The influence of arsenic alloying on the localised corrosion behaviour of magnesium

An in-situ scanning vibrating electrode technique is used to investigate the effect of alloyed arsenic on magnesium immersed in chloride containing aqueous solution, both in freely corroding and anodically polarised conditions. Arsenic is shown to strongly suppress cathodic activation of the corroding Mg even under circumstances where breakdown has occurred and subsequent propagation of dark filiform-like tracks is observed. Under galvanostatic anodic polarisation, rates of hydrogen evolution are significantly mitigated compared to pure Mg and no time-dependent evolution of local cathodic sites is detected. The findings support the theory that cathodic activation of the dark corroded Mg surface is associated with accumulated transition metal impurity, which in turn become poisoned towards cathodic hydrogen evolution by the presence of As. In addition, these preliminary studies suggest that alloying with a strong cathodic poison may provide a means of producing a more-charge effective anode material for primary sea-water activated Mg batteries

Quantifying the Role of Transition Metal Plating in the Cathodic Activation of Corroding Magnesium

This paper examines the effect that transition metal ions have on the corrosion behaviour of high purity magnesium. A series of experiments indicate the ability of the selected transition metal ions to accelerate corrosion through cathodic (re)plating aiming to gain an insight on the Mg corrosion activation described in recent publications. High purity magnesium samples were used in conjunction with known additions of Fe2+, Cu2+, Zn2+ and Mn2+ cations. The results indicate that the presence of transition metal ions in the corrosive electrolyte leads to transition metal (re)plating and to an increase of the magnesium corrosion rate. By systematically varying the transition metal ion concentration it was possible to determine the relative efficiencies of the selected metal cations. It was also observed that the metal (re)plating process and the efficiency of the cathodic activation were limited by the formation of insoluble transition metal (hydr)oxide precipitates and the time-dependent hydrolysis

The influence of Chloride Ion Concentration on Passivity Breakdown in Magnesium

An empirically derived dependence of an apparent breakdown potential (Eb) of magnesium (Mg) on chloride ion concentration is reported. In situations where spontaneous breakdown in the absence of external polarization is observed, leading to the subsequent propagation of localised corrosion, Eb can be determined by following time-dependent changes in free corrosion potential (Ecorr). Breakdown of temporary passivity is marked by a clear inflection in the time-dependent value of Ecorr, characterised by a sharp decrease in potential from a maximum value where Ecorr = Eb. Characterisation of localized corrosion behaviour by in-situ scanning vibrating electrode studies, prior to and following the point of breakdown, is employed to explain the observed Ecorr vs. time characteristics. Examples of typical behaviour upon immersion in aqueous solutions containing different chloride ion concentrations ([Cl-]) are given for commercially pure Mg and an AZ31 alloy. For high purity Mg, which remains passive at pH 11 at all chloride concentrations ≤ 2 mol dm-3, Eb values are determined as a function of chloride ion concentration [Cl-] by means of a potentiodynamic method. In both cases a dependence of Eb on [Cl-] is established such that Eb = A + B log10[Cl-], where the value of B is typically -0.11 V per decade. The practical implications of the existence of a breakdown potential for Mg is considered in terms of the selection of an appropriate chloride ion concentration in standard corrosion tests

Smart-release inhibition of corrosion driven organic coating failure on zinc by cationic benzotriazole based pigments

A novel cationic benzotriazole pigment (CBP) based on the benzotriazolium cation (BTAH2+) exchanged into a sulfonated organic resin has been synthesized and evaluated as a means of inhibiting the corrosion-driven cathodic disbondment of organic coatings from the surface of galvanized steel. The CBP is acidic in nature (BTAH2+ pKa ≈ 1.1) and is intended to be compatible with acidic coating formulations such as etch-primers. Delamination rates, as measured using a scanning Kelvin probe (SKP), were found to decrease monotonically with increasing CBP volume fraction (ΦCBP) and to approach zero when ΦCBP = 0.1. The mechanism of CBP operation is described

Combinatorial Studies into the Effect of Thermally Inter-Diffused Magnesium on the Kinetics of Organic Coating Cathodic Delamination from Zinc Galvanized Steel

This paper describes a high-throughput study into the role of Mg in preventing corrosion driven coating disbondment of organic coatings from Zn-Mg alloy galvanized steel. A graded Mg wedge is applied to a hot-dip zinc galvanised steel substrate using physical vapour deposition, and subsequently annealed to produce metallic inter-diffusion and formation of Mg2Zn11 intermetallic. An overcoat of electrically insulating polyvinyl butyral (PVB) is applied and corrosion is initiated from a penetrative coating defect using an aqueous electrolyte. The variation in Mg coating weight across the wedge facilitates a systematic investigation of the effect of Mg on Volta potential and the rate of corrosion driven cathodic coating disbondment using scanning Kelvin probe (SKP) potentiometry. The rate of cathodic disbondment is shown to decrease rapidly even at very low Mg coating weight (corresponding to 25 nm thickness before annealing). The results are explained in terms of the galvanic polarity of the corrosion cell formed between Zn exposed at the defect site, and the intact Zn-Mg layer at the metal-organic coating interface

A Method for Quantifying the Synergistic Inhibitory Effect of Corrosion Inhibitors When Used in Combination: A ‘Chromate Generating Coating’

Corrosion inhibitive pigments, based on the cations Ce4+ and Cr3+ exchanged into smart release resins, are dispersed in a polyvinyl butyral (PVB) model coating and applied to a hot dip galvanised steel (HDG) substrate. An investigation is made into the influence of different pigment volume fractions (øpig) of Ce(IV) and Cr(III) based inhibitors, used both in isolation and combination, on the kinetics and mechanism of corrosion driven cathodic coating delamination. The rate of coating delamination is obtained using scanning Kelvin probe (SKP) potentiometry and time lapse photography, and the efficiency with which each inhibitor combination is able to resist cathodic coating delamination is calculated. Isobolograms, commonly utilized within the field of drug interaction, are presented as an effective method for characterising corrosion inhibitor interactions. In some cases, the sum of the efficiencies calculated for Ce(IV) and Cr(III) based pigments is shown to be greater than the sum of their individual efficiencies. It is proposed that Ce4+, released upon electrolyte exposure, is able to oxidize the Cr3+ species resulting in the formation of transient CrO4−2

The Sacrificial Protection of Steel by Zinc-Containing Sol-Gel Coatings

The scanning vibrating electrode technique (SVET), electrochemical impedance spectroscopy, and salt spray testing are used to investigate the ability of Zn rich sol-gel coatings to provide sacrificial protection to carbon steel. Three types of coatings (containing either Zn powder, a colored pigment, or both) are applied to steel. Intact coatings are shown to act as barrier layers through which electrolyte ingresses over time. Under conditions where the substrate is exposed by an artificial coating defect, SVET is used to investigate the extent to which different coatings offer sacrificial protection when the defect size is systematically changed. The total anodic current, as derived using SVET, doubles when the defect covers 25% of the total area compared to when 12% of the area is exposed. This finding is consistent with efficient sacrificial protection of the steel by the zinc based coating. This sacrificial protection is observed for up to 24 hours for cases where the defect constitutes up to 52% of total area. The protection offered in the presence of a colored pigment is delayed and it is proposed that the pigment restricts the ability of Zn to couple with the underlying steel

The Use of Chromium and Chromium (III) Oxide PVD Coatings to Resist the Corrosion Driven Coating Delamination of Organically Coated Packaging Steel

This paper describes a systematic study into the role of chromium and chromium (III) oxide thickness in preventing corrosion driven coating disbondment of organically coated packaging steel. A graded wedge of chromium and chromium (III) oxide is applied to steel using physical vapour deposition (PVD). A polyvinyl butyral (PVB) overcoat is applied and corrosion is initiated from an artificial defect using NaCl. Scanning Kelvin probe (SKP) potentiometry is used to monitor coating delamination. Wedge thickness variation allows for high throughput investigations into the effect of both metallic chromium and chromium (III) oxide thickness, on coating disbondment rate. A linear reciprocal relationship is observed between chromium metal thickness and disbondment rate. Increasing chromium (III) oxide thickness (applied over chromium metal) results in a decrease in delamination rate. This work highlights the ability of PVD to produce chromium/chromium (III) oxide corrosion resistant coatings to use as alternatives to hexavalent chromium-based systems