Corrosion of high purity Mg, Mg2Zn0.2Mn, ZE41 and AZ91 in Hank’s solution at 37 °C

The CO₂ partial pressure required to maintain a synthetic body fluid (SBF) at a constant pH, based on the initial bicarbonate concentration, was evaluated to be 0.013 atm for Hank’s solution and 0.083 atm for SBF27. Corrosion of high purity Mg and three Mg alloys in Hank’s solution was studied using hydrogen evolution, weight loss and Tafel extrapolation. The solution pH was maintained constant by CO₂. There was initially an incubation period with a low corrosion rate, a period of increasing corrosion rate, and subsequently steady state corrosion. Some hydrogen dissolved in the Mg metal.     

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.     

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.     

Understanding corrosion via corrosion product characterization: II. Role of alloying elements in improving the corrosion resistance of Zn–Al–Mg coatings on steel

Corrosion products are identified on Zn, ZnMg and ZnAlMg coatings in cyclic corrosion tests with NaCl or Na₂SO₄ containing atmospheres. For Mg-containing alloys the improved corrosion resistance is achieved by stabilization of protective simonkolleite and zinc hydroxysulfate. At later stages, the formation of layered double hydroxides (LDH) is observed for ZnAlMg. According to thermodynamic modeling, Mg²⁺ ions bind the excess of carbonate or sulfate anions preventing the formation of soluble or less-protective products. A preferential dissolution of Zn and Mg at initial stages of corrosion is confirmed by in situ dissolution measurement. The physicochemical properties of different corrosion products are compared.     

Corrosion and electrochemical behavior of Mg–Y alloys in 3.5% NaCl solution

The corrosion mechanism of Mg–Y alloys in 3.5% NaCl solution was investigated by electrochemical testing and SEM observation. The electrochemical results indicated that the corrosion potential of Mg–Y alloys in 3.5% NaCl solution increased with the increase of Y addition. The corrosion rate increased with the increase of Y addition because of the increase of Mg₂₄Y₅ intermetallic amounts. The corrosion gradually deteriorated with the increase of immersion time. The corrosion morphologies of the alloys were general corrosion for Mg–0.25Y and pitting corrosion for Mg–8Y and Mg–15Y, respectively. The main solid corrosion products were Mg(OH)₂ and Mg₂(OH)₃C1.4H₂O.     

Corrosion behavior of rapidly solidified Mg-Zn-rare earth element alloys in NaCl solution

Rapidly solidified flaky powder metallurgy (RS FP/M) processing was applied for preparation of corrosion-resistant bulk Mg alloys with Zn and rare earth elements. The corrosion behavior of the melt spun Mg-Zn-La and Mg-Zn-Yb alloy ribbons in 1% NaCl solution was investigated in order to determine optimum composition of corrosion-resistant Mg alloys. The effect of heat-treatment on the corrosion behavior of RS Mg-Zn-La and Mg-Zn-Yb alloys also was studied. In the Mg-Zn-La alloys, as-quenched alloys showed good corrosion resistance in the NaCl solution, but heat-treatment led to degradation due to microstructure change, that is, reduction in dispersion of the Mg₁₇La₂-type intermetallic compound. In the Mg-Zn-Yb alloys, both as-quenched and heat-treated Mg_97.5Zn_0.5Yb₂ alloys exhibited low corrosion rates because fine distribution of Mg₂Yb-type intermetallic compound in α-Mg matrix was not largely changed by heat treatment.     

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 performance and mechanical properties of sputter-deposited MgY and MgGd alloys

Films of MgY and MgGd alloys were deposited on silicon wafers by magnetron sputtering. The microstructure, crystal structure and mechanical properties were evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and tensile testing. Corrosion was evaluated for immersion in 3.5% NaCl solution saturated with Mg(OH)₂. TEM, SEM and XRD indicated that the alloys were single phase. There was no significant change of corrosion rate with alloy content. The strength increased with alloying content, and ductility decreased concomitantly. Strengthening was consistent with solid solution strengthening or short-range order.     

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.     

Effects of Al content on the corrosion properties of Mg–4Y–xAl alloys

The corrosion performances of Mg–4Y–xAl (x = 1, 2, 3, and 4 wt%) alloys in the 3.5% NaCl electrolyte solution are investigated by electrochemical tests, weight loss measurement and corrosion morphology observation. The results indicate that corrosion modes for the alloys are localized corrosion and the filiform type of attack. With Al concentration increasing from 1 to 4 wt%, the corrosion rate of Mg–4Y–xAl alloys decreases firstly and then increases, and WA42 alloy shows the best corrosion resistance. The addition of Al element to Mg–4Y alloys leads to the formation of Al₂Y and Al₁₁Y₃ intermetallic compounds and reduces the proportion of Mg₂₄Y₅ phase. Corrosion resistance of the Mg–4Y–xAl alloys mainly depends on the size and distribution of the second phases. Besides, the addition of excessive Al can greatly consumes the Y element in the matrix, thus leading to a less protective film on the alloys. The effect of the relative Volta potential changes between the second phases and α-Mg on corrosion resistance of Mg–4Y–xAl alloys is insignificant. The main corrosion products of the Mg–4Y–xAl alloys are Mg(OH)₂, Mg₃(OH)₅Cl·4H₂O, Mg_0.72Al_0.28(CO₃)_0.15(OH)_1.98(H₂O)_0.48, and Mg₄Al₂(OH)₁₂CO₃·3H₂O.     

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.     

In situ infrared reflection spectroscopy studies of the initial atmospheric corrosion of Zn–Al–Mg coated steel

NaCl induced atmospheric corrosion of ZnAl2Mg2 coated, electrogalvanised (EG) and hot dipped galvanised (HDG) steel was studied using in situ infrared reflection absorption spectroscopy, XRD and SEM. Initial corrosion leads to the formation of Mg/Al and Zn/Al layered double hydroxides (LDHs) on ZnAl2Mg2, due to the anodic dissolution of Zn–MgZn2 phases and cathodic oxygen reduction on Zn–Al–MgZn2, Al-phases and on zinc dendrites. In contrast to EG and HDG, were no ZnO and Zn₅(OH)₈Cl₂⋅H₂O detected. This is explained by the buffering effect of Mg and Al which inhibit the ZnO formation, reduce the cathodic reaction and corrosion rate on ZnAl2Mg2.     

热处理对轧制态Mg5Gd合金腐蚀行为的影响

通过浸泡试验、电化学测试、扫描电化学显微镜和腐蚀形貌分析等手段研究热处理工艺对轧制态Mg5Gd合金在3.5 wt.％NaCl饱和Mg(OH)2溶液中腐蚀行为的影响及机理,以期达到提高镁合金耐蚀性的目的.结果表明:固溶处理能显著降低Mg5Gd合金的腐蚀速率,并且使其腐蚀变均匀,腐蚀坑变浅,这主要归因于固溶处理可以熔解镁基...     

Corrosion of Mg alloys EV31A,WE43B,and ZE41A in chloride- and sulfate-containing solutions saturated with magnesium hydroxide

This study studied corrosion in 0.1 M Na₂SO₄, 0.1 M NaCl, and 0.6 M NaCl, all saturated with Mg(OH)₂, using weight loss, hydrogen evolution, and electrochemical measurements. Corrosion was similar in all cases. Nevertheless, the corrosion rates were alloy-dependent, were somewhat lower in 0.1 M Na₂SO₄ than in 0.1 M NaCl, and increased with NaCl concentration. The corrosion damage morphology was similar for all solutions; the extent correlated with the corrosion rate. The corrosion rates evaluated by the electrochemical methods were lower than those evaluated from hydrogen evolution, consistent with the Mg corrosion mechanism involving the unipositive Mg⁺ ion.     

Limitations in the use of the potentiodynamic polarisation curves to investigate the effect of Zn on the corrosion behaviour of as-extruded Mg–Zn binary alloy

The effects of Zn on corrosion behaviour of as-extruded Mg-(1-4)Zn alloys were investigated using an immersion test, a zero resistance ammeter technique, and a potentiodynamic polarisation test. As a result, it was revealed that the solutionised Zn enhanced protectiveness of the passive film, and accelerated the H₂ evolution rate of the Mg–Zn binary alloys. The acceleration of the H₂ evolution rate by addition of Zn leads to an increase in the net corrosion rate of the Mg–Zn alloy. In this research, the polarisation test was found to have some limitations for evaluating the true corrosion behaviour of passive Mg–Zn alloy.     

The effect of substrate composition on the electrochemical and mechanical properties of PEO coatings on Mg alloys

Plasma electrolytic oxidation (PEO) is a unique surface treatment technology which is based on anodic oxidation forming ceramic oxide coatings on the surface of light alloys such as Mg, Al and Ti. In the present study, PEO coatings prepared on AZ91D, AZ31B, AM60B and AM50B Mg alloys have been investigated. Surface morphology and elemental composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). SEM results showed that the coating exhibited a porous top surface layer and a subsequent dense layer with micro-pores and shrinkage cracks. Phase analysis of coatings was carried out by X-ray diffraction (XRD). XRD analyses indicated that PEO coatings on AZ alloys had higher amount of Periclase (MgO) followed by the presence of Spinel (MgAl₂O₄) e.g. on the AZ91D alloy compared to that on AM series alloys. In order to examine the effect of substrate composition on adhesion strength of PEO coating scratch tests were carried out. Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization technique in 3.5% NaCl solution at room temperature (20 ± 2 °C). Corrosion test results indicated that the corrosion rates of coated Mg alloys decreased by nearly two orders of magnitude as compared to bare Mg alloys. PEO coatings on AZ series alloys showed better corrosion resistance and higher adhesion properties than AM series alloys. In addition to the PEO processing parameters, such are mainly attributes of the compositional variations of the substrate alloys which are responsible for the formation, phase contents and structural properties of the PEO coatings.     

The effect of T4 heat treatment on the microstructure and corrosion behaviour of Zn27Al1.5Cu0.02Mg alloy

The effect of heat treatment on the microstructure and corrosion behaviour of Zn27Al1.5Cu0.02Mg alloy was examined. The alloy was prepared by melting and casting route and then thermally processed (T4 regime). Corrosion behaviour of the as-cast and heat treated alloy was studied in 3.5 wt.% NaCl solution using immersion method and electrochemical polarization measurements. The applied heat treatment affected the alloy microstructure and resulted in increased ductility and higher corrosion resistance of the heat treated alloy. Electrochemical measurements of the corrosion rate at the free corrosion potential are in agreement with the results obtained using the weight loss method.     

Study of aging and electrochemical behaviour of Al?Li?Cu?Mg alloys

Aging study of 1441 and 8090 Al? Li? Cu? Mg alloys exhibited characteristic precipitation hardening phenomena. Potentiodynamic polarisation studies carried out on various tempers of the 1441 and 8090 Al? Li? Cu? Mg alloys in 3.5% NaCl, with a small amount of H₂O₂, and in 3.5% NaCl solution with pH 10 and 12 showed the shifting of open circuit potential (OCP) towards more negative potential and higher corrosion rate, with the increase of aging time. The OCP value has shifted anodically with addition of H₂O₂ in 3.5% NaCl solution. Further, passivity phenomenon has been observed for all the alloy tempers in 3.5% NaCl solution at pH 10 and 12. The observation of OCP shift towards more negative potential and the higher corrosion rate with the increase of aging time has been attributed to the presence of higher amounts of anodic δ (AlLi), S′ (Al₂CuMg) and T₁ (Al₂CuLi) phases, studied by TEM, XRD and DSC methods.     

Structural and corrosion characterization of biodegradable Mg–RE (RE=Gd,Y, Nd) alloys

Binary Mg–Gd (up to 5% Gd in mass fraction), Mg–Nd (up to 9% Nd in mass fraction) and ternary Mg–Gd-Y (up to 5% Gd, 1% Y) alloys with precisely determined contents of cathodic impurities (Fe, Ni, Cu, Co) were studied. The alloys were studied in the as-cast state (cooling rate of 500 K/min) and after solution heat treatment (T4). Structures were investigated by optical and scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction and glow discharge spectrometry. Structural investigation was completed by Vickers hardness measurements. Corrosion behavior in the simulated physiological solution (9 g/L NaCl) was assessed by immersion tests and potentiodynamic measurements. It was found that the structures of the as-cast alloys were dominated by fine α-Mg dendrites and eutectic Mg–RE phases. The dendrites exhibited RE-concentration gradients which were most pronounced in the Mg–Gd alloys. For this reason, the T4 heat treatment of the Mg–Gd alloy led to the formation of a new cuboidal Mg₅Gd phase. The corrosion resistance was significantly improved by Gd. The effect of Nd was weak and the addition of Y to Mg–Gd alloys had harmful effect on the corrosion resistance. The T4 heat treatment strongly accelerated the corrosion of Mg–Gd alloys. Its effect on the corrosion of Mg–Nd alloys was not significant. The observed corrosion behavior of the alloys was discussed in relation to their structural states and contents of cathodic impurities.     

The influence of yttrium (Y) on the corrosion of Mg-Y binary alloys

Corrosion of Mg-Y alloys was studied using electrochemical evaluations, immersion tests and direct observations. There were two important effects. In 0.1 M NaCl, the corrosion rate increased with increasing Y content due to increasing amounts of the Y-containing intermetallic. In 0.1 M Na₂SO₄, the corrosion rate decreased with increasing Y content above 3%, attributed to a more protective surface film, despite the intermetallic. The corrosion rate evaluated by electrochemical impedance spectroscopy was somewhat smaller than that evaluated from H evolution as expected from the Mg corrosion mechanism. A mechanism is proposed for filiform corrosion. Direct in situ corrosion observations revealed that a predominant feature was hydrogen evolution from particular parts of the alloy surface.