Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiCp)

The influence of silicon carbide (SiCp) proportion and matrix composition on four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) immersed in 1-3.5 wt% NaCl at 22 °C was investigated by potentiodynamic polarization. The kinetics of the corrosion process was studied on the basis of gravimetric measurements. The nature of corrosion products was analysed by scanning electron microscopy (SEM) and low angle X-ray diffraction (XRD). The corrosion damage in Al/SiCp composites was caused by pitting attack and by nucleation and growth of Al₂O₃ · 3H₂O on the material surface. The main attack nucleation sites were the interface region between the matrix and the reinforcement particles. The corrosion process was influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement and saline concentration.     

Enhancement of corrosion resistance of Mg-9 wt.% Al-1 wt.% Zn alloy by a calcite (CaCO3) conversion hard coating

A calcite (CaCO₃) coating on Mg alloy, formed by chemical conversion treatment, was investigated. Aqueous with Ca²⁺ concentration of ∼220 ppm was employed in the chemical conversion treatment. Cross-sectional microstructures of the coated sample after 2 h of treatment revealed a two-layer coating structure. The corrosion current density (I_corr) of the coated sample was approximately two orders of magnitude lower than that of the untreated sample. Electrochemical impedance spectroscopy (EIS) and an appropriate equivalent circuit suggested that each of the layers of the two-layer coating effectively protects Mg alloy against corrosion.     

Determination of corrosion potential of coated hollow spheres

Copper hollow spheres were created on porous iron particles by electro-less deposition. The consequent Ni plating was applied to improve the mechanical properties of copper hollow micro-particles. Corrosion properties of coated hollow spheres were investigated using potentiodynamic polarisation method in 1 mol dm⁻³ NaCl solution. Surface morphology and composition were studied by scanning electron microscopy (SEM), light microscopy (LM) and energy-dispersive X-ray spectroscopy (EDX). Original iron particles, uncoated copper spheres and iron particles coated with nickel were studied as the reference materials. The effect of particle composition, particularly Ni content on the corrosion potential value was investigated. The results indicated that an increase in the amount of Ni coating layer deteriorated corrosion resistivity of coated copper spheres. Amount of Ni coating layer depended on conditions of Ni electrolysis, mainly on electrolysis time and current intensity. Corrosion behaviour of sintered particles was also explored by potentiodynamic polarisation experiments for the sake of comparison. Formation of iron rich micro-volumes on the particle surface during sintering caused the corrosion potential shift towards more negative values. A detailed study of the morphological changes between non-sintered and sintered micro-particles provided explanation of differences in corrosion potential (E_corr).     

Corrosion evaluation of microarc oxidation coatings formed on 2024 aluminium alloy

Dense alumina ceramic coatings of 7 μm thickness were fabricated on 2024 aluminium alloy by microarc oxidation (MAO). The corrosion behaviour of the MAO coated alloys was evaluated using potentiodynamic polarisation and EIS measurements. The results show that the corrosion process of the coated alloy can be divided into three stages: (1) the initial stage (the first 2-6 h of immersion): penetration of corrosion medium into the aluminium alloy was inhibited by coating; (2) the second stage (after 24 h of immersion), corrosion medium penetrated to attack the interface between the substrate and the coating; (3) the final stage (after about 96 h): corrosion process was controlled by the diffusion of corrosion products.     

Corrosion protection of magnesium alloy AZ31 sheets by spin coating process with poly(ether imide) [PEI]

In the present study, the potential of poly(ether imide) as corrosion protective coating for magnesium alloys was evaluated using the spin coating technique. The influence of different parameters on the coating properties was evaluated and the corrosion behaviour of the coatings was investigated using electrochemical impedance spectroscopy. The best corrosion protection was obtained preparing the coatings under N₂ atmosphere, using 15 wt.% solution in N′N′-dimethylacetamide (DMAc) which resulted in a coating of approximately 2 μm thickness, with an initial impedance of 10⁹ Ω cm² and of 10⁵ Ω cm² after 240 h of exposure to a 3.5% NaCl solution.     

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.     

Microstructure and corrosion performance of a cold sprayed aluminium coating on AZ91D magnesium alloy

A pure Al coating was deposited on AZ91D magnesium alloy through cold spray (CS) technique. The microstructure of the coating was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the grain interfaces and subgrains formed close to the particle/particle boundaries. Electrochemical tests revealed that the cold sprayed pure Al coating had better pitting corrosion resistance than bulk pure Al with similar purity in neutral 3.5 wt.% NaCl solution. In addition, a mass-transfer step was found to be involved in the corrosion during 10 days immersion.     

Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy

Phosphate conversion coating, which is considered as an alternative to chromium conversion coating for improving the corrosion resistance of die-cast AZ91D magnesium alloy is studied. The structure and formation mechanism of the coating was investigated in details using ESEM/EDX, XRD, EPMA, ICP and electrochemical method. It was found that the conversion coating was composed of complex phosphate containing magnesium and aluminum and showed amorphous structure. The thickness of the coating was about 10 μm. A possible formation mechanism for phosphate conversion coating was proposed. Local pH rise due to the evolution of hydrogen in the vicinity of micro-cathode attributed to the movement of hydrolysis reaction balance for manganese dihydro phosphate in treating solution and resulted in the phosphate coating precipitated on metal surface. Initial depositing position was related to the microstructure of substrate. Galvanic effect between α phase and β phase caused the flower-like phosphate nuclei preferential depositing on β phase. Ball-like phosphate nuclei deposited mainly in α phase interior due to the micro-galvanic effect within α phase. The primary α phase was removed from the metal matrix due to the corrosion dissolution, which suggested the anodic dissolution mechanism during phosphate conversion coating growth. The inner stress in dried film resulted in random distribution of the network micro-cracks on film surface, the crack site did not correspond to β phase.     

Corrosion behaviour of magnetron sputtered α- and β-Ta coatings on AISI 4340 steel as a function of coating thickness

The corrosion behaviour of magnetron sputtered α- and β-Ta coated AISI 4340 steels was studied with potentiodynamic polarization and electrochemical impedance spectroscopy. The coating porosity was observed to decrease with increasing coating thickness. For coatings less than 10 μm thick (α- or β-Ta), porosity was significant and open pores resulted in severe localized corrosion of the steel substrate, coating delamination, and overall coating failure. Additionally, the β-Ta coatings were more susceptible than the α-phase to delamination. As for the 50 and 100 μm thick α-Ta coatings, the electrochemical impedance behaviour was comparable to that of Ta foil, demonstrating the coating viability and corrosion resistance.     

Effects of Ti/TiN multilayer on corrosion resistance of nickel-titanium orthodontic brackets in artificial saliva

The effects of multilayered Ti/TiN or single-layered TiN films deposited by pulse-biased arc ion plating (PBAIP) on the corrosion behavior of NiTi orthodontic brackets in artificial saliva are investigated. The multilayered Ti/TiN coating is found to exhibit a greater free corrosion potential, much lower passive current density, and no breakdown up to 1.5 V. Moreover, electrochemical impedance spectroscopy (EIS) results indicate that the multilayered Ti/TiN coating has a larger impedance and lower porosity which is believed to be responsible for the exceedingly low metal ion release rate during 720 h exposure in the test solution. Visual inspection of the surfaces reveals different corrosion processes for the TiN and multilayered Ti/TiN coatings.     

Effect of plasma electrolytic oxidation coating on the stress corrosion cracking behaviour of wrought AZ61 magnesium alloy

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10⁻⁶ s⁻¹ and 1 × 10⁻⁷ s⁻¹, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.     

Preparation and hot corrosion behaviour of a MCrAlY + AlSiY composite coating

A MCrAlY + AlSiY composite coating was prepared by arc ion plating. Hot corrosion behaviours of the composite coating and the reference NiCoCrAlYSiB coating were investigated. The results indicate that the composite coating consisted of β-(Ni,Co)Al with dispersed σ-NiCoCr and Cr₃Si in outer layer and Cr-rich phases in inner layer after annealing. As being corroded for 200 h, the composite coating proved better corrosion resistance than that of reference NiCoCrAlYSiB coating. The enhanced corrosion resistance of the composite coating was due to the gradient distribution of Al-enriched outer layer and Cr-enriched inner layer.     

Effect of high temperature heat treatment on intergranular corrosion of AlMgSi(Cu) model alloy

Copper containing 6000-series aluminium alloys may become susceptible to intergranular corrosion (IGC) as a result of improper thermomechanical processing. Effect of cooling rate after solution heat treatment on the corrosion behaviour of a model AlMgSi(Cu) alloy of nominal composition (wt%) 0.6 Mg, 0.6 Si, 0.2 Fe, 0.2 Mn and 0.1 Cu was investigated. Slow cooling rates were simulated by isothermal treatment for predetermined times in lower temperature baths immediately after solution heat treatment. Treatment for 10-100 s at temperatures below 400 °C introduced susceptibility to IGC. Longer heat treatment at the same temperatures introduced susceptibility to pitting. A corrosion resistant time zone was found between the zones of IGC and pitting at temperatures lower than 350 °C. Quenching in water after solution heat treatment prevented IGC. IGC was related to microgalvanic coupling between the noble Q-phase (Al₄Mg₈Si₇Cu₂) grain boundary precipitates and the adjacent depleted zone. Pitting was attributed to coarse particles in the matrix. Possible mechanisms causing the corrosion resistant intermediate zone are discussed. The results indicate possible methods for obtaining increased corrosion resistance of similar alloys by proper thermal processing.     

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.     

Evaluation of the anti-corrosive effect of acid pickling and sol-gel coating on magnesium AZ31 alloy

The effect of different acid pre-treatment procedures on the corrosion of magnesium AZ31 alloy was compared by measuring the amount of hydrogen gas formed when the surface was in contact with aqueous 5% sodium chloride solution. A 4-7 μm thick sol-gel coating prepared by phosphoric acid catalyzed sol-gel processing of a methyltriethoxysilane/tetraethoxysilane mixture was applied to the differently pre-treated magnesium surfaces. The corrosion rate of the alloy decreased by a factor of up to 60 by combination of acid pickling and sol-gel coating. The addition of triethylphosphate or 1,2,4-triazole as corrosion inhibitors led to further improvements. Composition and texture of the films was investigated by scanning electron microscopy and energy dispersive X-ray analysis.     

A study on mechanism of corrosion protection of polyaniline coating and its failure

Polyaniline (PANI) coatings were electrochemically deposited on substrates of stainless steel and platinum in solutions of 0.2 M H₂SO₄ and 0.1 M aniline by cyclic voltammetry. The corrosion protection of the PANI coatings and their failure were investigated in 0.2 M H₂SO₄ solution. It was observed that the corrosion protection ability of the coating to steel substrate was increased with the increase of the coating thickness. The corrosion protection ability was mainly attributed to the passivating effect of PANI due to its oxidizing ability in its emeraldine state. During its operation, the PANI coating in emeraldine state tended to gradually lose its corrosion protection ability. This gradual failure of the PANI coating, but faster than expected, was confirmed to be related to a gradual reduction of the emeraldine PANI and a gradually increased resistance between the PANI coating and the stainless steel substrate. These findings lead to a new mechanism for the corrosion protection of PANI coating and its failure.     

Corrosion behaviour of AIP NiCoCrAlYSiB coating in salt spray tests

NiCoCrAlYSiB coatings were deposited by arc ion plating (AIP) and annealed/pre-oxidised under various conditions. The corrosion behaviour of as-deposited and annealed/pre-oxidised coatings was studied by salt spray testing in a neutral mist of 5 wt% NaCl at 35 °C for 200 h. The results showed that the as-deposited NiCoCrAlYSiB coating behaved poorly while the annealed and pre-oxidised ones performed much better in salt spray tests. The dense microstructure in annealed coatings and formation of α-Al₂O₃ scales on the surface during pre-oxidation improved the corrosion resistance in salt spray test. The corrosion process was investigated from the aspects of corrosion products, and its electrochemical mechanism was proposed as well.     

Effect of surface nanocrystallization on the corrosion behaviour of AISI 409 stainless steel

The influence of surface mechanical attrition treatment (SMAT) on the corrosion behaviour of AISI 409 grade stainless steel in 0.6 M NaCl was studied. SMAT using 2 mm ∅ 316L stainless steel (SS) balls for 15, 30 and 45 min and 5 mm ∅ balls for 15 min offers a better corrosion protective ability. In contrast, treatment using 5 mm ∅ balls for 30 and 45 min and by using 8 mm ∅ balls for 15, 30 and 45 min, induces microstrain and defect density that results in a decrease in corrosion resistance.     

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.     

Langmuir-Blodgett films of dococyltriethylammonium bromide and octadecanol on iron. Deposition and corrosion inhibitor performance in CO2 containing brine

The stability and compressibility of Langmuir films of dococyltriethylammonium bromide (C₂₂TAB) and 1-octadecanol (C₁₈OH) and their mixtures on water surfaces were first investigated. Langmuir-Blodgett films were transferred onto iron substrate. Their effect on corrosion of iron in carbon dioxide containing brine were investigated by electrochemical methods. The C₁₈OH formed a thin homogenous film with molecular area 19.4 Å² at 36 mN m⁻¹ at water surface. The films of C₂₂TAB and C₂₂TAB/C₁₈OH mixtures were less dense, with 31 Å² molecular area at 36 mN m⁻¹ at water surface. The corrosion rate of iron substrate was reduced by 95% by deposition film of C₁₈OH, while the corrosion rate of iron was reduced by 60% for films of C₂₂TAB and C₂₂TAB/C₁₈OH mixtures.