Li leaching from Li carbonate-primer: Transport pathway development from the scribe edge of a primer/topcoat system

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.