Localized Corrosion of Binary Mg-Nd Alloys in Chloride-Containing Electrolyte Using a Scanning Vibrating Electrode Technique

To elucidate the role of noble metal impurities on corrosion of Mg, 3D time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging in combination with X-ray photoelectron spectroscopy and optical microscopy measurements were carried out on Mg samples (99.9%) before and after Mg polarization at E OCP +0.5 V in 0.1 M NaCl. A significant segregation of Fe (and of Mn and Al) metallic impurities at grain boundaries (GBs) was observed on the Mg surface by 3D ToF-SIMS. A 3-step mechanism of Mg corrosion was proposed, including a catalytic effect of Fe segregated at the GBs on the hydrogen evolution reaction (HER). In the 1 st stage, the initiation of Mg corrosion is accompanied by the HER occurring over Fe impurities segregated at GBs leading to formation of small circular defects, and propagation with occurrence of dark filiform-like pattern corrosion enriched in Cl − . Magnesium and its alloys exhibiting interesting physico-chemical properties such as excellent wide range of mechanical properties at room temperature and low weight (1.7 g.cm −3 ), become good alternatives to aluminum and iron-based alloys in many industrial applications such as automobile, aerospace, energy storage or biomedicine. 1,2Mg and its alloys are spontaneously covered by an oxide/hydroxide layer.3 When exposed to aqueous solution, the oxide layer has a duplex structure consisting of an inner magnesium oxide and an outer porous hydroxide layer 4,5 with a possible presence of magnesium hydride (MgH 2 ).6 At ambient temperature, this oxide layer formed on the Mg (or Mg alloys) surfaces is protective. 1,7 In most aqueous environments (or high humidity) and in a large range of pH, (0-10) as inferred from the potential-pH diagram, this oxide/hydroxide layer is not stable and Mg undergoes dissolution.8 Thus, this is one of the most important limiting factors for applications of Mg and its alloys. Mg dissolution is accompanied by the hydrogen evolution reaction (HER), as the major cathodic reaction. The well-known increase of the HER rate under anodic polarization of Mg seems to be in contradiction with the kinetic models of charge transfer electrochemical reactions (Butler-Volmer equation), predicting that the cathodic current should decrease exponentially with increasing anodic potential. This phenomenon called "the negative difference effect" (NDE) 9-16 and more recently "the anodic hydrogen evolution" process 17,18 has been also observed for other metals than Mg such as Al, Li. [19][20][21][22] Despite the abundant literature on Mg corrosion and many models of Mg corrosion (such as: the formation of Mg(I) as an intermediate product, 23,24 the formation and breakdown of the partially protective film, 11,[25][26][27] the formation of hydride intermediates, [28][29][30][31][32] formation of dark filiform patterns 13,16 and increase the hydrogen production in aqueous chloride solutions.12,15 Even a very low content of impurities (usually < 150 ppm in "pure" Mg) 41 can lead to formation of such surface film and the morphology of this film depen...

Section: Discussionmentioning

confidence: 99%

Role of Segregated Iron at Grain Boundaries on Mg Corrosion

Mercier

Światowska

Zanna

et al. 2018

“…Evidence of enhanced cathodic activity has been reported by Williams and coworkers through use of the scanning vibrating electrode technique (SVET) which has shown the temporal evolution of net cathodically active regions at prior net anodic sites on 99.9% 10 and 99.99% 6 pure Mg, Mg-Al-Zn alloy AZ31B, 11 and Mg-Nd binary alloy surfaces, 20 under free corrosion and during galvanostatic anodic polarization. As such, the NDE being a manifestation of enhanced cathodic activity affiliated with anodic sites may be considered non-controversial, however the important remaining question still remains as to what physical mechanism is responsible for the enhanced cathodic activity.…”

mentioning

confidence: 99%

The Role of Surface Films and Dissolution Products on the Negative Difference Effect for Magnesium: Comparison of Cl⁻versus Cl⁻Free Solutions

Cain

Gonzalez-Afanador

Birbilis

et al. 2017

Self Cite

The anodic dissolution and associated hydrogen evolution of high purity Mg (80 ppmw Fe) was studied as a function of potential in unbuffered 0.6 M NaCl (pH ≈ 8.5), 0.6 M NaCl saturated with Mg(OH) 2 (pH ≈ 10.25), 0.1 M MgCl 2 (pH ≈ 5.6), 0.1 M Na 2 SO 4 (pH ≈ 5.5), and a 0.1 M Tris(hydroxymethyl)aminomethane hydrochloride (TRIS, pH ≈ 7.25) buffer solution via simultaneous mass loss, hydrogen volume collection, potentiostatic and potentiodynamic polarization, and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The negative difference effect (NDE) was substantial in the unbuffered Cl − containing environments where Mg(OH) 2 formed on the surface and weak in 0.1 M Na 2 SO 4 . In contrast, the 0.1 TRIS buffered solution exhibited a positive difference effect and the absence of thick corrosion films. Mg(OH) 2 films formed on samples in Cl − spalled off easily whilst the Mg(OH) 2 films formed in 0.1 M Na 2 SO 4 were more tenacious, which suggests that film stability plays significant role in the NDE. Research and development of magnesium (Mg) and its alloys has greatly increased in the last 15 years primarily due to the demand for lightweight vehicles in the automotive and aerospace industries, 1 in addition to the exploration of Mg as an anode material in primary and secondary batteries.2 However, the poor corrosion resistance and complex dissolution behavior of Mg has limited its wider use as an engineering material.3-5 One aspect associated with Mg dissolution which has not to date been mechanistically elucidated in full, is the phenomenon known as the negative difference effect (NDE).3 This effect is characterized by an increase in the rate of hydrogen evolution with increasing anodic overpotential relative to the open circuit -which provides a local cathodic reaction for a significant portion of the anodic reaction.6 This phenomenon confounds the understanding of Mg dissolution mechanisms, establishment of a Tafel law and other reaction kinetic parameters, as well as other issues including accurate analysis of corrosion rates from electrochemical impedance spectroscopy (EIS). [7][8][9] There are several theories purporting to explain the origins of the NDE among which include noble impurity element enrichment, 10,11 non-faradaic mass loss via metal spalling, 12,13 formation and dissolution of hydrides and partially protective films, [14][15][16][17] and univalent Mg based dissolution followed by further oxidation of Mg + →Mg 2+ in solution (leading to hydrogen evolution from the reduction of water away from the electrode in the electrolyte);18 however, no consensus has been reached on the physical origins of the NDE.The contemporary paradigm for the manifestation of the NDE centers on evidence for the enhanced cathodic activity of dissolving Mg anodes;19 whereby numerous works confined within a relatively recent timeframe have elucidated such a phenomenon. Evidence of enhanced cathodic activity has been reported by Williams and coworkers through use of the scanning vibrating electrode techniqu...

“…The localized corrosion activity occurring over a commercial grade HDG surface immersed in aqueous sodium chloride electrolyte has been mapped using in situ SVET, which has been widely used in previous corrosion studies. [13][14][15][16][17][18][19][20][21][22] The effect of concentration and pH on corrosion inhibition by H 2 PP and an inorganic inhibitor, namely sodium phosphate (Na 3 PO 4 ), are assessed. A pH of 11.5 has been chosen to mimic the underfilm conditions found in a corrosion-driven delamination cell.…”

mentioning

confidence: 99%

Inhibition of Localized Corrosion of Hot Dip Galvanized Steel by Phenylphosphonic Acid

Glover

Williams

2017

Self Cite

Phenylphosphonic acid (H 2 PP) is investigated as a corrosion inhibitor of hot dip galvanized steel (HDG) fully immersed in a 5% (w/v) sodium chloride electrolyte. An in-situ Scanning Vibrating Electrode Technique (SVET) is used where concentrations of H 2 PP are systematically added to the electrolyte in neutral conditions. H 2 PP, at a concentration of 5 × 10 −2 mol dm −3 , is shown to effectively inhibit localized corrosion over a 24 h period with 96% efficiency. H 2 PP is compared with a sodium phosphate (Na 3 PO 4 ) inhibitor at the same concentration over a wide pH range. With current legislative pressure to eliminate Cr(vi)-based inhibitors from protective coating formulations, there is a pressing need to identify effective, environmentally-friendly alternatives. Chromatefree passivation treatments have been studied extensively 1-4 and the most widely used alternative to chromate is a phosphate passivating treatment. Such a system facilitates the precipitation of protective zinc compounds via the supply of soluble anions that react with the zinc ions generated in metal dissolution.5 A phosphate passivation treatment generates a Zn 3 (PO 4 ) 2 barrier to inhibit the anodic process, 4,5 the formation of which can act to extend the pH stability of the surface layer down to values of around pH 6. However, as zinc phosphates are 1000 times more soluble than Cr (III), inferior corrosion inhibition performance is observed. 7 The corrosion of a zinc surface generates a wide range of pH zones, whether in bulk electrolyte conditions, atmospheric conditions or, most notably, under a water droplet. 8,9 Corrosion-driven cathodic coating delamination phenomena produces an alkaline underfilm environment where values of pH 10-11.5 have been recorded previously on a coated zinc surface. 10In order to compete with an adsorbed chromium (III) oxide layer, an effective corrosion inhibitor system would have to be stable in a range of pH 4-14.7 It is therefore of great importance to examine potential chromate replacement inhibitors in non-neutral conditions.In a recent Scanning Kelvin probe study, we showed that in-coating additions of H 2 PP increased the initiation time for underfilm cathodic coating delamination by up to 20 h and reduced the delamination rate by up to 94% on hot dip galvanized steel (HDG) surfaces.11 An 'etch' effect was shown to be the dominant inhibition mechanism, whereby a reaction occurs at the point of first contact of the wet coating with the substrate producing an interfacial metal phosphate salt layer. We hypothesize that a second mode of inhibition occurs whereby the leaching of dissociated PP 2− ions from the coating into the substrate/coating interface forms a ZnPP layer, blocking the anodic reaction. H 2 PP has been shown previously to strongly adsorb onto surface oxide films, disfavoring chloride.12 The principal aim of this work is to determine the efficiency of H 2 PP as an inhibitor for HDG surfaces in bulk immersion conditions. The localized corrosion activity occurring over a commercial gr...