Physical Characterization of Cathodically-Activated Corrosion Filaments on Magnesium Alloy AZ31B

A sequential isotopic tracer study of corrosion film growth for Mg-3Al-1Zn-0.25Mn (AZ31B) and Mg-1.2Zn-0.25Zr-<0.5Nd (ZE10A) was conducted by 4 h immersion in H 2 18 O or D 2 16 O, followed by a 20 h immersion in a 0.01 wt% NaCl H 2 18 O or D 2 16 O solution. Sputter depth profiles were obtained for 16 O, 18 O, H, and D using secondary ion mass spectrometry (SIMS). Compared to the previous tracer study for these alloys in salt-free water, the addition of 0.01 wt% NaCl resulted in a transition from oxygen inward-dominated film growth to a component of mixed inward/outward film growth for both alloys. The hydrogen tracer behavior remained inward growing for AZ31B, and short-circuit, inward growing for ZE10A, in both pure water and in 0.01 wt% NaCl solution, with extensive penetration of D beyond the film and into the underlying alloy also observed for ZE10A. Analysis of the films by X-ray photoelectron spectroscopy (XPS) and cross-section scanning transmission electron microscopy (STEM) indicated intermixed Mg(OH) 2 and MgO, with the relative fraction of Mg(OH) 2 peaking near the center of the film. These findings suggest a decoupled film growth mechanism, with initial formation of oxide followed by NaCl-accelerated conversion to hydroxide, likely by both solid-state and dissolution-precipitation processes. Low density Mg alloys are of great interest for the production of lightweight vehicle parts to achieve increased fuel efficiencies and better performance.1-6 Key challenges to achieve widespread adoption in automotive vehicle applications include the cost of primary Mg metal, the formability of wrought Mg alloys (to expand application beyond current Mg cast parts), and the rapid corrosion of Mg in many aqueous and humid environments.6 From a corrosion perspective, the high reactivity of Mg, which leads to micro-and/or macro-galvanic corrosion coupling, and the inability of Mg alloys to sufficiently establish and/or maintain a protective surface film are major challenges in many environments. [7][8][9][10][11][12][13][14] Of particular detriment is the acceleration of Mg corrosion in salt-containing environments, which has been linked to increased local micro-galvanic coupling at second phases/metallic impurity sites and enhanced Mg dissolution, as well as disruption of the Mg(OH) 2 /MgO based surface film. The successful application of isotopic tracer studies employing secondary ion mass spectrometry (SIMS) to gain insights into the film growth mechanism of Mg alloys in room-temperature water and 85• C humid air was recently demonstrated. 45,46 Isotopic tracer studies have been widely used in oxidation studies of Al-, Fe-, Ni-, and Zr-base alloys, as well as SiC, with significant new insights gained regarding the growth mechanism of the oxide films. 45,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] However, application of this experimental approach to Mg is particularly challenging due to the formation of both MgO and Mg(OH) 2 products in the film, necessitating the use of both ...

Section: Resultssupporting

confidence: 77%

Section: Resultsmentioning

confidence: 99%

Tracer Film Growth Study of the Corrosion of Magnesium Alloys AZ31B and ZE10A in 0.01% NaCl Solution

Brady¹,

Fayek²,

Leonard³

et al. 2017

“…with significant populations of pores and cracks) and the ability of Al-Mn intermetallic particles to catalyze the cathodic reaction (H 2 gas evolution) resulting from the increased number of particles (and surface area) in contact with the electrolyte. 12 Recently Salleh et al 16 demonstrated that the kinetics of the H 2 gas evolution reaction on a Mg(OH) 2 covered surface were significantly higher than those on a pristine Mg surface, indicating that the Mg(OH) 2 corrosion product could be a significant source of cathodic activation. No evidence was found of crystalline Mg(OH) 2 within the fresh corrosion filament, which was found to exhibit significant cathodic activation.…”

Section: Discussionmentioning

confidence: 99%

“…enhances) the cathodic kinetics. 41 Thus, it would be interesting to utilize SVET measurements to probe the cathodic activity of aged corrosion filaments on AZ31B-H24 to determine if Zn-enrichment 12 leads to a secondary effect.…”

Section: Discussionmentioning

confidence: 99%

Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

Cano

McDermid

Kish

2015

The filiform-like corrosion of extruded Mg alloy AM30 immersed in a dilute near-neutral NaCl solution was investigated using electrochemical techniques coupled with transmission electron microscopy (TEM). Scanning vibrating electrode technique (SVET) measurements showed that the filament-like corrosion consisted of an intensely anodic propagation front supported by a cathodicallyactivated filament behind. The TEM examination of the corroded and intact surface films in cross-section using thin foils prepared by focused-ion-beam (FIB) milling indicated that the cathodic activity was likely a combined result of the formation of a thick, highly-defective MgO film and the ability of Al-Mn intermetallic particles to catalyze the cathodic H 2 gas evolution reaction. The formation of an Al-rich layer at the film/alloy interface with time, due to the incongruent dissolution of the alloy, was not able to stop the initiation and propagation of the filaments. Mg and Mg alloys have been shown to be susceptible to the formation of laterally-spreading products, which tend to manifest as thread-like "filaments" or radially-expanding "discs" during corrosion in aqueous NaCl solutions. [1][2][3][4][5][6][7][8][9][10][11][12] Williams et al. have offered critical insights into the electrochemical mechanism of this unique localized corrosion mode on pure Mg 1,5,7 and the Mg-Al-Zn-Mn alloy AZ31B 9 by employing the scanning vibrating electrode technique (SVET), where their SVET measurements showed that the corrosion filaments/discs became cathodically-activated following their formation. It was proposed that the net cathode on the locally corroded regions galvanically coupled with the adjacent intensely anodic regions to drive the lateral propagation of the corrosion products across the exposed surface. The formation of discs versus filaments was suggested to depend on whether the catalytic efficiency of the locally corroded region allowed for galvanic coupling with its perimeter, or merely along the most anodic pathways of the microstructure.7 This "differential electrocatalytic" mechanism was also shown to drive filiform corrosion on coated Mg as determined by scanning Kelvin probe (SKP) measurements. 1The mechanism, however, by which the cathodic reaction (H 2 gas evolution) is enhanced on corroded Mg or Mg alloys continues to be unclear. Proposed mechanisms include (i) the enrichment of noble secondary phase constituents such as iron (Fe) impurity particles (for pure Mg) or Al-Mn intermetallic particles (for AZ31B) within the corroded regions 5,9,13,14 (ii) an increased catalytic tendency of the corroded regions caused either by undissolved metallic Mg "chunks," 15 a significantly roughened surface film 5 or the formation of Mg(OH) 2 as a corrosion product. 16 The SVET measurements reported by Williams et al. suggested that a significantly reduced cathodic activation was exhibited by the as-cast AZ31B corrosion filaments 9 relative to the corrosion discs on pure Mg.5 This finding agrees well with the noble secondary phase theory,...

“…The decrease in anodic and cathodic reaction rate for PEDOT coated sample may be due to the PEDOT acting as a physical barrier impeding mass transport of Mg +2 ions, as discussed later. On the other hand, the increase in cathodic reaction rate of PEDOT coated sample after 24 hours immersion compared to 30 minutes immersion may be attributed to: (i) the fact that the Mg alloy surface naturally becomes more cathodically active during corrosion, 41,42 (ii) the increase in the pH which lead to precipitation of a more protective Mg(OH) 2 layer (see below), and (iii) the increased immersion time allowed further penetration of the electrolyte through the pores in the PEDOT coating, and thus increased the ability of Al-Mn particles to sustain the H 2 evolution reaction. This could explain why the E corr shift was much more dramatic for the PEDOT coating, since the uncoated AZ31B has a much thinner surface film, and thus a smaller number of Al-Mn particles embedded available in a given surface area.…”

Section: Resultsmentioning

confidence: 99%

Localized Corrosion Behavior of AZ31B Magnesium Alloy with an Electrodeposited Poly(3,4-Ethylenedioxythiophene) Coating

Tefashe

Dauphin‐Ducharme

Danaie

et al. 2015

Herein we report the characterization of localized corrosion of AZ31B magnesium alloy electrochemically coated with poly(3,4-ethylenedioxythiophene) (PEDOT) from ionic liquid electrolyte. Several scanning probe techniques including high resolution electron microscopy, SVET, SECM amperometric detection of H 2 fluxes, potentiometric SECM detection of local pH and localized potentiodyanmic measurements were used to evaluate the microstructure of the coating and its corrosion protectiveness. In order to examine long-term durability of corrosion protection due to the PEDOT coating, these measurements were performed after different immersion times. It was observed that PEDOT coating appears to lose its protective ability after localized coverage of corrosion products. The results of this study provide important information considering the interest in this coating for use in biomedical implants and prior indications of beneficial passivation. Replacement of heavy alloy components with light-weight materials is of prime concern for automotive and aerospace manufacturers in order to reduce the fuel consumption and hence polluting CO 2 emissions. Magnesium alloys are one of the potential candidates owing to their excellent physical and mechanical properties such as lower density, a higher strength-to-mass ratio and a higher vibrational damping capacity compared to steel and aluminum alloys.1 However, the high corrosion susceptibility is the major drawback preventing more widespread use of such Mg-based alloys.The addition of alloying elements, alternate processing technologies and surface coatings are effective in improving the corrosion performance of Mg-based alloys.2,3 Dip coating is particularly attractive to the automotive industry because of its simplicity, low costs and great coverage ability. 4,5 Coating processes vary from the widely used chromate/manganese dip treatment to anodic treatments in fluoridecontaining baths, 6,7 and are generally considered hazardous to human health and to the environment.2,3 More environmentally sustainable surface modification processes were identified in conductive polymers but remain challenging to implement with light metal alloys. 8,9 Conducting polymer films are readily formed by direct electrodeposition onto Pt and Au.10 They have been used for corrosion protection of structural metal like steel and aluminum with success but comprehensive investigations for magnesium materials is lacking. The deposition of conductive polymers onto Mg-based electrodes from aqueous solutions is generally not feasible because the positive potentials required for electropolymerization concurrently form thick surface oxide films and lead to copious hydrogen evolution 11,12 that prevent the formation of adherent and continuous conducting polymer films. 13Room temperature ionic liquids (RTIL's) are increasingly being applied in electrochemistry 14 and have recently been explored as solvents for the electropolymerization of conducting polymers. [15][16][17] Generally, conducting polymers prepared from...