Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

An exploratory study of the corrosion of Mg alloys during interrupted salt spray testing

A first systematic investigation was carried out to understand the corrosion of common Mg alloys (Pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt.% NaCl by measuring hydrogen evolution and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3 wt.% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)₂ surface film on Mg alloys is probably formed by a precipitation reaction when the Mg²⁺ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120-X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle. The electrochemical measurements of the corrosion rate, based on the “corrosion current” at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.     

Corrosion behaviour of die-cast AZ91D magnesium alloy in aqueous sulphate solutions

The corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions was investigated by SEM, FTIR and polarization measurements. For immersion times less than 48 h, no pitting corrosion occurred and only generalized corrosion was apparent. According to the polarization curves, the corrosion rate order of the die-cast AZ91D Mg alloy in three aqueous solutions was: NaCl > MgSO₄ > Na₂SO₄. The main corrosion products were Mg(OH)₂ and MgAl₂(SO₄)₄·22H₂O in the sulphate solutions and the product film was compact. Precipitation of MgAl₂(SO₄)₄·22H₂O required a threshold immersion time.     

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D

Magnesium alloy AZ91D was exposed in humid air at 95% relative humidity (RH) with a deposition of 70 μg/cm⁻² NaCl. The corrosion products formed and the surface electrolyte were analysed after different exposure times using ex situ and in situ FTIR spectroscopy, X-ray diffraction and Ion Chromatography. The results show that magnesium carbonates are the main solid corrosion products formed under these conditions. The corrosion products identified were the magnesium carbonates hydromagnesite (Mg₅ (CO₃)₄ (OH)₂4H₂O) and nesquehonite (MgCO₃ 3H₂O). The corrosion attack starts with the formation of magnesite at locations with higher NaCl contents. At 95% RH, a sequence of reactions was observed with the initial formation of magnesite, which transformed into nesquehonite after 2-3 days. Long exposures result in the formation of pits containing brucite (Mg(OH₂)) covered with hydromagnesite crusts. The hydromagnesite crusts restrict the transport of CO₂ and O₂ to the magnesium surface and thereby favour the formation of brucite. Analysis of the surface electrolyte showed that the NaCl applied on the surface at the beginning was essentially preserved during the initial corrosion process. Since the applied salt was not bound in sparingly soluble corrosion products a layer of NaCl electrolyte was present on the surface during the whole exposure. Thus, Na⁺ and Cl⁻ ions can participate in the corrosion process during the whole time and the availability of these species will not restrict the atmospheric corrosion of AZ91D under these conditions. It is suggested that the corrosion behaviour of AZ91D is rather controlled by factors related to the microstructure of the alloy and formation of solid carbonate containing corrosion products blocking active corrosion sites on the surface.     

Enhanced corrosion resistance of high strength Mg–3Al–1Zn alloy sheets with ultrafine grains in a phosphate-buffered saline solution

The corrosion behaviour of ultrafine grained AZ31Mg alloy sheets with very high strength, which were prepared by high-ratio differential speed rolling (HRDSR) technique, was studied in a phosphate-buffered saline solution. The corrosion resistance was greatly improved after HRDSR. This result was attributed to the enhanced stability of the Mg(OH)₂ layer due to the grain refinement and precipitation of various types of P-containing compounds on the stabilised Mg(OH)₂ layer. The HRDSR technique has a good potential to be used for the development of magnesium sheets with good combination of mechanical and biocorrosion properties.     

Influence of chloride ion concentration and temperature on the corrosion of Mg-Al alloys in salt fog

The influence of temperature and chloride concentration on the corrosion behaviour of Mg-Al alloys exposed to salt fog was evaluated. Corrosion attack increased with decreasing aluminium content in the alloy and increasing Cl⁻ concentration and temperature. The effect of Al-Mn inclusions, which revealed several stoichiometries and were up to 300 mV more noble than the magnesium matrix, was only noticeable in the early stages of corrosion of the AZ31 alloy. Aluminium segregation and β-phase distribution were the main controlling factors for the AZ80 and AZ91D alloys, the latter being more susceptible to variations in the saline concentration.     

Corrosion behaviour of a magnesium matrix composite with a silicate plasma electrolytic oxidation coating

The corrosion behaviour of AZ92 magnesium alloy reinforced with various volume fractions of silicon carbide particles (SiCp) and treated by alternating current (AC) plasma electrolytic oxidation (PEO) was investigated in humid and saline environments. For untreated composites, corrosion attack started around the Al-Mn inclusions and gradually developed into general corrosion without significant galvanic coupling between the matrix and the SiCp. PEO coatings consisted mainly of MgO and Mg₂SiO₄, and revealed increased hardness, reduced thickness and slightly higher corrosion resistance with increasing proportion of reinforcement. Pit formation and hydration of the outer layer were the main mechanisms of corrosion of PEO-treated specimens.     

Formation and breakdown of surface films on magnesium and its alloys in aqueous solutions

The influences of surface films formed by open-circuit exposure to neutral solutions on the corrosion and electrochemical behaviour of pure Mg and Mg alloys have been examined by in situ ellipsometric analysis and electrochemical measurements. Surface films mainly composed of Mg(OH)₂ grew rapidly during open-circuit exposure to 0.1 M NaCl and 0.1 M Na₂SO₄ solutions. These films had protective ability to passivate Mg in the solutions. However, they suffered local breakdown under anodic polarisation. The passive current density decreased and the breakdown potential increased with increasing immersion time and film thickness. Influences of purity and alloying elements on the passivity and its breakdown of Mg have been discussed.     

Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg–RE alloys: RE = Ce,La, Nd,Y, Gd

Corrosion behaviour was characterised in salt spray and in 3.5% NaCl solution saturated with Mg(OH)₂ of as-cast and solution heat-treated binary Mg–RE alloys. The corrosion rate in the immersion test for the solution heat-treated Mg–RE alloys was substantial, and was greater than that of high-purity Mg. These corrosion rates were probably caused by the particles in the microstructure and/or by Fe rich particles precipitated during the solution heat-treatment. The corrosion rate in the immersion tests for each as-cast Mg–RE alloy was greater than that of high-purity Mg, attributed to micro-galvanic acceleration caused by the second phase. Corrosion rates in salt spray had a general correlation with corrosion rates in the immersion tests, but there were differences. The values of apparent valence were always less than 2 consistent with Mg corrosion being only partly under electrochemical control.     

On the corrosion behaviour of novel high carbon rail steels in simulated cyclic wet-dry salt fog conditions

Eutectoid rail steels are prone to excessive corrosion in the coastal locations in India. In order to minimise this problem, four new rail steels with microalloying elements, Cu, Cr, Ni and Si were designed. The corrosion behaviour of these four rail steels were compared with the behaviour of three rail steels already in commercial application. Quantitative evaluation done by weight loss measurements after simulated wet-dry salt fog exposure test showed similar weight loss values for all rail steels. The FTIR spectra of rust samples revealed the presence of Fe_3−xO₄ as the major phase in both inner and outer layers of rusts on all the rail steels. Relative amounts of the different rust phases have been compared. SEM micrographs of the rusted samples revealed that the rust on Cr-Cu-Ni and Cr-Cu-Ni-Si rail steel was more compact than other rail steels. Impedance spectroscopy showed that the rust formed on Cr-Cu-Ni and Cr-Cu-Ni-Si rail steels resulted in the higher impedance in the high frequency region, compared to other rail steels.     

Corrosion resistance of zinc-magnesium coated steel

A significant body of work exists in the literature concerning the corrosion behaviour of zinc-magnesium coated steel (ZMG), describing its enhanced corrosion resistance when compared to conventional zinc-coated steel. This paper begins with a review of the literature and identifies key themes in the reported mechanisms for the attractive properties of this material. This is followed by an experimental programme where ZMG was subjected to an automotive laboratory corrosion test using acidified NaCl solution. A 3-fold increase in time to red rust compared to conventional zinc coatings was measured. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the corrosion products formed. The corrosion products detected on ZMG included simonkolleite (Zn₅Cl₂(OH)₈ · H₂O), possibly modified by magnesium uptake, magnesium hydroxide (Mg(OH)₂) and a hydroxy carbonate species. It is proposed that the oxygen reduction activity at the (zinc) cathodes is reduced by precipitation of alkali-resistant Mg(OH)₂, which is gradually converted to more soluble hydroxy carbonates by uptake of atmospheric carbon dioxide. This lowers the surface pH sufficiently to allow thermodynamically for general precipitation of insoluble simonkolleite over the corroding surface thereby retarding the overall corrosion reactions, leaving only small traces of magnesium corrosion products behind. Such a mechanism is consistent with the experimental findings reported in the literature.     

Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg–X alloys: X = Mn,Sn, Ca,Zn, Al,Zr, Si,Sr

Corrosion of binary Mg–X alloys (as-cast and solution heat-treated) was characterised by immersion tests in 3.5% NaCl solution saturated with Mg(OH)₂, and by salt spray. Alloys with high corrosion rates in immersion tests also had high corrosion rates in salt spray. Corrosion rates of the solution heat-treated alloys did not meet the expectation that they should be equal to or lower than those of high-purity Mg. There was circumstantial evidence that the higher corrosion rates were caused by the particles in the microstructure; the second phases had been dissolved. The corrosion rate of all alloys was faster than that of high-purity Mg.     

Baking treatment effect on materials characteristics and electrochemical behavior of anodic film formed on AZ91D magnesium alloy

The composition and microstructure of the anodic films formed on AZ91D Mg alloy, with or without baking, were investigated. The associated corrosion behavior of the anodized alloy in 3.5 wt% NaCl solution was also examined using electrochemical impedance spectroscopy (EIS). The results show that MgO was the main component in the anodic film which also contained some Mg(OH)₂, Al₂O₃, Al(OH)₃, and MgAl₂O₄. Both the amorphous and crystalline forms of anodic film were identified. The degree of crystallinity depended on baking temperature, which increased with increasing temperature in the range of 50-250 °C. The amounts of MgO and Al₂O₃ increased as a result of a dehydration reaction. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. An optimum value of polarization resistance of anodic film was obtained for the alloy baked at 150 °C for 2 h followed by air cooling.     

The in vivo and in vitro corrosion of high-purity magnesium and magnesium alloys WZ21 and AZ91

Corrosion was studied in vitro in Nor’s solution (CO₂ – bicarbonate buffered Hank’s solution) at 37 °C, and in vivo implanted in the lower back muscle of rats. Nor’s solution is a good model for HP Mg and WZ21, because (i) the pH is maintained by the same buffer as in blood and (ii) concentrations of corrosive chloride ions, and other inorganic constituents, are similar to those in blood. The higher in vitro corrosion rate of AZ91 was caused by micro-galvanic from second phases. The lower in vivo corrosion rate of AZ91 was tentatively attributed to suppression of micro-galvanic corrosion by tissue encapsulation.     

Corrosion of high purity Mg, AZ91, ZE41 and Mg2Zn0.2Mn in Hank’s solution at room temperature

This work compared the corrosion of typical Mg alloys (AZ91, ZE41 and Mg2Zn0.2Mn) and high purity (HP) Mg in Hank’s solution at room temperature and in 3% NaCl saturated with Mg(OH)₂. Corrosion was characterised by the evolved hydrogen and the surfaces after immersion. Corrosion in Hank’s solution was weakly influenced by microstructure in contrast to corrosion in the 3% NaCl solution. This is attributed to the formation of a more protective surface film in Hank’s solution, causing extra resistance between the α-Mg matrix and the second phase. The alloys with substantial Zn contents had a shorter incubation period in Hank’s solution.     

The relation between microstructure and corrosion behavior of AZ80 Mg alloy following different extrusion temperatures

Cast AZ80 alloy was subjected to conventional extrusion pressing at 250 °C, 300 °C, and 350 °C. In order to characterize the changes in their microstructure a thorough study was done through various microscopy analyses including Optical Microscope, SEM, and TEM.Corrosion performance of each condition was investigated in 3.5% NaCl solution saturated with Mg(OH)₂ (pH ≈ 10.5) using immersion and AC and DC polarization tests. The local potential difference on the surface resulting from different compositions of second phase particles to the matrix was investigated using scanning Kelvin probe force microscopy (SKPFM) technique.The results show grain refinement by a factor of about 15-20 and obvious evidence of dynamic recrystallization were identified leading to the formation of nano-sized grains after the extrusion process.The corrosion resistance of cast AZ80 alloy drastically decreases after the thermo-mechanical processes and the main factor is high dependence on different phase rearrangements before and after the extrusion process, especially β phase. For the extrusion conditions, different corrosion resistances are attributable especially to dislocation rearrangement results by grain growth after dynamic recrystallization.     

The influence of Mgon the formation of calcareous deposits on a freely corroding low carbon steel in seawater

The purpose of this study was to determine how magnesium in seawater influences the corrosion behaviour of freely corroding steel. This was done by studying if Mg(OH)₂ is formed and if calcite and aragonite differ in their protective properties. No Mg(OH)₂ was detected after immersion of steel in a Mg²⁺-containing artificial seawater. Magnesium seems to influence the corrosion behaviour of freely corroding steel by causing calcium carbonate to precipitate as aragonite. Aragonite is more effective in covering the surface than calcite and is therefore more functional in preventing oxygen from reaching the steel surface, thereby lowering the corrosion rate.     

Corrosion of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2

Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solution saturated with Mg(OH)₂. The intrinsic corrosion rate measured with weight loss, P_W = 0.25 ± 0.07 mm y⁻¹, was slightly smaller than that for high-purity Mg. Some specimens had somewhat higher corrosion rates attributed to localised corrosion. The average corrosion rate measured from hydrogen evolution, P_AH, was lower than that measured with weight loss, P_W, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism is suggested.     

Corrosion protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na₂SO₄ electrolyte, and accelerated atmospheric corrosion tests (humid, SO₂ polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce-Zr-Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce-Zr-Nb-treated samples, with or without an additional layer of epoxy-polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO₂, Ce₂O₃, ZrO₂, Nb₂O₅, MgO, and MgF₂ as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na₂SO₄. This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.     

Corrosion behaviour of dc magnetron sputtered Fe1−xMgx alloy films in 3 wt% NaCl solution

Fe_1−xMg_x alloy films (with x ? 43.4 at.% Mg) were deposited by dc magnetron sputtering onto glass slide substrates. The objective of this study was to characterise the corrosion properties of these alloys in saline solution for application as new friendly environmentally sacrificial coatings in the protection of steel structures. The morphological and structural properties of the alloys were systematically studied prior to electrochemical experiments, and then the degraded surfaces were analysed to determine the composition and nature of corrosion products. Alloys with <25  at.% Mg were single-phase body-centred cubic (bcc) with enlarged lattice parameters, whereas for magnesium contents above 25 at.%, amorphisation occurred. The reactivity of the alloys in saline solution is strongly dependent on the Mg content and the alloy structure. The incorporation of magnesium leads to an open circuit potential shift of the alloy towards more negative values, that confers an attractive interest of these alloys as sacrificial coatings. A transition in corrosion activity is observed at 25 at.% Mg from which the reactivity decreases with the magnesium content increase. The evolution of the alloy corrosion behaviour is discussed in terms of structural and corrosion products evolution versus magnesium content.