Corrosion Inhibition of AA 2024-T3 by Aqueous Silicate, Molybdate, and Praseodymium Species

Corrosion protection systems involving conversion coatings and pigmented organic coatings often act by releasing soluble inhibiting species into the local environment.  Therefore, as was done for chromate, it is of interest to understand the fundamental mechanism of inhibition provided by commercially employed non-chromate inhibitors including silicate, molybdate, and praseodymium species dissolved in aqueous solution.  The current study focuses on acquiring scientific understanding of the inhibiting mechanism of the aforementioned inhibitors utilizing various techniques including standard electrochemical experiments, bulk exposure of AA 2024-T3 coupons in quiescently aerated inhibitor-containing solution, SEM/EDAX analysis, in situ AFM scratching coupled with scanning Kelvin probe force microscopy (SKPFM) to study the corrosion morphology and kinetics at high-spatial resolution, and surface analytical and compositional techniques like Raman Spectroscopy and XPS.  Furthermore, the speciation of the inhibitor ions has been investigated using OLI Analyzer, a commercial thermodynamic modeling package capable of calculating physical and chemical properties of aqueous systems.  Early results show that silicate and molybdate impart anodic corrosion inhibition on AA 2024-T3.  For instance, silicate exhibited potent inhibition in alkaline solution where the passive current density is decreased by almost two orders of magnitude.  It is proposed that silicate ions form a thin-hydroxide layer over the dissolved aluminum-oxide forming a protective passive film.  Solutions containing molybdate at near-neutral pH suppress corrosion by increasing the pitting potential.  In acidic media, molybdate reduces in a fashion that is not self-inhibiting resulting in accelerated corrosion.  Preliminary studies on praseodymium show that it inhibits corrosion cathodically, however studies are ongoing.