The transpassive dissolution mechanism of highly alloyed stainless steels: I. Experimental results and modelling procedure

The transpassive dissolution of austenitic stainless steels (AISI 316L, AISI 904L, 254SMO and 654SMO) in a 0.5 M sulphate solution with pH 2 was studied by conventional and rotating ring-disc voltammetry, as well as electrochemical impedance spectroscopy. The main process in the transpassive potential region was found to be the release of soluble Cr(VI), while small amounts of lower-valency Cr or Mo species are released as well. Secondary passivation readily occurs for AISI 316L, whereas the remaining highly alloyed steels dissolve at high current densities in the whole potential range studied. The dissolution rate was found to increase in the order AISI 904L<254SMO<654SMO. Thus it can be correlated to the increase in the Cr and especially Mo content of the steel substrate. The impedance spectra contain contributions from the transpassive dissolution of Cr and secondary passivation, probably due to enrichment of Fe in the outermost layer of the surface film. A kinetic model of the process is proposed, including a two-step transpassive dissolution of Cr via a Cr(VI) intermediate and the dissolution of Fe(III) through the anodic film. The model was found to be in quantitative agreement with steady state current vs. potential curves and electrochemical impedance spectra. The kinetic parameters of transpassive dissolution were determined and the relevance of their values is discussed.     

The transpassive dissolution mechanism of highly alloyed stainless steels: II. Effect of pH and solution anion on the kinetics

The effect of pH and solution anion on the kinetics of transpassive dissolution of highly alloyed austenitic stainless steels (AISI 904L, 254SMO and 654SMO) was studied by a combination of electrochemical techniques. The experiments were performed in 0.5 M sulphate and 0.5 M chloride solutions, and in an equimolar mixture of the two. The transpassive dissolution was found to start at higher potentials in solutions with higher pH. The rate of transpassive dissolution was shown to decrease with increasing pH and to be the lowest in chloride solutions and the highest in sulphate electrolytes. The steady-state current vs. potential curves and the impedance spectra of the studied materials in the transpassive potential region were found to be consistent with a proposed kinetic model. The model describes the process as dissolution of Cr as Cr(VI) and Fe as Fe(III) through the anodic oxide film via parallel reaction paths. The kinetic parameters of the model in solutions with different pH values and different anions were determined. The role of pH and solution anion in the transpassive dissolution process is discussed in relation to changes induced by these parameters in the composition of the anodic passive film. The factors determining the efficiency of Fe as a secondary passivating agent are also considered.     

Electrochemical and XPS studies of the passive film formed on stainless steels in borate buffer and chloride solutions

The passivity of AISI 304L and AISI 316L stainless steels in a borate buffer solution, with and without the addition of chloride ions, was studied using cyclic voltammetry and potentiodynamic measurements. The passive layers formed by electrochemical oxidation at different passivation potentials on both the stainless steels were studied by X-ray photoelectron spectroscopy, their compositions were analysed as a function of depth, and the cationic fraction of the passive film was determined. The passive films established on the two stainless steels in the borate buffer solution at pH = 9.3 contained the oxides of two main elements, i.e., Fe and Cr. The oxides of the alloying elements Ni and, optionally, Mo, also contribute to the passive layer. In the presence of chloride ions a strong chromium enrichment was observed in the passive layers.     

不锈钢盐酸露点腐蚀行为的原位研究

利用一种新型的露点腐蚀模拟装置结合原位的电化学阻抗谱,电化学噪声等测试手段评价了304和316L两种不锈钢的盐酸露点腐蚀行为.结果表明,316L不锈钢表现出更优异的耐盐酸露点腐蚀性能,主要原因可归结为两点:一是316L不锈钢钝化膜中含有较高的Cr/(Cr+Fe) 比以及较低含量的Fe;二是316L不锈钢钝化膜中含有能改善抗点蚀性能的Mo.     

2205和316L不锈钢在氢氟酸中的电化学腐蚀行为

通过动电位极化和电化学阻抗方法考察了2205双相不锈钢和316L不锈钢在5%(体积分数)HF溶液中的电化学行为,借助Mott-Schokkty曲线分析了两种不锈钢表面钝化膜的半导体特性。结果表明：两种不锈钢在氢氟酸溶液中都能发生钝化,且2205双相不锈钢的钝化区间范围更宽,维钝电流密度更低。2205双相不锈钢表面钝化膜表现出更高的钝化膜电阻和电荷转移电阻,其抗氢氟酸腐蚀性能优于316L不锈钢,这主要与2205双相不锈钢中的Mo和Cr含量高、表面钝化膜缺陷少、钝化膜易修复等因素有关。     

Effect of austenite instability on the hydrogen-enhanced crack growth of austenitic stainless steels

Fatigue crack growth tests were performed to assess the fatigue behavior of AISI 316L and 254 SMO stainless steels (SSs) in air and gaseous hydrogen. 254 SMO SS generally exhibited a greater resistance to fatigue crack growth than 316L. Sensitization treatment had only a marginal effect on the fatigue crack growth behavior of both alloys in air. Moreover, 316L SS exhibited significant hydrogen-enhanced crack growth but 254 SMO, even sensitized 254 SMO specimens, did not. A thin layer of strain-induced martensite was formed on the fatigue-fractured surface of the 316L SS, and its content increased when raising the stress ratio. The thin martensite layer was responsible for the hydrogen-enhanced fatigue crack growth of the 316L SS. By contrast, the extremely stable austenite was responsible for the low susceptibility of 254 SMO SS to hydrogen-accelerated crack growth. The trapping of hydrogen at the grain boundaries and the transformed martensite in the sensitized 316L specimens led to increased fatigue crack growth rates and intergranular fracture of the material.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

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.     

The electrochemical behavior of austenitic stainless steel with different degrees of sensitization in the transpassive potential region in 1 M H2SO4 containing chloride

This paper investigates electrochemical methods to estimate the degree of sensitization (DOS) for as-rolled and solution-treated austenitic stainless steels (AISI 304). The change in DOS was introduced by heat treatment of various time periods at 700 °C, and the electrochemical experiments were conducted at 27 °C in 1 M H₂SO₄ + 0.2 wt.% NaCl. The results show that there is no obvious difference in the anodic polarization curves of the specimens with the differences in DOS. However, the DOS of specimens can be clearly differentiated with AC impedance response in a particular transpassive potential region. In the transpassive potential region of the anodic polarization curve, as overpotential is increased beyond the breakdown potential, three types of anodic dissolution, passive dissolution, grain-boundary attack, and pitting can be characterized. The DOS can only be correctly evaluated with AC impedance response in the middle of the transpassive potential region, where anodic dissolution is of grain-boundary attack type, from 1.05 to 1.1 V. The chromium-depleted zone was preferentially attacked and an intergranular corrosion was found for the serious sensitized specimen after AC impedance test. The healing effect was observed in as-rolled specimens as the specimens were heated more than 72 h, but the effect was not found in the solution-treated specimens sensitized for up to 480 h at 700 °C.     

Electrochemical corrosion behavior of LDX 2101® duplex stainless steel in a fluoride-containing environment

The effect of fluoride on the electrochemical corrosion behavior of an LDX 2101® duplex stainless steel (DSS) was studied. Open-circuit potential (E_OC) and electrochemical impedance spectroscopy (EIS) measurements were carried out in artificial saliva and with the addition of fluoride (1 wt% NaF). The electrochemical corrosion behavior of the AISI 316L austenitic stainless steel (SS) was also evaluated for comparison. Both open-circuit potential and EIS results indicate that DSS and austenitic SS undergo spontaneous passivation due to spontaneously formed oxide film passivating the metallic surface, in the simulated aggressive environments. However, LDX 2101® exhibits superior corrosion resistance as compared with AISI 316L, and this improvement is ascribed to the formation of a passive film which shows a higher protective effect than the one formed on AISI 316L.     

Comparison Study on the Semiconductive and Dissolution Behaviour of 316L and Alloy 625 in Hydrochloric Acid Solution

The corrosion behaviour of 316 L and Alloy 625 was investigated using cyclic polarization,electrochemical impedance spectroscopy,X-ray photoelectron spectroscopy,Auger electron spectroscopy and induced coupled plasma-optical emis sion spectrometer.The results indicated that Alloy 625 showed better corrosion resistance than 316 L and the prolonging immersion time could enhance corrosion resistance of the two alloys.The passive film formed on the surface of 316 L exhibited an electronic structure of p-p heterojunction,with Fe_3 O_4 and Cr_2 O_3 enriched in the outer and inner layers,respectively.However,Alloy 625 presented the electronic structure of n-p heterojunction dominated by the outer Fe_2 O_3/NiFe_2 O_4 and inner Cr_2 O_3.This resulted in the opposite semiconductive properties of the passive films formed on the two materials.In the acid solutions,Fe and Mo suffered from selective dissolution while Cr and Ni were relatively stable.The corrosion rates were mainly dominated by the dissolution of iron.Alloy 625 presented better corrosion resistance than 316 L due to the obviously lower content of Fe and the higher content of Cr and Ni in the passive film.The continuously selective dissolution of iron resulted in the increase in Cr/Fe ratio in the passive film,which was responsible for the enhancement in corrosion resistance.     

A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.     

Effect of p H on the Electrochemical Behaviour and Passive FilmComposition of 316L Stainless Steel

The effect of pH on the electrochemical behaviour and passive film composition of 316 L stainless steel in alkaline solutions was studied using electrochemical measurements and a surface analysis method. The critical pH of 12.5 was found for the conversion from pitting corrosion to the oxygen evolution reaction(OER). OER was kinetically faster than pitting corrosion when both reactions could occur, and OER could postpone pitting corrosion. This resulted in pitting being initiated during the reversing scan in the cyclic polarization at the critical pH. According to the X-ray photoelectron spectroscopy analysis, the content of Cr and Mo decreased with pH, while Fe content increased. This induced the degradation of the passive film, which resulted in the higher passive current densities under more alkaline conditions. The selective dissolution of Mo at high p H was found, which demonstrated that the addition of Mo in austenitic stainless steels might not be beneficial to the corrosion resistance of 316L in strong alkaline solutions.     

Seawater resistance of a second generation superaustenitic stainless steel

The first generation of superaustenitic 6Mo steels like 254 SMO® has been further developed into a new steel–654 SMO®–containing very high amounts of those alloying elements most beneficial for the resistance to localized corrosion. 654 SMO is compared with other stainless steels and nickel-base alloys in tests performed in various kinds of seawater. The new steel is much superior to the 6Mo steel under all conditions investigated and compares favourably with Alloy C–276 in most tests. The most corrosive condition among those investigated represents a seawater system where aerobic and anaerobic areas exist simultaneously.     

Corrosion and passivation mechanism of chromium diboride coatings on stainless steel

The corrosion resistance of fully crystalline CrB₂ coatings magnetron sputtered onto AISI 316L stainless steel was tested in acidic solutions. CrB₂ coatings showed excellent corrosion protection, but suffered a breakdown when an anodic potential of greater than about +1 V (SHE) was applied to the surface in a 1 M HCl electrolyte. The coating failure at high potentials is attributed to transpassive dissolution of the coating at volume defects, enabling the electrolyte to reach the underlying 316L substrate, resulting in its rapid corrosion and subsequent fracturing of the coating. Electrochemical data and potential-pH (Pourbaix) diagrams, constructed from thermodynamic data, indicate that the corrosion resistance of CrB₂ is due to the formation of a Cr(III) oxide passive film in the absence of activation corrosion.     

Corrosion of stainless steel grades in molten NaOH/KOH eutectic at 250 °C: AISI304 austenitic and 2205 duplex

The present paper focuses on the corrosion of an austenitic (AISI304) and a duplex (2205) stainless steel grade in molten KOH/NaOH 50 w/o eutectic at 250 °C. Experimental activities have been performed consisting in electrochemical measurements (linear sweep voltammetry and electrochemical impedance spectrometry) complemented by metallographic (in‐plane and cross‐sectional SEM micrography), structural (X‐ray diffractometry) and compositional (EDX line‐profiles) characterisation of the materials attacked under electrochemically controlled conditions. Electrochemical measurements have shown that AISI304 exhibits a passivating behaviour, characterised by two passivation peaks and a transpassive threshold, while duplex, does not yield a clear indication of passivation. AISI304 was found to fail by intergranular corrosion and to be covered in both passive and transpassive conditions, by an incoherent scale, containing electrolyte species. Duplex samples, instead tends to fail by homogeneous attack and exhibit a range of scale structures, depending on the applied potential.     

CEMS、AES和XPS研究361钢中Mo在抗高温高压水应力腐蚀中的作用

用内转换电子穆斯堡尔谱(CEMS)、俄歇电子能谱(AES)和X-光电子能谱(XPS)研究比较了316和321两种奥氏体不锈钢在含Cl~-2000ppm,含O_28～9ppm高温水中应力腐蚀开裂前后的氧化膜结构及其元素分布,发现合金氧化膜层组成和结构与其抗应力腐蚀性间有一定的对应关系。合金元素Mo在一定条件下与介质作用形成Fe_2(MoO_4)_3相。Mo能控制铁的氧化物相,降低Cr的过钝化溶解速率以及抑制Cl-向膜穿透,起着延长应力腐蚀孕育期的作用。所得结果为阐明含Mo的316钢较不含Mo的321钢有较高的抗应力腐蚀性、以及两者有不同的SCC断口形貌提洪论据。文中还根据合金热力学的作用进一步作了讨论。     

The corrosion behaviour of austenitic and duplex stainless steels in artificial saliva with the addition of fluoride

The evolution of the passive films on 2205 duplex stainless steel (2205 DSS) and AISI 316L stainless steel in artificial saliva, and with the addition of fluoride, was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements. The extent of the passive range increased for the 2205 DSS compared to the AISI 316L in both solutions. The formation of the passive film was studied by EIS at the open-circuit potential (OCP). The passive layers were studied at the OCP by X-ray photoelectron spectroscopy (XPS). The passive films on both materials predominantly contained Cr-oxides, whereas the Fe species were markedly depleted.     

Die Diagnostik der Korrosionsbeständigkeit von Cr?Ni? und Cr?Ni?Mo-Stählen in Citronensäure

A diagnosis of the corrosion resistance of CrNi and CrNiMo steels in citric acid The electrochemical behaviour yields criteria concerning the corrosion reside stance in citric mid (5, 25, 50 und 90%). In pure acid there is no active dissolution, while in the presence of sulfates or chlorides active and transpassive regions are found. The stability of the polarization current, observed in the vicinity of its zero value, yields criteria concerning the stability of the passive state in a given range. The corrosion losses measured in citric acid (24%/100°C or 90%/60°C), with sulfate (0.3 and 1.2%) and chloride ions (0.014 and 0.031%) are as follows (mmly): Cr18Ni9 1.7; Cr18Ni9Mo2 0.3; Cr18Ni10Mo2Ti 0.03; Cr21Ni38 Mo5Ti 0.009.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.