Polystyrene/TiO₂ nanocomposite coating for strength and toughness enhancement of aluminum alloy 2024-T3 in accelerated stress corrosion cracking

The polystyrene/TiO2 nanocomposite coating is evaluated for reducing the susceptibility of aluminum alloy AA2024-T3 to stress corrosion cracking (SCC). The as-synthesized titania nanoparticles of aspect ratio (AR) 1, 2, and 4 are individually loaded at 25 wt% to the acetone-dissolved polystyrene (PS) to make the nanocomposites. Coated compact tension specimens are tested under accelerated SCC testing that combines the constant displacement rate equivalent to 10(-7) s(-1) and 3.5 wt% NaCl solution dripped at the crack tip. The load and crack mouth opening displacement of each specimen are recorded and analyzed to determine the indices to the mechanical ductility, strength, toughness, and fracture toughness. The PS/TiO2 AR1 coated specimens exhibit the least SCC susceptibility, with a relatively high fracture toughness of 55 MPa-m(1/2) and more than 60 h of lifetime without apparent cracking under the SCC testing. Intriguingly, the PS-coated specimens perform similarly well. The results are explained by an SCC failure mechanism that considers the complex interaction among the agglomerated nanoparticles, polystyrene matrix, and the substrate in various aspects. The mechanism suggests that the nanocomposite coating with a higher yield strength and less permeability performs better. The highly loaded nanoparticles such as 25 wt% used herein make the nanocomposite only more marginally yieldable than polystyrene, but undesirably introduce morphological and electrochemical defects that count against the SCC susceptibility improvement.