Localized corrosion (pitting): A model of passivity breakdown including the role of the oxide layer nanostructure

A model of passivity breakdown including the role of the inter-granular boundaries of the barrier oxide layer on the redistribution of the potential at the metal/oxide/electrolyte interfaces in the passive state is presented. Different mechanisms of breakdown at the oxide grain boundaries are considered, depending on which interface governs the potential drop: (i) local thinning and dissolution of the oxide layer, (ii) metal voiding or (iii) particle growth at the metal/oxide interface followed by rupture of the barrier layer. The role of chloride ions is discussed in each case. The key experimental observations made at the nanometre scale and validating the model are discussed.     

Passivity: enabler of our metals based civilisation

Abstract

The origin of passivity, as viewed from the Point Defect Model, is presented in terms of the phase space analysis of the rate law for the growth of passive films. It is shown that passivity is due to the formation of a metastable oxide and hence, the occurrence of passivity is a kinetic phenomenon, in which the rate of film growth of the oxide at the metal/barrier layer interface into the metal at zero barrier layer thickness must exceed the rate of barrier layer dissolution at the barrier layer/solution (bl/s) interface, in order for the barrier layer to exist. If this relationship does not hold, the barrier layer disappears and the surface becomes depassivated. Depassivation is illustrated with respect to transpassive dissolution, acid depassivation, pitting corrosion, flow assisted corrosion, impingement attack, resistive depassivation and other phenomena. The theory also leads to the development of Kinetic Stability Diagrams (KSDs), in which the potential for depassivation is plotted against pH to define regions of depassivation and passivation, so that regions in potential versus pH space can be defined within which passive films may exist as protective, metastable entities. The author offers these diagrams as kinetically inspired alternatives to the classical Pourbaix diagrams. One form of depassivation that occurs at highly localised regions on a metal surface is pitting corrosion. The theory for pitting is now highly developed and certain aspects of the subject are reviewed here. It is argued that the theory of pitting is sufficiently well established that the deterministic prediction of pitting damage is practical, in many systems. The theory is illustrated here with respect to the pitting of Type 403 stainless steel (SS), which is used extensively for the manufacture of low pressure steam turbines in steam cycle systems for the generation of electrical power.     

Passivity breakdown on AISI Type 403 stainless steel in chloride-containing borate buffer solution

Passivity breakdown on AISI Type 403 stainless steel (SS), a commonly employed blade alloy in low pressure steam turbines, has been studied and the data are interpreted in terms of the point defect model (PDM). The near normal distribution in breakdown potential measured in deaerated borate buffer solution (pH = 8.1 ± 0.1) with different chloride concentrations is in satisfactory agreement with the quantitative characterization of the breakdown potential distribution using the PDM. The linear dependence of breakdown potential on the square root of potential scan rate (υ^1/2), as predicted by the PDM, yields an estimate of the critical areal concentration of condensed vacancies at the metal/film interface (ξ < 7.0 × 10¹⁴ cm⁻²) that leads to passivity breakdown. This is in excellent agreement with that calculated from the unit cell dimensions of the substrate Fe-Cr alloy (ξ ≈ 10¹⁵ cm⁻²) and the barrier layer oxide (Cr₂O₃) (ξ ≈ 10¹⁴ cm⁻²) for vacancy condensation on the alloy lattice or on the cation sublattice, respectively, of the film. These provide convincing evidences for the validity of the PDM for modeling passivity breakdown on Type 403 SS.     

The design of copper-coating overpack for the high-level radioactive waste disposal concept in Japan

For the geological disposal system in Japan, a vitrified waste will be contained in a metal overpack, which, in turn, will be surrounded by a thick bentonite buffer. The overpack is aiming to prevent the contact of groundwater to vitrified waste during the high radioactivity and heat generation period of the first 1,000 years at least after emplacement. Within the Japanese program, consideration for overpack candidate materials has included carbon steel, copper–steel composite, and titanium–steel composite. Within the extensive safety assessment conducted in 2000, steel was selected. This selection was partly based on the manufacturability of carbon steel, as well as its well-understood corrosion behavior. However, the understanding of copper corrosion and welding/manufacturing technologies have greatly progressed over the past two decades. In this study, we focus on the copper-coated container developed by the Nuclear Waste Management Organization, because this technology is seemingly effective to maintain very long-term containment of hundreds of thousands of years, with a low cost of manufacturing within the Canadian program. We are investigating the applicability of the copper-coating technologies in terms of the corrosion allowance and mechanical design.     

NiCoCrAlY/NiCoCrAlYN涂层的腐蚀磨损行为

目的在中碳钢表面制备金属+氮化物涂层以研究其在腐蚀、磨损耦合环境下的服役行为。方法利用电弧离子镀技术在42CrMo钢基体表面制备NiCoCrAlY/NiCoCrAlYN双层涂层,通过改变氮气流量来调控涂层中氮元素含量。采用显微硬度计以及特制的腐蚀磨损试验机,同时结合XRD、XPS、SEM等各类表征技术研究涂层力学及腐蚀磨损行为。结果NiCoCrAlYN涂层物相主要为γ-Ni、γ'-Ni₃Al、fcc-CrN和fcc-AlN。随着N₂流量的逐步引入,涂层中CrN/AlN陶瓷相含量增加,显微硬度从575HV_0.05逐渐提升至730HV_0.05,而结合力等级从HF 2下降至HF 3。NiCoCrAlY/NiCoCrAlYN双层涂层比NiCoCrAlY单层涂层具有更佳的腐蚀磨损性能,且随着N含量的增加而显著提升。当N2流量为400 mL/min时,涂层显示出最佳的腐蚀磨损性能。此外,涂层的失效形式也由磨损失效逐渐转变为腐蚀失效。结论由于氮化物表层的高硬度特性增强了涂层耐磨性,NiCoCrAlY-NiCoCrAlYN界面的阻挡作用提高了涂层耐蚀性,加之底部NiCoCrAlY层具有良好的韧性和延展性,三者的综合作用导致NiCoCrAlY/NiCoCrAlYN双层涂层的腐蚀磨损性能优于NiCoCrAlY单层涂层。     

The thermogalvanic corrosion of mild steel in alkaline solution—III. The factors controlling etching,passivation and pitting

The thermogalvanic behaviour of mild steel in an environment where active dissolution, passivation and breakdown are all possible has been investigated. The polarity of the electrodes and the magnitude of the cell currents vary with time as these processes occur successively but at different rates at each electrode. In the presence of an aggressive ion the onset of passivity is often undesirable since localized breakdown may ensue, any subsequent thermogalvanic corrosion being predominantly located at the pit sites giving rise to a disproportionate increase in metal penetration relative to the recorded current. Since maximum cell currents of approaching 5 A/m² are possible in these systems, the cells are usually under resistance control; any circumstance which leads to a lowering of this resistance will therefore increase the metal loss. However, thermogalvanic action does not always stimulate the overall corrosion rate of the anode but may, by polarization of this electrode, induce passivity and breakdown at a metal surface which would not undergo these changes spontaneously. Although the metal loss is often lowered, the penetration commonly increases since the corrosion damage which was previously uniform becomes highly localized.     

Alternative models of passive state of metals: II. Development of K. Vetter’s model

The passivity model of K. Vetter is further developed. An equation for the interface potential at a passive layer/electrolyte boundary is obtained with a due regard for its functional dependence on the current of passive metal dissolution. A process of metal conversion into passive oxide on the metal/passive oxide interface and the electrical properties of the interface are considered. This work was supported by the Program “Russian Universities— Fundamental Research.”     

General corrosion of carbon steel in a synthetic concrete pore solution

In this study, we reviewed our recent work on the general corrosion of carbon steel (P355QL2) overpack material for the isolation of high-level nuclear waste in Belgium's supercontainer concept. By using electrochemical impedance spectroscopy and by optimizing the mixed potential model, which incorporates quantum mechanical tunneling of charge carriers across the barrier layer to describe the kinetics of the partial cathodic process, we evaluated all parameters in the model as a function of independent variables such as voltage, temperature, and pH. By delineating the partial anodic and cathodic processes, we found that the corrosion rate (CR) is independent of voltage over the voltage range from 0.2 to −1.0 V_SHE, which is predicted to be experienced in the repository. Furthermore, the CR is found to increase strongly with decreasing pH and increasing temperature.     

异种金属摩擦焊工艺温度场数值模拟研究

基于摩擦焊接过程中热-力学现象,建立摩擦焊接过程中热力耦合模型,对陶瓷/金属焊接过程中温度分布进行数值模拟。所提出的模型能够预测金属陶瓷摩擦焊接过程中随时间增量的温度分布情况;焊接界面区域产生的摩擦热消耗中间层铝,并在氧化铝和低碳钢之间建立焊接层。由于氧化铝和低碳钢具有不同的温度属性,在界面处会产生更多的热应力。数值模拟用来预测氧化铝/低碳钢接口处残余应力的变化情况,进而避免异种金属摩擦焊接过程中不完全联锁、接头强度差的现象。     

Electrochemical behavior of carbon steel with bentonite/sand in saline environment

Current designs for the geological disposal of high-level radioactive waste in Japan use carbon steel overpack containers surrounded by a mixed bentonite/sand buffer material, which will be located in a purpose-built repository deep underground. Though there are many suitable sites for a repository in Japan, the coastal areas are preferable from a logistical point of view. It is, therefore, important to evaluate the long-term performance of the carbon steel overpack and the mixed bentonite/sand buffer material in the saline groundwaters of coastal areas. In the current study, the passivation behavior and initial corrosion rates of carbon steel with and without mixed bentonite/sand were tested as a function of the pH and salt concentration in representative saline groundwaters. The main findings indicate that the passivation of carbon steel encapsulated in a buffer material will be unlikely in a saline environment, even at high pH (12), and that the corrosion rate of carbon steel is more strongly affected by the presence of the buffer material than by the salt concentration.     

The influence of secondary phase carbide particles on the passivity behaviour of NiTi shape memory alloys

The current investigation aims at studying the passivity behaviour of NiTi shape memory alloys with different levels of secondary phase titanium carbide (TiC) particles in an electrolyte of 0.9% sodium chloride at 37 °C. The influence of carbides and thermo‐mechanical treatment/cold working on the passivity breakdown is highlighted. The polarisation studies on the as‐cast and cold worked NiTi with high (0.05 wt%) and low (0.005 wt%) carbon levels show a significant difference in oxide stability. The alloy with extremely low carbon content shows a higher breakdown potential. Higher carbon levels result in higher density of larger TiC and these carbide/matrix interfaces are more susceptible to pitting. The qualitative behaviour of passive layer formed at 0.5 V on the cold worked NiTi alloy with different carbon levels was ascertained by electrochemical impedance spectroscopy (EIS). The oxide on the NiTi alloy with high (0.05 wt%) carbon levels showed lower resistance and poor stability at this condition.     

18-8不锈钢在含Cl~-介质中钝化膜化学组份的特征

应用 ESCA 和 AES 研究了18-8不锈钢在含 Cl~-介质中,表面阳极钝化膜的化学组份及其深度分布。着重考察了介质中的 Cl~-对钝化膜化学组份的影响,并讨论了 Cl~-在电极表面可能的反应。作者认为介质中的 Cl~-虽可嵌入钝化膜的表面层,但 Cl~-的嵌入并非是导致膜破坏的唯一原因。     

预置间隙对铝/钢CMT接头强度与失效模式的影响

采用CMT熔钎焊方法,对汽车用镀锌低碳钢板与6061铝合金焊接接头的强度与破坏模式进行了研究.采用不同规格的塞尺控制铝/钢搭接接头的预置间隙,以改变铝/钢异种材料熔钎焊接头的熔合面积.结果表明,通过预置间隙的方法可以有效增大熔合线长度,从而提高焊缝接头的拉剪强度.在间隙为0.5 mm时失效模式发生变化,主要原因是焊趾处铝/钢界面层中的锌元素增大了界面层的脆性.     

过渡层对铝/钢FSB接头性能和组织的影响

通过镀覆镍及Ni/Cu复合过渡层,采用锌作为钎料,对工业纯铝和低碳钢板进行了搅拌摩擦钎焊(FSB)连接. 对焊接接头进行了拉剪和抗撕裂试验,并与无任何过渡层接头进行了对比. 用扫描电镜及能谱对接头界面形貌和微观组织进行了分析,研究分析了过渡层对铝/钢FSB接头力学性能和微观组织的影响. 结果表明,过渡层能有效阻止脆性Al-Fe金属间化合物的生成,增加了接头抗撕裂载荷强度,提高接头的力学性能.     

Oxidation mechanism of an Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy at 470 °C (Part II)

This paper is the second part of a global study on the oxidation process of an Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the growth mechanism of a duplex oxide scale. The oxide layer has a duplex structure composed of an internal Fe-Cr spinel layer and an external magnetite layer. The magnetite layer grows by iron diffusion until Pb-Bi/oxide interface whereas the Fe-Cr spinel layer grows, at the metal/oxide interface, inside the space kept “available” by the iron vacancies accumulation due to iron outwards diffusion for magnetite formation. This growth mechanism is close to the “available space model”. However, this model is completed by an auto-regulation process based on oxygen supply.     

CO2 laser welding–brazing characteristics of dissimilar metals AZ31B Mg alloy to Zn coated dual phase steel with Mg based filler

The influence of process variables including heating mode, flux, laser beam offset, and travel speed on the weld bead geometry and joint strength was investigated during laser welding–brazing (LWB) of AZ31B Mg alloy to Zn coated steel. The wettability of molten filler metal on steel surface was studied via a Charge-coupled Device (CCD) camera. The reaction layers along the interface were characterized and the failure mechanism was identified. Dual beam processing was found to preheat the steel substrate and promote the wettability of molten filler metal on the steel surface, thereby improving the corresponding joint strength. Utilizing a flux was found to produce a similar effect on molten filler metal. The optimized range of laser offset was found to be between 0.5 and 1.0 mm toward the steel side of the joint. These optimized parameters led to a maximum joint strength of 228 N/mm. The joint strength was however found to decrease with increasing travel speed. Cracking was identified with travel speeds greater than 1 m/min. Microstructural characterization showed that heterogeneous interfacial reaction layers were produced from the seam head to the seam root of the joint. The reaction layer thickness varied within a certain range when applying different process parameters, suggesting the growth of interfacial layer was not essentially related to the heat input. The primary failure mode of the lap specimens was interfacial fracture. Cracks propagated along the Mg–Zn reaction layer and steel interface. Original Fe–Al phase formed during the hot-dip galvanization process hindered the metallurgical bonding of Mg–Zn reaction layer and steel substrate, which was attributed to interfacial type failure.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature I: Mechanism of Void-Induced Duplex Oxide Formation

Under CO₂ exposure at an intermediate temperature, typically 550?°C, 9Cr–1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe–Cr rich spinel oxide layer. It is proposed that the inner Fe–Cr spinel layer grows according to a mechanism involving void formation at the oxide/metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO₂, which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.     

Detection and identification of concrete cracking during corrosion of reinforced concrete by acoustic emission coupled to the electrochemical techniques

The tenacious oxide passive film, which is formed on the surface of embedded reinforcing steel under high alkaline condition of concrete, protects the steel against corrosion. However, the condition of passivity may be destroyed, due to processes such as leaking out of fluids from concrete, atmospheric carbonation or through the uptake of chloride ions. Passive steel reinforcing corrosion induced by chloride is a well-known problem, especially where chloride-containing admixtures or chloride contaminated aggregate are incorporated into the concrete. The objective of this work is on one hand to study the effect of chloride ions on passivity breakdown of steel, respectively, in simulated concrete pore solution (SCP) and in concrete reinforcement, and on the other hand to reproduce the carbonation phenomena by applying to the concrete samples a heating–cooling cycles. In this context, the acoustic emission coupled to the electrochemical techniques (potentiodynamic and electrochemical impedance spectroscopy (EIS)) are used.

The results show clearly that [Cl⁻]/[OH⁻] ratio of 0.6 is the critical threshold where the depassivation set-up can be initiated. In addition, the carbonation process is very aggressive with chloride ions and shows a perfect correlation with acoustic emission evolution.

A physical model of the reinforcement/electrolyte interface is proposed to describe the behavior of the reinforcement against corrosion in chloride solution.     

The oxidation resistance of nitrided mild steel

The oxidation resistance of mild steel in a simulated boiler flue gas atmosphere has been shown in laboratory tests to be significantly enhanced by a surface nitriding treatment which forms a controllable fine-grained martensitic layer without precipitation of iron nitrides. A mechanism is suggested by which improved protection results from the refinement of oxide grain size and the condensation of cation vacancies in the nitrided layer instead of at the metal/oxide interface. A parallel is drawn with the development of improved oxidation resistance in superalloys by sub-surface dispersion of incoherent stable oxide particles.     

Gas metal arc brazing of galvannealed steel sheets

An advanced gas metal arc welding technique with controlled short-circuiting mode of metal transfer and arcing at low power was employed for brazing of galvannealed steels using CuSi3 filler wire. The brazed joints showed the presence of an interface layer with Fe–Si intermetallic compounds (IMC). Increase in heat input resulted in an increase of the interface layer thickness. The maximum failure load of 2.5?kN was achieved for a heat input of 105?J?mm^?1 with the corresponding interface layer thickness of 2.7?µm. Heat inputs beyond 105?J?mm^?1 led to excessive spatter of zinc and increase in interface layer thickness with hard Si-rich IMC. In contrast, heat input values lower than 80?J?mm^?1 produced inadequate wetting of the steel surface by the molten filler wire deposit and resulted in low joint strength.