Possible scaling modes in high-temperature oxidation of two-phase binary alloys. II: Low oxidant pressures

The main possible modes of the high-temperature corrosion of binary two-phase alloys by a single oxidant under gas-phase pressures sufficient to corrode only the most-reactive alloy component are examined to compare their behavior with that of single-phase alloys. In the absence of important diffusion processes of the metal components in the alloy, the scale structures expected are different from those typical of single-phase alloys. Moreover, when diffusion in the alloy becomes important, these systems may develop an outer single-phase layer depleted in the most-reactive component, which may lead to different possible scale structures. The conditions for the transition between the various oxidation modes as well as the effect of the various parameters of kinetic, thermodynamic, and structural nature over the corrosion behavior of two-phase alloys are also examined.     

Oxidation of multicomponent two-phase alloys

The high-temperature corrosion behavior of two-phase alloys presents a number of differences compared to that of single-phase alloys. These differences are mainly a consequence of the limitations that the presence of two phases impose on the diffusion of the alloy components. In this review, it is shown that the exclusive scale formation of the more stable, slow-growing oxide is more difficult on a two-phase alloy, requiring a higher concentration of the more reactive alloy component than for a corresponding single-phase alloy. The main types of corrosion behavior for binary two-phase alloys are also considered, showing that if diffusion in the alloy is slow the scale structure will closely reflect that of the starting material. When diffusion in the alloy is not negligible, the scale structure becomes similar to what forms on single-phase alloys. The oxidation of two-phase ternary alloys is shown to be even more complex than the two-phase binary alloys. The principal added complexity compared to the binary alloys is that diffusion in the ternary alloys may also occur in the presence of two metal phases, as a result of an extra degree of freedom in the ternary system. The oxidation behavior of two-phase ternary alloys is discussed in the context of a number of recent experimental results.     

The internal oxidation of two-phase binary alloys beneath an external scale of the less-stable oxide

The internal oxidation of two phase binary A-B alloys by a single oxidant at high temperatures, under partial pressures sufficient to also form external scales of the less-stable oxide, is examined by means of quantitative models and compared with the corresponding behavior of single-phase alloys. It is shown that, depending on various factors, particularly on the solubility and diffusivity of the most-reactive component B in the most-noble component A, this process may or may not involve a diffusion process of the alloy components, leading to different scale morphologies. It is also concluded that even when the solubility and diffusivity of B in A are sufficiently high, so that the internal oxidation of the common type occurs, the restriction to the diffusion of B in the alloy due to its limited solubility affects the kinetics of internal oxidation, producing an increase of the rate of internal oxidation and of the critical concentration of B in the alloy required for the transition to the external oxidation of B with respect to single-phase alloys under the same values of all the relevant parameters. The lower the solubility of B in A, the larger these effects.     

The internal oxidation of two-phase binary alloys under low oxidant pressures

The main features of the internal oxidation in two-phase binary alloys are examined for insignificant and important diffusion of the most-reactive component and are compared with the behavior of corresponding single-phase systems. It is shown that two-phase alloys may have two different types of internal oxidation, one of which is similar to that of the single-phase alloys (classical type), producing a uniform distribution of small oxide particles in the zone of internal oxidation, while another is typical of two-phase systems and involves the in situ conversion of the most-reactive component into its oxide. It is also shown that, under the same values of all the relevant parameters, the classical internal oxidation of two-phase alloys involves faster kinetics and smaller degrees of enrichment of the most-reactive component in the zone of internal oxidation than for single-phase alloys. As a consequence of this, the transition to the external oxidation of the most-reactive component in these systems involves higher overall concentrations of the most-reactive component than in corresponding single-phase alloys.     

Corrosion Behavior of AZ31 Magnesium Alloy in Highly Alkaline Environment

Industrial components made with a magnesium-aluminum alloy AZ31 are often used in diverse engineering applications where component weight,strength,and durability are critical.Similarly as other Mg alloys,however,the AZ31 is susceptible to corrosion in alkaline environments.In this work,corrosion of commercial grade AZ31 alloy plate was examined in potassium hydroxide(KOH) using immersion and potentiodynamic studies.The results suggest that the concentration of Al and Mn in the alloy may govern the kinetics of micro-galvanic corrosion and the initiation of corrosion pits.Further,trace amounts of Ni in the AZ31 alloy were seen to enable formation of Ni(OH)2 surface layer,which may have further accelerated alloy corrosion due to its cracking and void coalescence.The effect of pH on the corrosion behavior of AZ31 was studied with reference to Pourbaix diagrams.     

The Criteria for the Transitions Between the Various Oxidation Modes of Binary Solid-Solution Alloys Forming Immiscible Oxides at High Oxidant Pressures

The possible high-temperature corrosion modes ofbinary solid-solution alloys forming two immisciblecompounds by a single oxidant include (1) the exclusivegrowth of external scales of the most-noble component, which may or may not be associated with theinternal oxidation of the most-reactive component, (2)the formation of composite external scales containing amixture of the two compounds, or finally (3) the exclusive growth of the most-stable compound asan external scale. The conditions for the stability ofeach scale structure depend on a number of thermodynamicand kinetics parameters, whose effects are examined quantitatively in this paper. Theconditions for the stability of the various structuresand the criteria for the transitions among them are alsoexamined. The maximum number of possible scale structures is four, but it can reduce to threeand, in some cases, only to two. In particular, theinternal oxidation of the most-reactive component maynot occur if the stabilities of the two oxides are not sufficiently different from eachother.     

Corrosion Performance Based on the Microstructural Array of Al-Based Monotectic Alloys in a NaCl Solution

The aim of this study is to compare the electrochemical behavior of three monotectic Al-based alloys (Al-Pb, Al-Bi, and Al-In) in a 0.5 M NaCl solution at room temperature. Two distinct microstructure arrays were experimentally obtained for each Al monotectic alloy by using a water-cooled unidirectional solidification system. Results of electrochemical impedance spectroscopy (EIS) plots, potentiodynamic polarization curves, and impedance parameters obtained by an equivalent circuit analysis are discussed. It was found that the Al-Pb alloy has lower corrosion current density, higher polarization resistance, lower relative weight, and cost than the corresponding values of Al-Bi and Al-In alloys. It is also shown that the electrochemical behavior of the three alloys examined are intimately correlated with the scale of the corresponding microstructure, with smaller droplets and spacings (i.e., cell and interphase spacings) being associated with a decrease in the corrosion resistance.     

CO2 High Temperature Corrosion and Its Prevention of Chromia Forming Fe-base Alloys

High temperature corrosion of chromia forming Fe-base alloys by CO_2 produces not only oxidation but also carburisation. The corrosion kinetics in CO_2-rich gas is found to be increased compared with that in air or oxygen. As a result, higher alloy chromium levels are required to achieve protective chromia formation in CO_2. Corrosion reaction mechanisms in CO_2 are examined and the internal carburisation of alloys in low carbon activity CO_2 gases are analysed based on the variation of p_(O_2) at the interface of oxide and metal. Carbon penetration through chromia oxide scale has been revealed by atom probe tomography. The strategies to resist CO_2 corrosion are reviewed by alloying of Si and/or Mn, forming additional diffusion barrier layers, and by adding sulphur to modify oxide grain boundaries to reduce carbon diffusion along the grain boundaries.     

Oxidation kinetics of a Pb-64 at.% in single-phase alloy

The solid-state oxidation kinetics of a Pb-64 at. % In (50 wt. %) single-phase alloy were studied from room temperature to 150°C using an AES (Auger Electron Spectroscopy) depth profiling technique. The general oxidation behavior of this alloy is different from that of a Pb-3 at.% In alloy but similar to that of a Pb-30 at.% In alloy. The oxide formed on this alloy is almost pure In oxide (In₂O₃) with the possible existence of some In suboxide near the oxide/alloy interface. At room temperature, oxidation of the alloy follows a direct logarithmic law, and the results can be described by the model proposed previously by Zhang, Chang, and Marcotte. At temperatures higher than 75° C, rapid oxidation occurred initially followed by a slower parabolic oxidation at longer time. These data were described quantitatively by the model which assumes the existence of short-circuit diffusion in addition to lattice diffusion in the oxide as proposed by Smeltzer, Haering, and Kirkaldy. The effects of alloy composition on the oxidation kinetics of (Pb, In) alloy are also examined by comparing the data for Pb-3, 30, and 64 at. % In alloys.     

The steady-State corrosion kinetics of two-phase binary alloys forming the most-stable oxide

The steady-state kinetics in the high-temperature oxidation of binary A-B alloys containing a mixture of the conjugated solid solutions of B in A (alpha phase) and A in B (beta phase) with exclusive formation of the most-stable oxide BO_v have been examined, assuming that the external scale grows on top of a subsurface layer of alpha phase. The results obtained are compared with the corresponding behavior of alloys which are single phase in the whole range of composition. Under identical values of all the parameters involved the concentration of B at the alloy-scale interface is smaller for two-phase than for single-phase alloys under the same concentration of B in the alloy as a result of the restricted flux of B through the alpha-phase layer. As a consequence of this, the two-phase alloys corrode more slowly than single-phase alloys and this difference increases as the solubility of B in the alpha phase decreases. Finally, the simultaneous formation of BO_v both externally and as internal oxide is more likely for two-phase than for single-phase materials.     

Hot Corrosion Behavior of a Ni3Al-Based IC21 Alloy in a Molten Salt Environment

Ni₃Al-based alloys have become important candidates for hot components in turbine engines, owing to their low densities and outstanding mechanical properties in service environments. The hot corrosion behavior of a Ni₃Al-based IC21 alloy in a molten salt environment of 75 wt% Na₂SO₄ and 25 wt% NaCl at 900 °C was studied, via oxidation kinetics analyses, scanning electron microscope observations and energy dispersive as well as diffraction analyses by X-ray. A multilayer corrosion oxide scale and dendritic morphology internal corrosion zone formed after hot corrosion, and inter-phase selective corrosion phenomena were also observed. Salt fluxing and oxidation-sulfidation processes were inferred to be the essential hot corrosion mechanisms of the alloy. Moreover, additions of Cr and Y proved to be beneficial to the hot corrosion resistance of the IC21 alloy, while the Mo content should be strictly controlled.     

An Approximate Analysis of the External Oxidation of Ternary Alloys Forming Insoluble Oxides. II: Low Oxidant Pressures

The construction and the properties of three-dimensional diagrams showing the regions of stability of the various compounds, which can form as a result of the oxidation of ideal ternary A–B–C alloys by a single oxidant at a constant temperature (kinetics diagrams) are examined for oxidant pressures insufficient to oxidize all possible alloys within the system (low oxidant pressures). For the calculation it is assumed that the various oxides do not dissolve into each other and do not form double oxides and that the alloy has an ideal behavior, while internal oxidation of the most-reactive components is disregarded. The range of meaningful oxidant pressures is divided into six intervals, which correspond to the formation of different types of scales. The simplified two-dimensional (2D) kinetics diagrams presented are obtained by projecting the appropriate three-dimensional (3D) lines of equilibrium between the alloy and the various oxides on the base triangle, which gives the composition of the system in terms of the three metal components only. The kinetics diagrams are correlated with the corresponding equilibrium phase diagrams for the same quaternary A–B–C–O systems.     

On the mechanism of high-temperature sulphur corrosion of binary alloys

Taking into account the phase composition and morphology of the scales three groups of alloys may be distinguished. The first of them contains alloys forming monophase scales which constitute solid solutions of sulphides of both alloy components (AgCu type). The mte of corrosion of these alloys is a monotonous function of their composition, and the mechanism of scale formation depends on the geometrical configuration of the reaction system. The second group includes alloys the components of which form sulphides of limited mutual solubility or sulphides having spinel structures (FeCr type); the scale may then be a mono- or double phase one and the exact structure is a function of the concentration of the alloying element. The metals belonging to the third group form mutually immiscible sulphides (CuZn type). The scales form double layers, the external layer being made up by sulphides of the base metal, while the internal layer is a heterophase mixture of the sulphides of both alloy components. The kinetics and the mechanism of the corrosion of these alloys is largely independent from the atmosphere (elemental sulphur or H₂? H₂S mixture).     

Thermodynamic characteristics and numerical modeling of internal nitridation of nickel base alloys

Kinetics and thermodynamics of the process of internal nitridation of various nickel‐base alloys have been investigated in oxygen‐free nitrogen atmospheres. Furthermore, the influence of formation and spalling of a protective oxide scale on the internal nitridation behavior of the alloys was studied by isothermal and cyclic oxidation tests in air. In general, nitridation kinetics of model nickel‐base alloys of the system Ni‐Cr‐Ti was found to obey a parabolic rate law indicating that the nitridation process is diffusion‐controlled. The temperature dependence of the nitridation rate constants is well described by an equation of the Arrhenius type. A thermodynamic calculation of the Ni‐Cr‐Ti‐Al‐N system was used to determine the nitrogen solubility in respective alloys as a function of temperature and alloy composition. The results show that a higher chromium content gives rise to an increase in the nitrogen solubility of Ni‐Cr‐Ti alloys leading to an increased nitridation rate in accordance with the experimental observations. From the calculated values for the nitrogen solubility, the diffusion coefficients of nitrogen were assessed using Wagner's classical theory of internal oxidation. A computer model of internal nitridation was developed that combines a commercial thermodynamic software (ChemApp) with a finite‐difference diffusion calculation. It was found that this model describes the internal nitridation process in reasonable agreement with the experimental results and allows to treat the case of simultaneous formation of different nitrides. The dependence of internal nitridation behavior on spalling and cracking of the oxide was incorporated into the simulation on the basis of simple assumptions showing that this calculation method successfully applies also to complex internal corrosion processes.     

Analysis of Pore Formation at Oxide–Alloy Interfaces—II: Theoretical Treatment of Vacancy Condensation for Immobile Interfaces

Void generation at scale–alloy interfaces during the oxidation of binary alloys as a result of matter transport through the scale and of diffusion of the metal components in the alloy is examined under a number of simplifying assumptions. In particular, it is assumed that the scale–alloy interface does not displace with time and that all the metal vacancies at this interface condense there to form voids, which amounts to calculating the maximum rate of interface void production. The analysis is developed both for the case of the parabolic rate law and for a general kinetics behavior, using the experimental information concerning the kinetics of weight gain. Application of this treatment to the oxidation of FeAl at 1000°C, taking into account the gradual transformation of the initial form of alumina into the stable alpha form, predicts the presence of a maximum in the overall void volume, in qualitative agreement with experimental observations. However, the overall volume of voids measured is much smaller than the values calculated under various conditions, suggesting that an effective mechanism of vacancy annihilation must be present at the scale–alloy interface.     

海洋工程用铝合金的腐蚀与防护研究进展

海洋工程用铝合金部件在服役环境下引发的点蚀、晶间腐蚀等已成为困扰机器装备使用寿命和稳定性的关键问题。目前,阴极保护、缓蚀剂、阳极氧化和保护涂层是针对海洋环境中铝合金腐蚀的常用防护措施。阐述了海洋工程装备常用的铝合金类型和使用场所,发现5系和6系铝合金是船舶制造和海洋平台搭建的首选材料,其中,具备优异力学性能、耐腐蚀性能的5系铝合金一般用来制作甲板、储存装置等大型主要承力构件。重点综述了铝合金在海洋大气区、浪花飞溅区、海水全浸区的腐蚀行为和腐蚀机制,经对比发现,与钢不同,铝合金在海水全浸区的腐蚀最严重,而在环境最恶劣的浪花飞溅区腐蚀损伤相对较轻;点蚀、晶间腐蚀是2种典型的铝合金腐蚀类型,同时应力腐蚀、微生物腐蚀也制约着铝合金在海洋工程领域的应用。最后分析了当前在海洋环境中对铝合金腐蚀防护采取的几种措施,指出工程实际中采用的防护方式为2种及2种以上措施的联合使用,并提出铝合金未来在失效行为分析、性能优化和涂层材料选择等方面的发展趋势,以期为研发在极端海洋环境下服役的铝合金及其防护材料提供参考。     

实海暴露黄铜脱锌腐蚀行为及抑制脱锌机理研究

采用金相－扫描电子显微镜和电子探讨Ｘ射线能谱成分分析方法研究了黄铅实海暴露脱锌腐蚀特征及抑制机理。通过对长期实海暴露试样的腐蚀产物和合金基体进行成分分析及形貌观察,研究表明加Ａｓ的Ｈ６８Ａ黄铜晶界优先发生脱锌腐蚀,而加Ｓｎ后,由于-了晶界,　大大提高了单相α黄铜(HSn70-1A)的耐蚀性能.而对双相HSn70-1A黄铜,添加Sn,可在一定程度上抑制脱锌,但并不足以阻止腐蚀沿β相之间的α相晶界的连通.     

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2 at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5at.% aluminum has been studied at 800 and 900°C under 1atm O₂. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.     

Corrosion behaviour of different hot rolled steels

The oxidation-corrosion behaviour of hot rolled alloys was examined by electrochemical impedance spectroscopy. The corrosion behaviour of the non-oxidised alloys was first determined in order to have a reference behaviour. Then, each alloy was oxidised for 1 and 3 days at 650 °C in air and its corrosion behaviour was also determined. For all the alloys, Fe₂O₃ was formed at the scale-gas interface. However, the Fe₂O₃ crystallographic structures varied as a function of the alloy composition. Differences in the corrosion behaviour are due to the thickness, the microstructure and the porosity of the scale. The new graphite chromium iron alloy (Hi-Cr + C) have an oxidation-corrosion behaviour close to the indefinite chill double paired (ICDP) one and is therefore thought to be a good candidate to replace the ICPD alloy. In the case of the high speed steel (HSS) alloy, the oxidation-corrosion kinetics are too slow to prevent sticking problems. The Co addition decreases the corrosion-oxidation rates for the non-oxidised and oxidised samples but this effect is limited in time.     

Factors affecting the high-temperature oxidation behavior of some dilute nickel- and cobalt-base alloys

Oxidation of nickel- and cobalt-base alloys, containing small additions of a higher valent second metal, in oxygen or air at high temperatures results in the formation of relatively complicated scale morphologies which change subtly with increasing additions of the second element and its characteristics. The various factors that can influence the oxidation behavior of such alloys are assessed and correlated with the oxidation kinetics and scale morphology types. For very dilute alloys the increase in oxidation rate compared with that of the corresponding pure metal (nickel or cobalt) is largely due to doping of the external oxides. However, once the solubility limit of the second metal in this oxide is exceeded, additional increases in second metal content of the alloy can either increase further or decrease the oxidation rate. The exact behavior depends on the relative interplay of factors such as internal oxide formation and coalescence, blocking effects of incorporated internal oxide or pores in the scale, short-circuit paths through the scale, doping, and the relative diffusion rates of the two metals in the scale. Probable rate-determining steps for oxidation of different alloy composition ranges are proposed.