The effect of using surfactant as additive in corrosion protection of aluminum 2024-T3 by hard anodizing method

Aluminum has found a wide variety of applications thanks to its remarkable combination of characteristics, such as low density, easy workability and high corrosion resistance [1]. In atmospheric corrosion environments the most common form of corrosion is pitting corrosion, which is further enhanced when chlorides (Cl–) are present in the corrosive atmosphere [2]. Anodizing is the method which is often applied to improve the corrosion resistance of aluminum alloys [3]. Anodization of aluminum in aqueous electrolytes forms anodic oxide films (AAO) with two different morphologies, that is, the nonporous barrier-type oxide films and the porous-type oxide films. The chemical nature of the electrolytes mainly determines the morphology of AAOs [4]. The term “Hard Anodizing” (HA) indicates a process of production of anodic coating for engineering applications. Their primary characteristic is high abrasion resistance or hardness and thickness [3]. The HA process is characterized by the use of sulfuric acid at relatively low temperatures and high current densities, and has been widely used for various industrial applications [5]. Also, two-step potentiostatic anodizing is widely used to fabricate highly ordered porous alumina structures from the top to the bottom. In this method, potentiostatic anodizing under the appropriate conditions gradually leads to self-ordering of the pore distribution [6]. Each alloy has a special feature that is used in a specific industry. Alloys of the 2xxx series which is rich in copper, are sensitive to localized corrosion in the form of pitting corrosion. The presence of copper by creating particles called intermetallic particles like Al2CuMg and Al2Cu causes cracking and reduces corrosion resistance by creating oxides. Naturally formed oxide film on the surface of aluminum reduces corrosion rate to significant extent, but this layer may not be able to fully protect aluminum from corrosion due to the presence of corrosive agents [7]. In this research, a two-step method was used for 2024-T3 alloy. First, after Mild Anodizing (MA) at 35°C and 18 V and removing the oxide layer with phosphoric acid, hard anodizing was done at 55 V and Sulfuric Acid 2 M (SA 2M) and -2 °C. Then 10% v/v of Ethylene Glycol (EG) was added to the sulfuric acid solution and anodizing was done. In the final stage, 0.04, 0.08 and 0.16 mM of SDS surfactant was added to the electrolyte containing SA 2M and EG 10% v/v and anodizing was done again. The impedance test showed that the corrosion resistance of the 0.04 mM SDS has improved more than other samples so that Rp = 101 kΩ.cm2 was observed as seen in Fig 1. In this way, with the increase in surfactant concentration, the amount of corrosion resistance decreases because the tendency of the surfactant to the surface decreases and the anodizing is disturbed.