Lokale Korrosion und elektrochemisches Rauschen

Localized corrosion and electrochemical current noise The analysis of current transients resulting from metastable pitting events is a useful method for the investigation of the mechanisms of localized corrosion. Until recently, the noise of the passive current limited the detection of smaller metastable and pit initiation events. The analysis of the noise itself was not very successful, and was only a disturbing factor in the detection of the current transients. Improvements made in electrochemical data acquisition instrumentation and the appearance of novel methods of analysis, such as microelectrochemistry, now enables the analysis of the electrochemical noise. The investigation of localized corrosion on stainless steels reveals, that the current noise is influenced by numerous small pit initiation events due to the dissolution of non‐metallic inclusions. This observation and the dependence on surface area of the current noise help in clarifying the mechanistic aspects of pitting corrosion.     

Characteristics of the random transients caused by clustered pitting corrosion for Al–Mg microelectrode

The clustered pitting corrosion process has been investigated by electrochemical noise measurement using two nominally identical Al–Mg microelectrodes in aerated 3.5 wt% NaCl solution (pH 3.5). Current transients associated with the metastable pitting were detected and the pits were examined by scanning electron microscopy (SEM) observation. The results show that two types of clustered pits can be characterized by current transients separately. Pit interaction is confirmed by one transient with multiple peaks. Considering the nonFaradic current and cathodic current of H₂ evolution inside the pit, the average volume determined from an integrated charge associated with individual single peak current transient is lower than that of actual pits, while the single peak transient count is more than that from SEM observation. It thus implies that reactivation of one metastable pit occurs and the relevant millisecond transient demonstrates only a single peak associated with nanopit for each reactivation. The results from this work shed insight into the use of statistical analysis of current transients to forecast the corresponding pit morphology.     

Depassivation current transients measured between identical twin microelectrodes in open circuit

Current transients between identical twin titanium microelectrodes have been measured in open circuit. The electrodes showed transient breakdown of passivity in 1.5 M HCl although the potential is far below the pitting potential. Measurement of the current between the twin microelectrodes in sodium sulphate solution containing no chloride showed no detectable breakdown transients above the background noise. The current transients represent the breakdown of passivity and associated nucleation of corrosion pits. Some of the open circuit breakdown events show evidence of brief metastable pit propagation in the titanium surface.     

Cellular automaton model for simulation of metastable pitting

Abstract

As a new simulation method, cellular automata (CA) approach had been used in a number of areas, including electrochemistry and corrosion science research. In this work, a probability CA model was developed to simulate the growth of metastable pits. The simulation imaged the morphology of the pit and calculated the time dependences of pitting current and pitting current density. Results demonstrated that the CA approach is capable of simulating corrosion pit in terms of the pit morphology and pit growth kinetics. During growth of a hemispherical metastable pit controlled by diffusion, the time dependence of pitting current follows I?=?f(t²) law. Furthermore, it is determined that, for the simulation results to be consistent with the experimental results, an optimal range for variable d should be 0·1–0·25.     

Halbleitereigenschaften von Passivfilmen und ihre Bedeutung für die Lochkorrosion

Semiconductive properties of passive films and susceptibility to localized corrosion The semiconductor properties of the passive films on AISI 304 were studied by Mott-Schottky analysis and photoelectrochemical methods. The results were compared with the number of current transients during passivation at different potentials which describe the metastable pitting events. The results show that the onset potential for pit nucleation coincides with the potential at the point of zero charge in the passive film (flat-band potential). A simple electrostatic model is presented showing that the first step of the pit nucleation event - the adsorption of the pit inducing anions - is not possible until a region of positive space charge is present in the passive film. Furthermore the model is confirmed by the appearance of a maximum in the transient number occurring in the same potential range where drastic changes in the electronic properties of the passive film take place.     

Evolution of current transients and morphology of metastable and stable pitting on stainless steel near the critical pitting temperature

Type 904L austenitic stainless steel with a critical pitting temperature (CPT) of 56 °C was polarized at 750 mV (Ag/AgCl) in 1 M NaCl at 45, 49 and 54 °C. Current transients due to metastable pitting were recorded at these three temperatures, and a smaller number of stable pit current transients were recorded at 62 °C. The metastable pit current, I, followed a power law relationship I ∼ tⁿ during initial growth prior to repassivation. The exponent n increased with temperature, from 0.5 to 1.5. To grow at the lower temperatures, the pits need to be more occluded (lower value of n; lower pit current density). The repassivated metastable pits showed two morphologies: small occluded cavities associated with the small transients, and open ones with polished surfaces for large transients recorded close to the CPT. A new type of deterministic repassivation was identified for the latter type of pit. The stable pit currents rose in proportion to t^3/2, just like the faster-growing metastable pits, but only for a few seconds; then there was a drop in current and a further noisy increase occurred with a lower exponent. These pits undercut the metal surface, producing a lacy metal cover. The results are broadly consistent with a model that attributes the CPT to the onset of passivation in the saturated pit solution. A new feature requiring explanation is that, very close to the CPT, pits can precipitate a salt film but fail to undercut the surface. It is hypothesized that since a higher anodic current density is required at the undercutting site than at the pit base, there will be a range of temperatures where the former but not the latter would exceed the critical current density for passivation and is thus disallowed.     

Combined anodic/cathodic transient currents within nucleating pits on Al–Fe alloy surfaces

Pit nucleation and metastable growth on microelectrodes of aluminium and some Al alloys in chloride solution below the pitting potential are described by recurring current transients. Pit nucleation is a sharp, violent event and is observed repeatedly on all materials tested: pure aluminium, titanium and iron binary aluminium alloys, and alloy 1050A. At the imposed electrode potential, current transients due to pit nucleation recorded for pure aluminium and a solid solution titanium binary alloy are exclusively anodic. Those from alloys containing iron intermetallics sometimes exhibit both anodic and cathodic wings. The cathodic wing is due to transient hydrogen evolution at sites which border iron-rich intermetallic particles: these are preferred pit nucleation sites.     

In Situ Characterization of Pitting Corrosion of Stainless Steel by a Scanning Electrochemical Microscopy

In this work, a scanning electrochemical microscopy (SECM) was used to characterize in situ the metastable and stable pitting processes occurring on a stainless steel in the chloride solution. It was found that metastable pitting would occur on the steel that was at corrosion potential and passive potential. The positive shift of potential would enhance the metastable pitting current. On application of a potential exceeding pitting potential, the pit became stabilized and maintained a continuous growth. The SECM is capable of detecting the microdissolution event and provides a “visual” observation of the pitting processes.     

Stochastic modeling of pitting corrosion in aluminum alloys

The stochastic theory of pitting corrosion has been successfully used to analyze the statistical nature of pitting. In this paper a Monte Carlo model is used to predict the extent of damage accumulation in aluminum alloys. This model uses experimental parameters obtained by electrochemical noise measurements on electrodes arrays. The algorithm is based on the random occurrence of the metastable pit birth/death or the stable pit growth. Simulated pit depth distributions are compared to experimental data obtained by Optical Profilometry (OP), leading to an improvement of the model and challenging the existence of a metastable/stable transition in free corrosion conditions.     

基于Hilbert-Huang变换的电化学噪声解析及其应用

借助Hilbert-Huang变换 (HHT) 研究了Q345B碳钢在模拟混凝土孔隙液中的电化学噪声信号的时频谱,发现相比离散小波变换 (discretewavelet transform,DWT),HHT在噪声信号识别过程中具有更高的时频分辨率和稳定性,能够从本质上提高对电化学噪声中耦合的亚稳态点蚀信号的解析精度。针对Q345B碳钢处于钝化态、亚稳态点蚀萌发和稳态点蚀生长等不同阶段的噪声特点,提出了一套基于HHT边界谱的腐蚀状态量化指数与腐蚀特征识别方法。借助于在线电化学噪声监测装置,HHT算法将可用于诊断工业环境的腐蚀形态和腐蚀发展趋势。     

Determining the probability of stable pit initiation on aluminium alloys using potentiostatic electrochemical measurements

A method is described that determines the probability of stable corrosion pits being formed on aluminium alloys. The method employs potentiostatic measurements of metastable pit equivalent volumes, in coulombs, at the open circuit potential. The metastable pit volumes are statistically analysed using an extreme value statistics procedure that returns an expected distribution of metastable pit volumes. This enables the probabilities of the occurrence of metastable pits of given volumes with time to be calculated. The size requirement for a metastable pit to transition into a stable pit was then determined using both optical microscopy and electrochemical tests. This size requirement when combined with the metastable pit size distributions was used to determine the probability of the formation of a stable pit. The method is demonstrated for aluminium alloy AA2024-T3 in chloride media with and without chromate additions and validated against optically measured pitting rates.     

Pitting Kinetics of 304 Stainless Steel Using ESPI Detection Technique

The electronic speckle pattern interferometer was used to in situ monitor the pitting corrosion of 304 stainless steel at anodic polarization.The pitting current and pitting current density of a single pit were obtained.The pit growth was controlled by the corrosion products diffusion.The pit morphology was observed by a scanning electron microscope.The results showed that the pit was dish shaped,and the geometric parameters and pit growth time conformed to the function of Y = A+B1t+B2t2+B3t3.     

Evaluation of metastable pitting on titanium by charge integration of current transients

The metastable pitting of titanium has been studied under potentiostatic control in solutions containing chloride ions. An approach based on the charge integration of current transients was proposed for a quantitative determination of metastable pitting. A pit density (d_mpit) was defined as the number of metastable pits per unit area per unit time (cm⁻² h⁻¹) with a typical size, instead of a size distribution. The calculated d_mpit of titanium at 0.5 V_SCE in 0.6 M NaCl was about 1.0 × 10³ cm⁻² h⁻¹ with a typical radius of 0.12 μm. An exponential potential dependence of d_mpit was obtained through the integration approach.     

Using pit solution chemistry for evaluation of metastable pitting stability of austenitic stainless steel

The stability of a corrosion pit is directly related to the pit solution chemistry. In order to know the stability of a meastable pit, the product of corrosion current density and diffusion depth (pit radius for an open hemisphere pit) is compared to the stability product obtained from the artificial pit electrode results. In this paper the stability of a metastable pit recorded on a 904L stainless at 54°C (2°C below the alloy CPT) is investigated. Assuming the geometry result of such a transient is an open hemisphere, pick current density and pit radius were calculated from Faraday's equation and then pit stability product (i·a) was calculated. This value was compared with the product of i·a associated with the pit solution chemistries (critical, saturation and supersaturation concentrations) which were obtained from the investigation on artificial pit electrode at the same temperature. SEM investigations on pit cavity generated from large metastable pit current transients with the stability product greater than the stability product associated with supersaturation concentration of pit environment proves that measured pit solution chemistry from artificial pit electrode is a reliable step for evaluation of pit stability.     

Effect of Mo species on metastable pitting of Fe18Cr alloys—A current transient analysis

The analysis of current transients occurring on Mo containing ferritic Fe-18Cr steels in chloride containing electrolyte in absence or presence of molybdate anions as external inhibitor reveals insight into the role of Mo-species in metastable pitting events, consisting of pit initiation, pit propagation including some metal dissolution, and pit repassivation. A mathematical method on the basis of statistical distribution functions was developed in order to meet the requirements of a systematic data analysis which allows to compare different systems of alloys and electrolytes. The results clearly indicate the beneficial effect of alloyed Mo as well as in the electrolyte in inhibiting (metastable) pitting corrosion. However, the mechanism of inhibition is unlike for alloyed Mo and added into the electrolyte.     

The inhibition of pitting corrosion of stainless steels by chromate and molybdate ions

The inhibitive effects of chromate and molybdate on pitting corrosion in stainless steel AISI 304 and AISI 316 were studied in acidified chloride solution. The results presented show that these known inhibitors affect both the nucleation of pitting and metastable pitting by reducing the numbers and sizes of these events. This makes attainment of stable pit growth more difficult.     

304不锈钢表面TiN涂层的耐蚀性能

目的 提高304不锈钢的耐腐蚀性能.方法 采用磁控溅射技术在304不锈钢表面沉积TiN涂层,并采用SEM、XRD及GDOES对涂层的表面形貌、成分进行测试.通过极化曲线和电化学噪声技术评价TiN涂层和基体在pH=2.5的3.5%(质量分数)NaCl溶液中的腐蚀行为,并研究涂层的失效机制.结果 在304不锈钢表面沉积了厚约1μm且均匀、致密的TiN涂层.极化曲线分析表明,基体和TiN涂层试样出现了自钝化和点蚀现象,其中304不锈钢基体的腐蚀电位为-0.41 V,腐蚀电流密度为8.01×10-6 A/cm2,与之相比,TiN涂层的腐蚀电位(-0.28V)明显增大,腐蚀电流密度(6.34×10-8 A/cm2)显著降低.电化学噪声分析显示,在浸泡初期,TiN涂层电极电流暂态峰数量较少,强度较大,噪声电阻较低,而随着浸泡时间的延长,其电流暂态峰数量增加,强度降低,噪声电阻明显大于304不锈钢基体.腐蚀形貌观察表明,304不锈钢和TiN涂层表面均出现了点蚀.结论 TiN涂层能够明显改善基体的耐蚀性能.TiN涂层主要起物理阻碍作用,涂层的主要失效形式是涂层表面的微观缺陷和破裂.     

Effect of Alloying Tin on the Corrosion Characteristics of Austenitic Stainless Steel in Sulfuric Acid and Sodium Chloride Solutions

The effect of tin on general and pitting corrosion behaviors of the austenitic stainless steel in sulfuric acid and sodium chloride solutions was investigated by potentiostatic critical pitting temperature, cyclic potentiodynamic polarization, electrochemical impedance spectroscopy, and scanning electron microscopy. The results showed that there is an optimal tin addition which is around(0.062–0.1) wt%, and the general corrosion resistance of B316 LX with 0.08 wt% tin addition in boiling H2SO4 increased remarkably with a corrosion rate of an order of magnitude lower than that of 316 L.Hydrolyzation of tin ions induces more metastable pit occurrence on the material surface. However, the pitting resistance of B316 LX increases because tin oxides improve the density and uniformity of the passive film, and hydroxide and oxide of tin inhabit the process of pit growing. The effect of tin on pitting corrosion process is illustrated schematically.     

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.     

Pitting transients analysis of stainless steels at the open circuit potential

Metastable pitting of stainless steels in chloride containing oxidising electrolytes is investigated at rest potential using a new experimental technique allowing to record simultaneously the potential and corrosion current variations. Different industrial surface conditions (BA and 2B) are tested for both ferritic (FeCr type) and austenitic (FeCrNi type) stainless steels. It was shown that the number of pitting events decreases with the exposure time and that BA condition provides better resistance to pitting than 2B. As far as pitting mechanisms are concerned, the potential recovery after pitting does not reflect the pit repassivation but rather refers to the discharge of the surface capacity. Analysing the pitting transients provides quantitative information on the cathodic reaction through the passive film (transfer resistance and surface capacitance). Differences in pitting transient shapes are discussed as well.