Some concepts of corrosion fundamentals

The corrosion process involves acid-base reactions as well as electrochemical reactions. It is the acid-base reactions that make metallic corrosion diversified depending not only on the electrode potential but also on the acid-base species present in the environment. Surface defects, molecular fluctuation of adsorbed species and mass transport perturbation provide the non-uniform characteristics of the immediate zone of corroding metal surfaces. Localized corrosion results from the growth of electrochemical perturbation into the formation of a dissipative structure, which begins when the perturbation exceeds a marginal size. Selective ion transport through corrosion precipitates on the metal surface often plays a decisive role in the development of localized corrosion and passivation.     

Enhanced predictive corrosion modeling with implicit corrosion products

An advanced mathematical approach to describe the influence of corrosion products on the corrosion rate is presented here. The related model can be used as input equation for numerical predictive corrosion simulations or simply as an empirical model, to extrapolate experimental data of corrosion tests to longer times and to interpret the physical parameters behind. This semiempirical model assumes that a constant share of the dissolved metal precipitates on the surface and hinders the diffusion processes. Hence, the effective corrosion rate decreases exponentially with increasing dissolution. The explicit corrosion progress over time is derived by time integration on a newly developed, time dependent corrosion rate equation. The derived expression can be effortlessly implemented in existing for example finite element method, which is demonstrated for the uniform corrosion of a zinc surface. Furthermore, this approach is qualitatively compared with other empirical models for corrosion products and the validity is demonstrated by fitting of experimental data. A very good agreement between experiment and theory can be achieved for various materials and environments considering no change of the driving corrosion mechanism.     

The localized corrosion of Cr-Ni stainless steel

The kinetics of localized corrosion of Cr-Ni stainless steel is characterized by the quadratic time dependence of the total current density, by the linear increase in the number of pits and by the decrease of the current density in the pits. The pits have a shape of a rotary ellipsoid. The Tafel slope of the metal dissolution in the pits is 0.50 V. The dissolution rate is the highest on the pit bottom and the lowest at the mouth of the pit. The different rate of metal dissolution is caused by the different concentration of chloride ions over the pit surface.     

AZ91D镁合金电火花堆焊组织及腐蚀性能

采用与母材同质的电极材料,在AZ91D镁合金母材上进行电火花堆焊,研究了焊缝的组织、界面特征及腐蚀性能.结果表明,通过优化的工艺参数可以获得组织均匀、致密的电火花堆焊焊缝.焊缝组织晶粒尺寸在1~5μm之间,由过饱和的α-Mg相、Mg₁₇Al₁₂相以及亚稳态Al Mg相所组成;焊缝与母材之间为冶金结合,形成超薄层熔化互扩散结晶型结合界面,母材一侧没有形成明显热影响区,焊缝保持电极原有的成分;焊缝耐蚀性能优于母材,细化的晶粒组织提高腐蚀的均匀性,过饱和的α-Mg相和晶界上网状连续分布的β相降低腐蚀速率,是其耐蚀性能提高的主要因素.     

The stability of localized corrosion

Chloride pitting of iron group metals at noble potentials proceeds in the polishing state dissolution, provided that metal chloride in the pit solution is maintained above a critical concentration. It ceases to progress by pit repassivation if the pit is smaller than a critical size, or transforms into the active state pitting if the pit size is greater. The boundary potential between the polishing state and the active state pitting may be represented by the passivation-depassivation potential in the pit solution of the critical chloride concentration. Crevice corrosion is characterized by the crevice protection potential, at which the hydrogen ion concentration in the crevice solution is equivalent to pH_pd—the passivation-depassivation pH of the crevice metal. It continues to corrode at more noble potentials than the protection potential, where the crevice solution is more acidic than pH_pd, but is inhibited in the less acidic crevice solution at less noble potentials.     

The corrosion of tin under thin electrolyte layers containing chloride

The corrosion of tin under thin electrolyte layers containing chloride was investigated by electrochemical measurements and surface characterization. The results show that the corrosion rate increases as the electrolyte layer thickness deceases in the initial stage. Moreover, the corrosion rate under the same electrolyte layer thickness increases firstly and then decreases with the exposure time. In the later stage, the anodic processes under thin electrolyte layers of 50 and 100 μm are inhibited due to the difficult diffusion of dissolved metal ions. The corrosion products formed on tin surface are composed of stannous and stannic oxides and hydroxides.     

The corrosion kinetics of mild steel in the presence of cyanoguanidine formaldehyde resins

The corrosion inhibition efficiency of resins produced by condensation of cyanoguanidine and formaldehyde has been demonstrated for the acid corrosion of mild steel. The percentage inhibition was found to increase with resin concentration up to a limiting value of ca. 96%. The presence of resin was found to decrease both the rate of hydrogen evolution and the corrosion rate to the same degree. The activation energy of hydrogen evolution and of the overall corrosion reaction were found to be equal and characteristic of diffusion controlled processes. It is suggested that cyanoguanidine formaldehyde resins act as cathodic inhibitors of the acid corrosion of mild steel. It is concluded that the resin forms an insoluble film on the metal surface, diffusion through which is the rate determining step of the corrosion reaction. The degree of coverage of electrode surface was calculated from the shift in the cathodic Tafel lines caused by the presence of resin. The percentage inhibition and the degree of coverage were found to be higher in HCI than in H₂SO₄, for comparable resin concentrations. This is attributed to the adsorption of chloride ions on the metal surface forming oriented dipoles which enhance the adsorption of the resin.     

碳钢焊接接头腐蚀行为分析

通过对焊接接头各区域显微组织观察以及开路电位、极化曲线和电偶腐蚀电流的测量,研究了两种热输入所得焊接接头各区域的腐蚀行为. 结果表明,热输入小的接头各区域耐蚀性由小到大依次为母材,焊缝,热影响区;热输入大的接头各区域耐蚀性由小到大依次为母材,热影响区,焊缝. 该结果与接头不同区域的显微组织相关. 两种接头总腐蚀速率相比,热输入大的接头耐蚀性较好.     

The role of chelating agents on the corrosion mechanisms of aluminium in alkaline aqueous solutions

The influence of 1,2-diaminoethane (DAE) on the mechanism of aluminium corrosion in KOH solutions at pH 13 was investigated by combining time-resolved inductively coupled plasma optical emission spectrocopy, open-circuit potential measurements and X-ray photoelectron spectroscopy. In pure KOH solutions, a very slow corrosion rate is initially observed, corresponding to the dissolution of the native oxide layer. Following this incubation stage, the corrosion rate is increasing due to the formation and oxidation of Al hydride, until a steady state is reached. DAE behaves as a strong initial corrosion accelerator, due to synergistic effects with hydroxyl ions and a dissolution mechanism in three successive steps has been proposed: (i) a rapid initial dissolution induced by the formation and detachment from the surface of bidentate (chelate) Al-DAE metal bound surface complexes; (ii) a slower step ascribed to the formation and release of monodentate Al-DAE metal bound surface complexes and (iii) a final step dominated by direct oxidation of surface aluminium hydride by hydroxyl species as in pure KOH.     

碳钢焊接接头腐蚀的有限元模拟

由于焊接接头各区域之间耐蚀性和力学性能的差异,在接头处容易发生局部腐蚀,严重威胁着管线的安全运行,需要对接头腐蚀行为进行预测. 基于电偶腐蚀原理建立了碳钢焊接接头腐蚀的有限元模型,结合COMSOL软件,对三种温度下的接头腐蚀形貌进行了预测. 并利用该模型研究了焊缝余高、焊缝与母材的面积比以及焊缝缺陷对焊缝金属腐蚀速率的影响规律. 结果表明,该有限元模型不但能进行焊接接头腐蚀行为预测,而且可为输油气管道的焊接接头设计提供理论依据.     

The problem of sulfur in high-temperature corrosion

The role of defect and transport properties of transition metal sulfides on the kinetics and mechanism of high-temperature sulfide corrosion of metals and alloys is discussed. It has been shown, that due to the very high concentration of defects in common metal sulfides, not only pure metals but also conventional high-temperature alloys (chromia and alumina formers) undergo very rapid degradation in highly sulfidizing environments. Refractory metals, on the other hand, are highly resistant to sulfide corrosion, their sulfidation rates being comparable with the oxidation rate of chromium. Pioneering work of Douglasset al. has shown that alloying of common metals by niobium or molybdenum, and in particular combined alloying by molybdenumand aluminum, dramatically decreases the sulfidation rate. A novel Fe–30Mo–9Al alloy has been proved to be highly resistant to sulfide corrosion, its sulfidation rate being comparable with that of pure molybdenum. Even better resistance to highly-sulfidizing environments show new amorphous Al–Mo and Al–Mo–Si alloys, these materials also being simultaneously oxidation resistant. Thus, new prospects have been created for the development of a new generation of coating materials, resistant to multicomponent sulfidizing-oxidizing atmospheres, often encountered in many branches of modern technology.     

2195铝锂合金摩擦搅拌焊接头组织与腐蚀行为

研究了5 mm厚度2195-T8铝锂合金板材摩擦搅拌焊(FSW)接头不同位置的显微组织、腐蚀电化学行为以及在晶间腐蚀介质中的腐蚀特征. 结果表明,母材为纤维状晶粒组织,而焊核区为晶粒尺寸小于10 μm的完全再结晶组织. 2195-T8铝锂合金母材时效强化相为T1相(Al₂CuLi)和θ'相(Al₂Cu),热力影响区大部分T1相和全部θ'相已重新溶解至基体中,而焊核区所有T1相和θ'相均重新溶解. 从母材至焊核区,电位逐渐增加,相应地FSW接头上表面发生宏观平面电偶腐蚀,焊核区承担阴极电流,而母材部分则承担阳极电流. 在晶间腐蚀介质中,焊核区表面发生程度较轻的均匀腐蚀,母材部分发生深入基体内部的腐蚀.     

The relationship between the corrosion resistance and impurity content of aluminium oxide layers

In the last few years there has been an increased production and application of aluminium foil of under 20 μm thickness all over the world. Household foil is often stored in a wet environment, where the surface is only protected against corrosion by an aluminium oxide layer produced during manufacturing. In this study it has been shown that the aluminium-water vapour reaction is greatly affected by the composition and structure of the amorphous oxide layer on the surface and this depends on the composition of base metal and on the annealing of the foil. The incubation period of aluminium hydroxide nucleation is shortened and its growth rate accelerated by alkali and alkaline earth metal cations incorporated in the oxide layer. In particular the corrosion resistance is decreased by the presence of alkali (Li) and alkaline earth metal (Mg) cations in the oxide layers.     

A review of the theoretical modelling of crevice and pitting corrosion

A metal surface exposed to a corrosive environment may, under certain conditions experience attack at a number of isolated sites. If the total area of these sites is much smaller than the surface area then the metal is said to be experiencing localized corrosion. The rate of dissolution in this situation is often much greater than that associated with uniform corrosion and structural failure may occur after a very short period. Several different modes of localized corrosion may be identified. These are dependent on the type of metal undergoing corrosion and its environment at the time of attack. One of the most destructive forms is pitting corrosion which is characterized by the presence of a number of small pits on the exposed metal surface. The geometries of the pits depend on many factors such as the metal composition and the surface orientation. A similar mode of corrosion is crevice attack which occurs in situations where two or more surfaces in close proximity lead to the creation of a locally occluded region in which enhanced dissolution may occur. The rapidity with which localized corrosion can lead to the failure of a metal structure and the extreme unpredictability of the time and place of attack, has led to a great deal of study of this phenomenon. There are many difficulties associated with the experimental measurements necessary to understand the process and as a result a number of theoretical models of the various stages of localized corrosion have been developed. In this paper, a review of the literature on this modelling is presented with particular reference to crevice and pitting corrosion. The accuracy of the predictions of the models and the validity of the various approximations are discussed and both the achievements and the weak areas of the current state of corrosion modelling are demonstrated.     

The role of rusts in corrosion and corrosion protection of iron and steel

The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) are modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to elucidate the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, in the rusts, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the rate of air-oxidation at this pH is very fast, and that dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to the dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe²⁺ and OH⁻ ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH⁻ ions results in strongly acidic solutions (pH 1.41), causing acid-corrosion. It is proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, delaying the crevice- and acid-corrosion stages. Further, it is argued that iron compounds with negative charges due to the non-stoichiometric proportions of the lattice oxide ions and metal ions (solid oxoanions of iron) exhibit stable cation-selective permeability even with a drop in pH. Rust films including such compounds would stop the passage of aggressive anions and act to protect iron and steel.     

Effect of nitrogen on corrosion of duplex stainless steel weld metal

Abstract

A type 329Jl duplex stainless steel was gas tungsten arc welded without filler material in an Ar–N₂ gas mixture atmosphere with the aim of changing only the nitrogen content in the weld metal. The effect of nitrogen on the microstructure and corrosion properties of the weld metal was examined. An increase in nitrogen partial pressure increased the nitrogen content of the weld metal and brought reductions in the ferrite content and the quantity of Cr₂N nitride precipitates. Three corrosion parameters, namely, critical pitting temperature (CPT), pitting potential, and corrosion rate, were measured for weld metals having different nitrogen contents. The CPT and pitting potential increased and corrosion rate decreased with increasing nitrogen content of the weld metal. The corrosion behaviour was explained in terms of changes in microstructure and pitting index depending on the nitrogen content of the weld metal.     

焊后钎剂残渣腐蚀行为分析

采用扫描电子显微镜、电化学工作站等手段对同一型号不同药芯钎料钎剂残渣的腐蚀行为进行了系统研究分析。研究结果表明：钎料和基体在钎剂溶液中均发生了腐蚀,基体的腐蚀速率整体明显高于钎料的腐蚀速率;腐蚀速率随着时间的延长不断降低,但腐蚀并未停止。钎料组织越细,晶界越多,晶界处非规则排列的原子将发生较多腐蚀反应,耐腐蚀性下降。高温加速试验后Fe基体和镀锌层间出现微观裂纹,钎剂中强腐蚀性物质已渗透到界面层;在镀锌层区域,锌元素大量被氧化,形成了ZnO;在残留钎剂层区域,Fe,Zn元素含量较高,耐蚀性较差的Fe元素部分已扩散到残留钎剂层区域,形成铁的氧化物,同时又与NaCl等发生反应,形成FeCl3等物质。 创新点： 探明了钎剂残渣的腐蚀行为,明确其对钎焊接头可靠性的影响程度。     

The initiation of single corrosion sites on iron by focused laser illumination

An experimental method has been developed for selective depassivation of small regions on an otherwise passive metal surface with the use of a focused laser radiation. A nitrogen pumped dye laser provided a pulsed beam which was focused to a diameter of approximately 5 μm. The method was used to create corrosion sites on iron immersed in a solution of 0.1 M Na₂B₄O₇ containing NaCl in the range of 0.01 to 0.03 M. It was found that the corrosion rate was controlled by transport through a covering of corrosion products which precipitated over the site. The corrosion current from single sites increased with the square root of time, and with a proportionality factor that depended upon both the applied potential and chloride ion concentration.     

Prediction of corrosion behavior using neural network as a data mining tool

A supervised neural network (NN) method was used as a data mining tool to predict corrosion behavior of metal alloys. The NN model learned the underlying laws that map the alloy’s composition and environment to the corrosion rate. Existing corrosion data on corrosion allowable as well as corrosion resistive alloys were collected for both DC and AC corrosion experiments. The data mining results allow us to categorize and prioritize certain parameters (i.e. pH, temperature, time of exposure, electrolyte composition, metal composition, etc.) and help us understand the synergetic effects of the parameters and variables on electrochemical potentials and corrosion rates.     

Atmospheric corrosion of magnesium alloys AZ31 and AZ61 under continuous condensation conditions

This paper studies the corrosion rate of magnesium alloys AZ31 and AZ61 exposed in humid air under continuous condensation conditions. The shape of the gravimetric curves for corrosion progress suggests that the process is controlled by factors related with the corrosion product layer growing on the metallic surface according to gravimetric results there is an initial period in which only a small part of the corroded metal is incorporated in the corrosion product layer, but after longer testing times the proportion of metal that comes to form part of this layer tends to increase very significantly.