Behaviour of zinc in electropolished and etched Al-Zn alloys and effect on corrosion potential

The enrichments of zinc developed in binary, solid solution Al-0.3at.%Zn, Al-0.4at.%Zn and Al-1at.%Zn alloys by electropolishing and alkaline etching are examined using Rutherford backscattering spectroscopy and medium energy ion scattering with additional interest in how such enrichments affect the corrosion potentials of the alloys. During alkaline etching in 0.1 M sodium hydroxide solution, significant enrichments of zinc arise in the alloy, similar to that achieved by an anodizing treatment. However, enrichment is unusually low following electropolishing in perchloric acid solution. Contrary to the effect of enriched copper in Al-Cu alloys, zinc enrichment has minor influence on the corrosion potentials of etched alloys in ammonium pentaborate solution, which remain roughly within ±100 mV of those of non-enriched alloys.     

Enrichment behaviour of gallium in heat and surface treatments of Al–Ga foils

The behaviour of gallium is examined after heat and surface treatments of aluminium foils containing either 120 or 1300 ppm gallium, with low enrichments of gallium at the surfaces arising from foil manufacture. Vacuum heat treatment at 823 K for 20 ks caused negligible additional enrichment, probably associated with the high solubility of gallium in aluminium. Subsequent alkaline etching in 0.25 M sodium hydroxide for 60 s at 348 K increased significantly enrichments, to about 3.2×10¹⁴ and 3.6×10¹⁵ Ga atoms cm⁻² for the foils containing 120 and 1300 ppm gallium respectively. The enrichments were probably located in both the metal and the overlying etching products. In contrast, electropolishing in perchloric acid/ethanol eliminated pre-existing enrichments, probably due to activation of the foil, with no enrichment developing during the electropolishing procedure. Barrier-type anodic films formed on the alkaline-etched foils contained gallium species, with increased concentrations at the film surface. Gallium enriched the metal region immediately beneath the anodic film during anodizing and led eventually to detachment of the growing film from the etched foil, followed by growth of a second film beneath the detached film. The combined amounts of gallium in the anodic film and enriched metal layer were similar to the levels at the surface regions of the etched foils. The anodic films formed on the electropolished foils, with no pre-existing enrichment, were free of gallium species and remained attached to the foils, since insufficient gallium could enrich in the metal beneath the anodic film during the relatively short period of anodizing.     

Generation of copper nanoparticles during alkaline etching of an Al-30 at.%Cu alloy

A mechanism of formation of copper nanoparticles is proposed for alkaline etching of a sputtering-deposited Al-30 at.%Cu alloy, simulating the equilibrium θ phase of 2000 series aluminium alloys. Their formation involves enrichment of copper in the alloy beneath a thin alumina film, clustering of copper atoms, and occlusion of the clusters, due to growth of alumina around the clusters, to form nanoparticles. The proposed mechanism is supported by medium energy ion scattering, Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy of the alloy following immersion in the sodium hydroxide solution, which disclose the enrichment of copper and the generation of the nanoparticles in the etching product of hydrated alumina. The generation of the nanoparticles is dependent upon the enrichment of copper in the alloy in a layer of a few nanometres thickness, with no requirement for bulk de-alloying of the alloy.     

Influence of surface treatment on detachment of anodic films from Al–Mg alloys

Previous work of the authors has revealed detachment of anodic films during anodizing of electropolished, solid-solution Al–3 at.%Mg alloy at constant current density, with the possibility of the electropolishing pre-treatment having a significant influence on the loss of the film material. Here, anodic films were formed on a similar alloy, with pre-treatment by electropolishing, alkaline etching or mechanical polishing. The results disclose the detachment of the anodic film for all pre-treatments, thus demonstrating that prior electropolishing is not essential for initiation of detachment. However, the occurrence of detachment was sensitive to the current density, with lower current densities promoting detachment at earlier stages of growth of the anodic film for a particular pre-treatment.     

Initial corrosion protection of Zn–Mn alloys electrodeposited from alkaline solution

The effects of a deposition current density (c.d.) on the corrosion behaviour of Zn–Mn alloy coatings, deposited from alkaline pyrophosphate solution, were investigated by atomic absorption spectrophotometry (AAS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical microscopy, electrochemical impedance spectroscopy (EIS) and measurement of corrosion potential (E_corr). XRD analysis disclosed that zinc hydroxide chloride was the main corrosion product on Zn–Mn coatings immersed in 0.5 mol dm⁻³ NaCl solution. EIS investigations revealed that less porous protective layer was produced on the alloy coating deposited at c.d. of 30 mA cm⁻² as compared to that deposited at 80 mA cm⁻².     

Inhibitive effect of sodium eperuate on zinc corrosion in alkaline solutions

The effect of sodium eperuate prepared from Wallaba (Eperua falcata Aubl) extract on zinc corrosion was investigated in alkaline solutions with chloride ions (i.e., simulated concrete pore solutions) by using electrochemical techniques. Sodium eperuate inhibits the corrosion of zinc in 0.1 M NaCl solutions with pH 9.6. As its concentration increases to 1 g/L, the inhibition efficiency reaches approximately 92%. In alkaline solutions with pH 12.6, sodium eperuate has no adverse effect on passivity of zinc, and retards the chloride attack. These suggest that sodium eperuate is an effective inhibitor for the protection of zinc in alkaline environments.     

The effect of water content on the electropolishing behavior of Inconel 718 alloy in perchloric–acetic acid mixtures

The electropolishing behavior of Inconel 718 alloy was studied in perchloric–acetic acid mixtures using a rotating disc electrode. The electropolishing behavior of an Inconel 718 weld, which was prepared with electron beam welding, was also investigated. A leveled but not brightened surface can be achieved when Inconel 718 alloy is potentiostatically polished in the acid mixture with 20 vol.% perchloric acid. Interestingly, a brightening effect could be obtained in this acid mixture by adding 10–50 ml l⁻¹ water or by being at rest at room temperature for several days. When electropolishing in acid mixture with 40 vol.% perchloric acid, leveling and brightening of the Inconel 718 surface can be detected. When electropolished in this acid mixture, the fusion zone of the Inconel 718 weld cannot be leveled together with its nearby base metal. Nevertheless, a good polished surface of the Inconel 718 weld can be achieved with the acid mixture with 20 vol.% perchloric acid by adding 40 ml l⁻¹ of water. Electropolishing was performed in the limiting diffusion current region where the transport of water to the anode seemed to be the rate-determining process.     

Electrodeposition of a protective copper/nickel deposit on the magnesium alloy (AZ31)

An environmental-friendly Cu electrodeposition process was proposed for the Magnesium alloy (AZ 31). Experimental results show that a good bonding between Cu deposit and Mg alloy surface can be achieved with a pretreatment of galvanostatic etching and then copper electrodeposition in the alkaline copper-sulfate plating bath. Microstructures between Cu deposit and Mg alloy substrate were examined with scanning electron and energy-filtering transmission electron microscopes (SEM and EF-TEM). The Cu-deposited Mg alloy can be further electroplated in acidic Cu and Ni plating baths to acquire a protective Cu/Ni deposit.     

Microbially influenced corrosion and inhibition of nickel–zinc and nickel–copper coatings by Pseudomonas aeruginosa

The present study describes the effect of Pseudomonas aeruginosa on the corrosion rate of nickel–zinc and nickel–copper alloy coatings. The presence of bacteria was associated with decreases in R_ct values, suggesting that P. aeruginosa promoted the corrosion of nickel–copper alloy coatings. However, R_ct values of nickel–zinc coatings increased in response to bacterial inoculation, corresponding to a decrease in corrosion rate for nickel–zinc alloy coatings. Our results suggest that the activity of P. aeruginosa facilitated the corrosion of nickel–copper alloy, while serving a protective function for the nickel–zinc alloy.     

Enrichment factors for copper in aluminium alloys following chemical and electrochemical surface treatments

Surface treatments, including chemical polishing, alkaline etching, acid pickling, and electropolishing, of aluminium and copper-containing aluminium alloys lead to enrichment of solid solution copper in the metal just beneath the residual oxide films of the treatment processes. The paper presents the enrichment factor for copper as a function of the copper content of the bulk matrix material. The factor is defined as the ratio of the copper enrichment, measured in units of 10¹⁵ atoms cm⁻², to the copper content of the matrix in at.%. Although absolute levels of enrichment increase with increase in copper content of the alloy, the enrichment factor increases in the opposite sense.     

Evaluation of hydrogen absorption behaviour during acid etching for surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys

The hydrogen absorption behaviour during acid etching for the surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys has been investigated on the basis of the surface morphology, electrochemical behaviour and hydrogen thermal desorption analysis. To simulate the conventional acid etching for the improvement of the biocompatibility of Ti alloys, the specimens are immersed in 1 M HCl, 1 M H₂SO₄ or 0.5 M HCl + 0.5 M H₂SO₄ aqueous solution at 60 °C. Upon immersion, commercial pure Ti absorbs substantial amounts of hydrogen irrespective of the type of solution. In H₂SO₄ or HCl + H₂SO₄ solutions, the hydrogen absorption occurs for a short time (10 min). For Ti-6Al-4V alloy, no hydrogen absorption is observed in HCl solution, whereas hydrogen absorption occurs in other solutions. For Ni-Ti superelastic alloy, the amount of absorbed hydrogen is large, resulting in the pronounced degradation of the mechanical properties of the alloy even for an immersion time of 10 min, irrespective of the type of solution. The hydrogen absorption behaviour is not necessarily consistent with the morphologies of the surface subjected to corrosion and the shift of the corrosion potential. The hydrogen thermal desorption behaviour of commercial pure Ti and Ni-Ti superelastic alloy are sensitively changed by acid etching conditions. The present results suggest that the evaluation of hydrogen absorption is needed for each condition of acid etching, and that the conventional acid etching often leads to hydrogen embrittlement.     

Sodium enrichment of an Al-Mg alloy surface after alkaline etching

This paper reports a case of sodium enrichment of the surface of an Al-Mg alloy that contains sodium impurities in its bulk and has been subjected to etching treatment in an alkaline solution. This phenomenon has only occurred in the case of the aforementioned Al-Mg alloy and has not been observed in other tested Al-Cu, Al-Mg-Si or pure Al type alloys, despite the fact that these also contain sodium as an impurity. The origin of the sodium ions incorporated in the aluminium oxide film that covers the surface of the Al-Mg alloy is discussed, and an explanation is suggested for this difference in behaviour compared with the other alloys.     

Large-area self-ordered aluminium sub-micrometre dot arrays prepared by electropolishing on polycrystalline aluminium at constant current

The effects of electropolishing on the sub-micrometre structure pattern on the surface of polycrystalline aluminium were investigated. Under constant current, the electropolishing process results in large-area uniform sub-micrometre dot arrays despite the polycrystalline aluminium surface. Effects of temperature on the pattern of the surface structures were also investigated. Experimental temperature was shown to have a great influence on the structure pattern. When the electropolishing temperature increases, the structure of aluminium surface tends to transform from a dot pattern to stripe one and then to a corrosion structure.     

Electropolishing of high-purity aluminium in perchloric acid and ethanol solutions

The electropolishing behaviour of high-purity aluminium with ultrasonic agitation was investigated in ethanol-perchloric acids by using anodic polarization measurement, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), and atomic force microscope (AFM). The results show that the electropolishing behaviour of the aluminium is under mass-transfer control and Al³⁺ is suggested as the governing species for the salt film mechanism during anodic dissolution in limiting current plateau.     

The corrosion of Fe3Al alloy in liquid zinc

A systematic study of the isothermal corrosion testing and microscopic examination of Fe₃Al alloy in liquid zinc containing small amounts of aluminum (less than 0.2 wt.%) at 450 °C was carried out in this work. The results showed the corrosion of Fe₃Al alloy in molten zinc was controlled by the dissolution mechanism. The alloy exhibited a regular corrosion layer, constituted of small metallic particles (diameter: 2-5 μm) separated by channels filled with liquid zinc, which represented a porosity of about 29%. The XRD result of the corrosion layer formed at the interface confirmed the presence of Zn and FeZn_6.67. The corrosion rate of Fe₃Al alloy in molten zinc was calculated to be approximately 1.5 × 10⁻⁷ g cm⁻² s⁻¹. Three steps could occur in the whole process: the superficial dissolution of metallic Cr in the corrosion layer, the new phase formation of FeZn_6.67 and the diffusion of the dissolved species in the channels of the corrosion layer.     

Electrochemical corrosion characteristics of ZrNi5−xCox alloys

The electrochemical corrosion behaviour of a series of ZrNi_5−xCo_x alloys with x=0-4 has been tested using potentiokinetic polarisation technique. The polarisation curves were measured in deaerated 0.5 M sulphate solutions with pH=0.2-7 and in strong alkaline solution of KOH (pH=15). It is shown that the presence of greater amounts of Co in the alloy (x?2) worsens the passivating properties of the alloy in acidified sulphate solutions. On the other hand, in strong alkaline solutions, both low- and high-cobalt alloys undergo stable passivation. The degree of Ni substitution by Co in the alloys does not generally affect the shape of cathodic polarisation curves.     

The alkaline stability of phosphate coatings I: ICP atomic emission spectroelectrochemistry

Inductively coupled plasma (ICP) spectroelectrochemistry was used to measure the alkaline resistance of zinc phosphate conversion layers on galvanized steel. The release of phosphate, zinc, manganese and nickel were measured quantitatively as a function of time when exposed to 0.1 M NaOH at ambient temperature. It was found that phosphate and zinc are removed in a 1:1 ratio during the early stages of the reaction so as to produce a residual hydroxide layer. The presence of Mn²⁺, Fe²⁺, or Ni²⁺ ions in the phosphate layer reduces the rate of phosphate leaching proportional to their degree of incorporation in the layer.     

Modelling the relationship between microstructure of Galfan-type coated steel and cut-edge corrosion resistance incorporating diffusion of multiple species

This paper describes a considerable extension to a previously documented [S.G.R. Brown, N.C. Barnard, 3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel, Corrosion Science 48 (2006) 2291-2303], first-order model used to simulate the localized degradation experienced in Zn-4.5 wt% Al steel coatings exposed to 5% NaCl aqueous solution. The temporal localization and intensity of discrete corrosion effects are predicted using established relationships and, in contrast to earlier models, the evolution of multiple concentration fields is included and calculated using straight-forward finite difference techniques. Changes in composition are included in the quantification of both anodic and cathodic processes involved in the corrosion of steel coatings in contact with aerated saline solutions. Reported [J. Elvins, J.A. Spittle, D.A. Worsley, Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion, Corrosion Science 47 (2005) 2740-2759] and modelled performances of typical Galfan composition coated steels are evaluated for different coating microstructures undergoing so-called cut-edge corrosion. In summary, this latest model successfully matches measured rates of metal loss during localized corrosion. Additionally, the inclusion of multiple species diffusion functionality has greatly improved the simulation of the cathodic reaction in particular and the overall form of the current density distribution near the corroding surface.     

Copper–nickel alloys modified with thin surface films: Corrosion behaviour in the presence of chloride ions

A strong influence of the Ni content in on the barrier properties of the surface films, spontaneously formed on Cu–Ni alloys (containing 10–40 at.% of Ni) in a slightly alkaline NaCl solution or previously produced by forming bis(diethyldithiocarbamato)Cu(II) and Ni(II) surface complexes, were studied under stagnant and solution impinging conditions using polarization and EIS methods. An unexpected behaviour that the corrosion resistivity of alloys with x(Ni) ? 20 at.% is lower than that of the Cu–10Ni alloy was explained using solid-state processes within the surface films and enhanced electronic conductivity of the surface film formed on the Cu–10Ni alloy.     

Synthesis, matching and deconstruction of polarization curves for the active corrosion of zinc in aerated near-neutral NaCl solutions

SYMADEC 1, used previously to synthesise, match and deconstruct polarization curves for the Fe/H₂O/H⁺/O₂(4e⁻) corrosion system, has been updated to SYMADEC Multimetal. The polarization curve of a zinc anode alloy corroding with formation of a non-protective layer of ZnO in fully-aerated, near-neutral NaCl solution was modelled. To test the SYMADEC Multimetal model four published polarization curves of zinc actively corroding in similar salt solutions are deconstructed. The polarization curve current/voltage contribution from the ZnO reduction/oxidation peak overlaid on the polarization curve is estimated by a bi-Gaussian relationship.