Corrosion Behavior Of Polypyrrole Coatings Modified With Functionalized Inhibitor Doped Silanes On AA2024-T3

For aluminum and its alloys,chromate conversion coatings have been the industrial pretreatment of choice for some decades now due to their excellent corrosion protection and ease of formulation.But,due to their carcinogenic nature there is an immediate need to find suitable,environmentally friendly alternatives.Among the investigated alternatives,silanes and electroactive conducting polymers have indicated promising results.In this project we explored the possibility of synergistic coupling of silanes and polypyrrole coatings for corrosion protection of AA2024-T3.Silanes are monomeric silicon molecules,which once adequately hydrolyzed can form stable bonds with both organic and inorganic substrates.Upon curing they also undergo selfcondensation form a dense layer that act as a barrier against aggressive environment.Silane layers only offer passive protection but active self-healing may be introduced by using appropriate inhibitors and nanoparticles.Pyrrole is an aromatic heterocyclic organic compound that once polymerized via oxidation acquires conductivity.Due to their redox nature,they can offer anodic protection and some barrier protection against corrosion.But problems related to dissolution of metal before coating deposition and low adhesion to active metals still need to be resolved.In this research work,we chose two silanes(APS and BTMS)for depositing silane layers.Cerium ions were selected as inhibitor ions and ceria nanoparticles were used to host self-healing properties to these silane layers.Deposition of polypyrrole coatings from aqueous solutions of sulfuric acid and different configurations of silane-polypyrrole coatings were studied.We also studied the deposition of polypyrrole coatings from other aqueous solutions and the effect of anodizing on silane layer performance.Results indicated that BTMS coating performs better as it can form a much stronger interface with the alloy surface.Excessive self-condensation and presence of hydrophilic functional group leads to poor performance of APS coating.Inclusion of cerium ions led to improved performance of BTMS coating and a self-healing phenomenon is observed due to the deposition of insoluble cerium hydroxide deposits at pitting sites.Polypyrrole coatings deposited on untreated or silane treated alloy samples at +1.0V vs.SCE from sulfuric acid solution presented a characteristic current-time transient with three stages.An important decrease in dissolution current for silane treated samples was observed.Deposition of polymeric coatings was not possible when the alloy was alkaline cleaned.Polarization and impedance spectroscopy testing revealed better performance of coating have a silane base layer.Even better corrosion protection could be achieved when the polypyrrole layer was sandwiched between two silane layers.Bottom silane layer improved adhesion while top silane layer provided denser barrier by filling the pores of the polypyrrole coating.Best performance was obtained for the polypyrrole coating deposited on top of a silane layer from a mixed silane-pyrrole solution.A dense interfacial layer provided better protection to the substrate as evidenced by electrochemical and morphological studies.FTIR analysis revealed characteristic polypyrrole peaks.Polypyrrole coatings deposited from various aqueous solutions at varying anodic potentials revealed more uniform morphology for the coatings deposited at lower potentials.The size of the anion also plays an important role in determining the adhesion as delamination was observed for sodium dodecyl sulfate coatings having a very large anion.Improved adhesion is observed for sodium salicylate coatings while polarization tests reveal better protection from uniform,dense sulfate doped coatings.Anodizing the substrate prior to silane deposition led to significant improvement in polarization and impedance spectroscopy performance.The BTMS molecules are able to penetrate the porous oxide layer forming a very compact barrier against the aggressive electrolyte.Incorporation of optimized concentration of inhibitor ions and nanoparticles further improved the coating performance while excessive amount of nanoparticles led to decrease in corrosion protection performance.