Praseodymium-based inhibitors in epoxy polyamide primers

4 Praseodymium-based inhibitors in epoxy polyamide primers 

A number of different rare earth-based compounds were evaluated as potential replacements for strontinm chromate corrosion inhibitors in epoxy polyamide 

PrPjj contained only trace amounts of Pr(OH)3 and was largely unaffected by exposure to atmospheric conditions. Analysis of Pr(OH)3 and Prj(C03)3 xHp was also performed to confirm the compositions of the commercial Pr-0 compounds following exposure to the ambient atmosphere. Although Pr(OH)3 was not affected by exposure, Pr3(C03)3.xHp partially converted to Pr(C03)0H. The studies provided insight into the rate at which Pr-0 compounds will or will not convert to other species when exposed to atmospheric conditions. Unlike Pr203 and Pr fCOjlj xHp, PrfOHlj was not affected by ejq)osure to H O or CO. In general, once Pr species are in the hydroxide phase, they will not adsorb CO. Likewise, Prp, is stable and did not readily react with ambient H O or CO.  

A representative schematic summarizing the species driven off and phase transformations that take place during the thermal decomposition of Pr O, Pr Oj, Pr(OH)3, and Pr3(C03)3 xH30 can be seen in (Fig. 6.3). Each arrow represents the species driven off and nnmber of stages required for the starting material to reach the final stable phase as determined by XRD. Solid state reactions in the bulk and on the surface of rare earth sesquioxides induce morphological modifications in addition to compositional changes. 

As dissolution, migration and formation of the rare earth materials from the primer to active electrochemical sites is the most likely process for mitigating corrosion, precipitation studies were conducted to study the effect of CO on phase stability in the Pr-H O system. Solvated praseodymium species were precipitated under various conditions including temperatures from ambient up to 85 °C in 25% tricholoroacetic acid saturated with gases including Ar, and COj. 

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