The effect of Ca on corrosion behavior of heat‐treated Mg–Al–Zn alloy

The effect of 0.5, 1.0, and 1.5 wt% Ca additions on the microstructure and corrosion resistance of the heat‐treated Mg–Al–Zn alloy was investigated. Addition of 0.5 wt% Ca did not form any new phase but suppressed the discontinuous precipitation of the β ‐Mg₁₇Al₁₂ phase by being dissolved in both the second phase and magnesium matrix. In the materials containing higher amounts of Ca, however, metallographic investigation shows that Ca added to Mg–Al–Zn can obviously decrease the size of β ‐Mg₁₇Al₁₂ and forms Al₄Ca intermetallic compounds in the shape of bone‐like morphology. The corrosion tests used include constant immersion technique, and potentiodynamic polarization experiments and salt spray test. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX, and XRD. The results of corrosion tests show that magnesium alloy Mg–Al–Zn with 1.0 wt% Ca addition has the best corrosion resistance behavior.     

Research on intercrystalline corrosion,exfoliation corrosion,and stress corrosion cracking of Al–Zn–Mg–Sc–Zr alloy

The intercrystalline corrosion, exfoliation corrosion (EXCO), and stress corrosion cracking (SCC) of Al–Zn–Mg–Sc–Zr alloy were investigated by means of constant temperature immersion corrosion method, optical microscopy, transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS). The results show that intercrystalline corrosion, and EXCO susceptibility of Al–Zn–Mg–Sc–Zr alloy decrease gradually with increasing of aging time. Corrosion susceptibility order from low to high is as follows: OA > PA > UA > NA. The SCC susceptibility index of PA temper is more than OA temper at the same strain rate. According to TEM observation, with aging time prolonging, a part of η′ phases transform to η equilibrium phases, which become coarse gradually. The distribution discontinuity of the grain boundary precipitates increases. In addition, for Al–Zn–Mg–Sc–Zr alloy without EXCO, the EIS is comprised by a capacitive impedance arc at high frequency and an inductive impedance arc at low frequency. Once EXCO occurs, the EIS is composed of two capacitive impedance arcs at high frequency and at low frequency, respectively.     

Effect of second-phase particle on localized corrosion in Al–Zn–Mg–Cu–Mn alloy friction stir welded joint

The microstructure and corrosion behavior of a high-strength Al–Zn–Mg–Cu–Mn alloy friction stir welded (FSWed) joint were investigated using scanning electron microscope, transmission electron microscope, open-circuit potential test, and potentiodynamic polarization. The weld nugget zone and heat-affected zone (HAZ) of the FSWed joint showed diverse microstructural characteristics including micron-scale intermetallic particles and nanoscale precipitates, leading to different localized corrosion sensitivities. The results of electrochemical tests confirmed the regional difference of corrosion in FSWed joints. Detailed research on the corrosion process revealed that micron-scale Al(Fe,Mn,Si) particles promoted the dissolution of the vicinal matrix to induce pitting corrosion. The directionally distributed intermetallic particles and grain boundary precipitates in HAZ conduced to the development of intergranular microcracks into exfoliation. The matrix precipitates affected the localized corrosion tendency due to regional variation of the redissolution degree.     

Exfoliation corrosion of Al–Zn–Mg–Cu–Zr alloy containing Sc examined by electrochemical impedance spectroscopy

The exfoliation corrosion behavior of an Al–Zn–Mg–Cu–Zr alloy containing Sc artificially aged at 120 °C for 24 h is studied by macroscopic observation techniques and electrochemical impedance spectroscopy (EIS) measurements. After 48 h immersion, the blisters start bursting and delamination initiates, along with the appearance of two time constants in the impedance diagrams. According to the simulation by equivalent circuit, the corrosion rate decreases sharply and then reaches a steady state, which is due to the change of the solution pH and oxide layer thickness, as well as the accumulation of corrosion products.     

Electrochemical corrosion behavior of biomedical Ti–22Nb and Ti–22Nb–6Zr alloys in saline medium

The aims of this study were to investigate the effects of Zr addition and potentiodynamic polarization on the microstructure and corrosion resistance of Ti–22Nb and Ti–22Nb–6Zr alloy samples. The corrosion tests were carried out in 0.9% NaCl at 37 °C and neutral pH value, utilizing the OCP, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. The results of XRD and optical microscopy indicated that the addition of Zr stabilized the β phase, which plays a crucial role in the corrosion resistance improvement of the Ti–22Nb–6Zr alloy. From the polarization curves, it can be seen that the alloys exhibited a wide passive region without the breakdown of the passive films and also low corrosion current densities. In addition, the values of the corrosion current densities and passive current densities decreased with the addition of 6 at% Zr into the Ti–22Nb alloy. The EIS results of these two alloy samples after 1‐h immersion in 0.9% NaCl solution, and being fitted by R_S(Q_PR_P) model, suggested that the corrosion resistance of the passive films improved with the addition of Zr and only a single passive film formed on the surfaces. However, two time constants were observed for the Ti–22Nb and Ti–22Nb–6Zr alloy samples after potentiodynamic polarization, the spectra of which can be fitted using the R_s(Q_o(R_o(Q_bR_b))) model. In addition, the corrosion resistance of the two alloy samples was reinforced significantly because of polarization when compared to the immersed samples. All these observations suggested a nobler electrochemical behavior of the titanium alloys with the addition of Zr element and after polarization.     

Mechanism of (Mg, Al, Ca)-oxide inclusion-induced pitting corrosion in 316L stainless steel exposed to sulphur environments containing chloride ion

The mechanism of oxide inclusion-induced pitting corrosion in 316L stainless steel exposed to sulphur environments containing chloride ions was investigated by scanning electron microscope analysis, electrochemical measurements and scanning Kelvin probe force microscopy (SKPFM). Two inclusion types were observed. The (Mg, Al, Ca)-oxide inclusions play an important role in pitting formation. SKPFM measurement results show that the inclusion sites exhibited a lower surface potential than the matrix. Finally, the schematic representation of the initiation and propagation process of the (Mg, Al, Ca)-oxide inclusion-induced pitting corrosion in 316L stainless steel exposed to sulphur environments containing chloride ions was established.     

Enhancement of the corrosion properties of cold sprayed Ti–6Al–4V coatings on mild steel via silica sealer

The pore formation associated with the cold spray process requires the development of an economical sealer to enhance the corrosion resistance of Ti–6Al–4V coatings on a mild steel substrate. Herein, a sound method is developed to seal pores in the cold sprayed Ti–6Al–4V coatings with silica sealer. Potentiodynamic polarization tests in 3.5 wt% sodium chloride electrolyte were employed to investigate the corrosion resistance of cold sprayed Ti–6Al–4V coatings fabricated at two standoff distances (30 and 70 mm) before and after the sealing process. The polarization resistance of cold sprayed Ti–6Al–4V coatings significantly increased by >80% after the sealing process. The electrochemical responses of cold sprayed Ti–6Al–4V were dependant on sealing the pores with agglomerated silica nanoparticles as observed by scanning electron microscopy and energy-dispersive X-ray analysis. The increase in polarization resistance makes the sealer an effective treatment for cold sprayed Ti–6Al–4V coatings used in marine environments and other engineering applications. This sophisticated sealing process can reduce the deposition cost by reducing the thickness of Ti–6Al–4V coatings and increasing their lifetime on metal components.     

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.     

Corrosion behaviour of Mg–Al alloys with Al–11Si thermal spray coatings

The corrosion resistance of AZ31, AZ80 and AZ91D Mg–Al alloys with Al–11Si thermal spray coatings was evaluated by electrochemical and gravimetric measurements in 3.5 wt% NaCl solution. The changes in the morphology and corrosion behaviour of the Al–11Si coatings induced by a cold‐pressing post‐treatment under 32 MPa were also examined. The as‐sprayed Al–11Si coatings revealed high degree of porosity and poor corrosion protection, which resulted in galvanic acceleration of the corrosion of the magnesium substrates. The application of a cold‐pressing post‐treatment produced more compact Al–11Si coatings with better bonding at the substrate/coating interface and slightly higher corrosion resistance. However, interconnected pores remained in the cold‐pressed coatings due to the low plasticity of the Al–11Si powder and galvanic corrosion of the substrate was observed after immersion in 3.5 wt% NaCl for 10 days.     

A zinc dipping technique for Mg–16Li–5Al–0.5RE alloy at room temperature

In order to obtain a high quality protective plating coating on Mg–16Li–5Al–0.5RE alloy, a zinc dipping technique at room temperature was investigated. The zinc dipping technique included two immersion processes, the primary immersion process and the secondary immersion process. Primary zinc transition layers (PZTLs) and secondary zinc transition layers (SZTLs) were obtained after the primary and secondary immersion processes, respectively. The polarization curves, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and X‐ray diffraction (XRD) were employed to characterize PZTLs and SZTLs. The results indicated that immersion time had obvious effects on PZTLs and SZTLs, the optimum primary immersion time and secondary immersion time were 5 min and 30 s, respectively. Then nickel‐plating coating deposited on the SZTL of Mg–16Li–5Al–0.5RE alloy was investigated via EDS, SEM, polarization curves, and EIS. The results demonstrated that the nickel‐plating coating obviously improved corrosion resistance of Mg–16Li–5Al–0.5RE alloy and had good adherence property with the alloy because of the presence of zinc transition layer on the alloy.     

The electrochemical behavior of Mg–9Al–0.5Zn,Mg–9Al–0.7Zn,and Mg–9Al–1.0Zn in a NaCl solution

In this study, the electrochemical behavior of Mg–9Al–0.5Zn, Mg–9Al–0.7Zn, and Mg–9Al–1.0Zn electrodes in a 0.7 mol L^?1 NaCl solution is evaluated by using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and potentiostatic oxidation. The utilization efficiencies of these materials are also determined. The results show that the Mg–9Al–1.0Zn alloy has the highest corrosion resistance and that Mg–9Al–0.5Zn displays the largest discharge current in the 0.7 mol L ^?1 NaCl solution at 25°C. In addition, the utilization efficiencies of the alloys decrease as follows: Mg–9Al–1.0Zn > Mg–9Al–0.7Zn > Mg–9Al–0.5Zn. This study illustrates that doping Zn into Mg‐Al electrodes increases the corrosion resistance and utilization efficiency but decreases the discharge activity of Mg–Al–Zn anodes when the Zn content is between 0.5% and 1.0%.     

Electrochemical methods study on corrosion of 5% Al–Zn alloy‐coated steel under thin electrolyte layers

The corrosion behavior of a 5% Al–Zn alloy (GF) coated steel was investigated under cyclic wet–dry condition using electrochemical techniques. The wet–dry cycle was conducted by exposure to alternate condition of 1 h immersion in seawater and 7 h drying at ambient temperature. The polarization resistance, R_p of the coating was monitored during the wet–dry cycles by two points AC impedance method and the corrosion potential, E_corr was measured only when the coating was immersed in seawater. Simultaneously, the electrochemical impedance spectroscopy (EIS) of the coating was obtained after it was immersed in different cycles of wet–dry condition. The results obtained by two points AC impedance method had good agreement with those achieved from EIS technique, which proved that the two points AC impedance method was correct and an effective method for atmospheric corrosion study. The monitoring results indicated that the corrosion rate of GF coating firstly increased, then decreased slowly with time, and at last reached a relative steady state with local corrosion under the cyclic wet–dry alternate condition.     

SKPFM and SEM study of the deposition mechanism of Zr/Ti based pre-treatment on AA6016 aluminum alloy

The formation of Zr/Ti based pre-treatments is strongly affected by the microstructure of 6xxx alloys for application in the automotive industry. AA6016 was pre-treated using a fluotitanate/fluozirconate acid based model solution at room temperature. In order to study the mechanism of formation of the pre-treatment, the open circuit potential was measured during the layer formation. The effect of the microstructure was investigated at different stages of the deposition by means of SEM-EDS and scanning Kelvin probe force microscope (SKPFM). The electrochemical behaviour of the pre-treated alloys was characterized by means of open circuit potential measurements and potentiodynamic polarization in aggressive solutions containing chlorides.The deposition of the Zr and Ti containing oxide is an electrochemically driven process. The existence of cathodic sites on the alloy surface is the driving force for the formation of the conversion layer. During the initial stages of dipping in the conversion bath the naturally formed oxide film is removed or thinned due to the presence of fluorides in the bath. Successively, the deposition of the conversion layer initiates onto cathodic intermetallics. The film exhibits lateral growth in the region surrounding the intermetallics, progressively covering the entire surface.The formation of the conversion layer progressively reduces the Volta potential difference between intermetallics and matrix. This potential difference is completely eliminated for relatively long immersion times. This is associated to an improvement of the corrosion behaviour of AA6016, as shown by potentiodynamic polarization curves. Besides, the deposited conversion layer improves the adhesion of an acrylic paint on the alloy.     

Mechanical properties,corrosion behavior,and microstructure of Sr modified Al–9.2Mg–0.7Mn alloys

The influence of strontium (Sr) additions in the form of Mg–Sr master alloys from 0 to 0.6 wt% on the mechanical properties, corrosive nature, and microstructure of Al–9.2Mg–0.7Mn alloys is investigated. The material is studied in a fully annealed (O‐temper) and a sensitizing treatment at 150°C for 7 days. Here we demonstrate that there will be a new phase which might be (Al, Mg)₁₇Sr₂ formed in the as‐cast microstructure. When the Sr content is 0.2 wt%, under the premise that the mechanical properties of completely annealed alloy change little (relative to the matrix: the ultimate tensile strength increases by 8 MPa and the elongation only decrease by 1.6%), the intergranular corrosion resistance is significantly improved. The specific performance is that the mass loss from intergranular corrosion decreases by more than 53% from the addition of 0.2 wt% Sr after sensitizing.     

Corrosion of twin belt and twin roll cast AlMg3Mn alloys

Corrosion of twin belt and twin roll cast AlMg3Mn sheet samples was investigated. The AlMg3Mn sheet samples submitted to immersion tests undergo alkaline pitting around Al–Fe and α_c-Al(Fe,Mn)Si intermetallic particles in the twin roll and twin belt cast samples respectively. The weight loss is higher in the latter and increases with increasing homogenisation temperature for both groups. The twin roll cast AlMg3Mn samples reveal very few and small alkaline corrosion pits and hence much less weight loss in the immersion tests. The pitting activity is governed in the immersion tests by the microgalvanic corrosion activities between the intermetallic particles and the matrix while the anodic particles were inactive.     

Effect of different aging processes on the corrosion behavior of new Al–Cu–Li–Zr–Sc alloys

The intergranular corrosion and exfoliation corrosion behaviors of Al–Cu–Li–Zr–Sc alloys under different aging effects, such as single‐stage aging, strain aging, and double‐stage aging, were studied. Among the three aging treatments, single‐stage aging resulted in the best resistance to corrosion, followed by double‐stage aging; strain aging resulted in the worst corrosion resistance. A 3.5% precooling strain could increase the dislocation density, which promoted the precipitation of corrosion‐prone T₁ phase and increased the corrosion driving force of the alloy. Double‐stage aging made the precipitated T₁ phases finer and more uniform and reduced the number of equilibrium phases at grain boundaries, thus improving the corrosion properties of the alloy. The corrosion susceptibility of the alloy was attributed to the T₁ phase and precipitate‐free zone (PFZ), and the underlying corrosion mechanism was revealed as preferential dissolution of the equilibrium phase at grain boundaries and its surrounding distortion zone, followed by expansion of the PFZ along the grain boundaries, resulting in the development of corrosion from the grain boundaries to the intragranular regions.     

Corrosion behaviour of as-cast ZK40 with CaO and Y additions

The microstructures of as-cast ZK40, ZK40 with 2% (mass fraction) CaO and ZK40 with 1% (mass fraction) Y were investigated, and the intermetallic phase morphology and the distribution were characterised. By having discrete intermetallic particles at the grain boundaries for the ZK40, the microstructure was modified to a semi-continuous network of intermetallic compounds along the grain boundaries for the ZK40 with CaO or Y additions. The CaO was not found in the microstructure. However, Ca was present in Ca₂Mg₆Zn₃ intermetallic compounds which were formed during casting. Hydrogen evolution and electrochemical impedance spectroscopy tests revealed that the addition of CaO slightly enhanced the corrosion resistance whereas Y had a negative effect on the corrosion resistance of ZK40. Immersion tests showed that severe localised corrosion as well as corrosion along the intermetallic compounds played an important role in the corrosion process of ZK40–Y whereas the localised corrosion was not pronounced for ZK40 or ZK40–CaO alloys. Micro-segregation in the α-Mg matrix was notably higher for the ZK40 alloy compared with the modified alloys. The combination of this effect with a possible formation of a more stable corrosion layer for the ZK40–CaO was attributed as the main reason for an improved corrosion resistance for the ZK40–CaO alloy.     

Corrosion performance of Zn–Mg–Al coated steel in accelerated corrosion tests used in the automotive industry and field exposures

The corrosion performance of Zn–Mg(1–2%)–Al(1–2%) (ZMA) coatings has been compared to zinc–iron alloy (galvannealed, GA) and zinc–aluminum coating (Zn–5Al, Galfan) as well as to conventional zinc coatings produced by hot‐dip galvanization (HDG) and electrogalvanization (EG). For this purpose, cosmetic samples (painted and uncoated) and hem‐flange panels were produced. Their corrosion performance was compared in three different accelerated corrosion tests, as regularly used by the automotive industry, e.g., VDA621‐415, N‐VDA (VDA233‐102), and Volvo STD 423‐0014. As can be concluded from our results, the behavior of ZMA coatings was strongly dependent on the testing conditions as well as on the configuration of the samples. The advantageous effect of ZMA coating was more pronounced in open situations than in confined ones, irrespective of the testing conditions. ZMA coatings provided a significant improvement in comparison to conventional coatings in tests involving a significant salt load such as VDA621‐415 or neutral salt spray especially on cosmetic configurations. By contrast, the beneficial effect of ZMA coatings was less obvious in tests with lower salt load (VDA233‐102, Volvo STD423‐0014), particularly when considering cosmetic corrosion on painted samples and corrosion in confinement. Interestingly, no significant differences were observed between samples with varying Al and Mg content in the metallic coating (1–2% each). The results were compared to data from field exposure at stationary sites.     

Experimental and theoretical research on interfacial reaction of solid Co with liquid Al

Interfacial reaction between solid Co and liquid Al was investigated with immersion tests and theoretical modeling. The microstructure characterization indicated that a Co₂Al₅ intermetallic layer was formed at solid–liquid interface during the immersion test. The modeling results indicated that the corrosion rate of a solid metal in a liquid metal was controlled by both the formation and dissolution of the intermetallic layer. In Co–Al reaction system, the formation and dissolution of the Co₂Al₅ layer reached an equilibrium state in a very short time, and the corrosion of the Co matrix was mainly dominated by the dissolution of the Co₂Al₅ layer.     

Impedance spectroscopy studies of electroless Ni–P matrix,Ni–W–P,Ni–P–ZrO2, and Ni–W–P–ZrO2 coatings exposed to 3.5% NaCl solution

Ni–P matrix, ternary Ni–W–P and Ni–P–ZrO₂ coatings, and quaternary Ni–W–P–ZrO₂ coatings were deposited using electroless method from a glycine bath. Their corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) for various immersion times in a 3.5% NaCl solution. From among the investigated coatings, the ternary Ni–W–P coatings show the highest resistance to corrosion in the first hour of exposure to the 3.5% NaCl medium. An addition of ZrO₂ adversely affects the performance of both the Ni–P coatings and the Ni–W–P coatings. For all the coatings, including the ones containing tungsten, a marked decrease in pore resistance (R_por) over time is observed. This means that their corrosion resistance and capacity to protect the substrate decline. On the other hand, after 24 h immersion in the 3.5% NaCl solution the Ni–W–P coating shows the highest low‐frequency impedance modulus (|Z|_{f = 0.01 Hz}). As regards corrosion resistance, the Ni–P coatings and the Ni–W–P coatings perform best.