An investigation into the corrosion behaviour and effect of inhibitor additions on commercial Zn-Mg-Al alloys

• Main Author: Lewis, Thomas A.
• Other Authors: Sullivan, James H.
• Published: Swansea University 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.752376

The general premise of this work was to better understand the corrosion behaviour of newer-generation zinc-magnesium-aluminium galvanising alloys. In addition to this, the impact of both novel and established corrosion inhibitor additions dosed into solution were studied to assess the effects to prospective alloy lifetimes and the underlying mechanisms of inhibitor action. This was through means of microstructural characterisation and analysis, and accelerated corrosion testing under immersion conditions; this included the use of the SVET, time-lapse optical microscopy, open-circuit potential, potentiodynamic polarisation, and gravimetric analysis. Accordingly, the microstructural attributes of three commercial grade zinc-magnesium-aluminium alloys were studied according to the differing quantities of magnesium and aluminium included in each alloy. The primary zinc-rich dendritic phases were observed to diminish in both volume fraction percentage and in size for increasing alloying addition. This was accompanied by a corresponding increase in eutectic phase volume fraction, which consisted of a binary and ternary lamellar eutectic, as confirmed by SEM-EDS. Alongside the microstructural changes, corrosion performance was noted to improve as alloying additions were increased. This was realised by SVET-measured metal loss values, of which SVET revealed fewer anodic sites and a lessened extent of anodic evolution. Time-lapse microscopy data demonstrated that corrosion was initiated in eutectic phases, attacking the MgZn2 phase in the first instance. The improved corrosion resistance for higher alloyed samples was associated with the preferential attack of magnesium-rich phases, forming beneficial corrosion products which enabled a reduction in corrosion reaction kinetics. The remaining work utilised a selected alloy of Zn-2wt.% Mg-2wt.% Al composition for experimental studies. The effect of solution pH was next considered to understand the impact to corrosion behaviour in such environments. For neutral and alkaline conditions, a characteristic localised attack was noted, with improvements in corrosion performance corresponding to higher pH conditions. Acidic conditions instead led to a generalised corrosion mechanism, illustrating a more widespread and more pronounced corrosive attack on the alloy surface. The increased corrosion resistance associated with higher pH conditions was attributed to an enhanced presence and stability of beneficial corrosion products. Further work was performed to assess the effectiveness and mechanisms of action for both established and more novel corrosion inhibitor additions on the selected zinc-magnesium-aluminium alloy. This was performed by dosing designated concentrations of the inhibitor species into solution. The addition of sodium phosphate was recognised to progressively reduce the formation and evolution of anodic sites, providing enhanced levels of corrosion resistance accordingly. The growth of anodic sites was observably restricted through the local formation of insoluble metal phosphate precipitates, predicted to be tertiary phosphate species according to solubility calculations. An anodic inhibition effect was suggested via reaction of phosphate anions with that of metal cations in solution, to produce insoluble metal phosphate species at regions of anodic activity. An amino acid, L-tryptophan, was studied as a prospective corrosion inhibitor for the designated zinc-magnesium-aluminium alloy coated steel. The addition of this compound at higher concentrations revealed a beneficial impact to the corrosion performance, whereby metal loss values were reduced and localised anodic activity was curtailed. This was realised to transpire via the formation of a film on the sample surface, precipitating predominantly in cathodic regions and eventually extending to moderate coverage of anodic regions, according to time-lapse microscopy. The data suggested that this inhibitor species acted primarily as a cathodic inhibitor, restricting mass transport of oxygen at the sample surface. The mechanism of action was not definitively demonstrated, and several mechanisms were discussed. A rare earth metal compound in the form of cerium(III) chloride was also studied as a corrosion inhibitor for the zinc-magnesium-aluminium alloy in question. An inverse relationship between extent of corrosion and concentration of inhibitor addition was realised, whereby higher concentrations enabled favourable corrosion resistance levels. The development of anodic activity was hindered by the deposition of films at the sample surface, and these were noted to form only in regions of cathodic activity. It was proposed that these films were of a cerium oxide/hydroxide composition, and limit adsorption of oxygen at the sample surface, regulating corrosion kinetics and thus the rate of anodic growth. Accordingly, the overall data suggested that this compound acted through means of cathodic inhibition. The combination of techniques has enabled valuable insights to be gained into the corrosion behaviour of commercial zinc-magnesium-aluminium alloys in different environments; this has also aided in the understanding of the underlying mechanisms of action for a range of prospective corrosion inhibitors within the zinc-magnesium-aluminium system.