Effect of rapid solidification on the microstructure and corrosion behaviour of Al-Zn-Mg based material

The corrosion behaviour of Al-5Zn-3Mg-0.6Cu-0.8Zr-0.25Cr-0.15Ni-0.15Ti alloys, produced by traditional and powder technologies, with similar thermo-mechanical treatments, in 3% sodium chloride solution, has been examined by electrochemical methods, scanning electron microscopy, transmission electron microscopy and X-ray microanalysis. The alloys reveal similar precipitation but of different shape, size and distribution; further, both alloys experience localized corrosion. Copper-rich precipitates initiate the dissolution of surrounding particles, enriched in Zn and Mg. As a result, the surface is enriched with other alloying elements after a full polarisation run. Cast material has lower corrosion properties because of the higher heterogeneity of the structure. The structure heterogeneity of the cast material involves a more non-uniform distribution of the precipitates, larger Zn- and Mg-rich particles, and depletion of the matrix and areas around the precipitates by alloying elements compared with the powder material.     

Pitting corrosion in cast 7XXX aluminium alloys and fibre reinforced MMCs

The corrosion behaviour of cast and heat-treated Al-6%Zn-1%Mg and Al-6%Zn-1%Mg-1%Ag alloys and metal matrix composites (MMCs) were investigated using dynamic polarisation techniques, followed by scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The addition of silver and the incorporation of continuous Altex fibres into the alloys have a positive effect on the materials’ corrosion resistance. It was observed that due to the presence of second phase particles and microsegregation, pitting occurs preferentially at the grain boundary and fibre/matrix interface regions in the cast alloys and composites, respectively.     

Pitting corrosion of spray formed Al-Li-Mg alloys

The pitting corrosion behaviour of two new spray formed alloys, designated OX24 and OX27, has been compared with spray formed AA7034 and more corrosion resistant AA5083 by polarisation tests. The new alloys have been designed for use in the aerospace industry where good corrosion resistance and specific mechanical properties are required. OX24 and AA5083 had good corrosion resistance; OX27 pitted immediately showing poor corrosion resistance. However, both OX24 and OX27 show better corrosion resistance than AA7034. Scanning electron microscopy showed that corrosion was associated with intermetallic particles, except in OX24 where initiation appeared not to be associated with the Zr-rich intermetallics.     

Corrosion behaviour of several aluminium alloys in contact with a thermal storage phase change material based on Glauber’s salt

This article presents the results of a study about the corrosion behaviour of four aluminium alloys (EN AW 2024, 3003, 6063, and 1050) in contact with a commercial thermal storage material based in the Glauber’s salt (Na₂SO₄·10 H₂O). Results indicate that the Al 2024 alloy is not compatible with this material due to the extense formation of NaAlCO₃·(OH)₂ in contact with air. The aluminium alloys 3003, 6063 and 1050 showed to be fully compatible with the material.     

Electrochemical behaviour of amorphous and nanoquasicrystalline Zr-Pd and Zr-Pt alloys in different environments

The corrosion behaviour of melt spun amorphous and nanoquasicrystalline Zr₇₀Pd₃₀ and Zr₈₀Pt₂₀ alloy ribbons has been investigated using potentiodynamic polarization study in NaCl, H₂SO₄ and NaOH solutions at different concentrations. The amorphous and nanoquasicrystalline alloys show better corrosion resistance than Zr in all the solutions studied. Both the alloys are susceptible to chloride attack and pitting has been observed. Complete passivation has been observed in H₂SO₄, while gradual break down of passivating layer occurs in NaOH. In general, nanoquasicrystalline state in both the alloys shows better corrosion resistance than amorphous state in all the solutions studied.     

The effect of molybdenum on the corrosion behaviour of the high-entropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments

The purpose of this study is to investigate the electrochemical properties of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x high-entropy alloys in three aqueous environments which simulate acidic, marine, and basic environments at ambient temperature (∼25 °C). The potentiodynamic polarisation curves of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x alloys, obtained in aqueous solutions of H₂SO₄ and NaOH, clearly revealed that the corrosion resistance of the Mo-free alloy was superior to that of the Mo-containing alloys. On the other hand, the lack of hysteresis in cyclic polarisation tests and SEM micrographs confirmed that the Mo-containing alloys are not susceptible to pitting corrosion in NaCl solution.     

An electrochemical study of repassivation of aluminium alloys with SEM examination of the pit interiors using resin replicas

The pitting and repassivation behaviour of two aluminium alloys, AA5083 and spray formed Al-Mg-Li were investigated. The alloys did not exhibit classic repassivation on the back scan instead showing a transition to high current, potential independent behaviour. A series of experiments were performed to establish a clearer understanding of the mechanisms occurring in this region. The switch from pit growth to one-dimensional tunnelling is now believed to cause the transition. This is supported by SEM observation of tunnels using a resin replica technique. Tunnels were seen after the transition, but there was no evidence that tunnels were present before the transition.     

Effect of temper on the distribution of pits in AA7075 alloys

A comparative study of pitting severity in T6 and T73 tempers of AA7075 in 0.1 M NaCl is reported here. Pitting was more severe for T6 tempers compared to T73 tempers. This could be attributed to higher pit nucleation sites in the former. The statistical distributions for pit areas indicated nucleation saturation in T73. The extreme value distributions showed differences for the two tempers. These differences in pit distribution are explained on the basis of the role of constituent as well as strengthening particles as well as the role of grain boundary η precipitates.     

Corrosion behaviour of friction stir welded AA7108 T79 aluminium alloy

The corrosion behaviour of a friction stir welded AA7108 T79 aluminium alloy has been investigated using accelerated testing (ASTM G34 EXCO) and electrochemical measurements. The welded alloy showed the expected zones associated with friction stir welding, namely nugget, thermomechanically affected zone and heat affected zone. Corrosion testing revealed that the edge regions of the thermomechanically affected zone were most susceptible to corrosion. The localized corrosion occurs intergranularly due to the non-uniform distribution of η/η′ (MgZn₂) precipitates within the thermomechanically affected zone.     

Assessing the kinetics and mechanisms of corrosion of cast and HIPed Stellite 6 in aqueous saline environments

Cobalt-base (Stellite) alloys have seen extensive use in wear environments mainly due to their high strength, corrosion resistance and hardness. Co-base superalloys rely primarily on carbides, formed in the Co matrix and at grain boundaries, for their strength and the distribution, size and shape of carbides depends on processing conditions. Currently use of Stellite alloys has extended into various industrial sectors (e.g. pulp and paper processing, oil and gas processing, pharmaceuticals, chemical processing) and the need for improved information regarding corrosion (and often tribo-corrosion) of Stellite alloys has increased. It has been recognised that processing changes, which affect the microstructure of Stellite alloys, will most probably affect the corrosion performance.In this paper the corrosion behaviour of Stellite 6 in the as-cast and the HIP consolidated forms has been compared and contrasted using DC electrochemical techniques in static saline conditions. It has been shown that there is a significant difference in the corrosion performance of HIP consolidated Stellite 6 and it is possible to link the corrosion mechanisms to the microstructure. The benefits of using HIPing as a manufacturing process for the corrosion performance of Stellite 6 are discussed.     

Corrosion of heat treated magnesium alloy ZE41

This paper investigates the effect of heat treatment upon the corrosion morphology and mechanism of ZE41 alloy. The results of optical and scanning electron microscopy (SEM) together with potentiodynamic polarisation reveal the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The corrosion of the heat-treated alloy is significantly altered due to changes in the microstructure, specifically the Zr-rich regions and the grain boundary T-phase.     

Pit growth behaviour of aluminium under galvanostatic control

The pit growth process on (1 0 0) aluminium under anodic pulse current in a mixed solution of 1 M HCl and 0.1 M H₂SO₄ at 30 °C has been evaluated using potential transient measurements and pit size distributions obtained by scanning electron microscopy. Sustained pit growth is observed for all pits during the initial anodic potential rise before reaching a steady-state etch potential, whereas a substantial fraction of the pits passivate at the steady-state etch potential. The pit growth rate during the initial potential rise is 3.4 μm s⁻¹, which is similar to that at the steady-state etch potential. The growth rates of active pits are potential-independent.     

Characterisation and properties of synthetic patina on copper base sculptural alloys

Copper-based statuary alloys were submitted to accelerated patina formation and degradation in well-defined laboratory conditions. Analogies and differences between these patinas and patinas developed on monuments during long-term outdoor exposure are discussed. A very simple patina growth and degradation technique is proposed to better understand the electrochemical processes involved. Patinas were analysed in many replica samples and using destructive techniques not applicable to monuments due to their historic and cultural value. The best quantitative information obtained by “in situ” open circuit potential and current density measurements demonstrated the significant contribution of these electrochemical tools to Cultural Heritage maintenance.     

Corrosion behaviour of intermetallic aluminide coatings on nitrogen-containing austenitic stainless steels

The present work reports the effect of aluminide layers on the aqueous corrosion behaviour of four different 316L stainless steels containing various nitrogen contents (0.015%, 0.1%, 0.2% and 0.56% N). Diffusion annealed aluminide layers are generated over the surface by heat treatment of the aluminium precoated alloys at 750 °C for 25 h in nitrogen atmosphere. X-ray diffraction patterns of the surface modified samples showed the presence of AlN, Al₁₃Fe₄ and FeAl₂ phases. Diffusion of aluminum into the alloy, and the formation of AlN by the reaction of aluminium with matrix nitrogen, was identified using secondary ion mass spectrometry (SIMS). The nitrogen peak in the diffused layer was found to increase with increasing nitrogen content of the base alloy. SEM observation of cross-sectionally mounted alloys showed the presence of spherical AlN phase in addition to iron aluminide intermetallic phases. The role of such a composite surface layer containing intermetallic aluminides and nitride on the corrosion resistance of austenitic stainless steels in 0.5 M NaCl and 0.5 M sulphuric acid is discussed in greater detail based on open circuit potential (OCP)–time measurements, potentiodynamic polarisation studies and electrochemical impedance spectroscopy (EIS) investigations. The aluminide layered alloy with 0.1% N content showed better corrosion performance. The presence of nitrogen was found to have a positive effect in enhancing the hardness of the composite layer. Role of matrix nitrogen on the microstructure and microchemical distribution at the surface, and its role on corrosion resistance in acidic and chloride media are discussed in detail.     

Effect of second phases on the corrosion behaviour of wrought Mg-Zn-Y-Zr alloy

The corrosion behaviour of wrought Mg-Zn-Y-Zr alloy was investigated by corrosion morphology observation and electrochemical measurement. The results indicate that the corrosion process can be divided into three stages, corresponding to three types of corrosion features. At the initial stage, corrosion occurred surrounding the second phases, which was driven by galvanic couple effect; at the middle stage, filiform corrosion was found in the central regions of α Mg matrix; at the final stage, pitting corrosion presented around the second phases. The second phases have a great effect on the corrosion process of Mg-Zn-Y-Zr alloy.     

The effect of impurity inclusions on the pitting corrosion behaviour of beryllium

An electrochemical polarisation study has been conducted to determine the effect of impurity inclusions on the pitting corrosion behaviour of beryllium. Three grades of commercial beryllium differing in impurity levels were used. This work showed a strong relationship between the pitting potential of the beryllium and the level of impurity inclusions, the grade with the least number of overall inclusions being the most resistant to pitting corrosion. The work also revealed the preferred sites for pit initiation, with most corrosion pits initiating at intermetallic Fe/Al/Be inclusions, and to a smaller extent at inclusion sites containing either elemental silicon or carbides.     

Corrosion behaviour of Al/Al3Ti and Al/Al3Zr functionally graded materials produced by centrifugal solid-particle method: Influence of the intermetallics volume fraction

Intermetallic particles, Al₃Ti and Al₃Zr were formed in Al–5mass%Ti and Al–5mass%Zr alloys, respectively, by centrifugal casting, in order to create functionally graded materials (FGMs). At present, no information is available on the influence of the amount of intermetallics on the electrochemical properties of these alloys.In this paper, the corrosion resistance of Al/Al₃Ti and Al/Al₃Zr FGMs was investigated by open-circuit measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. Results suggests that the corrosion resistance of the FGMs is affected by galvanic effects between the intermetallic particles and the metallic matrix. Lower centrifugal forces resulted in an improvement of the electrochemical properties.     

Corrosion behaviour of thermal sprayed nitinol coatings

NiTi alloy is here investigated as an alternative coating to stainless steel since it is considered to possess good corrosion properties. Three different thermal spray techniques (high velocity oxy-fuel -HVOF-, vacuum plasma spray -VPS- and atmospheric plasma spray quenching -APS+Q-) have been used for building the coatings, and electrochemical tests have been carried out for corrosion evaluation. Open-circuit tests have revealed that the VPS-coating shows fairly good corrosion resistance, both in the as-sprayed and polished forms. The HVOF coatings however, showed a strong dependence on the surface conditions and APS+Q is dominated by electrolyte penetration through coating cracks, thus exhibiting a higher i_corr.     

Effect of sulfuric acid on pit propagation behaviour of aluminium under AC etch process

Pit propagation on high purity aluminium electrode in 2 M HCl solutions with and without H₂SO₄ under an alternating current (AC) has been examined. Pit development and potential transients were dependent on the H₂SO₄ concentration. In the sulfate-free etchant, most pits developed from the pretreated surface, with little tendency to form clusters of pits. With increasing H₂SO₄ concentration the size of the pit clusters increased. There is an optimal H₂SO₄ concentration, which is 0.01 M H₂SO₄ in this study, to form a deep etched layer of uniform thickness with high surface area. At H₂SO₄ concentrations higher than 0.01 M, the pit propagation proceeded on limited foil surface sites and deep etched regions were formed locally, since sulfate ions assisted passivation and reduced the number of pit nucleation sites on foil surface. Analysis of potential transients during the anodic half-cycle supports the hypothesis that sulfate ions retarded the pit nucleation.     

Corrosion behaviour of sintered NdFeB deposited with an aluminium coating

A protective, pure Al coating was deposited by direct current (DC) magnetron sputtering onto sintered NdFeB magnets. Separated, single phases of sintered NdFeB (the Nd-rich phase, the B-rich phase and the matrix phase) were prepared by arc melting for open circuit potential (OCP) tests. The corrosion process of the sintered NdFeB magnets coated with Al (Al/NdFeB) was studied experimentally. It was found that the corrosion process can be divided into three different stages. The Al coating cannot provide complete sacrificial protection for the sintered NdFeB magnets.