Immersion and electrochemical corrosion properties and combined corrosion mechanism of TC4 alloy by keyhole TIG welding

In this study, the effect of heat input on the microstructure and its correlation with the immersion corrosion properties of TC4 welded joints by Keyhole tungsten inert gas (TIG) process was researched. The combined corrosion mechanism between immersion corrosion and electrochemical corrosion was established. The results showed that immersion corrosion formed a passivation film on the surface of the welded joint and formed a minimal number of pitting pits. After immersion, electrochemical corrosion testing was carried out. Due to the existence of the passivation film, the self-corrosion potential increased from-0.6 V ∼ −0.2 V–0.1 V–0.6 V, and the self-corrosion current density decreased and the fitting impedance of the film increased. These all showed that the corrosion resistance of the joint was enhanced after immersion. However, due to the combined effect of immersion corrosion and electrochemical corrosion, the potential difference between the surface passivation film and the internal metal was sufficiently large to induce pitting corrosion on the surface. After the breakdown of the passivation film, spot corrosion pits formed. Microcracks propagated along the primary β grain boundary and the α′ phase interface, and continuous corrosion for a long time may even lead to cracking failure of the welded joints. With the increase of heat input, the self-corrosion potential and the self-corrosion current density of immersion corrosion increased firstly and then decreased. The fitting film resistance value of the impedance decreased firstly and then increased. The aforementioned findings indicated that as the heat input increased, the corrosion properties of the joints firstly deteriorated and then improved.