Depletion depths of inorganic components from a scribe edge in a polyurethane primer containing Li₂CO₃, MgO, BaSO₄ and TiO₂ beneath a topcoat, were determined using a range of techniques including SEM/EDS and proton induced X-ray and γ-ray emission spectroscopies. SEM of sections cut using an ion beam revealed scribe damage penetrating 20–25 μm away from the scribe edge prior to leaching. After neutral salt spray (NSS) exposure a leached zone developing from the scribe edge was observed. For longer NSS exposure times (>96 h) this leached zone of nearly complete Li and Mg depletion did not develop any deeper than the scribe damaged region indicating that the depletion zone was caused by mechanical damage due to scribing. At short times small voids were formed in Li₂CO₃ particles within the primer well away from the scribe (100–260 μm) whereas a mixture of void and detachment in and around Li₂CO₃ particles was observed at longer times. The detachment was assumed to be part of a channel network within clusters of particles. Internal stresses within the primer resulting from buildup of inhibitor dissolution product within the voids were modelled using finite element analysis. It was found that strains related to von Mises stresses were concentrated around the inorganic particles and developed preferentially within the plane of the primer beneath the topcoat with some indication of concentration towards the primer/metal interface. These stresses resulted from osmosis and swelling related to the voids. They were also attributed to the observed cracking of the binder at some locations. Leaching experiments showed that Li was released very rapidly from the primer. The leaching data was modelled using a power law where the mass released is proportional to tⁿ where the n is an index that reflects the kinetic behavior dictated by the evolving primer porosity. In this study n values between 0 and 1 were observed for all species, with Li starting at around 0.7 but rapidly decreasing to close to zero.

Studies of Li depletion in sections of a Li ₂ CO ₃ -primer comprising a polyurethane binder, MgO, TiO ₂ , BaSO ₄ in addition to Li ₂ CO _3, were performed using a combination of particle induced γ-ray and X-ray emission spectroscopies along with SEM/EDS analysis. A mixture of depletion behaviours was observed. At the earliest stages (to around 48 h)initial release was confined to the surface. At longer times (168 h)voids developed deeper into the primer and after 500 h Li ₂ CO ₃ dissolution was observed at places throughout the thickness of the primer to the metal/primer interface. Microscopic transport pathways formed which involved all large inorganic particles. SEM showed that rupture of the polyurethane matrix contributed to network formation. Finite element analysis indicated that rupture may be due to internal stresses around particles isolated in the polyurethane matrix and associated with water uptake. Thus the transport network seemed to be generated by chemical dissolution at the particle/polymer interface and may be enhanced by mechanical degradation due to internal mechanical stresses. The release kinetics of the Li ₂ CO ₃ inhibitor from the primer was followed as a function of time and the data analysed according to a release behaviour of t ⁿ . There was very rapid initial release of Li followed by a slower release of Mg and to a lesser extent Ba. The value of n varied significant with time, but showed a mixture of Fickian release and direct dissolution for Mg and Ba at intermediate times, but transport through a pore network at longer times. The leaching data was interpreted in terms of local transport networks that developed in the primer with time.

Lithium based inhibitors in aerospace coatings are seen as excellent replacements for their chromium counterparts which are both carcinogenic and heavier. However, Li is generally difficult to detect and following changes in its distribution due to corrosion is impossible with many standard techniques. Combining MeV Particle Induced Gamma and X-ray emission provides a powerful tool and in this paper we summarise some recent experiments on such coatings using the CSIRO Nuclear Microprobe. PIGE mapping of the LiCO₃ particles and their patterning illustrates how the method will be extremely useful in monitoring surface corrosion.