The failure mechanism of model organic coatings from Mg alloy surfaces is characterised by a combination in-situ scanning Kelvin probe analysis and time lapse photography. Initiation of underfilm corrosion by application of group I chloride salts to a penetrative coating defect produces an apparent cathodic-driven coating delamination, where the disbondment distance increases linearly with time at high relative humidity, although filiform corrosion (FFC) is also observed in the vicinity of the defect. The disbondment process occurs both in the presence and absence of oxygen, indicating that hydrogen evolution comprises the predominant underfilm cathodic reaction. Post-corrosion elemental analysis of the delaminated region shows an abundance of group I cation, but no chloride. When magnesium chloride or HCl are used to initiate corrosion, then only FFC is produced. The mechanism is discussed in terms of net anodic dissolution at the defect coupled with underfilm cathodic hydrogen evolution, producing organic coating disbondment under conditions where cations are able to transport ionic current within a zone of increased pH.

The localized corrosion behavior of E717 magnesium alloy immersed in chloride-containing electrolyte is investigated using an in-situ scanning vibrating electrode technique (SVET), coupled with time-lapse imaging (TLI). It is shown that initiation of localized corrosion in chloride-containing electrolyte is characterized by the appearance of discrete local anodes, corresponding with the leading edges of dark, filiform like features, which combine with time to produce a mobile anodic front. The size and growth rate of these features are highly dependent on the chloride ion concentration of the electrolyte. SVET-derived current density maps reveal that the corroded surface left behind the anodic front is cathodically activated, where cathodic current density values progressively decline with increasing distance away from the anodic leading edge. The intensity of localized anodes is highly dependent on the chloride ion concentration, where progressively higher local anodic current density values are observed with increasing chloride ion concentration along with progressively higher rates of volumetrically-determined hydrogen evolution. Breakdown potential, measured using time-dependent free corrosion potential transients and potentiodynamic polarization at neutral and elevated pH respectively, is shown to vary with the logarithm of chloride ion concentration and the time for localized corrosion initiation is progressively increased with decreasing chloride concentration. From the combination of results which are presented herein, the underlying reasons for the influence of chloride ion concentration on the localized corrosion characteristics of E717 alloy will be discussed.

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The corrosion-driven organic coating failure of Mg alloys and in particular the E717 alloy was investigated using a combination of in-situ Scanning Kelvin Probe (SKP) analysis and time-lapse photography where two principal failure mechanisms were identified: cathodic delamination and filiform corrosion. Initiation of underfilm corrosion by application of group I chloride salt to a coating defect produced a cathodic-driven coating delamination. The delamination distance increased linearly with time and the delamination occurred both in the presence and absence of oxygen. Post-corrosion elemental analysis of the delaminated regions using secondary-ion mass spectrometry revealed an abundance of group I cation, but no chloride. Experiments using the SKP and Stratmann-type Mg samples showed that the delamination rates remained linear even at protracted holding times and were insensitive to the type of group I cation present in the initiating electrolyte. Additional experiments on AZ31 and AZ91 Mg alloys revealed that both alloys are susceptible to organic coating delamination with the latter alloy being the most resistant to coating deadhesion. The mechanism was discussed in terms of anodic dissolution at the defect coupled predominantly with underfilm hydrogen evolution, producing organic coating disbondment under conditions where cations are able to transport ionic current within a region of increased pH. The second focus of this thesis was to study the filiform corrosion (FFC) of organic coated E717, AZ31 and AZ91 Mg alloys. The FFC was inoculated by applying MgCl2, HCl and FeCl2 in a coating defect and the FFC propagation rates were quantified by determining the underfilm corroded area with time, which were shown to increase as a function of log[Cl-], remain unaffected by the absence of oxygen, but strongly dependent on the relative humidity of the holding environment. SEM-EDX surface analysis of FFC affected regions was used in combination with in-situ SKP mapping to elucidate the mechanism of FFC propagation, where chloride-induced anodic dissolution at the disbondment front is coupled with the reduction of water on a cathodically activated corroded surface behind with progressive Cl- entrapment in the FFC tail. Finally, the localised corrosion behaviour of the E717 Mg alloy immersed in chloride-containing electrolytes was investigated using an in-situ Scanning Vibrating Electrode Technique (SVET) coupled with Time-lapse Imaging (TLI). The localised corrosion was characterised by discrete local anodes corresponding with the leading edges of dark filiform-like features that combine with time to produce a mobile anodic front leaving a cathodically activated corroded surface behind similar to what was observed with the FFC mechanism. Breakdown potential, measured using time-dependent free corrosion potential transients and potentiodynamic polarisation at neutral and high alkalinity respectively, were shown to vary with the log[Cl-] and the time for corrosion initiation was progressively decreased with increasing chloride concentration.