Localized Corrosion Behavior of Al-Si-Mg Alloys Used for Fabrication of Aluminum Matrix Composites

The relationship between microstructure and localized corrosion behavior in neutral aerated chloride solutions was investigated with SEM/EDAX, conventional electrochemical techniques, and with scanning Kelvin probe force microscopy (SKPFM) for two custom-made alloys with Si/Mg molar ratios of 0.12 and 0.49. In this order, Al₃Fe, Al₃Mg₂, and Mg₂Si intermetallics were identified in the first alloy and Al(FeMn)Si and Mg₂Si particles in the second one. Anodic polarization curves and corrosion morphology showed that the alloy with higher Si/Mg molar ratio exhibited a better corrosion performance and evidence was shown that it had a more corrosion-resistant passive film. The corrosion process for both alloys in aerated 0.1 M NaCl solutions was localized around the Fe-rich intermetallics. They acted as local cathodes and produced dissolution of the aluminum matrix surrounding such particles. Mg₂Si and Al₃Mg₂ exhibited anodic behavior. SKPFM was successfully used to map the Volta potential distribution of main intermetallics. The localized corrosion behavior was correlated with a large Volta potential difference between the Fe-rich intermetallics and the matrix. After immersion in the chloride solution, such Volta potential difference decreased.     

Degradation processes in Al/SiCp/MgAl2O4 composites prepared from recycled aluminum with fly ash and rice hull ash

The degradation characteristics of Al/SiC_p/spinel composites prepared with fly ash (FA) and rice hull ash (RHA) under environmental conditions were investigated. Composite specimens were prepared with recycled aluminum via reactive infiltration in the temperature range 1050–1150 °C for 50–70 min and, in argon atmosphere at a pressure slightly above that of the atmospheric pressure. Results reveal that although both FA and RHA help in preventing SiC_p dissolution and the subsequent chemical degradation of the composites, due to the interaction of native carbon in FA with liquid aluminum, FA‐composites are susceptible to corrosion via Al₄C₃. Moreover, this phase accelerates the degradation process and increases the damage severity. The primary corrosion mechanism in both types of composites is attributed to microgalvanic coupling between the intermetallic Mg₂Si and the matrix. Accordingly, an appropriate control of the Si/(Si + Mg) molar ratio in the aluminum alloy hinders the Mg₂Si corrosion mechanism in both types of composites and a proper FA calcination prevents chemical degradation in FA composites.     

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.     

Cerium‐based thermal conversion treatments on silicon carbide reinforced 2009 aluminum alloy composites

The conversion coatings on SiC_p/2009 aluminum (Al) composites and 2009 Al alloy were obtained by immersing the samples into the cerium (Ce)‐based solutions with varying the chemical concentration (CeCl₃·7H₂O, NaCl), solution pH/temperature, immersing time, and drying temperature. The corrosion inhibition mechanism and kinetics were studied by using the electrochemical techniques and surface analyses. Potentiodynamic polarization tests showed that coatings improved corrosion resistance as compared to untreated samples and environmental factors played important role in the formation of conversion layer. Scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS) analysis revealed that relatively high inhibition efficiency for composite with 25 vol% SiC_p was due to the existence of rather high surface area of cathodic intermetallics/SiC_p, which support to deposit Ce oxides/hydroxides on the entire surface.     

Corrosion behavior of twinning‐induced plasticity (TWIP) steel

In this study, the corrosion behavior of twinning‐induced plasticity (TWIP) steel i.e. Fe‐Mn‐Al‐Si steel subjected to cold‐working (0, 20, and 35%), was examined in acidic (0.1 M H₂SO₄), alkaline (0.1 M NaOH) and chloride‐containing (3.5% NaCl) environments, using potentiodynamic polarization experiments. Interestingly, cold‐working did not show any significant change in the corrosion susceptibility of TWIP steel in all the three environments. However, TWIP steel showed the highest corrosion susceptibility in acidic environment and the lowest in alkaline environment. Scanning electron microscope analysis of the corroded TWIP steel samples revealed high‐localized attack in acidic environment and some pitting corrosion in chloride‐containing solution.     

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.     

Near-net shaped Al/SiCP composites via vacuum-pressure infiltration combined with gelcasting

To solve the problem of difficult machining, the near-net shaped Al/SiC_P composites with high volume fraction of SiC particles were fabricated by vacuum-pressure infiltration. The SiC_P preform with a complex shape was prepared by gelcasting. Pure Al, Al4Mg, and Al4Mg2Si were used as the matrices, respectively. The results indicate that the optimal parameters of SiC_P suspension in gelcasting process are pH value of 10, TMAH content of 0.5 wt.%, and solid loading of 52 vol.%. The Al matrix alloyed with Mg contributes to improving the interfacial wettability of the matrix and SiC particles, which increases the relative density of the composite. The Al matrix alloyed with Si is beneficial to inhibiting the formation of the detrimental Al₄C₃ phases. The Al4Mg2Si/SiC_P composite exhibits high relative density of 99.2%, good thermal conductivity of 150 W·m⁻¹·K⁻¹, low coefficient of thermal expansion of 10.1×10⁻⁶ K⁻¹, and excellent bending strength of 489 MPa.     

Corrosion behavior of under‐, peak‐, and over‐aged 6063 alloy: A comparative study

The mechanical property of age‐hardenable Al‐alloys is governed by the state of ageing, which determines the microstructure and consequently, their corrosion behavior which is a vital aspect for a number of applications. This article presents a comparative assessment of corrosion behavior of under‐, peak‐ and over‐aged Al‐Mg‐Si alloy. Corrosion characteristics have been determined via immersion tests in 0.1 M ortho‐phosphoric acid solution and intergranular corrosion (IGC) tests. Corroded surfaces are examined by field emission scanning electron micrographs‐energy dispersive spectroscopy and 3D optical profilometer. The obtained results reveal that the corrosion rate at a specific immersion time as well as the depth of IGC increases in the order for under‐, peak‐, and over‐aged states. Irrespective of the state of ageing, corrosion loss increases linearly but the rate of corrosion decreases rapidly with increasing immersion time. The dominant mode of corrosion in under‐aged alloy is identified as localized pitting, while peak‐aged is highly susceptible to IGC in contrast extensive pitting corrosion is observed for over‐aged alloy. The observed differences in corrosion behavior are explained considering characteristics of precipitates. Formation of β (Mg₂Si) in case of over‐aged alloy and presence of inclusions like AlFeMnSi particles are found to accelerate pitting corrosion.     

Effect of nano Al (Scx−1Zrx) precipitates on the mechanical and corrosion behavior of Al‐2.5 Mg alloys

Microanalytical, mechanical, and corrosion studies were undertaken to investigate the effect of nano‐precipitates of Al(Sc_x−1Zr_x) on the mechanical and corrosion characteristics of Al 2.5 alloy containing 0, 0.15, 0.3, 0.6, and 0.9 wt% of Sc with 0.15 wt% Zr. Addition of 0.3% Sc significantly increased the yield strength due to small precipitates sizes (5–19 nm) and the high coherency of the nano‐particles. Largest contributor to the strength was grain boundary strengthening caused by pinning of grain boundary precipitates. The alloys showed a good resistance to corrosion in 3.5 wt% neutral chloride solution. The alloy offered a high passivation tendency because of homogeneous coherent nano Al(Sc_x−1Zr_x) precipitates. The nano precipitates interfaces and homogeneously distributed Al₃Sc precipitates offer a high degree of corrosion resistance to Al 2.5 Mg Sc alloys compared to conventional aluminum alloys, such as Al 6061 and Al6013.     

Synthesis and passivation behavior of electroless Ni–P‐CNT composite coating

Carbon nanotubes (CNTs) have high chemical stability, unique hollow nanotube structure, and are believed to be ideal materials for fabricating composites. In this study, Ni–P and Ni–P‐CNT composite coatings were fabricated by electroless plating. Scanning electron microscopy was used to characterize the coatings. The corrosion behavior of Ni–P and Ni–P‐CNT coated samples were evaluated by polarization curves and electrochemical impedance spectroscopy in 3.5 wt% NaCl and 0.1 M H₂SO₄ aqueous solutions at room temperature. The results indicated that incorporation of CNTs in the coating significantly increased corrosion resistance. The role of CNTs in improvement of corrosion resistance of the coating was also discussed.     

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.     

Influence of In Situ Reaction on the Microstructure of SiC<Subscript>p</Subscript>/AlSi7Mg Welded by Nd:YAG Laser with Ti Filler

The effect of in situ reaction on the microstructure of Nd:YAG laser welded joints of aluminum matrix composite SiC_p/AlSi7Mg was studied. Results showed that the laser welding with Ti filler improved the tensile strength of welded joints. Moreover, the laser welding with in situ reaction effectively restrained the pernicious Al₄C₃ forming reaction in the interface between aluminum matrix and reinforcement particles. Simultaneously, the reaction-formed TiC phase distributed uniformly in the weld. This permitted SiC_p/AlSi7Mg composite to be successfully welded by Nd:YAG laser.     

Short‐term oxidation and hot corrosion resistance of a gradient CrN/Cr1−xAlxN coating

A CrN/Cr_1?xAl_xN coating comprised of an inner layer of CrN and an outer layer of Cr_1?xAl_xN with a gradient distribution of Al was deposited on two different alloys by a reactive sputtering method. Oxidation and hot‐corrosion tests of the gradient CrN/Cr_1?xAl_xN coating were performed at different temperatures. The phase compositions and morphologies of the as‐deposited coating and the corrosion products were investigated by using XRD and SEM/EDS. The results showed that the gradient CrN/Cr_1?xAl_xN coating exhibited good oxidation resistance at temperatures above 1000 °C owing to the formation of an α‐Al₂O₃‐rich oxide scale. The coating possessed good hot‐corrosion resistance in molten sulfate because the inner CrN layer could supply enough Cr to form a relatively protective Cr₂O₃ after the Al₂O₃‐enriched scale failed due to its dissolution in the molten sulfate.     

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.     

Corrosion behaviour of carbon steel coated with Zr‐based metallic glass

Zr₅₅Cu₃₀Al₁₀Ni₅ glassy alloy samples were prepared by the single roller melt spinning technique in an argon atmosphere. Amorphous ribbons were coated on carbon steel by the electron beam technique. Subsequently, the coated region was remelted to study its effect on the corrosion performance. The microstructure of the ribbons and of coated steel samples was characterized by X‐ray diffraction (XRD) and optical microscopy (OM). Potentiodynamic polarization studies were carried out in 0.1 M Na₂SO₄ electrolytes at pH = 8 and pH = 2. The coated samples showed a significant increase in corrosion resistance compared to the base carbon steel in both sulphate electrolytes. The pitting corrosion behaviour of the coated samples has been examined in 0.01 M NaCl electrolyte and was found to be poor. Furthermore, the galvanic and crevice corrosion behaviour of these alloys has been evaluated in the chloride electrolyte.     

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.     

Corrosion prevention of passivated iron in 0.1 M NaCl by coverage with an ultrathin polymer coating and healing treatment in 0.1 M NaNO3

An ultrathin, ordered and two-dimensional polymer coating was prepared on a passivated iron electrode by modification of 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. Subsequently, the electrode was healed in 0.1 M NaNO₃. Protection of passivated iron against passive film breakdown and corrosion of iron was examined by monitoring of the open-circuit potential and repeated polarization measurements of the polymer-coated and healed electrode in an aerated 0.1 M NaCl solution during immersion for many hours. Localized corrosion was markedly prevented by coverage with the polymer coating and the healing treatment in 0.1 M NaNO₃. Prominent protection of iron from corrosion in 0.1 M NaCl was observed before the breakdown occurred. The electrode surface covered with the healed passive film and polymer coating was analyzed by contact angle measurement, X-ray photoelectron spectroscopy and electron-probe microanalysis.     

Corrosion Behavior of Novel Al-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composites in Aerated 3.5% Chloride Solution

The corrosion behavior of novel Al-Al₂O₃ MMCs was evaluated in aerated 3.5% NaCl solution through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). These materials corrode almost spontaneously by pitting in aerated 3.5% NaCl solution. Observations indicate that intermetallic particles in these composites appear to play an important role in this pitting corrosion behavior. Addition of Al₂O₃ particles to the base alloys did not appear to increase their corrosion resistance significantly, although corrosion rate was affected by these reinforcement particles. In cyclic polarization experiments, the small difference between the pitting potentials and the repassivation potentials for these MMCs indicated their low resistance to pitting corrosion. EIS measurements indicate adsorption/diffusion phenomena at the interface of the composites. Electrically equivalent circuits are proposed to describe and substantiate the corrosion processes occurring in these materials.     

Electron beam welding of SiCp/2024 and 2219 aluminum alloy

SiC_p/2024 matrix composites reinforced with SiC particles and 2219 aluminum alloy were joined via centered electron beam welding and deflection beam welding, respectively, and the microstructures and mechanical properties of these joints were investigated. The results revealed that SiC particle segregation was more likely during centered electron beam welding (than during deflection beam welding), and strong interface reactions led to the formation of many Al₄C₃ brittle intermetallic compounds. Moreover, the tensile strength of the joints was 104 MPa. The interface reaction was restrained via deflection electron beam welding, and only a few Al₄C₃ intermetallic compounds formed at the top of the joint and heat affected zone of SiC_p/Al. Quasi-cleavage fracture occurred at the interface reaction layer of the base metal. Both methods yielded a hardness transition zone near the SiC_p/2024 fusion zone,and the brittle intermetallic Al₄C₃compounds formed in this zone resulted in high hardness.     

Electrochemical corrosion behaviour of Sn–Ag–Cu (SAC) eutectic alloy in a chloride containing environment

The corrosion behaviour of the Sn_94.5Ag_3.8Cu_1.5 (SAC) eutectic alloy was investigated in 0.1 M NaCl solution by potentiodynamic polarization and impedance spectroscopy measurements and compared with that of the conventional Sn_73.9Pb_23.1 eutectic solder employed for a long time in the packaging of microelectronic components and devices. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to characterize the SAC eutectic alloy prior to and after the electrochemical tests. The electrochemical results indicated that the Sn–Ag–Cu eutectic alloy exhibits better corrosion behaviour than the Sn–Pb eutectic solder in NaCl solution. The presence of a corrosion products layer constituted by tin oxy‐chloride was detected at the surface of both alloys investigated after the electrochemical tests. The better corrosion behaviour of SAC eutectic alloy compared to Sn–Pb eutectic solder is ascribed to the formation of a more compact surface film of corrosion products with improved protective properties owing to the presence of copper and silver, as revealed by EPMA.