Formation of smooth anodic nanoporous iron oxide film for enhancing photocathodic protection on plain carbon steel

Abstract

It is essential to fabricate smooth nanoporous iron oxide film with fewer defects on plain carbon steel (CS) to improve photocathodic protection of the film in corrosive media. For that, high temperature annealing heat treatment was chosen to manipulate the metallographic structure of CS in the muffle furnace at 900 °C with the duration of 30 min. Following that, the annealed CS was anodized in a mixture of aqueous ammonium fluoride solution and ethylene glycol to fabricate smooth nanoporous oxide film on CS. Finally, as-anodized oxide film was calcinated at 450 °C for 4 h in nitrogen atmosphere to form crystalline iron oxide film. Several electrochemical techniques were applied to analyze the photoelectron property and photocathodic protection performance of nanoporous iron oxide films. On the contrary, rough nanoporous iron oxide film with more striking defects was fabricated on untreated (no annealed) CS by using the same anodization and calcination processes. The results show that the metallographic structure of annealed CS behaved more uniform ferrite grains and fewer pearlite precipitate than untreated CS. Thereby, a smooth nanoporous iron oxide film with fewer defects formed on annealed CS. On the contrary, a rough nanoporous iron oxide film with more defects, such as dimples and cracks were fabricated on untreated CS. Both of iron oxide films were composed of hematite as the main phase and magnetite as the secondary phase. Therefore, the smooth iron oxide film displayed comparatively better conductivity and higher donor density under the condition of illumination than the rough film. On account of those, smooth film exhibits comparatively higher anodic and cathodic reaction activities and more excellent and stable photocathodic protection to the counter.