Effect of heat treatment on anodic activation of aluminium by trace element indium

The presence of trace elements in Group IIIA-VA is known to activate aluminium anodically in chloride environment. The purpose of this paper is to investigate the surface segregation of trace element In by heat treatment and resulting surface activation. Model binary AlIn alloys, containing 20 and 1000 ppm by weight of In, were characterized after heat treatment at various temperatures by use of glow discharge optical emission spectroscopy, electron microscopy and electrochemical polarization. Heat treatment for 1 h at 300 °C gave significant segregation of discrete In particles (thermal segregation), which activated the surface. Indium in solid solution with aluminium, obtained by 1 h heat treatment at 600 °C, also activated by surface segregation of In on alloy containing 1000 ppm In, resulting from the selective dissolution of the aluminium component during anodic oxidation (anodic segregation). The effect of anodic segregation was reduced by decreasing indium concentration in solid solution; it had negligible effect at the 20 ppm level. The segregated particles were thought to form a liquid phase alloy with aluminium during anodic polarization, which in turn, together with the chloride in the solution destabilized the oxide.     

Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

The role of chromium and molybdenum in passivation of amorphous Fe-Cr-Mo-P-C alloys in deaerated 1 M HCl

The effect of open circuit immersion on the passivation behavior of amorphous Fe-8Cr-Mo-13P-7C alloys with 0, 2 and 6 at.% molybdenum by potentiostatic polarization in de-aerated 1 M HCl was investigated. The polarization was made at 400 mV (SCE) which is in the passive region of chromium but in the transpassive region of molybdenum, whereas the open circuit potential of these alloys is in the passive region of molybdenum but in the active region of chromium. Open circuit immersion of molybdenum-bearing alloys results in the formation of molybdenum-enriched passive film, but passivation at 400 mV is based on the formation of chromium-enriched passive film and the molybdenum-enriched film formed by open circuit immersion is converted to the chromium-enriched film at 400 mV. Polarization at 400 mV immediately after immersion leads to rapid enrichment of chromium to the level of 50% cations within 100 s, but polarization after open circuit immersion of molybdenum-bearing alloys for 10 min results in a gradual increase in chromium content to the level of 50% of cations in 5 h. The current density of molybdenum-bearing alloys after open circuit immersion is always higher than that of the alloys polarized immediately after immersion. Open circuit immersion of molybdenum-free alloy for 10 min gives rise to serious damage of the alloy surface and hence prevents passivation even if prolonged polarization is carried out.     

Numerical modelling of the galvanic coupling in aluminium alloys: A discussion on the application of local probe techniques

A discussion is proposed on the determination of the input values and the experimental validation of finite element modelling of the galvanic coupling in aluminium alloys by local probe techniques such as the Scanning Vibrating Electrode Technique (SVET) and the microcapillary electrochemical cell (microcell). Polarization curves obtained by the microcell were introduced as input conditions in the model based on Laplace or Nernst-Planck equation. SVET measurements were performed to determine the coupling current distribution on an Al/Al4%Cu bimetallic system. Agreement was found between simulated and experimental current distributions depending on the input conditions and the solved equation.     

Electrochemical response of AlNiLa amorphous and devitrified alloys

The electrochemical response of Al_94−xNi₆La_x alloys (x = 4, 5, 6, 7) after different stages of devitrification was studied in 0.05 M Na₂SO₄ as well as in different concentrations [0.001 M, 0.01 M and 0.1 M] NaCl solutions. Complementary crystallization studies were carried out to elucidate the composition dependent phase evolution in these alloys. It was observed that the primary crystallization did not cause any deterioration in the corrosion resistance of the alloys as compared to the amorphous alloys. In the case of Al₈₇Ni₆La₇, there was actually an improvement in the passivating ability in benign media. The various primary crystalline phases in the different alloys investigated did not cause different electrochemical responses. However, the onset of secondary crystallization caused a reduction in the corrosion resistance in the NaCl media through a loss in passivating ability of all the alloys. This is due to increased galvanic activity as well as the loss of the amorphous phase.     

Influence of rare earth metals on the characteristics of anodic oxide films on aluminium and their dissolution behaviour in NaOH solution

The characteristics of oxide films on Al and Al1R alloys (R = rare earth metal = Ce, Y) galvanostatically formed (at a current density of 100 μA cm⁻²) in borate buffer solution (0.5 M H₃BO₃ + 0.05 M Na₂B₄O₇·10H₂O; pH = 7.8) were investigated by means of electrochemical impedance spectroscopy. EIS spectra were interpreted in terms of an “equivalent circuit” that completely illustrate the Al(Al1R alloy)/oxide film/electrolyte systems examined. The resistance of the oxide films was found to increase on passing from Al to Al1R alloys while the capacitance showed an opposite trend. The stability of the anodic oxide films grown in the borate buffer solution on Al and Al1R alloys was investigated by simultaneously measuring the electrode capacitance and resistance at a working frequency of 1 kHz as a function of exposure over a period of time to naturally aerated 0.01 M NaOH solution. Analyses of the electrode capacitance and resistance values indicated a decrease in chemical dissolution rate of the oxide films on passing from Al to Al1R alloys.     

Effect of excimer laser surface melting on the microstructure and corrosion performance of the die cast AZ91D magnesium alloy

Excimer laser surface melting (LSM) of the die cast AZ91D alloy has been investigated in terms of microstructure and corrosion behaviour. Excimer LSM of the alloy resulted in a highly homogeneous and refined melted microstructure, which improved the corrosion resistance of the alloy. The latter was associated with the large dissolution of intermetallic phases and the enrichment of aluminium within the melted layer. An increased number of laser pulses resulted in thicker melted layers, but also in enhanced porosity and the formation of micro-cracks at the overlapping area. Both factors diminished the corrosion resistance of the laser-treated alloy.     

Passivity of polycrystalline NiMnGa alloys for magnetic shape memory applications

The corrosion and passivation behaviour of bulk polycrystalline martensite Ni₅₀Mn₃₀Ga₂₀ and austenite Ni₄₈Mn₃₀Ga₂₂ alloys was compared in electrolytes with different pH values. Linear anodic and cyclic potentiodynamic polarisation methods and anodic current transient measurements have been conducted for the alloys and their constituents to analyze free corrosion, anodic dissolution and passive layer formation processes. Electrochemically treated alloy surfaces were characterized with scanning electron microscopy (SEM) and angle-resolved x-ray photoelectron spectroscopy (XPS). The electrochemical response of both alloys is in principal similar and is dominated by the Ni oxidation. In acidic solutions (pH 0.5 and 5) a slightly higher reactivity is detectable for the martensitic alloy which is mainly attributed to enhanced dissolution processes at the multiple twin boundaries. In weakly acidic to strongly alkaline solutions (pH 5-11) both alloys exhibit a low corrosion rate and a stable anodic passivity. While air-formed films comprise NiOOH, Ga₂O₃ and MnO₂, passive films formed in near neutral media (pH 5-8.4) are composed of Ni(OH)₂, NiOOH and Ga₂O₃ in the outer region and of NiO, MnO₂ and MnO in the metal-near region.     

The polarization characteristics of Pb-free Sn-8.5Zn-XAg-0.1Al-0.05Ga alloy in 3.5% NaCl solution

The polarization characteristics of Pb-free Sn-8.5Zn-XAg-0.1Al-0.05Ga alloy with varying Ag content were investigated in 3.5% NaCl solution. The value of Ag content i.e X varied from 0.1 wt% to 2 wt%. An increase in the Ag content up to 2 wt% resulted in a progressive increase in the corrosion current density and shifted the corrosion potential (E_corr) towards more active values. These changes were also reflected in the corrosion rate of the solder alloy. However the linear polarization resistance values decreased with increase in the weight content of silver. Increasing the Ag content from 0.1 to 2 wt% did not show a significant improvement in the passivation behaviour of the solder alloy as reflected in the passivation current density (i_p) and critical current density (i_cc) values. XPS and SIMS depth profile results revealed that the oxides of Zn and Sn were present in high concentrations on the outer surface of the solder alloy along with very low concentration of Al₂O₃.     

Electrochemical corrosion behaviour of Bi-11Ag alloy for electronic packaging applications

This study investigated the electrochemical corrosion properties of the solders for die-attach applications in 3.5% NaCl solution. Compared with Pb-5Sn and Zn-40Sn, Bi-11Ag exhibited higher corrosion potential and relatively low corrosion current density. The ductile Ag-rich phase which dispersed in the Bi matrix was able to accommodate the stress arising from the formation of a passive layer and contributed to the two-stage passivation. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results confirm that the corrosion products comprised BiOCl, Bi₂O₃ and AgCl.     

Electrochemical effects of S accumulation on ion-implanted Alloy 22 in 1 M NaCl solutions

Electrochemical and surface analysis studies on the passivation of Alloy 22, implanted with S at concentrations up to 2 at.%, in deaerated 1 M NaCl solutions indicated that, although there was very little influence from the S in the near-surface region on the corrosion current in the short term, the presence of S caused a small reproducible effect on the corrosion potential that was related to the amount of S present. However, there was strong evidence that S enriched on the surface of Alloy 22 during corrosion under normally passive conditions in deaerated NaCl solutions.     

Achieving a chromium rich surface upon steels via FBR-CVD chromising treatments

Surface alloying via fluidised bed reactor (FBR) chemical vapour deposition (CVD) can be used to produce adherent and highly corrosion resistant surface layers. In this work, 304, 316, and 409 stainless steels and 1020 steel were chromised, producing distinct surface layers tens of micrometres thick. The composition and surface of these layers was characterised using microscopy, GDOES, and XPS. Surfaces were highly enriched in Cr and displayed improved corrosion resistance as determined by electrochemical evaluations. This work demonstrates that it is possible to form continuous, functional and corrosion resistant Cr-rich surface layers via FBR-CVD on a variety of steels.     

Electrochemical and XPS studies of the passive film formed on stainless steels in borate buffer and chloride solutions

The passivity of AISI 304L and AISI 316L stainless steels in a borate buffer solution, with and without the addition of chloride ions, was studied using cyclic voltammetry and potentiodynamic measurements. The passive layers formed by electrochemical oxidation at different passivation potentials on both the stainless steels were studied by X-ray photoelectron spectroscopy, their compositions were analysed as a function of depth, and the cationic fraction of the passive film was determined. The passive films established on the two stainless steels in the borate buffer solution at pH = 9.3 contained the oxides of two main elements, i.e., Fe and Cr. The oxides of the alloying elements Ni and, optionally, Mo, also contribute to the passive layer. In the presence of chloride ions a strong chromium enrichment was observed in the passive layers.     

Corrosion and passivation of low-temperature nitrided AISI 304L and 316L stainless steels in acidified sodium sulphate solution

304L and 316L steels were nitrided at 425 °C for 30 h and examined at various depths in 0.1 M Na₂SO₄ acidified to pH 3.0. In the near-surface region with about 7-14 wt% N, at potentials of active state anodic currents were much higher than those for untreated steels, whereas in deeper regions with <7 wt% N the currents were only slightly increased in comparison with untreated steels or they were even lower in passive and transpassive states. Surface films were composed of oxygen-containing species on top and of Cr-N species in deeper layers. It is suggested that strong corrosion of near-surface regions is associated with nitride precipitates. Beneficial effect of low nitrogen concentrations can be due to initially accelerated corrosion which leads to larger amounts of passivating species and to the accumulation of corrosion resistant chromium nitrides.     

The valence state of copper in anodic films formed on Al-1at.% Cu alloy

During anodising of Al-Cu alloys, copper species are incorporated into the anodic alumina film, where they migrate outward faster than Al³⁺ ions. In the present study of an Al-1at.% Cu alloy, the valence state of the incorporated copper species was investigated by X-ray photoelectron spectroscopy, revealing the presence of Cu²⁺ ions within the amorphous alumina film. However, extended X-ray irradiation led to reduction of units of CuO to Cu₂O, probably due mainly to interactions with electrons from the X-ray window of the instrument and photoelectrons from the specimen. The XPS analysis employed films formed on thin sputtering-deposited alloy/electropolished aluminium specimens. Such an approach enables sufficient concentrations of copper species to be developed in the anodic film for their ready detection.