Corrosion behavior of thermal-sprayed WC cermet coatings in SO4 2− environment

A series of the electrochemical and long-term corrosion tests was carried out in a 3.5 wt% Na2SO4 solution on thermal-sprayed WC-17Co and WC-10Co-4Cr cermet coatings in order to examine the effect of composition of binder materials on the corrosion behavior. The results reveal that the overall corrosion resistance of the WC-17Co coating is inferior to that of the WC–Co–Cr coatings due to the corrosion of binder materials which induce WC particles to fall off. CoO and WO3 oxide films form on the surface of WC-17Co coating in Na2SO4 solution electrochemical corrosion process, which will protect the coating in the process of corrosion. Cr2O3 oxide film formed on the WC-10Co-4Cr coating surface has a strong hindered role to corrosion. The corrosion mechanism of WC-17Co coating in Na2SO4 solution is entire corrosion of Co matrix, while it is film-hole corrosion mechanism for WC-10Co-4Cr coating.     

The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water

Oxide films formed on Alloy 690 exposed to 290 °C water containing 3 ppm O₂ were investigated. It was found that Cr rich oxides form initially through solid-state reactions. Ni–Fe spinels gradually develop on surface layer by precipitation with increasing immersion time. Initially formed Cr rich oxides react with outwards diffusing Ni and Fe to form small spinel particles which then vanish gradually. An inner layer develops from oxide/matrix interface through inward diffusion of oxidant. Cr is preferentially oxidized and tends to dissolve into solution. The resultant inner layer consists of predominant NiO which cannot serve as a protective barrier layer.     

The influence of alloying elements on the chlorine-induced high temperature corrosion of Fe-Cr alloys in oxidizing atmospheres

The effect of the alloying elements Al, Cr, Mn, Mo, Si and Ti on the corrosion behaviour of ferritic Fe-15Cr model alloys was studied in a N₂/He-5 vol.% O₂ gas mixture with and without additions of 500–1500 vppm HCl at 600°C. The main corrosion mechanism is “active oxidation”, characterized by the formation of volatile metal chlorides at the metal/oxide interface. Volatilization and subsequent conversion of the chlorides into oxides results in the formation of porous and poorly adherent oxide scales. Large mass gains were observed for Fe-15Cr, Fe-35Cr and Fe-15Cr with additions of 5 wt.% Ti, 10 wt.% Mn or 10 wt.% Mo. The specific morphology of the corrosion products depends strongly on the alloying elements. For the Fe-Cr alloys, a model for the formation of the scales, which are characterized by alternating dense and porous layers, is presented. The addition of 5 wt.% Si or Al to Fe-15Cr leads to much better corrosion resistance by the formation of protective Cr₂O₃/Al₂O₃-layers, however in the case of Al addition the behaviour depends strongly on the experimental conditions, as surface treatment and flow velocity. In Fe-15Cr-10Mo preferential removal of the more reactive metals Fe and Cr was observed resulting in a Mo-enriched porous metal zone underneath the metal-oxide interface. The effect of carbon on the corrosion behaviour was examined by addition of 0.3–0.8 wt.% C to the model alloys. Cr-rich M₂₃C₆-carbides were attacked preferentially while Mo-rich M₆C-carbides are very stable relative to the matrix and the attack occurs in regions surrounding the carbides.     

Oxidation Behavior of Ni–Cr–Fe-Based Alloys: Effect of Alloy Microstructure and Silicon Content

The oxidation behavior of Ni–Cr–Fe-based alloys in a low oxygen partial pressure atmosphere (H₂–H₂O) was investigated in terms of the effect of alloy microstructure and their silicon content. It was found that the formation and growth kinetics of the oxide scale are rather sensitive to the alloy microstructure and their corresponding Si contents. Oxide ridges were found to form in areas with eutectic structure, while a thin and homogeneous oxide scale formed on austenite matrix. The thicknesses of the oxide ridges and the oxide layer on the austenite matrix were dependent of their corresponding Si contents. The austenite/carbide phase boundaries in eutectic structure can offer fast diffusion paths for metal outward diffusion, which leads to the formation of ridge-like oxide features. The continuous SiO₂ sub-layer formed at the oxide scale/metal interface on the austenitic matrix acted as an effective diffusion barrier to metal outward diffusion, resulting in rather thin and uniform oxide scales.     

The high temperature oxidation of 11% chromium steel: Part I – Influence of pH2O

The oxidation of 11% Cr steel (X20 11Cr1MoV) in the presence of dry O₂ and O₂ + 10 and 40% H₂O was investigated at 600°C. The exposure time was between 1 and 672 hours. The oxidized samples were investigated by a number of surface analytical techniques including GI‐XRD, SEM/EDX, GDOES and Auger spectroscopy. X20 steel (11Cr1MoV) forms a protective chromium rich α‐(Cr,Fe)₂O₃ oxide in dry O₂ at 600°C. In mixtures of oxygen and 10 or 40% H₂O, at the same temperature, the material is affected by chromium vaporization because of the formation of CrO₂(OH)₂(g). The loss of chromium tends to deplete the oxide in chromium. The formation of a more iron‐rich oxide may result in a loss of the protective properties of the oxide scale. The loss of chromium and the tendency to destabilize the protective oxide increases with the concentration of water vapour. The material suffers breakaway corrosion after 336 hours in an O₂/H₂O (60/40) mixture while the rate of oxidation is only marginally increased in the presence of 10% H₂O. The thick oxide formed in O₂/H₂O (60/40) environment features an inner layer consisting of FeCr spinel and an outer layer which is almost pure hematite.     

TEM and ToF-SIMS studies on the corrosion behavior of vanadium and chromium containing WC-Co hard metals in alkaline solutions

The corrosion behavior of hard metals with VC and Cr₃C₂ grain growth inhibitors was investigated in alkaline solutions by electrochemical methods. The two inhibitors have opposite effects on the corrosion behavior: Cr₃C₂ significantly improves the corrosion behavior, whereas VC-containing alloys show a poor resistance. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and analytical transmission electron microscopy (TEM) analyses of the distributions of Cr and V in the composite material, as well as in the surface layers formed during corrosion were employed to clarify the influence of these elements on the corrosion behavior. The measurements showed that VC is precipitated mostly along the WC/binder interface after the liquid-phase sintering process, while Cr₃C₂ is almost homogeneously dissolved in the binder. As VC is chemically instable in alkaline solutions, it completely dissolves out of the binder. In accordance with this observation no V was found in the corrosion product layer on the surface. As WC is more noble than Co, galvanic coupling between the two phases reinforces the Co dissolution, while the WC-phase is protected cathodically. Contrary to VC, chromium is stable in alkaline environments and forms a passivating Cr₂O₃ layer. Enrichment of Cr in the corrosion product layers was detected by TEM and ToF-SIMS. Due to surface passivation by Cr₂O₃, galvanic coupling effects between Co and WC play a much less important role in the corrosion process of the composite material.     

Passivation of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys in 1 M H2SO4 and 1 M NaOH solutions

The corrosion mechanisms of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys have been investigated by potentiodynamic and potentiostatic polarization. Very good passivation of the Al–Cr–Fe surface is exhibited from 1 M H₂SO₄ to 1 M NaOH solutions, which was confirmed by ICP-OES analysis over a period of 273 days. Potentiostatically formed passive films analysed by XPS revealed chromium enrichment in the outermost layer of the aluminium oxy-hydroxide film. Although Al–Cu–Fe–Cr showed passivation during potentiodynamic polarization, heavy active corrosion at OCP was revealed by ICP-OES. For the Al–Cu–Fe–Cr alloy, the 10% content of Cr is insufficient to maintain a protective “chemically stable” Cr oxide/hydroxide.     

The corrosion behavior of Fe-Cr alloys containing Co,Mn, and/or Ni and of a Co-base alloy in the presence of molten (Li,K)-carbonate

The corrosion behavior or commercial Fe ana Co base alloys and Fe-Cr model alloys with different contents of Co and/or Mn was investigated by continuous exposure tests in the presence of a thin carbonate film. All alloys studied form multi-layered corrosion scales consisting of outer Li containing oxides and inner Cr rich oxides, i.e. spinels or LiCrO₂. The LiCrO₂ is formed on alloys with high Cr contents (≤ 20 wt.%), whereas mixed (Fe,M)_3-x Cr_xO₄ spinels (M = Co, Mn, Ni) were found on alloys with lower Cr content (15–20 wt.%). Insoluble Cr containing oxides occur only in the inner layers of the corrosion scale, whereas on the surface of corroded specimens soluble chromates were detected. Alloys with Mn contents greater than 15 wt.% form Mn₂O₃ in the initial stages of the experiments, this oxide reacts with the melt and formation of Li₂MnO₃ takes place. In exposure tests up to 500 h Fe-Cr alloys with low contents of Mn and Co (10 wt.% Co or Mn) form iron rich oxides (LiFeO₂ and LiFe₅O₈) with varying amounts of dissolved Mn or Co. In the later corrosion stages outward diffusion of Mn and/or Co takes place and LiCoO, and Li₂MnO₃ are formed on top of LiFeO₂, whereby the concentration of Mn and/or Co in the inner layers (LiFeO₂ and spinel) decreases. The outer Li containing oxides LiFeO₂, LiCoO₂ and Li₂MnO₃ are nearly insoluble in the melt and when present at the surface protect the metallic material from further corrosive attack. Fe-Cr model alloys containing Co and Mn form multi-layered corrosion layers after 2000 h of exposure. These layers consist of four oxides in the following sequence from the metal-scale to the scale-melt interface: (Fe,Cr,Co,Mn)₃O₄ spinel, LiFeO₂, Li₂MnO₃ and LiCoO₂.     

Effect of Mn/Mo incorporated oxide layer on the corrosion behavior of AA 2024 alloy

The oxide layer formed over AA 2024 using 10 wt.% H₂SO₄ (plain oxide, PO) was modified by Mn/Mo oxyanions (permanganate/molybdate modified oxide, PMMO) as an alternative to Cr(VI) ions to enhance the corrosion resistance. The corrosion current density values obtained for PMMO was found to be 2.8% and 1.4% of hydrothermal treated oxide (HTO) and PO respectively after 168 h immersion in 3.5% NaCl solution. The electrochemical studies showed the higher barrier layer resistance for PMMO. The improved corrosion behavior of PMMO was observed based on the damage function calculated. Similar observations were confirmed by continuous salt spray test.     

Corrosion behavior of Ni-Cr weld overlay alloys with dispersed carbide particles in sodium chloride solution

Ni-Cr weld overlay alloys with dispersed carbide particles were prepared by a plasma transferred arc welding process. The corrosion behavior of the 40 vol% carbide/Ni-Cr materials was studied in sodium chloride solution by electrochemical techniques, scanning electron microscopy, x-ray diffraction, and electron dispersive spectroscopy. The materials under investigation were powder blends of Ni-50 mass % Cr powder mixed with NbC, TaC, TiC, WC, O₃C₂, or VC. The NbC, TaC, TiC, and WC particles partially dissolved in the molten alloy, causing the crystallization of M₂₃C₆. Blends with Cr₃C₂ and VC particles produced the crystallization of M₇C₃, V_xCr_yC_z and M₂₃C₆. The VC/Ni-Cr alloy combination caused preferential localized corrosion and the carbide remained as V_xCr_yC_z. The Cr₃C₂/Ni-Cr alloy combination became studded with the Cr-Cl corrosion product compound. With the WC/Ni-Cr alloy combination, the WC particles became oxidized and corrosion product spread over the matrix. Other carbides were not oxidized, but the matrix of the alloys with these dispersed particles did become covered with Cr-rich corrosion products. The NbC/Ni-Cr material exhibited the best corrosion resistance.     

CLAM钢焊缝在550℃流动的铅铋合金中的腐蚀行为

为了研究中国低活化马氏体(CLAM)钢TIG焊缝在流动的铅铋共晶合金(LBE)中的腐蚀行为,对CLAM钢TIG焊缝及母材在550 ℃,不同相对流速(1.70,2.31,2.98 m/s)的LBE中进行1 500 h的腐蚀试验. 结果表明,腐蚀试样表面均存在双层结构的氧化层,外氧化层由疏松的Fe₃O₄组成,内氧化层由致密的(Fe, Cr)₃O₄组成;随着LBE相对流速的增加,提高了CLAM基体材料中的Fe,Cr元素向LBE中的溶解速率和LBE中的O元素向CLAM基体材料中的扩散迁移速率,加剧了试样表面的腐蚀程度,最终导致试样表面氧化层的厚度不断增厚;经过相同条件的腐蚀试验后,CLAM钢焊缝试样的抗腐蚀性能比母材试样低.     

Cr3C2-NiCr and WC-Ni thermal spray coatings as alternatives to hard chromium for erosion-corrosion resistance

Cr₃C₂-NiCr and WC-Ni coatings are widely used for wear applications at high and room temperature, respectively. Due to the high corrosion resistance of NiCr binder, Cr₃C₂-NiCr coatings are also used in corrosive environments. The application of WC-Ni coatings in corrosive media is not recommended due to the poor corrosion resistance of the (pure Ni) metallic matrix. It is well known that the addition of Cr to the metallic binder improves the corrosion properties. Erosion-corrosion performance of thermal spray coatings is widely influenced by ceramic phase composition, the size of ceramic particles and also the composition of the metallic binder. In the present work, two types of HVOF thermal spray coatings (Cr₃C₂-NiCr and WC-Ni) obtained with different spray conditions were studied and compared with conventional micro-cracked hard chromium coatings. Both as-sprayed and polished samples were tested under two erosion-corrosion conditions with different erosivity. Tungsten carbide coatings showed better performance under the most erosive condition, while chromium carbide coatings were superior under less erosive conditions. Some of the tungsten carbide coatings and hard chromium showed similar erosion-corrosion behaviour under more and less erosive conditions. The erosion-corrosion and electrochemical results showed that surface polishing improved the erosion-corrosion properties of the thermally sprayed coatings. The corrosion behaviour of the different coatings has been compared using Electrochemical Impedance Spectroscopy (EIS) and polarization curves. Total material loss due to erosion-corrosion was determined by weight loss measurements. An estimation of the corrosion contribution to the total weight loss was also given.     

Development of boron white cast iron

Abstract

With boron substituting for carbon in cast iron composition and eutectic borides substituting for eutectic carbides in microstructure as the hard wear resistant phase, a new kind of wear resistant white cast iron has been developed. The microstructure and mechanical properties of this new white cast iron both in the as cast state and after appropriate heat treatments were studied. The results show that the as cast microstructure of the boron white cast iron comprises a dendritic matrix and interdendritic eutectics, and the eutectic compound is that of M₂B or M′_0˙9Cr_1˙1B_0˙9 type, where M represents Fe, Cr or Mn and M′ represents Fe or Mn. The morphology of the eutectic borides is much like that of carbide in high chromium white cast iron, but the hardness of boride is higher than that of carbide. The matrix in as cast microstructure comprises martensite and pearlite. After austenitising and quenching, the matrix mostly changes to lath type martensite and the eutectic borides remain unchanged. In addition, two different sizes of particles, with different forming processes during heat treatment, appear in the matrix. The boron white cast iron possesses higher hardness and toughness than conventional white cast iron and nickel hard white cast iron, and has a better balance between hardness and toughness than high chromium white cast iron.     

Chemical mechanism of chemical mechanical polishing of tungsten cobalt cemented carbide inserts

To explore the chemical mechanism of tungsten‑cobalt cemented carbide inserts in H₂O₂-based polishing fluid. Before and after the YG8 cemented carbide inserts were corroded, surface phase, element and structure were characterized by XRD and SEM/EDS. The chemical mechanism of tungsten‑cobalt carbide inserts during chemical mechanical polishing (CMP) was analyzed. XPS was utilized to analyze the corrosion products formed on the surface of YG8 cemented carbide inserts during chemical reaction to determine the chemical reaction equation. In the H₂O₂ environment, the electrode potential of the Co layer at the boundary between the binder phase with larger crystal domains and the hard phase is greater than the electrode potential of the intermediate layer γ(Co-W-C solid solution) phase and WC, which creates a potential difference between the three, and occurs galvanic corrosion. The hard phase WC is protected as the cathode of the entire battery and has a tendency to stabilize. The Co layer at the phase boundary is the most anode feature to be corroded and dissolved first. The γ phase of the intermediate layer serves as the secondary anode feature and serves also as the cathode of the Co layer. When the Co layer at the phase boundary is corroded to a certain extent, a galvanic couple is formed between the γ phase and the testing phase WC to cause corrosion. In addition, the binder phase with smaller crystal domains directly forms galvanic corrosion with WC. The chemical products created on the surface of the blades are Co₃O₄ and WO₃. However, Co₃O₄ and WO₃ oxide films are small in size and have little effect on material removal during polishing. When the binder phase corrosion on the blades surface reaches a critical point, the stress exerted by the polishing abrasive is basically concentrated on the WC particle surface. The strength of the WC particles that have lost the supporting effect of the binder phase becomes low and the structure becomes brittle. Under the mechanical scratching and compressive stress of the abrasive particles of the polishing solution, the smaller WC particles are directly pulled out. The surface layer of the larger WC particles is broken into WC grains, and then the surface layer is mechanically removed.     

Initial stages of the high-temperature corrosion of commercial Fe-Cr-Ni alloys in a carburizing atmosphere of low oxygen partial pressure

The early stages of corrosion of AISI 314, HK 40, and Alloy 800H have been studied in a strongly carburizing (a_C=0.8), weakly oxidizing atmosphere at 1098 K. Samples with electropolished and cold-worked surfaces were exposed for up to 400 min. at temperature, in a conventional corrosion rig or in a reaction vessel which was installed within an X-ray photoelectron spectrometer. The latter facility allowed the effects of the specimen heating rate and the rat of gas flow to be investigated. Examination of the corrosion products was accomplished with the aid of XPS, SEM, TEM, and conventional metallography. Initially, surface layers comprised of -Cr₂O₃, (Mn, Cr)₃O₄, and SiO₂ formed, with layer structure, microstructure, and composition being functions of alloy composition and surface condition. Only on the cold-worked surfaces did a well-developed duplex oxide, consisting of an outer, Cr-rich oxide layer and an inner, SiO₂ layer, form. In good agreement with the predicted value of 1.9 wt.%, between 1.4 and 2 wt.% Si in the alloy was required to form a complete SiO₂ layer. After an incubation period, -Cr₂O₃ became unstable and transformed to M₇C₃; the carbides then grew by diffusion of metal from the alloy substrate. The presence of manganese, as (Mn, Cr)₃O₄, in the surface oxide influenced the mode of carbide growth, whereas the rate of carbide growth was severely suppressed by a continuous SiO₂ layer which acted as a diffusion barrier both to metal and to carbon. It is argued that the SiO₂ layer is most effective in reducing carburization when it is free from or contains very few structural defects.     

ESCA study of oxidation and hot corrosion of nickel-base superalloys

A study of the high-temperature oxidation and Na₂SO₄-induced hot corrosion of some nickel-base superalloys has been accomplished by using ESCA to determine the surface composition of the oxidized or corroded samples. Oxidation was carried out at 900 or 1000°C in slowly flowing O₂ for samples of B-1900, NASA-TRW VIA, 713C, and IN-738. Oxidation times ranged from 0.5 to 100 hr. Hot corrosion of B-1900 was induced by applying a coating of Na₂SO₂ to preoxidized samples, then heating to 900° C in slowly flowing O₂. Corrosion times ranged from 5 min to 29 hr. For oxidized samples, the predominant type of scale formed by each superalloy was readily determined, and a marked surface enrichment of Ti was found in each case. For corroded samples, the transfer of significant amounts of material from the oxide layer to the surface of the salt layer was observed to occur long before the onset of rapidly accelerating weight gain. Some marked changes in surface composition were observed to coincide with the beginning of accelerating corrosion, the most striking of which were a tenfold decrease in the sulfur to sodium ratio and an increase in the Cr(VI) to Cr(III) ratio. Supported by NASA Grant No. NSG-3009     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiCp)

The influence of silicon carbide (SiCp) proportion and matrix composition on four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) immersed in 1-3.5 wt% NaCl at 22 °C was investigated by potentiodynamic polarization. The kinetics of the corrosion process was studied on the basis of gravimetric measurements. The nature of corrosion products was analysed by scanning electron microscopy (SEM) and low angle X-ray diffraction (XRD). The corrosion damage in Al/SiCp composites was caused by pitting attack and by nucleation and growth of Al₂O₃ · 3H₂O on the material surface. The main attack nucleation sites were the interface region between the matrix and the reinforcement particles. The corrosion process was influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement and saline concentration.     

Interfacial reactions between molten Al and a Co–Cr–Mo alloy with and without oxidation treatment

The effect of the formation of an oxide layer on the interfacial reactions between a Co–Cr–Mo alloy (CCM alloy) and molten Al is investigated. In the absence of an oxide layer, the CCM alloy reacted rapidly and an interfacial two-layer structure formed between the CCM alloy and molten Al. Upon oxidation, a uniform (Cr,M)₂O₃ (M: doped elements) oxide layer formed on the CCM alloy surface, which effectively inhibited the reaction between the CCM alloy matrix and molten Al for more than 2 h.     

The corrosion behaviour of WC-VC-Co hardmetals in acidic media

The effect of increasing vanadium carbide (VC) content on the corrosion behaviour of tungsten carbide - 10 wt% cobalt hardmetals was investigated in 1 M hydrochloric (HCl), and sulphuric (H₂SO₄) acids solutions. Increasing VC content makes the open circuit potential (OCP) in the test solutions more negative than the base alloy. Specimens exhibited pseudo passivation in all the test solutions. Increasing VC led to decreasing corrosion current density. However, the corrosion current densities during chronoamperometric tests were lower for 0 wt% VC. XRD and Raman spectroscopy showed that hydrated WO₃ formed in the surface films of all specimens in hydrochloric acid (HCl), while hydrated vanadyl sulphate also formed for higher VC content specimens in sulphuric acid (H₂SO₄).