Uniform and pitting corrosion events induced by SCN anions on Al alloys surfaces and the effect of UV light

The influence of the alloying elements on the uniform and pitting corrosion processes of Al-6061, Al–4.5%Cu, Al–7.5%Cu, Al–6%Si and Al–12%Si alloys was studied in 0.50 M KSCN solution at 25 °C. Open-circuit potential, Tafel polarization, linear polarization resistance (LPR) and ICP-AES measurements were used to study the uniform corrosion process on the surfaces of the tested alloys. Cyclic polarization, potentiostatic current-time transients and impedance techniques were employed for pitting corrosion studies. Obtained results were compared with pure Al. Passivation kinetics of the tested Al samples were also studied as a function of applied potential, [SCN⁻] and sample composition by means of potentiostatic current transients. The induction time, after which the growth of stable pits occurs, decreased with increasing applied potential and [SCN⁻]. Regarding to uniform corrosion, alloyed Cu was found to enhance the corrosion rate, while alloyed Si suppressed it. Alloying elements of the tested samples diminished pitting attack to an extent depending on the percentage of the alloying element in the sample. Among the investigated materials, Al–Si alloys exhibited the highest corrosion resistance towards uniform and pitting corrosion processes in KSCN solutions. The passive and dissolution behaviour of Al was also studied under the conditions of continuous illumination (300–450 nm) based on cyclic polarization and potentiostatic techniques. The incident photons had a little influence on pit initiation and a marked effect on pit growth. These explained in terms of a photo-induced modification of the passive film formed on the anode surface, which render it more resistant to pitting. The effects of UV photons energy and period of illumination on the morphology of the pitted surfaces were also studied.     

Pitting corrosion of Al and Al–Cu alloys by ClO4 ions in neutral sulphate solutions

The influence of various concentrations of NaClO₄, as a pitting corrosion agent, on the corrosion behaviour of pure Al, and two Al–Cu alloys, namely (Al + 2.5 wt% Cu) and (Al + 7 wt% Cu) alloys in 1.0 M Na₂SO₄ solution was investigated by potentiodynamic polarization and potentiostatic techniques at 25 °C. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) examinations of the electrode surface. In free perchlorate sulphate solutions, for the three Al samples, the anodic polarization exhibits an active/passive transition. The active dissolution region involves an anodic peak (peak A) which is assigned to the formation of Al₂O₃ passive film on the electrode surface. The passive region extends up to 1500 mV with almost constant current density (j_pass) without exhibiting a critical breakdown potential or showing any evidence of pitting attack. For the three Al samples, addition of ClO₄⁻ ions to the sulphate solution stimulates their active anodic dissolution and tends to induce pitting corrosion within the oxide passive region. Pitting corrosion was confirmed by SEM examination of the electrode surface. The pitting potential decreases with increasing ClO₄⁻ ion concentration indicating a decrease in pitting corrosion resistance. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al > (Al + 2.5 wt% Cu) alloy > (Al + 7 wt% Cu) alloy. Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO₄⁻ ion concentration and applied step anodic potential, while it decreases with increasing %Cu in the Al samples. The inhibitive effect of SO₄²⁻ ions was also discussed.     

Corrosion, passivation and breakdown of passivity of Al and Al-Cu alloys in gluconic acid solutions

The pitting corrosion of Al, and two Al-Cu alloys, namely (Al-2.5% Cu) and (Al-7.0% Cu) was investigated in gluconic acid (HG) solution through linear polarization and cyclic voltammetry techniques complemented with ex situ EDX and SEM examinations of the electrode surface. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method of chemical analysis was used to study the effect of alloyed Cu on the rate of the uniform corrosion of Al in these solutions. Results obtained from ICP were compared with those obtained from polarization measurements. For the three Al samples, the anodic responses did not involve active/passive transition due to spontaneous passivation. Addition of HG induced pitting (confirmed from SEM) within the passive oxide film due to the aggressive attack of gluconate (G⁻) anions. Relationships between pitting potential (E_pit), HG concentration, temperature, pH and potential scan rate were established. Alloyed Cu was found to enhance uniform corrosion, while it suppressed pitting attack. Local acidification model is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of G⁻ anions.     

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.     

Electrochemical behaviour of heat-treated AlZnMg alloys in chloride solutions containing sulphate

The electrochemical corrosion and passivation of Al5Zn1.7Mg0.23Cu0.053Nb alloys, submitted to different heat treatments (cold-rolled, annealed, quenched and aged, and quenched in two steps and aged), in sulphate-containing chloride solutions, has been studied by means of cyclic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). The cyclic polarization curves showed that sulphate addition to the chloride solution produced a poor reproducible shift of the breakdown potential to more positive potentials. The repassivation potentials, much more reproducible, and practically separating the passive from the pitting potential region, were slightly displaced in the negative direction with that addition. When the alloys were potentiodynamically polarized in the passive potential region, sulphate was incorporated in the oxide film, thus precluding chloride ingress. In addition, Zn depletion was favoured, whereas Mg losses were avoided. Different equivalent circuits corresponding to different alloys and potentials in the passive and pitting regions were employed to account for the electrochemical processes taking place in each condition. This work shows that sulphate makes these alloys more sensitive to corrosion, increasing the fracture properties of the surface layer and favouring the pitting attack over greater areas than chloride alone.     

Pitting corrosion of friction stir welded aluminum alloy thick plate in alkaline chloride solution

The pitting corrosion of different positions (Top, Middle and Bottom) of weld nugget zone (WNZ) along thickness plate in friction stir welded 2219-O aluminum alloy in alkaline chloride solution was investigated by using open circuit potential, cyclic polarization, scanning electron microscopy and atomic force microscope. The results indicate that the material presents significant passivation, the top has highest corrosion potential, pitting potential and re-passivation potential compared with the bottom and base material. With the increase of traverse speed from 60 to 100 mm/min or rotary speed from 500 to 600 rpm, the corrosion resistance decreases.     

The corrosion behavior of sputter-deposited aluminum-tungsten alloys

Thin films on aluminum-tungsten alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source, on glass and sapphire substrates. Completely amorphous films were obtained in the Al₈₀W₂₀-Al₆₇W₃₃ composition range. Passivity and corrosion behavior of amorphous Al-W alloys were investigated in 1 M deaerated hydrochloric acid solution using polarization and impedance spectroscopy measurements and have been correlated with the properties of pure alloy components. Tungsten and sputter-deposited Al-W thin films are inherently passive materials while aluminum undergoes pitting corrosion in hydrochloric acid solution. The passive film formed at the OCP on each alloy possesses excellent electric and dielectric properties comparable to those of the isolating film on tungsten. The absolute impedance increases with increasing tungsten content in the alloy. According to electrochemical polarization measurements, alloying Al with W in solid solution significantly enhances the material's resistance to pitting corrosion by shifting the breakdown potential above 2000 mV (Al₆₇W₃₃) and lowering the corrosion rate at the OCP by more than two orders of magnitude. The most likely mechanism explaining the passivity of amorphous Al-W alloys, the Solute Vacancy Interaction Model (SVIM), involves the formation of complexes between highly oxidized solute atoms (W⁺⁶) and mobile cation vacancies, which restrict the transport of Cl⁻ through the oxide film and inhibit its breakdown in hydrochloric acid solution. The role that film stress relaxation effects and microscopic defects in amorphous Al-W films, of the some composition, and deposited on various substrates play in their corrosion resistance is discussed.     

Pitting corrosion of AZ91D and AJ62x magnesium alloys in alkaline chloride medium using electrochemical techniques

Pitting corrosion of AZ91D-DC (die cast), AZ91D-ESTC (electromagnetically-stirred billets; thixocast), AZ91D-SFTC (billets solidified freely; thixocast) and AJ62x-DC (die cast) specimens was studied in alkaline chloride medium (0.1 M NaOH + 0.05 M NaCl + 2 ml H₂O₂) at 25 °C and pH 12.3. Electrochemical noise (EN) measurements have confirmed to some extent the polarization results (passive zone, pitting current and average corrosion rate). AZ91D-ESTC specimens have shown the best corrosion resistance followed by AZ91D-SFTC and AZ91D-DC. Intense corrosion rate was observed at the beginning of experiment and it decreased with immersion period. Localized corrosion with dense pitted areas was observed during a 16 h immersion period for AZ91D-SFTC and AZ91D-ESTC specimens. The best passive zone was observed for AJ62x-DC because of the corrosion products formed at the surface. After a 6 h of immersion, EN analyses in the frequency domain indicated a change in the sub-mode of pitting, becoming a classical pitting type, for AJ62x and AZ91D die cast specimens. Analysis with the scanning reference electrode technique (SRET) has showed that AJ62x specimen presented the biggest potential difference between the most active anode and the most active cathode and more numerous zones of intense localized corrosion. It was also found that the lifetime of the pit appeared after 8:20 h of immersion was longer for AJ62x and AZ91D die cast specimens being associated to a classical pitting.     

Pitting corrosion of lead in sodium carbonate solutions containing NO3 ions

Pitting corrosion of Pb in Na₂CO₃ solutions (pH=10.8) containing NaNO₃ as a pitting corrosion agent has been studied using potentiodynamic anodic polarization, cyclic voltammetry and chronoamperometry techniques, complemented with scanning electron microscopy (SEM) examinations of the electrode surface. In the absence of NO₃⁻, the anodic voltammetric response exhibits three anodic peaks prior to oxygen evolution. The first anodic peak A₁ corresponds to the formation of PbCO₃ layer and soluble Pb²⁺ species in solution. The second anodic peak A₂ is due to the formation of PbO beneath the carbonate layer. Peak A₂ is followed by a wide passive region which extends up to the appearance of the third anodic peak A₃. The later is related to the formation of PbO₂. Addition of NO₃⁻ to the carbonate solution stimulates the anodic dissolution through peaks A₁ and A₂ and breaks down the dual passive layer prior to peak A₃. The breakdown potential decreases with an increase in nitrate concentration, temperature and electrode rotation rate, but increases with an increase in carbonate concentration and potential scan rate. Successive cycling leads to a progressive increase in breakdown potential. The current/time transients show that the incubation time for passivity breakdown decreases with increasing the applied anodic potential, nitrate concentration and temperature.     

Effect of Ni content on the corrosion behavior of Cu-Ni alloys in neutral chloride solutions

The effect of systematic increase of Ni content on the electrochemical behavior of the Cu-Ni alloys in neutral chloride solutions was investigated. The pitting corrosion behavior of Cu-Ni alloys with different Ni contents, namely, 5, 10, 30 and 65 mass% Ni, in a stagnant 0.6 mol dm⁻³ NaCl solution of pH 7.0 was studied. The effect of chloride ion concentration on the electrochemical behavior of these alloys was also investigated. The results show that the increase in nickel content decreases the corrosion rate of the alloys in the neutral chloride solution. The increase of chloride concentration up to 0.3 mol dm⁻³ increases the corrosion rate. At higher concentrations ([Cl⁻] > 0.3 mol dm⁻³) the corrosion rate decreases due to the hydrolysis of Cu(I) chloride to form the passive Cu(I) oxide film. The breakdown potential depends on the chloride ion concentration and the nickel content of the alloy. For these investigations conventional electrochemical techniques and electrochemical impedance spectroscopy (EIS) were used. The impedance measurements have shown that the increase of the Ni content and the immersion time of the alloys in the chloride solution increase the corrosion resistance of the alloys. The experimental impedance data were fitted to theoretical values according to a proposed equivalent circuit model.     

Integrated AFM and SECM for in situ studies of localized corrosion of Al alloys

Rolled 3xxx series Al alloys, e.g., EN AW-3003, are generally used as fin or tube material in heat exchangers for automobiles. With reducing fin thickness, maintaining fin material integrity is of increasing importance. This study aimed at exploring the differences in intrinsic corrosion properties between EN AW-3003 and a newly developed Al-Mn-Si-Zr fin alloy using state-of-the-art local probing techniques. Volta potential mapping of both alloys by scanning Kelvin probe force microscopy (SKPFM) indicates a cathodic behaviour of constituent intermetallic particles (>0.5 μm) relative to the alloy matrix. Compared to EN AW-3003, the Al-Mn-Si-Zr alloy has a smaller number of particles with large Volta potential difference relative to the matrix. In situ atomic force microscopy (AFM) measurements in slightly corrosive solutions showed extensive localized dissolution and deposition of corrosion products on EN AW-3003, and only a small number of corroding sites and “tunnel-like” pits on Al-Mn-Si-Zr. Probing the ongoing localized corrosion process by integrated AFM and scanning electrochemical microscopy (SECM) revealed more extensive local electrochemical activity on EN AW-3003 than on Al-Mn-Si-Zr. In all, the lower corrosion activity and smaller tunnel-like pits resulted in lower material loss of the Al-Mn-Si-Zr alloy, a beneficial property when striving towards thinner fin material.     

Pitting and stress corrosion cracking behavior in welded austenitic stainless steel

The effect of microstructural changes in 304 austenitic stainless steel induced by the processes of gas tungsten arc welding (GTAW) and laser-beam welding (LBW) on the pitting and stress corrosion cracking (SCC) behaviors was investigated. According to the in situ observations with scanning reference electrode technique (SRET) and the breakdown potentials of the test material with various microstructures, the GTAW process made the weld metal (WM) and heat-affected zone (HAZ) more sensitive to pitting corrosion than base metal (BM), but the LBW process improved the pitting resistance of the WM. In the initiation stage of SCC, the cracks in the BM and HAZ propagated in a transgranular mode. Then, the crack growth mechanism changed gradually into a mixed transgranular + intergranular mode. The cracks in the WM were likely to propagate along the dendritic boundaries. The crack initiation rate, crack initiation lifetime and crack propagation rate indicated that the high-to-low order of SCC resistance is almost the same as that for pitting resistance. High heat-input (and low cooling rate) was likely to induce the segregation of alloying elements and formation of Cr-depleted zones, resulting in the degradation in the corrosion resistance.     

Role of alloyed silicon and some inorganic inhibitors in the inhibition of meta-stable and stable pitting of Al in perchlorate solutions

The study of both meta-stable and stable pitting events on the surface of pure Al and three Al–Si alloys, namely (Al + 6%Si), (Al + 12%Si) and (Al + 18%Si) alloys, was carried out in deaerated neutral NaClO₄ solutions of various concentrations (10⁻⁴–10⁻² M). Measurements were carried out under the effect of various experimental conditions using potentiodynamic anodic polarization and potentiostatic techniques. The results presented below showed that meta-stable pits (appeared as oscillations in current) form at potentials close to the pitting potential (E _pit) and during the induction time for stable pit formation. Various factors affecting the rate of meta-stable and stable pits were studied. The presence of Si as an alloying element in Al reduces the rate of formation of meta-stable pits, corresponding to a reduction in the probability of developing stable pits, and an increase in the pitting potential results. The inhibitive effects of chromate, silicate, molybdate and tungstate on pitting corrosion in Al were also studied. Results obtained showed that these known inhibitors retard both meta-stable and stable pitting events. This makes attainment of stable pit growth more difficult in presence of these inhibitors.

Influence of microstructure and composition on the corrosion behaviour of Mg/Al alloys in chloride media

The influence of the microstructure and aluminium content of commercial AZ31, AZ80 and AZ91D magnesium alloys was evaluated in terms of their corrosion behaviour in an aerated 3.5 wt.% NaCl solution at 25 °C. The corrosion process was monitored by electrochemical impedance spectroscopy (EIS). The surface was characterized by scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM) and low-angle X-ray diffraction (XRD). The extent of corrosion damage was strongly dependent on the aluminium content and alloy microstructure. Two key factors were observed for the lowest corrosion rates, which occurred for the AZ80 and AZ91D two-phase alloys: the aluminium enrichment on the corroded surface for the AZ80 alloy, and the β-phase (Mg₁₇Al₁₂), which acted as a barrier for the corrosion progress for the AZ80 and AZ91D alloys. Surface potential maps suggested that, between the β-phase and the α-matrix, the galvanic coupling was not significant.     

Complexities of corrosion behaviour of copper-nickel alloys under liquid impingement conditions in saline water

Flow-assisted electrochemical corrosion of Cu-Ni disc electrodes, subjected to a submerged impinging jet in saline water, was evaluated and quantified using the anodic current densities (j_a), corrosion potentials (E_corr), interfacial capacitances (C) and polarization resistances (R_p). The Reynolds numbers in the range of applied impinging velocities suggested that the flow between the solution and the electrode is turbulent in the systems investigated.The role of Ni content in the resistance of spontaneously formed barrier oxide film and diethyldithiocarbamate inhibitor film on Cu-Ni alloys (10, 20, 30 and 40 at.% Ni) was examined and evaluated using electrochemical impedance spectroscopy in dependence of the impinging velocity and the immersion time.The results are discussed in terms of the corrosion mechanisms and their relevance to the use of Cu-Ni alloys in desalination plants and in the marine engineering field because fluid hydrodynamics plays an important role in Cu-Ni alloys application. Based on the analysis presented in the paper, it is anticipated that the Cu-10Ni alloy is the best choice for application in the marine environment.     

Evaluation of corrosion and erosion-corrosion resistances of mild steel in sulfide-containing NaCl aerated solutions

This paper reports results of potentiodynamic polarization, electrochemical impedance measurements and erosion-corrosion of mild steel in aerated sulfide containing 3.5% NaCl solutions at room temperature. The pitting corrosion behavior was studied in NaCl solution containing 0.001, 0.005 and 0.010 M Na₂S, using potentiodynamic polarization and electrochemical impedance spectroscopy. The erosion-corrosion resistance was evaluated after rotating the samples in sulfide polluted NaCl solution for 24 h at a velocity of 300, 600 and 900 ppm using a rotating disc electrode. Results showed that the presence of sulfide ions in NaCl solution resulted in a significant increase in the corrosion attack due to the local acidification caused by iron sulfide formation. The localized replacement of the protective Fe-oxide film by a non-protective iron sulfide film is responsible for the pitting and erosion-corrosion attack. The study concluded that the higher the concentration of sulfide in NaCl solution, the lower the resistance to pitting and erosion-corrosion. Moreover, increasing the solution rotating velocity affects negatively the erosion-corrosion resistance.     

Electrochemical corrosion properties of metal alloys used in orthopaedic implants

This study concerns an investigation of the corrosion behavior of 316 stainless steel, CoCrMo and Ti6Al4V alloys in simulated body conditions (ringer lactate) at 37 °C by the use of Tafel plots, mixed potential and electrochemical impedance spectroscopy (EIS). Ti6Al4V alloy has the highest corrosion resistance followed by CoCr alloy. Ti6Al4V–CoCrMO was the best couple for galvanic corrosion with the minimum galvanic potential and current values according to mixed potential theory and Tafel method. It was concluded that Ti6Al4V was the most suitable material for implant applications in the human body.     

Pitting and repassivation processes of Al---Zn---Mg alloys in chloride solutions containing sulphate

The mechanism of localized corrosion of Al—5.03%Zn—1.67%mg—0.23%Cu alloy submitted to different heat treatments (annealed (A); cold-rolled (ST); quenched (F); quenched and aged (B); and quenched in two stages and aged (C)) in NaCl, Na₂SO₄ and in NaCl---Na₂SO₄ solution has been studied by cyclic polarization (CP), open circuit potential measurements, chronocoulometry, SEM and EDX. The electrochemical results in NaCl were consistent with the assays of SCC resistance of alloys B and C previously reported in the literature. Sulphate addition to chloride solutions produced a considerable shift of the breakdown potential in the positive direction. However, the repassivation potential, much more reproducible than the latter, was only slightly shifted in the negative direction. The role of sulphate was related with the oxide presence, its incorporation in the film precluding significant chloride penetration. However, the film conductivity apparently increased with potential in the presence of sulphate, finally making pit propagation possible and resulting in a much more intense pitting attack.     

Corrosion rates and negative difference effects for Al and some Al alloys

The rotating disc-ring technique with a Pt ring was used to quantitatively detect hydrogen evolved on an Al-disc electrode during spontaneous corrosion at open-circuit potential or during anodic polarization of an Al disc to study the unusual increase of H2 evolution with increase in anodic current (the so called negative difference effect). Experiments were made with high purity Al (99.999%), technical grade Al (99.5%) (Al–T) and Al–In alloy (0.074% In) in deaerated 0.5m aqueous NaCl solution (pH6.5) and NaCl solutions acidified with HCl to pH values of 3.0, 2.0 and 1.0. It was shown, in neutral solutions, that the corrosion rates of all three materials are controlled by the rate of water molecule dissociation or in more acid solutions, by the combined water molecule dissociation rate and H+ ion discharge rate. The corrosion rates of all three materials in the range of 3–10Acm2, was the lowest for 99.999% Al. The impurities in Al–T and Al–In increase the hydrogen evolution rate at cathodic polarizations in a similar manner, but very differently affect the negative difference effect. High purity Al and Al–T have a similar negative difference effect (8–15%), while the presence of In decreases it to only about 1%. No pitting and no negative difference effect were observed at potentials more negative than Epit. Therefore, the negative difference effect is connected with the anodic pitting dissolution mechanism. Several aspects of this problem are discussed in more detail.     

Effect of Al content on the corrosion behavior of Mg-Al alloys in aqueous solutions of different pH

The effect of systematic increase of Al content on the electrochemical behavior of the Mg-Al alloys in aqueous solutions of different pH was investigated. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy, EIS, were used to investigate the electrochemical behavior of the alloys in aqueous solutions. The results have shown that Mg-5Al is easily corroded due to the microgalvanic effect between α-phase and β-phase, its corrosion rate is even higher than that of Mg itself. The increase of Al content increases the corrosion resistance of the alloy due to the formation of the β-phase (Mg₁₇Al₁₂) together with the Mg α-phase. The ranking of the corrosion rate of these alloys was Mg-5Al > Mg > Mg-10Al ≅ Mg-15Al. The corrosion rates of the alloys in acidic solutions are pronouncedly high compared to those measured in neutral or basic solutions. The impedance measurements are in consistence with the polarization techniques and the impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface.