Corrosion of austenitic stainless steels in hot concentrated aqueous NaOH solutions

When austenitic stainless steel is exposed to a hot concentrated aqueous NaOH solution, two corrosion products are formed at the surface: (i) nickel, building up as a superficial metallic layer, and (ii) mixed oxide of the type NaMO₂, finely dispersed in the underlying austenite. The electrochemical and crystallographic factors which govern the growth of these products have been studied. The thermodynamics and kinetics of their formation are discussed. The morphology of the mixed oxide small crystals and their orientation relationships with the metallic matrix are such that they may be liable to initiate cracks. This possibility of crack initiation by the corrosion products is presented as a working hypothesis for the extension of this study.     

Cathodic behaviour of zirconium and its alloys in concentrated hot sulphuric media

Potentiokinetic investigations show a rapid increase in the rate of corrosion of zirconium and its alloys in H₂SO₄ as its concentration rises above 70%. The rate of dissolution increases with temperature. The apparent activation energy is approximately 83.6 kJ/mol. The cathodic behaviour is modified as a function of the initial cathodic potential value and the length of time that it is applied. These modifications are interpreted through the relative positions of the elementary anodic and cathodic curves. A protective film, principally composed of ZrO₂ and ZrH₂, develops during the prolonged application of the initial cathodic potential.     

Mechanism and kinetics of coagulation of γ′-phase in nickel alloys at temperatures of hot plastic deformationin nickel alloys at temperatures of hot plastic deformation

It is known that hot plastic deformation of hard-to-deform complexly alloyed heat-resistant nickel alloys is easier to conduct at temperatures of the double-phase region than at those of the single-phase region. In heating such alloys before deformation and in the process of deformation cooling of them the amount of strengthening γ^′-phase and its sizes and distribution change, which exerts a substantial effect on their susceptibility to hot plastic deformation in the double-phase range. For this reason, knowledge of the laws of variation of the amount, dispersity, and distribution of the strengthening γ^′-phase in heating to the deformation temperature is very important, because it makes it possible to control these processes. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 11–13, March, 1997. Deceased.     

Investigation of titanium corrosion in concentrated NaOH solutions

The rate of both spontaneous and polarization assisted formation of oxide films on commercially grade titanium, DIN 3.7025, was studied in concentrated, 1–5M NaOH solutions, with particular attention to the early stages of oxide formation. Simultaneous chemical dissolution of oxides was considered. The rate of spontaneous oxide formation was examined with cyclic voltammetry as an in‐situ method. The shape of potentiodynamic curves recorded in different alkaline solutions indicated that the electrolyte concentration affects the oxide composition. Titanium electrodes were covered with oxide films of different thickness, and the time dependence of polarization resistance was measured under a fixed anodic polarization. The polarization resistance may either increase or decrease tending to attain a stationary value, which, for a fixed polarization, depends on both the initial thickness and the NaOH concentration.     

Effect of carbon on corrosion and passivation of iron in hot concentrated NaOH solution in relation to caustic stress corrosion cracking

Quenched Fe-C materials with up to 0.875 wt.% C were examined in 8.5 M NaOH at 100 °C to better understand the effect of carbon on caustic stress corrosion cracking (SCC) of plain steels. Carbon at contents up to about 0.23 wt.% C accelerated anodic dissolution of iron, whereas at high contents it hindered corrosion and promoted the formation of magnetite. It is suggested that carbon particles on the corroding surface form confined regions with an increased concentration of H⁺ and HFeO₂⁻, thereby favouring the formation of Fe₃O₄. Intergranular SCC can be explained by preferred anodic dissolution of grain boundary material enriched in carbon.     

Properties of fluoride film and its effect on electroless nickel deposition on magnesium alloys

The physical characteristics and microstructure of the fluoride film formed during activation were investigated using SEM,XPS and SAM,and its stability in electroless nickel(EN) bath was analyzed.The effects of the fluoride film on EN deposition were studied additionally.The results show that the fluoride film on magnesium alloys is a kind of porous film composed of MgF2 with thickness of 1.6-3.2 μm.The composition of the activation bath and pretreatment of EN processing have influence on the composition of the fluoride film.The fluoride is stable and dissolves little in EN bath;as a result,the fluoride film can protect magnesium substrate from the corrosion of EN bath.The composition of fluoride determines the initial deposition of EN and part of the fluoride film finally exists as inclusion in EN coating.     

XPS determination of compositions of alloy surfaces and surface oxides on mechanically polished iron-chromium alloys

Using the XPS technique, the compositions of the alloy surface and the surface oxide on mechanically polished iron-chromium alloys were studied. For quantitative analysis, the ratio of photo-ionization cross section of Fe $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons to that of Cr $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons was estimated as 1.35 for Al Kα_1,2 radiation. Assuming a homogeneous oxide layer, the thickness of the oxide layer and compositions of the alloy surface and the surface oxide were simultaneously determined. The results showed no enrichment of chromium or iron either in the oxide layer or the alloy surface. Results on binding energies were also given.     

碳纤维表面镍镀层的XPS分析

采用电镀法制备镀镍碳纤维,应用X射线光电子能谱(XPS)分析技术研究涂层化学成分、元素化学状态及其随镀层深度的变化.结果表明:镀层表面的镍被氧化成NiO;中间镀层由单质镍组成,同时吸附少量O2;在镀层与碳纤维的界面处形成Ni-C-O键,此化学键为镀层与纤维之间提供了强大的界面结合力.基于对镀层的XPS分析,探讨镍在纤维表面的沉积过程.     

Galvanostatic transient study of anodic film formation on iron in concentrated chloride media

Film formation on iron dissolving anodically in concentrated NaCl and FeCl₂ solutions was investigated galvanostatically in a diffusion cell allowing for uniform current distribution. Induction times for film formation were determined and results were compared to previously reported limiting current data obtained with a rotating disk electrode. In all solutions studied, anodic film formation was found to be induced by precipitation of ferrous chloride at the electrode surface. The surface concentration leading to precipitation was found to be higher than the saturation concentration indicating that supersaturation is required to initiate film formation.     

铅锡合金表面钝化层的XPS研究

采用x射线光电子能谱(XPS)对铅锡合金表面钝化层的组分与结构进行分析,目的是要确定钝化层中各元素的化学价态和存在形式,并试图明确锡对钝化膜导电能力的影响.结果表明:钝化膜具有分层结构,表层由SnO2,SnO,PbSO4,PbOx(1＜x＜2)和PbOx·Sn1-xO2组成,内层为SnO2,SnO,PbO,PbOx和PbOx·Sn1-xO2.内层钝化膜的导电性优于表层,锡能促进生成电阻远小于PbSO4和PbO的铅的过渡氧化物PbOx(1＜x＜2),从而提高其电导率.但是,内层的物质构成和分布并不均匀,出现了非导电区(极少量)和导电区并存的现象,而外层并没有类似情况发生.钝化膜自外向内含铅量和含锡量均呈上升趋势,含氧量呈下降趋势,锡的增值幅度随母体含锡量的减少而增大,但铅浓度的变化规律与之相反.     

Powder high-temperature materials based on nickel alloys for strengthening parts of hot zones of metallurgical units

A considerable part (up to 50%) of the cost of metallurgical production is determined by the repair cost of equipment and spare parts. In this connection the problem of increasing the wear resistance of rapidly worn parts of metallurgical units operating under extreme conditions under the effect of heating, stresses, aggressive media, and other harmful factors will always remain urgent. One of the ways for solving the problem consists in creating a protective layer with elevated wear and oxidation resistances on the functional surfaces of such parts. The present work is devoted, to the determination of the service life of heavily loaded parts of hot zones of continuous casting machines (CCM). Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 34–38, January, 1998.     

Effect of chromium content and sweep rate on passive film growth on iron-chromium alloys studied by EQCM and XPS

Growth of passive films on Cr and Fe-Cr alloys with Cr concentrations ranging from 15% to 54% was studied in situ with the electrochemical quartz crystal microbalance (EQCM). Mass change and current transients were measured as the response to a potential change in the passive region from 0 to 800 mV (SHE) in a pH 1.5 sulfate electrolyte. From these measurements, the thickness change as well as integral and differential growth fractions were calculated. The growth fraction expresses the fraction of oxidized metal that remains in the film. It was found that higher Cr contents gave thicker films and an increase in the growth fraction. The thickness change curves were compared to film growth models assuming rate control at either a film interface or in the film itself through high field conductivity. The integral growth fraction for the potential change experiment was found to vary approximately linearly with the Cr bulk composition. The fraction of dissolved Cr calculated from EQCM data matched well with previous solution analysis results.     

三水铝石在氢氧化钠溶液中的平衡溶解度热力学模型(英文)

针对拜耳法生产氧化铝过程中最为重要的NaAl(OH)4-NaOH-H2O体系,构建三水铝石溶解于氢氧化钠溶液的反应平衡常数的理论模型,并得出电解质NaAl(OH)4新的Pitzer模型参数和NaOH-NaAl(OH)4-H2O体系的热力学混合参数(温度范围298.15～373.15K)。将所得模型和参数对NaAl(OH)4-NaOH-H2O体系进行适用性分析。结果表明:将该理论模型用来计算三水铝石溶解反应的平衡常数是适用且准确的;电解质NaAl(OH)4的Pitzer模型参数β(0)(NaAl(OH)4)、β(1)(NaAl(OH)4)和CΦ(NaAl(OH)4),Al(OH)4-与OH-的二离子相互作用参数θ(OH-Al(OH)4-),以及Al(OH)4-与OH-和Na+的三离子相互作用参数ψ(Na+OH-Al(OH)4-)均与温度相关;当温度在298.15～373.15K时,预测三水铝石溶解于氢氧化钠溶液的平衡溶解度是可行的。     

Machining induced surface integrity in titanium and nickel alloys: A review

Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high reliability levels, surface integrity is one of the most relevant parameters used for evaluating the quality of finish machined surfaces. The residual stresses and surface alteration (white etch layer and depth of work hardening) induced by machining of titanium alloys and nickel-based alloys are very critical due to safety and sustainability concerns.This review paper provides an overview of machining induced surface integrity in titanium and nickel alloys. There are many different types of surface integrity problems reported in literature, and among these, residual stresses, white layer and work hardening layers, as well as microstructural alterations can be studied in order to improve surface qualities of end products. Many parameters affect the surface quality of workpieces, and cutting speed, feed rate, depth of cut, tool geometry and preparation, tool wear, and workpiece properties are among the most important ones worth to investigate. Experimental and empirical studies as well as analytical and Finite Element modeling based approaches are offered in order to better understand machining induced surface integrity. In the current state-of-the-art however, a comprehensive and systematic modeling approach based on the process physics and applicable to the industrial processes is still missing. It is concluded that further modeling studies are needed to create predictive physics-based models that is in good agreement with reliable experiments, while explaining the effects of many parameters, for machining of titanium alloys and nickel-based alloys.     

Characterization of air-formed surface oxide film on Ti-29Nb-13Ta-4.6Zr alloy surface using XPS and AES

The surface oxide film on a Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) was precisely characterized using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) to understand the composition and chemical state of the surface oxide film of TNTZ. For comparison, the component metals, titanium, niobium, tantalum, and zirconium, were also characterized to consider the effect of those on the formation of the surface oxide film on their alloy. The characterization of the surface oxide films on TNTZ and its component revealed the following issues. The surface oxide film on TNTZ consists of a composite oxide that contains titanium, niobium, tantalum, and zirconium but forms continuous layer and is very thin, ca. 3.7 nm. The oxide film is not completely oxidized because it contains various valences of cations. In particular, the oxidation of tantalum is inhibited in the oxide. Tantalum is enriched in the substrate in TNTZ just under the surface oxide because of this inhibition in the oxidation. The formation of the surface oxide film in TNTZ is predominantly governed by titanium. The preferential oxidation of an element is not always dependent on the initial oxidation potential of that element, the relationship between the oxidation energy from a smaller valence to a larger valence, and the dehydration process. In other words, a complicated competition governs the resultant composition of surface oxide.     

The effect of surface deformation on the hot corrosion behavior of some nickel-chromium alloys

The resistance of some NiCr alloys to Na₂SO₄-induced hot corrosion at 900°C has been found to be strongly affected by surface treatments such as grit-blasting and mechanical indentation. These cold-work procedures, when followed by recrystallization anneal, produced a large number of new grain boundaries in the deformed region. When deformed-and-annealed specimens of NiCr, Ni15 Cr and IN 738 were placed directly in contact with molten Na₂SO₄, the grain boundaries facilitated sulphur penetration and enhanced metal wastage. The deleterious effects of mechanical treatments could, in the case of Ni15 Cr, be offset by suitable thermal treatments such as pre-oxidation prior to Na₂SO₄ exposure.