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.     

Anodic behaviour of stainless steel S43000 in concentrated solutions of sulphuric acid

Electrochemical, AES and XPS techniques were employed to characterize the anodic behaviour of S43000 stainless steel in concentrated sulphuric acid (90.0-96.4 wt.%). Electrochemical experiments showed that passivity is not spontaneous and requires anodic polarization in the acids studied. Rotating cylindrical electrode experiments showed that the corrosion rate is controlled by the mass transfer rate of FeSO₄ from a saturated surface salt. AES and XPS analyses provided evidence that passivity involves the formation of a chromium-rich oxide-hydroxide film. The passivation mechanism and passive state stability are considered to relate to the manner in which undissociated H₂SO₄ molecules participate in the corrosion process. The findings have meaningful implications regarding the development of more corrosion resistant stainless steels for acid service.     

Electrochemical deposition of a carbon nanotube-poly(o-phenylenediamine) composite on a stainless steel surface

Electrodeposition of a nano-composite made of oxidized carbon nanotubes (CNTs) and a conductive polymer such as poly(o-phenylenediamine) (PoPD) on a stainless steel surface from aqueous solution was carried out by cyclic voltammetry. The presence of the CNTs enhanced the deposition of the PoPD and this enhancement was more significant in the presence of single walled carbon nanotubes (sCNT) by comparison to multi-walled carbon nanotubes (mCNT). Scanning electron microscope images indicated the incorporation of the CNTs in the PoPD layer. The nano-composite layer as well as the pure PoPD layer keeps the stainless steel in a passive state in acidic solution. The oxide film underneath the nano-composite layer is unique and showed high corrosion resistance in concentrated chloride solutions, which was confirmed by the presence of high contents of iron and chromium components. These findings suggest that the CNTs indirectly assist the passivation of the stainless steel by catalyic oxygen reduction and polymer oxidation process.     

The effect of chromate in the corrosion behavior of duplex stainless steel in LiBr solutions

The electrochemical behavior of duplex stainless steel (DSS) in LiBr media was investigated by anodic cyclic polarization curves and AC impedance measurements. The effect of bromide concentration and chromate presence in the solutions on the corrosion behavior of DSS was studied. It was found, by cyclic polarization curves analyses, that there was different pitting susceptibility of passive films formed on DSS depending on the chromate/bromide ratio: pitting corrosion susceptibility highly decreased from a chromate/bromide ratio lower than 0.01.The comparative investigations carried out in LiBr and LiBr + 0.032Li₂CrO₄ verify the assumption that the halide ions facilitate inhibitor adsorption. The addition of halides increased inhibition efficiency to a considerable extent. Passive film becomes more resistant when bromide concentration increases, although film thickness decreases.     

不锈钢载波钝化参数对钝化膜层耐蚀性的影响

运用电化学交流阻抗谱、原子力显微镜研究了在不同的载波钝化参数下形成的不锈钢钝化膜的耐蚀性,测定了极化电阻值Rp和相应的钝化膜的表面形貌。结果表明：载波钝化条件参数对膜层的耐蚀性有较大的影响,各参数(频率、占空比、高电位、低电位)的变化使得电极表面钝化膜的形貌发生很大的变化,从而影响到膜层的耐蚀性。     

Characterisation of anodic oxide films formed on titanium and two ternary titanium alloys in hydrochloric acid solutions

The purpose of this paper is to characterise the anodic oxide films formed on titanium and two ternary titanium alloys, Ti‐15Mo‐5Al and Ti‐10Mo‐10Al in 1%, 10% and 20% hydrochloric acid solutions at 25, 50, and 75°C. The anodic film on titanium in hydrochloric acid is stable between + 0.6 V (SCE) and + 2.0 V (SCE). For our new ternary titanium alloys, the passive film is formed at about + 0.6 V (SCE) and is stable to + 2.0 V (SCE). The anodic polarization curves for alloys differ from the base metal curve, presenting two peaks for the critical passivation current density in the active‐passive potential range. At the first current peak (the first critical passivation potential E_cr1) a porous titanium pentaoxide (Ti₃O₅) is formed. When the potential reaches the second current peak (the second critical passivation potential E_cr2) the compact and protective titanium dioxide (TiO₂) is formed. The impedance spectra exhibit the typical behaviour for a passive film i.e. a near capacitive response illustrated by a phase angle close to − 90 ° over a wide frequency range. The oxide film on titanium and its alloys in hydrochloric acid solutions exhibits a high resistance and a low capacitance (with the increase of the potential) attributed to the surface roughness decrease as the oxide layer thickens.     

Electrochemical behaviour of high nitrogen stainless steel in acidic solutions

The electrochemical behaviour of high nitrogen stainless steel in acidic solutions was studied by potentiodynamic polarization, EIS, Mott-Schottky and XPS. The passive film formed in neutral NaCl solution was very stable, but the stability of the film decreased with the addition of H₂SO₄ into the solution. The passive film formed in acidic Na₂SO₄ has a superior protective ability than that in acidic NaCl solution. The stability of the film formed in tested solution decreased with increase of applied potentials. The film formed on steel surface was of n-type semiconductor. Chloride penetration mechanism was proposed for the observed passive film breakdown.     

Electropolymerization of PANI coating in nitric acid for corrosion protection of 430 SS

A smooth and adherent polyaniline (PANI) coating was electropolymerized in HNO₃ solution by cyclic voltammetry in order to protect the ferritic and economic 430 stainless steel (SS) from corrosion. The corrosion resistance was evaluated in 3.5 wt.% NaCl solution by anodic polarization, potentiodynamic polarization, and electrochemical impedance spectroscopy. The PANI coating shifts the corrosion potential of SS to the passive region and provides a strong and steady corrosion resistance. A low growth rate leads to a compact PANI coating that acts as a good physical barrier. However, the strong corrosion inhibition is mainly attributed to the dense oxide layers formed by the catalytic effect of PANI at the polymer/metal interface. The composition of the oxide layers was analyzed via depth profiling using X-ray photoelectron spectroscopy alternating with sputtering. Compared with layers formed on uncoated SS, the iron and chromium oxide layers under the PANI are thinner and denser, absorbing less oxygen, and providing a stronger corrosion inhibition.     

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。     

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.     

Influence of pH on electrochemical properties of passive films formed on Alloy 690 in high temperature aqueous environments

Passive films formed on Alloy 690 in different pH solutions at high temperatures were studied by potentiodynamic polarization, Auger electron spectroscopy, thermodynamic diagrams and the Mott–Schottky relation. The chemical compositions and electronic structures of the passive films were found to be strongly pH-dependent. In alkaline solutions, a secondary passivation was clearly observed on potentiodynamic polarization curves. The passive films were a mixture of Cr₂O₃ and FeCr₂O₄ below the flat band potential of nickel oxide and were NiFe₂O₄ above this potential. Electronic structure models, describing the electrochemical properties of the passive films, are proposed and discussed.     

硼酸-硼砂介质中硫离子对不锈钢钝化膜的侵蚀性

 Mott-Schottky图、Nyquist图及阳极极化曲线测定研究了硼酸-硼砂缓冲溶液中硫离子对不锈钢钝化膜耐蚀性能的影响，结果表明：随浸泡时间增加不锈钢电极阻抗值增大，但硫离子加入后阻抗值快速降低；阳极极化曲线测定显示硫离子使不锈钢钝态电流增大；硫离子浓度的增加使不锈钢电极的Mott-Schottky图中体现p-型半导体(铬氧化物)性质的直线段发生较大变化，说明硫离子影响了钝化膜中铬氧化物的性质，使其耐蚀性能降低. 钝化膜      

AN AES STUDY OF ANODIC OXIDE FILM ON 1Cr18Ni9 STAINLESS STEEL

用Auger电子能谱仪(AES)研究1Cr18Ni9不锈钢表面阳极氧化膜的组分分布。用Ar~+溅射剥层所作的剖面分析发现过钝化阳极氧化膜的表面层中含有大量的Mo与Cr的离子,中间层基本上是一个富Cr的氧化层,Fe处于中性态;然后逐渐过渡到基体内,在过渡层中氧逐渐减少到零。与硝酸钝化膜和自然钝化膜中元素的剖面分布相比,过钝化阳极氧化膜的富Cr区要比后二种膜宽得多,膜中Cr的氧化程度也比较完全。根据这些特点讨论了过钝化阳极氧化膜抗腐蚀性能比硝酸钝化膜优良的原因。     

γ_N相在硼酸缓冲溶液中的腐蚀与钝化性能

采用等离子体源渗氮技术在304L奥氏体不锈钢表面制备高氮面心结构的γ_N相层,利用阳极极化曲线和电化学阻抗谱(EIS)研究γ_N相在pH=8.4硼酸缓冲溶液中的腐蚀行为。结果表明:γ_N相的阳极极化曲线呈现出自钝化-过钝化溶解过程,自腐蚀电位Ecorr较原始不锈钢提高了75mV,维钝电流密度Jp降低近一个数量级,耐蚀性能明显提高。与原始不锈钢钝化膜相比,γ_N相钝化膜的EIS容抗弧直径及|Z|值增大,相位角平台变宽,其电荷转移电阻Rct增至1.064×107Ω·cm2,双电层电容Cdl降至65.4μF/cm2,说明γ_N相钝化膜更致密,表现为近电容特性。随着浸泡时间增加,γ_N相钝化膜的Rct稳定在107Ω·cm2量级,具有良好的稳定性。     

Influence of second phases on the electrochemical behavior of hot dipped Al-Mg-Si coated steel

The influence of second phases on the electrochemical behavior of hot dipped Al-Mg-Si coated steel has been investigated using an open circuit potential (OCP) and anodic polarization measurements. The results of OCP monitoring data for Mg₂Si particles show fast ennoblement of OCP then slowly rise until the last stage of monitoring. The localized corrosion starts from the surrounding matrix of Mg₂Si particles and propagates in the coating surface. The passivation of the Al-Mg-Si coating surface during anodic polarization can be attributed to the formation of the surface oxide film, which inhibits further dissolution of the coating.     

High corrosion resistance of austenitic stainless steel alloyed with nitrogen in an acid solution

Passivity of austenitic stainless steel containing nitrogen (ASS N25) was investigated in comparison with AISI 316L in deareated acid solution, pH 0.4. A peculiar nature of the passivation peak in a potentiodynamic curve and the kinetic parameters of formation and growth of the oxide film have been discussed. The electronic-semiconducting properties of the passive films have been correlated with their corrosion resistance. Alloying austenitic stainless steel with nitrogen increases its microstructure homogeneity and decreases the concentration of charge carriers, which beneficially affects the protecting and electronic properties of the passive oxide film.     

Corrosion of 304 stainless steel exposed to nitric acid-chloride environments

In an effort to examine the combined effect of HNO₃, NaCl, and temperature on the general corrosion behavior of 304 stainless steel (SS), electrochemical studies were performed. The corrosion response of 304 SS was bifurcated: materials were either continuously passive following immersion or spontaneously passivated following a period of active dissolution. Active dissolution was autocatalytic, with the corrosion rate increasing exponentially with time and potential. The period of active corrosion terminated following spontaneous passivation, resulting in a corrosion rate decrease of up to five orders of magnitude. The length of the active corrosion period was strongly dependent on the solution volume-to-surface area ratio. This finding, coupled with other results, suggested that spontaneous passivation arises solely from solution chemistry as opposed to changes in surface oxide composition. Increasing NaCl concentrations promoted pitting, active dissolution upon initial immersion, a smaller potential range for passivity, longer active corrosion periods, larger active anodic charge densities preceding spontaneous passivation, and larger corrosion current and peak current densities. In contrast, intermediate HNO₃ concentrations promoted active dissolution, with continuous passivity noted at HNO₃ concentration extremes. During active corrosion, increased HNO₃ concentrations increased the anodic charge density, corrosion current density, and peak current density. The time required for spontaneous passivation was greatest at intermediate HNO₃ concentrations. Susceptibility to pitting was also greatest at intermediate HNO₃ concentrations: the pit initiation and repassivation potentials decreased with increasing HNO₃ concentration until the HNO₃ concentration exceeded a critical concentration beyond which susceptibility to pitting was entirely eliminated. Increasing solution temperature increased the susceptibility to both pitting and active dissolution.     

304L不锈钢在两种高温高压水溶液中形成的钝化膜半导体性质研究

304L不锈钢在ZnSO_4和Na_2SO_4两种高温高压水溶液中腐蚀后表面形成一层钝化膜,对腐蚀后样品在硼酸缓冲溶液(pH8.4)中进行动电位扫描,并绘制其Mott-Schottky(M-S)曲线;利用光电流法,绘制(I_(ph)hv/I_O)~(1/2)-光子能量曲线,详细分析表面钝化膜半导体性质。结果表明:含锌样品表面钝化膜呈现多层结构;钝化膜的半导体类型为n型(不含锌样品钝化膜呈p型);平带电位负移;载流子浓度降低;Zn~(2+)对304L不锈钢钝化膜半导体的结构及性质有较大的影响。     

Ellipsometry of passive oxide films on nickel in acidic sulfate solution

Nickel passive film has been studied in acidic sulfate solutions at pH 2.3 and 3.3 by ellipsometry. During anodic passivation followed by cathodic reduction, the roughness increases with dissolution of nickel, being indicated by gradual decrease of reflectance. However, the ellipsometric parameters, Ψ (arctan of relative amplitude ratio) and Δ (relative retardation of phase), are relatively insensitive to the roughness increase. From the change of Ψ and Δ, δΨ and δΔ, during the anodic passivation and reduction, thickness of the passive oxide film was estimated with assumption of refractive index of n_f = 2.3 of the film. The thickness estimated is a range between 1.4 and 1.7 nm in the passive potential region from 0.8 to 1.4 V vs. RHE, having a tendency of thickening with increase of potential. Cathodic reduction at constant potential induces a change of the oxide film to an oxide film with lower refractive index of n_f = 1.7, accompanied by thickening of the film about 30% more in the initial stage of reduction for 30 s. The gradual decrease of thickness takes place for the oxide with the lower refractive index in the latter stage. The potential change from the passive region to cathodic hydrogen evolution region may initially cause hydration of the passive oxide of NiO, i.e., NiO + H₂O = Ni(OH)₂, and during the latter stage of reduction, the hydrated nickel oxide gradually dissolves.     

Corrosion assessment of nitric acid grade austenitic stainless steels

The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO₃ media. The results in 6 N HNO₃ show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO₃, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO₃ with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (R_P) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest R_P value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO₃ revealed that the passive films were mainly composed of Cr₂O₃ and Fe₂O₃ for all the alloys. The corrosion resistance of different NAG SS to HNO₃ corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper.