impact toughness and corrosion resistance. This precipitation and the quantitative determinations of the phases have received considerable attention and different precipitation sequences (σ phase, χ phase, and carbides) have been suggested. This study investigates the phase transformation during continuous cooling and isothermal treatments in commercial duplex stainless steel grades and the effects on alloy properties, and compares the most common techniques of analysis.     

Effect of nitrogen on corrosion behavior of 28Cr-7Ni duplex and microduplex stainless steels in air-saturated 3.5 wt% NaCl solution

The corrosion behavior of 28Cr-7Ni-O-0.34N duplex stainless steels in air-saturated 3.5-wt% NaCl solution at pH 2, 7, 10 and 27 °C was studied by the potentiodynamic method. Two types of microstructures were investigated: the as-forged duplex and microduplex (average austenite grain size 5-16 μm) structures. The austenite volume fractions of the tested steels were between 0.35 and 0.64. The nitrogen effect on corrosion behaviors of both duplex and microduplex stainless steels were the same. At pH 2, the corrosion potential increased when the nitrogen content increased, however, corrosion current density as well as corrosion rate decreased. At pH 7 and 10, the effect of nitrogen on corrosion potential and corrosion rate could not be observed. Corrosion potential at pH 10 was lower than at pH 7. Pitting potential increased when the nitrogen content in the tested steels increased at all tested pH. For the nitrogen effect on the passive current density, it seemed that only at pH 2, the average passive current densities reduced when the nitrogen content increased. Nitrogen may have participated in the passive film or has been involved in the reaction to build up passive film. The ammonium formation and nitrogen enrichment at the interface metal/passive film with adsorption mechanism were discussed. The dissolute nitrogen might have combined with the hydrogen ions in solution to form ammonium ions, resulting in increasing solution pH. The steel could then easily repassivate, hence the corrosion potential and pitting potential would increase. However, the ammonium formation mechanism could not explain the decrease of corrosion potential in basic solution. Nitrogen enrichment at the metal/passive film interface with adsorption mechanism seemed to be an applicable consideration in increasing pitting potential. However, this mechanism did not involve the ammonium ion formation. In general, for the duplex and microduplex stainless steels tested, nitrogen increased the general corrosion resistances in acid solution and pitting corrosion resistance at all solution pH. Metallographic observation in both tested duplex and microduplex steels after pitting corrosion at all tested pH revealed that, the corroded structure in the tested steels without nitrogen alloying was austenite, but those with nitrogen alloying was ferrite. Even though ferrite had a higher chromium content than austenite but higher dissolved nitrogen in austenite than in ferrite may have increased the pitting resistance equivalent number (PRE) of austenite to be higher than that of ferrite.     

6005A铝合金搅拌摩擦焊接头的晶间腐蚀行为

对6005A铝合金搅拌摩擦焊接头的晶间腐蚀行为进行了研究.结果表明,母材的晶间腐蚀倾向最大,热影响区(HAZ)次之,焊核区(NZ)和热力影响区(TMAZ)的晶间腐蚀倾向最低.结合场发射扫描电镜、高分辨透射电镜分析解释了接头不同区域的腐蚀行为:母材的晶间腐蚀是两组微电池效应的结果,即晶界析出相/沉淀无析出带(PFZ)和铝基体/PFZ;HAZ内晶界析出相的数量的减少、间距的变大及晶内Q'相的析出显著改善了该区的晶间腐蚀性,但晶内Q'相的析出也引起了点蚀的发生;NZ和TMAZ内绝大部分的合金元素固溶于基体,抑制了晶间腐蚀的发生.     

超级双相不锈钢焊接接头的耐蚀性能

通过SEM和EDS研究了采用不同焊接工艺后超级双相不锈钢UNSS32750焊接接头的两相比例及成分变化,并采用临界点蚀温度和浓硝酸法测试比较了不同焊接工艺接头的耐点蚀和晶间腐蚀性能.结果表明,焊接中较高的热输入、加填焊丝和背面采用氮气保护焊的方法可以稳定焊接接头中的奥氏体相的比例,并且较高的热输入,使得焊接接头冷却速度相对较慢,有助于铬的扩散而消除晶界贫铬现象,减小晶间腐蚀倾向;而与此相反的是较高的热输入,会导致两相中元素分配不均衡使铁素体相优先发生腐蚀,从而恶化材料的整体耐点蚀性能.     

Die Bedeutung des Oxalsäure-Tests für die Prüfung der Korrosionsbeständigkeit nichtrostender Stähle

The meaning of the oxalic acid etch test for testing the corrosion resistance of stainless steels In the oxalic acid etch test according to ASTM A 262 practice A, precipitations of phases rich in chromium and molybdenum which can occur in stainless steels, are preferentially dissoved. The behaviour of such phases in the oxalic acid etch test was investigated taking precipitations of carbide M₂₃C₆, s?-phase, χ-phase and Laves-phase in stainless steels AISI 304 L and 316 L as examples. The chemical composition of these was evaluated with a scanning transmission electron microscope (STEM) by EDS. With coarser precipitations, it was possible to support this analytical method by EDS of metallographic cross sections in a scanning electron microscope (SEM). In oxalic acid, critical threshold potentials exist above which the above mentioned phases are preferably attacked, furthermore critical pH values, below which no selective attack of the precipitated carbides and intermetallic phases occurs. The numerical values of the threshold potentials as well as the critical pH values were evaluated. When testing stainless steels in the oxalic acid etch test, the steel specimens are polarized to a highly positive potential in the very trans passive range. In this potential range the corrosion rate of stainless steels increases with increasing chromium content, while in the active and passive range the corrosion rate decreases with increasing chromium content. Other than the nitric-hydrofluoric acid test, the copper-copper sulfate-sulfuric acid test, and the ferric sulfate-sulfuric acid test, the oxalic acid etch test does therefore not indicate any chromium depletion. Hence, an intergranular attack also occurs when precipitations of carbides rich in chromium are present at the grain boundaries of austenitic stainless steels with the carbides being precipitated without any chromium depletion of the areas adjacent to the grain boundaries. Sensitized austenitic stainless steels which are susceptible to intergranular corrosion due to the precipitation of chromium rich carbides and chromium depletion of the areas adjacent to the grain boundaries, can suffer intergranular SCC in high temperature aqueous environments when additionally critical conditions with respect to the mechanical stress level and the oxygen concentration in the environment are given. For the detection of sensitized microstructures, the oxalic acid etch test must be valued critically due to the dependence of the corrosion rate on the chromium content mentioned above, and is obviously by far less suited than the conventional tests for establishing resistance to intergranular corrosion in sulfuric acid-copper sulfate solutions with additions of metallic copper (Strauß test, severe Strauß test).