The electrochemical corrosion mechanism can be better explained by the galvanic cell principle. When two metals with different reactivity (such as copper and zinc) are immersed in the electrolyte solution, a potential difference will be generated between the two metals, and a current will flow through the connection with a wire. During this process, the active metal (zinc) will be consumed. That is, electrochemical corrosion. Different from chemical corrosion (such as oxidation of metal in air, hydrogen evolution of zinc in acid solution), electrochemical corrosion must generate current, and the magnitude of the current is directly related to the generation of corrosion products, that is, the higher the current density, the corrosion faster.
The degree of activity of various metals in the electrolyte solution can be expressed by their standard electrode potential, that is, the metal ions and the metal ions containing unit activity (activity and concentration are positively correlated, when the concentration is less than 10-3mol/L, the two values are considered to be the same). , at the temperature of 298K (25℃) and the equilibrium electrode potential under the gas partial pressure of 1.01MPa.
The lower the standard electrode potential is, the more active the metal or alloy is, and the more likely it is to be corroded when it forms a galvanic pair with a high-potential metal. It can be seen that in addition to the nature of the metal, the factors that determine the potential of the metal standard electrode are: solution metal ion activity (concentration), temperature, and gas partial pressure.
Another important factor is the film that covers the metal surface.Except for a few precious metals such as gold and platinum, most metals can form oxide films with certain protective effect in air or water, otherwise most metals cannot exist in nature. The nature of the metal surface film has an important influence on its corrosion occurrence and corrosion rate.
