Influence of the surface state on the Initiation of Crevice Corrosion on stainless steels

Results from sea-water and laboratory exposure tests with multiple crevice assemblies are presented. The results from tests on austenitic and ferritic-austenitic stainless steel samples subjected to various surface treatments demonstrate that an acid treatment greatly improves the crevice corrosion resistance of previously ground surfaces. Pickling in dilute sulphuric acid and passivation in nitric acid has thereby virtually the same beneficial effect as the common nitric + hydrofluoric pickling acid. The beneficial effect of pickling and passivation is related to the removal from the surface of sulphide inclusions, being potential nucleation sites for the initiation of discrete pitting attacks preceding the onset of crevice corrosion. Potentiodynamically determined pitting potentials and results from testing in 10% FeCl₃ · 6 H₂O according to ASTM G 48–76 show poor correlation with results from sea-water exposures.     

Improvement on the pitting corrosion resistance of 304 stainless steel via duplex passivation treatment

A remarkable improvement in the pitting corrosion resistance of 304 stainless steel was attempted using a novel duplex passivation treatment method. First, chemical passivation in nitric acid followed electrochemical passivation via potential polarization of step cycling in sodium nitrate electrolyte. Compared with traditional chemical passivation, breakdown potential was increased from 0.31 V_SCE to positive than 0.9 V_SCE at 70°C in a solution bearing 0.6 M [Cl^?] concentration. The critical pitting temperature was enhanced from 21.5°C to above 70°C in a solution with 6 M [Cl^?] concentration. Impedance analysis and X‐ray photoelectron spectroscopy results show that a more compact passive film with a higher ratio of chromium oxide on iron oxide was achieved by electrochemical passivation compared with chemical passivation. Morphology observation suggested that the potential polarization of step cycling slightly increased the dissolution of inclusions after being subjected to chemical passivation. The probable reason for the improvement on pitting resistance is discussed in detail based on inclusion dissolution and the protectiveness in passive film.     

Effects of nitric acid passivation on the pitting resistance of 316 stainless steel

Samples of 316 stainless steel have been subjected to passivation treatments at ambient temperature for 1 h in solutions of up to 50 wt% nitric acid. Pitting potentials of the treated samples were measured in 1 M NaCl at 70°C and were shown to vary with the concentration of the pre-treatment; increasing with concentration up to 25 wt% and then decreasing as the acid concentration was further increased. The corrosion potential reached during the passivation treatment increased with acid concentration, such that the highest measured pitting potential was associated with a final passivation potential of 300–400 mV versus SCE. MnS inclusions were at least partially removed by treatment with any nitric acid concentration, whilst chromium enrichment of the film reached a peak value for an acid concentration of 25 wt%. The rate of metastable pitting was also found to vary with the concentration of the acid used in the passivation treatment, as did the probability of a metastable pit becoming stable. A probabilistic pitting model suggests that acid treatment reduces the number of possible pit initiation sites at low potentials, but the most dangerous sites in corrosion terms are also the most difficult to remove by acid treatment.     

Effects of nitric acid passivation on the corrosion behavior of ZG06Cr13Ni4Mo stainless steel in simulated marine atmosphere

ZG06Cr13Ni4Mo martensitic stainless steel was nitric acid-passivated to improve its corrosion performance. The effects of nitric acid passivation on the surface morphology, chemical composition, electrochemical properties, semiconductor behavior, and long-term corrosion performance of the stainless steel were investigated using various analytical techniques. An in-depth analysis of X-ray photoelectron spectroscopy (XPS) showed that the passive film formed after the acid passivation process showed high thickness and a duplex character as it consisted of a hydroxide layer and an oxide layer. The oxide layer affected the corrosion resistance and thickness of the passive film. The thickness of the passive film was calculated theoretically as well as experimentally by fitting the electrochemical impedance spectroscopy and XPS results. The electrochemical tests revealed that the dramatic increase in the corrosion resistance of the stainless steel after the passivation was due to the formation of a thick, low-disorder passive film rather than Cr enrichment. The removal of inclusions resulted in higher pitting resistance, whereas the increased roughness showed a negative effect on the corrosion behavior of the stainless steel. During the wet–dry cyclic tests, the modification of the passive film was examined. The passivated stainless steel exhibited good corrosion resistance for up to 50 days of exposure in the simulated environment.     

304不锈钢环保型酸洗钝化工艺及其性能研究

采用化学浸泡法和动电位扫描法研究了304不锈钢在环保型酸洗钝化工艺下所得钝化膜的耐点蚀性能,运用X射线光电子能谱(XPS)分析了钝化膜的组成和结构.结果表明:由正交试验优选出的最优配方和工艺可大大提高304不锈钢表面钝化膜的耐点蚀性能;304不锈钢表面钝化膜中的Cr、Fe、Ni分别以Cr2O3、FeO、NiO的形式存在;钝化处理后,试片表面钝化膜中的铬元素和镍元素的含量明显增加.综合考虑,不锈钢柠檬酸钝化的最佳工艺为:柠檬酸质量分数4%,氧化剂X体积分数5%,乙醇体积分数2.5%,温度40℃,钝化时间60min.     

Cerium chemical conversion coatings for corrosion protection of stainless steels in hot seawater environments

In order to replace the hazardous chromate‐based surface treatment, a new cerium chemical conversion coating was developed on 316L stainless steel through a mixed solution of hydrated cerium nitrate, citric acid, and hydrogen peroxide. The chemical composition was characterized by energy‐dispersive spectroscopy, X‐ray photoelectron spectroscopy and atomic force microscope. The dense conversion coating is composed of CeO₂ with a small amount of Ce₂O₃ and has small grain size lower than 50 nm. Its thickness is about 47.4 nm as determined by spectroscopic ellipsometry analysis. Potentiodynamic polarization was used to study the corrosion behavior of the coatings in the concentrated artificial seawater at 72 °C. In comparison with the conventional nitric acid‐chromate passivated specimens, the cerium conversion coatings show much higher pitting potentials. It is suggested that the cerium conversion treatment is more effective than the nitric acid‐chromate passivation to improve the pitting resistance of 316L stainless steel used in the hot seawater environments.     

316L不锈钢柠檬酸钝化工艺及其耐点蚀性能研究

采用正交试验方法研究了316L不锈钢柠檬酸钝化工艺,利用电化学测试方法测量了不锈钢焊接接头各部位在钝化前后点蚀电位的变化,并以此评价钝化工艺对不锈钢耐点蚀性能的影响.研究结果表明,由正交试验优选出的最优配方和工艺为:柠檬酸、双氧水、乙醇的质量分数分别为3%、10%、5%,温度25℃,钝化时间90 min.此工艺配方可大大提高316L不锈钢整体的耐点蚀性能.     

Eis and XPS study of surface modification of 316LVM stainless steel after passivation

The effects of surface finish, nitric acid passivation and ageing in air on corrosion resistance of 316LVM stainless steel in 0.5% H₂SO₄ have been investigated by EIS, potentiodynamic polarization measurements and XPS. The results indicate that a smoother surface exhibits to a higher corrosion resistance. The effectiveness of the passivation treatment strongly depends on nitric acid concentration, passivation time and temperature. The passivation treatment significantly increases the corrosion resistance due to a high Cr content in the passive film and increased film thickness. Ageing after passivation increases the corrosion resistance whereas ageing before passivation has little effect.     

Einfluß von Passivierungs- und Kohlenstoffschichten auf austenitischen CrNiMo-Stählen auf die Beständigkeit gegen Lochfraß und Spannungsrißkorrosion

Effects of passivation and carbon films on austenitic CrNiMo steels on their piting and stress corrosion resistance The influence of passive film and combinations of a passivation and a carbon layer on the resistance to pitting and SCC of austenitic CrNiMo steels has been investigated in physiological sodium chloride solution (Tyrode solution) at pH 6.9 to 7.4 at 37 ± 1°C. The passive film was obtained after electrolytic polishing in H₃PO₄ + H₂SO₄ + C₆H₅NHCOCH₃ + oxalic acid + corrosion inhibitor CS by treatment with 40% nitric acid the carbon film was obtained by CVD. Impurities in the steel (non-metallic inclusions) and the different metallic phases were investigated and the chemical composition of the passive film was determined by quantitative analysis. The resistance to pitting of the steel with and without passive film was determined potentiodynamically in Tyrode's solution at 37 ± 1°C. The resistance to SCC was determined in Tyrode's solution at 37 ± 1°C, in neutral glycerole and in boiling magnesium chloride solution at 154 ± 1°C and evaluated in terms of K_σ and K_τ. The corrosion damage was investigated by optical and scanning electron microscopy. The investigations have revealed that the different surface conditions considerably improve the pitting and SCC resistance of the steels in the media used in this work, so that they make possible the use of these materials as surgical implants.     

HCO3–对超级13Cr马氏体不锈钢钝化行为及点蚀行为的影响

目的研究在0.1 mol/L NaCl硼酸缓冲溶液中,研究HCO_3~–对超级13Cr马氏体不锈钢的钝化及点蚀行为的影响。方法采用动电位极化、恒电位极化、Mott-Schottky曲线、电化学阻抗谱等电化学测试手段,并结合3D超景深显微镜进行点蚀形貌观察,研究超级13Cr马氏体不锈钢的电化学腐蚀行为。结果随着HCO_3~–浓度的增加,超级13Cr的钝化区间变宽,点蚀电位向正向移动,稳态点蚀发生的敏感性降低。HCO_3~–减少了超级13Cr亚稳态点蚀数量,降低了亚稳态点蚀电流密度峰值的平均值。随着HCO_3~–浓度的增大,超级13Cr钝化膜电阻Rf升高,电荷转移电阻Rct升高,钝化膜电容Cf逐渐减小。HCO_3~–使得超级13Cr钝化膜半导体特性由n型转变为n+p型双极性,且随着溶液中HCO_3~–浓度的增大,钝化膜中的施主密度ND和受主密度NA减小。结论 HCO_3~–的加入使得超级13Cr不锈钢钝化膜厚度增大,钝化膜内点缺陷密度降低,对基体的保护作用增强,抑制了超级13Cr的亚稳态和稳态点蚀发生。     

316L不锈钢无铬钝化工艺研究

利用动电位扫描法和化学浸泡研究了316L不锈钢在两种无铬钝化工条件下所形成的钝化膜的耐点蚀性能,结果表明,钝化液组分Wb具有良好的钝化效果。     

不同方式注入Ta对Cr4Mo4V和Cr12MoV抗点蚀性能的影响

研究了用不同方式注入Ta对Cr4Mo4V和Cr12MoV抗点蚀性能的影响。注入方式包括直接离子注入、离子束混合和离子束辅助沉积。阳极极化行为表明不同方式注入都使两种材料在0．1MNaCl醋酸缓冲溶液（pH=5.9）中的钝化电流密度降低了一个数量极，并在大范围钝化区保持低电流钝化。试验亦表明所有注入方式都显著地提高了两种基材的钝化和抗点蚀性，而其中IBAD方式最为有效。用AES和XPS分析了注入原子的分布和化学成分，并讨论分析了提高耐蚀性的机理。     

Real time pit initiation studies on stainless steels: The effect of sulphide inclusions

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.     

Vergleich des elektrochemischen Verhaltens der Stähle CrMnN 18 12 und CrNi 18 10 in Salpetersäure

Comparison of the electrochemical behaviour of the steels CrMnN 18 12 and CrNi 18 10 in nitric acid The electrochemical behaviour of the steels CrMnN 1812 and CrNi 18 10 in nitric acid has been studied by current-potential curves atomic absorption analysis of the solution and impedance spectroscopy. It has been found that in nitric acid solutions the corrosion resistance of the steel CrMnN 18 12 is comparable with that of the steel CrNi 18 10. Passivation of the steels starts at the corrosion potential. The two-step passivation range is probable attributable to a phase transformation in the oxidelayers formed on the steel surface. The analysis of the impedance curves allowed the establishment of an equivalent circuit for the passivation process. The double layer capacity, the polarization resistance, the Ohmic resistance and the capacity of the passive layer have been calculated.     

Electrochemical investigations of pitting corrosion

By using chronopotentiostatic and stepwise potential change experiments with potentiokinetic and galvanostatic testing, the following types of pitting corrosion of stainless steels in chloride-containing solutions have been investigated: sulphate inhibition of pitting; 35%Cr-Fe alloy; Cr-Ni-Mo stainless steels. The circuit resistance was found to be of fundamental importance. The pit passivation potential depends on the intensity of a corrosion attack before potential change only in the case of inhibited solutions. In uninhibited solutions passivation and formation potentials are nearly equal only in the case of potentiostatic circuit conditions. The Cr-Fe alloy and the Cr-Ni-Mo stainless steels show a potential range of repassivating pitting. Of practical interest is the critical potential of stable pitting which decreases with increasing circuit resistance. The beneficial effect of Mo is only valid for the pitting potential obtained potentiostatically and not at higher circuit resistances. Considering the practical meaning of the addition of Mo it may be concluded that this element is probably essentially connected with repassivation of pits and conditioning effects of the passive layer.     

The electrochemical behaviour of mild steel in phosphate-borate-sulphide solutions

The localized corrosion of mild steel in neutral buffered solutions containing sodium sulphide is studied using potentiostatic and potentiodynamic techniques complemented by scanning electron microscopy and energy dispersive X-ray analysis. The breakdown potential depends linearly with the logarithm of the sodium sulphide concentration. At potentials close to ? 0.70 V (SCE), the nucleation and growth of a ferrous sulphide layer replacing the initial ferrous hydroxide film occurs prior to pit growth. In the ? 0.70 to ? 0.60 V range the logarithm of the induction time for pit initiation decreases linearly with the reciprocal of the applied potential. At potentials more positive than ? 0.60 V the oxide formation hinders pit growth leading to a partial passivation. The results suggest that two-dimensional salt islands are formed before pitting as it has been postulated for iron in halide-containing solutions.     

Application of surface response analysis to the optimisation of nitric passivation of cp titanium and Ti6Al4V

Passivation of Ti6Al4V and cp Ti by mean of nitric acid treatment are used to reduce their surface reactivity, and consequently increasing resistance to corrosion, in physiological media.

In this work, the response surface methodology was employed to know the effects of operating parameters of the nitric passivation (HNO₃ content, temperature of solution and passivation period) on the corrosion resistance of commercially pure (cp) titanium and Ti6Al4V alloy in Ringer's solution. The experiment included two 3³ factorial designs. According to the responses from the experimental designs, the effects of each variable were calculated and the interactions between them were determined. The statistical test has revealed that the main effect of the nitric acid content is the most significant factor. The surface response methodology was applied to graphically determine the optimal working conditions.

Different electrochemical techniques (OCP, Rp and cyclic polarisation tests) were used to compare the electrochemical behaviour of cp Ti and Ti6Al4V, with and without passivation.     

化学镀镍层缓蚀工艺研究

目的提高化学镀Ni-P镀层的耐蚀性能。方法在钢铁基体表面化学镀Ni-P镀层,采用植酸、硅酸钠、钼酸铵和双氧水中的一种或多种复配对镀层进行钝化处理。通过硝酸点滴法、贴滤纸法分析镀层耐蚀性的变化,并通过正交实验确定四种物质复配的最佳浓度。结果以植酸、硅酸钠、钼酸铵和双氧水为主要成分配制的复合缓蚀剂能使镀层耐硝酸效果良好,电化学实验表明,复合钝化膜的腐蚀电流密度降低和阻抗值增加都超过1个数量级。结论所研究的复合缓蚀剂各组分间存在协同效应,最佳配方为:6 g/L植酸,6 g/L硅酸钠,4 g/L钼酸铵,30 mL/L双氧水。     

南海大气环境下304不锈钢的点蚀特性研究

目的 研究南海大气环境中服役的304不锈钢的点蚀原因与机理。方法 以不同暴露周期的304不锈钢试样为研究对象,采用动电位极化、电化学交流阻抗谱、扫描电镜和Kelvin 探针技术对其进行分析研究。结果 随着暴露时间的延长,304不锈钢表面的点蚀坑数量和深度均逐渐增加,其极化曲线中,钝化区间缩短,点蚀电位负移,阳极极化曲线斜率明显减小,并出现反复再钝化现象,钝化曲线逐渐消失,同时阻抗测试中的膜层电阻越来越小。SEM分析表明,304不锈钢暴露初期出现不连续的点蚀,点蚀坑向纵深发展,暴露后期出现点蚀群,局部有多个点蚀坑连成一片形成溃疡状的腐蚀表面,同时随暴露时间的延长,扫描试样微区的Kelvin表面电位不均匀性逐渐增强。结论 304不锈钢试样的耐蚀能力随暴露时间的增加而不断下降,点蚀现象不断增加。暴露初期,点蚀坑主要向纵深发展;暴露后期,点蚀坑的宽度达到一定程度后,本体溶液向坑内迁移,稀释了坑内溶液的酸度,点蚀坑向纵深和横向同时发展。     

Korrosion nichtrostender austenitischer Stähle in (kondensierender) chloridhaltiger Salpetersäure

Corrosion of austenitic stainless steels in (condensing) nitric acid containing chlorides The corrosion parameters involved in the behaviour of austenitic stainless steels and nickelbase alloys in chloride containing nitric acid are being stated. Investigation of the corrosion resistance of the austenitic stainless steel 1.4306, ESR grade for application in nitric acid, in nitric acid condensates containing small amounts of chlorides. The condensates were formed from boiling nitric acids of molarities 6 to 10 containing 15 to 45 mg chlorides/1. The calculated corrosion rates of < 1 μm/y can be considered rather small. Especially in cases of insufficient wetting of the heat exchanger surfaces, however, local roughening and pitting corrosion is to be expected, mostly under rust-colored, chloride-containing deposits. The intensity of pitting corrosion increases with increasing chloride contents of the nitric acid solutions that evaporate. The sealing surfaces consisting of steel 1.4306 and PTFE did not exhibit any crevice corrosion. Nitric acid grade ESR-1.4306 definitely does not suffice requirements as described in this paper. It is suggested to use stainless steels which exhibit an improved resistance against chlorides and, at the same time, a high resistance against nitric acid attack. The following steel grades may be considered: X 1 CrNi 25 21 (Mat. No. 1.4335), X 2 CrNiMoN 25 22 2 (Mat. No. 1.4466) and/or X 1 NiCrMoCuN 31 27 4 (Mat. No. 1.4563).