Corrosion behaviour of Zn–Al–Mg coated steel sheet in sodium chloride-containing environment

Conventional hot-dip galvanised zinc coated (Z) and novel hot-dip galvanised Zn–Al–Mg alloy coated (ZM) steel sheet samples with a coating thickness of 7 μm each were exposed to standardised salt spray test and cross-sections of the corrosion samples were analysed by using SEM and EDS. On Z corrosion proceeds very fast and the steel substrate is attacked even after 100 h of exposure. ZM samples showed a different behaviour. The entire metallic ZM coating is converted into a stable, adherent aluminium-rich oxide layer, which protects the steel substrate against corrosive attacks. This layer is the main reason for the enhanced corrosion resistance of the ZM coating in sodium chloride-containing environment.     

Microstructure effects on the electrochemical corrosion properties of Mg – 4.1% Ga – 2.2% Hg alloy as the anode for seawater-activated batteries

Mg–Ga–Hg alloy is a new material with special electrochemical corrosion properties that make it ideal for use in seawater-activated battery anodes. The effects of microstructure and phase transformation on the electrochemical properties of the Mg – 4.1% Ga – 2.2% Hg alloy were studied and compared with Mg–Al system alloys. The results show that the Mg – 4.1% Ga – 2.2% Hg alloy, when used as an anode, has an appropriate corrosion potential in a half-cell test and superior electrochemical properties in a single cell assembled with CuCl. The Mg₃Hg and Mg₂₁Hg₅Ga₃ phases of the alloy influence its corrosion behaviour and provide a steady corrosion potential during the discharge process.     

The effect of Mg alloy substrate on “electroless” E-coating performance

The corrosion performance of “electroless” E-coating pre-film on eight different Mg alloys is compared in a 5 wt.% NaCl. The results show that the alloys have different levels of surface alkalization effect, resulting in different thickness of films formed on the alloys. The alloying elements in a Mg alloy do not directly influence the film formation and corrosion performance. Instead, the corrosion resistance of substrate has a significant effect on the degradation or corrosion process of the coated Mg alloy. The corrosion resistance of the substrate Mg alloy can influence the porosity of pre-film during “electroless” E-coating deposition, and it is also a critical factor determining the film corrosion degradation.     

An investigation of the corrosion mechanisms of WE43 Mg alloy in a modified simulated body fluid solution: The influence of immersion time

The corrosion mechanisms and kinetics of WE43 Mg alloy in a modified simulated body fluid (m-SBF) are investigated by electrochemical, hydrogen evolution and analytical techniques. The changes in the impedance response over time are related to four corrosion stages involving the formation of a partially protective corrosion layer and adsorption of Mg intermediates, formation of an inner passive MgO layer with increasing coverage over time, rupture of the corrosion layer and lateral growth of stable pits. ATR-FTIR, XRD and XPS results show the presence of an amorphous carbonated apatite/Mg(OH)₂ mixed corrosion layer.     

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.     

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.     

Corrosion processes of Mg–Y–Nd–Zr alloys in Na2SO4 electrolyte

The corrosion behavior of Mg–Y–Nd–Zr (WE43 commercial alloy) was investigated in Na₂SO₄ electrolyte using potentiodynamic polarization curves, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiles, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) analyzes. SEM and EDS data show that Nd-rich precipitates are mainly located at the grains boundaries. Zr/Y-rich zones are distributed inside the most of the grains. XPS study indicates a depletion of Mg on surface that could be attributed to Mg dissolution and an enrichment of the addition element oxides. XPS and ToF-SIMS analyzes demonstrate that the corrosion films are made up of a magnesium hydroxide (Mg(OH)₂) outer layer and an inner layer containing magnesium oxide (MgO), yttrium oxide (Y₂O₃) and hydroxide (Y(OH)₃), mixed with a small amount of MgH₂, zirconium oxide (ZrO₂) and neodymium oxide (Nd₂O₃). The Y₂O₃ and Y(OH)₃ signals increase slightly in the inner layer towards the corrosion film/alloy interface. Unlike these compounds, ZrO₂ and Nd₂O₃ compound signals are constant inside the inner layer. It is concluded that: (i) neodymium, zirconium and yttrium play a key role in the slightly improved corrosion resistance of the alloy and (ii) the cathodic reaction is slower on WE43 than on pure Mg and AZ91.     

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.     

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.     

Understanding corrosion via corrosion product characterization: I. Case study of the role of Mg alloying in Zn-Mg coating on steel

In the present work the corrosion inhibitive role of Mg in Zn-Mg coatings is considered for different stages of corrosion. Corrosion product characterization was carried out using XRD, IRRAS, MEB-FEG-EDS on technical Zn-Mg coatings after various exposure times in a standardized cyclic corrosion test. The results are compared with artificial corrosion products obtained by chemical and electrochemical synthesis. The importance of the ageing and the role of the atmospheric CO₂ on the nature and morphology of the corrosion products are discussed. The corrosion resistance of Zn-Mg alloy is correlated with the stabilization of simonkolleite against its transformation into smithsonite, hydrozincite, and zincite during ageing cycles in presence of CO₂. The stabilization appears to be due to the preferential formation of magnesium carbonates. Thermodynamic modeling and titrometric analysis demonstrate that Mg²⁺ enhances simonkolleite during dry-wet cycling by (1) removing carbonate from the environment and thereby limiting of the transformation of simonkolleite into zincite, smithsonite, and hydrozincite and by (2) buffering the pH of the electrolyte around 10.2 due to the precipitation of Mg(OH)₂ preventing the dissolution of zinc based corrosion products into soluble hydroxide complexes.     

Duplex-layered manganese phosphate conversion coating on AZ31 Mg alloy and its initial formation mechanism

A duplex-layered phosphate conversion coating was obtained on AZ31 Mg alloy by substituting NaF bath with a citric bath. The morphology, composition and corrosion resistance of the coating were investigated using SEM, EDS, SPM and electrochemical methods. A three-stage mechanism for initial formation of the coating was proposed: Dissolution of the loose oxide film and deposition of Mg₃(PO₄)₂ and AlPO₄, formation of a composite intermediate layer of Mg₃(PO₄)₂, AlPO₄ and Mg(OH)₂, and deposition of manganese phosphate nuclei followed by growth and lamination of the nuclei. The nuclei preferentially deposit at the Al–Mn phase surface and near the grain boundary.     

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.     

An innovative specimen configuration for the study of Mg corrosion

Based on an analysis of galvanic corrosion research, the research reported herein was formulated to examine the measurement of polarisation curves for Mg to develop a methodology whereby reliable polarisation curves can be measured for Mg. Cathodic polarisation curves were measured for high purity Mg in 3.5% NaCl saturated with Mg(OH)₂ using three specimen types: (i) mounted specimens, (ii) specimens hung by fishing line and (iii) plug-in specimens. Cathodic polarisation curves were evaluated to yield the corrosion current density i_corr and the corresponding corrosion rate P_i, which was compared with the corrosion rate evaluated from hydrogen evolution measurements, P_H, and the corrosion rate evaluated by weight loss measurements, P_W. Mounted specimens produced values of corrosion rate, P_i, three times larger than values of corrosion rate, P_i, for plug-in specimens, attributed to crevice corrosion in the mounted specimens. Crevice corrosion in Mg is totally unexpected from prior research. The plug-in specimen configuration was designed to have no crevice and to allow simultaneous measurement of P_H and P_i; P_i was consistently less than P_H and indicated an apparent valence for Mg of 1.45 in support of the Mg corrosion mechanism involving the uni-positive Mg⁺ ion. The plug-in specimen has advantages for the study of Mg corrosion.     

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.     

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.     

Chemistry of corrosion products on Zn-Al-Mg alloy coated steel

Zn-Al-Mg alloy (ZM) coating provides a decisively enhanced corrosion resistance in a salt spray test according to DIN EN ISO 9227 (NSS) compared to conventional hot-dip galvanised zinc (Z) coating because of its ability to form a very stable, well adherent protecting layer of zinc aluminium carbonate hydroxide, Zn₆Al₂(CO₃)(OH)₁₆·4H₂O on the steel substrate. This protecting layer is the main reason for the enhanced corrosion resistance of the ZM coating. Surface corrosion products on ZM coated steel consist mainly of Zn₅(OH)₆(CO₃)₂, ZnCO₃ and Zn(OH)₂ with additions of Zn₅(OH)₈Cl₂ · H₂O and a carbonate-containing magnesium species.     

Corrosion behaviour of silicon-carbide-particle reinforced AZ92 magnesium alloy

The corrosion behaviour of silicon-carbide-particle (SiCp) reinforced AZ92 magnesium alloy manufactured by a powder metallurgy process was evaluated in 3.5 wt.% NaCl solution, neutral salt fog (ASTM B 117) and high relative humidity (98% RH, 50 °C) environments. The findings revealed severe corrosion of AZ92/SiC/0-10p materials in salt fog environment with formation of corrosion products consisting of Mg(OH)₂ and (Mg,Al)_x(OH)_y. The addition of SiCp increased the corrosion rate and promoted cracking and spalling of the corrosion layer for increasing exposure times. Composite materials revealed higher corrosion resistance in high humidity atmosphere with almost no influence of SiCp on the corrosion behaviour.     

Influence of alloyed Nd content on the corrosion of an Al–5Mg alloy

The influence of neodymium (Nd) additions from 0 to 0.17 wt.% on the electrochemical response, corrosion, and hardness of a model 5xxx alloy (Al–5Mg) was studied. The combination of SEM, polarisation, constant immersion and nitric acid mass loss testing, followed by optical profilometry, revealed that Nd had no significant effect on pitting or general corrosion of Al–5Mg; however with Nd additions the extent of intergranular corrosion following sensitisation was decreased substantially. Nd additions also increased alloy hardness and thus microalloying with Nd was shown to improve the properties of Al–5Mg.     

Investigation of the corrosion behaviour of laser-TIG hybrid welded Mg alloys

The paper presents the corrosion behaviour of the laser-tungsten inert gas welded Mg alloy. The effects of microstructure variations of Mg alloy joint on the corrosion behaviour and reliabilities of joint are investigated. The results demonstrate that the effects of some weld defects and precipitated phases on the corrosion behaviour of weld joint are very little, and corrosion resistance of joint is predominantly influenced by grain refinement or interactions of grain refinement and continued net-shaped β phases. Moreover, the corrosion resistance of weld joints and welding mode (butt and lap joint) keep a close relation, which must not be ignored.     

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.