Dilute Harrison’s solution

With other metals, in particular with aluminium and its alloys, the ennobling mechanism is not clear-cut. Recent work by Yan et al. [60] carried out with PPy doped with electroinactive 1,3-benzene-disulfonic acid and electroactive sodium 4,5-dihydroxy-l,3-benzene-disulfonate and using a two-compartment cell similar to that of Rammelt et al. [64] seems to prove that the anodic protection model cannot be applied to the aluminum surface. Although the recharging of PPy by O2 was demonstrated, they found that a small area of exposed aluminium (AA 2024-T3) simulating a defect in a coating and immersed in dilute Harrison s solution (0.35 wt% (NH4)2S04 + 0.05 wt% NaCl in water) did not... 

Electrochemical impedance spectra for scribed hybrid coatings at different immersion times in dilute Harrison s solution (A) without inhibitor and (B) with MBI/h-cyclodextrin complex. Inset optical images of the samples after 4 weeks of immersion [10]... 

This concept is illustrated in Fig. 13.17. The measurements were taken from epoxy primers containing either non-conductive as-received aluminium flake or a conductive PPy/Al flake hybrid pigment from reaction 1. Before EIS measurements were taken, the samples were immersed in dilute Harrison s solution for 3 h. Notice that for the conductive PPy/Al flake primer, there is a large drop in impedance between a PVC of 30% and 35% while this same drop is not observed for the as-received flake until a PVC of 45%. The drop observed at the lower PVC for the conductive pigment would be indicative of the percolation threshold while the impedance drop observed at the higher PVC for the non-conductive pigment would be more closely associated with the CPVC. Also it is important to note that at a PVC of 35%, the impedance of the PPY/Al flake hybrid primer drops below the impedance observed for bare aluminium 2024. This is indicative of some interaction between the aluminium and PPy at the interface between the aluminium substrate and the coating. 

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