The ability of Mg2Ge crystals to behave as ‘smart release’ inhibitors of the aqueous corrosion of Zn-Al-Mg alloys
Introduction
Zinc and zinc based alloy coatings are able to provide sacrificial protection to underlying steel substrates and are heavily utilised within the automotive and construction industries. Despite the superior corrosion properties afforded by zinc coatings, there is a growing demand to both improve coating performance, and to lower coating weights. One approach is to introduce alloying additions into galvanized zinc coatings, and the kinetics and mechanism of zinc alloy corrosion has been investigated extensively in a range of environments [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. Active inhibition can also be provided by corrosion inhibitors, which are released from the organic coating matrix upon exposure to stimuli (for example contact with a corrosive electrolyte) [25,26]. Active corrosion inhibitors are primarily added to organic components of an organic coating system (e.g. pre-treatment, primer, topcoat) and their incorporation into metallic coatings is, to date, limited. The aim of this paper is therefore to investigate the ability of Mg2Ge crystals, formed within metallic Zn-Al-Mg (ZAM) alloys, to behave as corrosion inhibitors and provide active corrosion protection.
The effect of alloying additions on the corrosion rate of ZAM alloys has been extensively researched [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. The MgZn2 and MgZn11 intermetallic particles, formed in zinc-magnesium and ZAM alloys, have been shown to undergo preferential anodic attack [5,12,22]. and the magnesium oxide formed can locally replace zinc oxide [5,12,22]. Magnesium oxide is a wide band gap (∼ 7 eV) semiconductor (effectively an insulator by comparison to zinc oxide (∼ 3 eV) [27], and has been shown to reduce rates of the cathodic oxygen reduction reaction (ORR) [8,12]. Mg2+ ions, released during anodic dissolution of intermetallic particles, can also react with OH− (formed at the cathode) to form magnesium hydroxide (Mg(OH)2) which has the effect of ‘buffering’ surface pH to mildly alkaline values (∼pH 10.2) [1]. At these values of pH, protective zinc hydr(oxides) and simonkolleite are stable [2,7] but aluminium, present within ZAM, is predicted to become anodically active. Anodic dissolution of aluminium produces aluminates which can react with Mg2+ to form protective layered double hydroxides (LDH) [2,7,16].
Other alloying elements under investigation include Ge, [28] and the addition of 0.3−0.5 wt.% Ge to industrially relevant Mg-1 wt.% Zn binary alloys [29] has led to the formation of ‘inert’ Mg2Ge intermetallic particles [30] which are cathodically polarised during OCP and potentiondynamic polarisation exposure of Mg alloys [29].
In comparison, active corrosion inhibition is typically achieved by the direct incorporation of inhibitive pigments into an organic coating. However, interaction between inhibitors and the surrounding matrix can lead to loss of inhibition capability, reduced lifespan and complete organic coating failure. As well as effectively inhibiting corrosion, inhibitive pigments must be able to leach (release and transport) to defective areas, where they can form protective layers [31]. The use of low solubility pigments, which are unable to release and transport to coatings defects, can result in incomplete coating inhibition. High solubility pigments can rapidly leach out of the organic coating and cause osmotic blistering [26,[32], [33], [34], [35], [36], [37], [38]]. In recent years, significant effort has instead been focused on the development of ‘self-healing’ organic coatings, whereby a protective corrosion inhibitor is encapsulated by ‘nano-containers’ so that matrix-inhibitor interaction cannot occur. The containers are evenly distributed throughout the coating and the controlled release of the inhibitive pigment is instead triggered by external mechanical (e.g. coating abrasion or cracking) or chemical (e.g. pH) stimuli [25,26]. To date, several attempts at nano-container design have been made. These include: polymer based capsules [39], halloysites (aluminosilicates) [[40], [41], [42]], polyelectrolyte shells [43], layered double hydroxides (LDH) [44], ion exchange resins [45], conducing polymers [46], and mesoporous inorganic materials (e.g. silica) [37,38,41,47]. Other factors to consider are the control of the capsule permeability by variations in external stimuli (for example pH, temperature, ionic strength etc), as well as container size [25,26]. Nano-containers should be less than 300−400 nm in size to avoid the creation of hollow cavities and risk to coating integrity [26].
To date the majority of research into the use of nano-containers is focused on their inclusion into organic coatings as opposed to metallic coatings [54]. More recently, inorganic nano-containers such as halloysites and mesoporous silicon have become the topic of increasing research [[47], [48], [49], [50], [51], [52], [53], [54], [55], [56]]. Both types of nano-container are commercially available in large quantities and are more economically feasible than alternatives [57]. They also offer the additional potential benefits of mechanical and thermal stability [40,42] which means that it is possible for them to be incorporated into metallic coating systems. However, in order to embed these nano-capsules in metallic coatings, it is necessary to functionalize their surfaces and much of the published work in this area focuses on the modification of nano-containers [[47], [48], [49], [50], [51], [52], [53], [54], [55], [56]]. In order that these smart release vehicles provide adequate corrosion inhibition it is vital that the nano-containers are compatible with other components present in the metal matrix and both the zeta potential of particles, and their electrostatic interaction with the surface [[48], [49], [50], [51], [52], [53]], have been identified as crucial factors which influence their incorporation into the matrix.
Given 1.) the ability to form Mg2Ge intermetallic particles within zinc alloys, [30,58] and 2.) the ability of Mg2+ ions (released during the dissolution of Mg based intermetallics) to reduce corrosion rates, [[59], [60], [61]] it seems plausible that the addition of Ge to ZAM alloys would result in the formation of thermally and mechanically stable, intermetallic ‘smart release’ capsules, and a reduction in ZAM corrosion rate.
This paper describes an investigation into the effect of Ge additions (0–1.8 wt.%) on the kinetics and mechanism of ZAM corrosion, as it occurs in 0.17 mol.dm−3 NaCl. In so doing the amount of Ge, added to a ZAM (1.6 wt.% Al, 1.6 wt.% Mg) master alloy, is systematically changed and the resultant microstructure is characterised using SEM. The relative nobility of the phases present are determined using scanning Kelvin probe force microscopy (SKPFM). The scanning vibrating electrode technique (SVET) derived mass loss is used to assess the corrosion performance of ZAM-Ge alloys, and time lapse microscopy (TLM) is used to provide mechanistic information regarding the corrosion at a microstructural level. Complimentary open circuit measurements (OCP) measurements are also conducted.
Section snippets
Materials
ZAM-Ge samples were produced in an inert (argon) environment. ZAM (96.8 wt.% Zn, 1.6 wt.% Mg and 1.6 wt.% of Al) pieces, obtained from Tata Steel UK, were heated to 650 °C in a crucible. Varying amounts of Ge were added to produce four different alloy compositions and the mixture was further heated to 1000 °C. Samples were then air cooled within the crucible. A IR thermometer optris CT laser 3 M pyrometer was used to measure the temperature on the alloy surface.
Chemicals including nitric acid
Materials characterisation
The composition of the alloys under investigation are presented in Table 1.
The SEM image of an industrially produced Zn-1.6 wt.%Al-1.6 wt.% Mg coating is shown in Fig. 1. The coating is composed of three different phases; primary zinc dendrites (∼30−50 μm) binary eutectic and ternary eutectic. Binary phases are lamellar structures made up of primary zinc and MgZn2, whilst the ternary eutectic consists of primary zinc, MgZn2 and aluminium nodules. SEM images of the four different ZAM-Ge cast
Discussion
Although the exact mechanism by which ZAM-Ge alloys are able to provide corrosion protection is yet to be fully discerned, it would seem reasonable to propose that the principal microstructural components responsible for the increased resistance ZAM- 1.8 Ge observed in this work, are the Mg2Ge crystals. Once in contact with the corrosive electrolyte it is believed that Mg2Ge undergoes preferential anodic de-alloying (Fig. 5) by loss of Mg2+, leaving behind a surface enriched in Ge (Fig. 8).
Conclusions
A systematic study into the effect of Ge additions on the corrosion resistance afforded by Zn-Al-Mg alloy coatings has been completed to so show that;
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the addition of Ge into the ZAM alloy system resulted in the formation of a new Mg2Ge phase within the microstrusture. This phase existed in two different forms. At all levels of Ge addition, a Mg2Ge plate like structure was observed and at the highest concentration (ZAM-1.8 Ge) large Mg2Ge crystals were also observed. The area fraction of the Mg
CRediT authorship contribution statement
N. Wint: Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing. A.D. Malla: Investigation, Data curation, Writing - original draft. N. Cooze: Investigation, Data curation. T. Savill: Investigation, Data curation. S. Mehraban: Investigation, Data curation, Methodology. T. Dunlop: Investigation, Data curation. J.H. Sullivan: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft,
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The authors would like to thank Tata Steel UK and EPSRC for funding via the COATED2 Centre for Doctoral Training (EP/L015099/1).
The authors would like to acknowledge the financial support of the RFCS (Research Fund for Coal and Steel, grant number: RFSR-CT-2015-00011) support and all the partners: ArceloMittal (France), OCAS (Belgium), Max Planck Institut (Germany), Tata steel (Netherlands), Voestalpine (Austria) for the supply of samples; Chimie ParisTech (France), University of Chemistry and
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2023, Corrosion ScienceCitation Excerpt :In these sites, the selective dissolution of Zn-rich phases (such as Al-Zn eutectic and η-Zn(Al)) occurred because of the relatively low corrosion potential [52], resulting in pitting on the surface of weld seam as shown in Fig. 6c. The accumulation of corrosion products around the pits inhibited the diffusion of dissolved oxygen and Zn-rich phases (such as Al-Zn eutectic and η-Zn(Al)) hydrolyzed to produce H+, which decreased the local pH [53]. Therefore, this deep orange area was mainly related to the severe localized corrosion.