The fundamentals of electrochemistry are presented, including sections that are key to understanding metal corrosion. The processes at the metal-medium interface are examined. The nature of the electrochemical cell as a model for measuring properties and controlling redox reactions is explained. The principles of thermodynamics and prediction of the direction of electrode reactions are provided, including Gibbs energy, the Nernst equation, Purbe diagrams, and polarization curves. This is a bridge to understanding the corrosion destruction of the welded joint as a process in a multi-electrode electrochemical cell.