Under a certain acid concentration and hydrogen pressure, the hydrogen electrode can establish the following equilibrium: 2H+ +2e=H2. The potential of this hydrogen electrode is called the equilibrium potential of hydrogen, which is related to hydrogen ion concentration and hydrogen partial pressure.
If the anode potential in a corroded battery is more positive than the equilibrium potential of hydrogen, the equilibrium potential of the cathode is definitely more correct than that of hydrogen, so the corrosion potential is more positive than that of hydrogen, and hydrogen evolution corrosion will not occur.
If the anode potential is negative than the equilibrium potential of hydrogen, the corrosion potential may be negative than the equilibrium potential of hydrogen, and hydrogen depolarization and hydrogen evolution corrosion may be realized. In a word, the equilibrium potential of hydrogen becomes an important criterion for hydrogen evolution corrosion, and EEH =-0.059 PH. The stronger the acidity, the smaller the pH value, the more positive the equilibrium potential of hydrogen and the more negative the anode potential, which has a considerable effect on increasing the possibility of hydrogen depolarization.
Therefore, the reason why many metals do not undergo hydrogen evolution corrosion in neutral solution is because the concentration of hydrogen ions in the solution is too low, the equilibrium potential of hydrogen is low, and the anode potential is higher than that of hydrogen. However, when metals (magnesium and alloys) with larger negative potentials are selected as anodes, hydrogen evolution corrosion sometimes occurs, even in alkaline solutions, because their potentials are more negative than the equilibrium potential of hydrogen.
Hydrogen evolution corrosion:
When a metal is electrochemically corroded in a strong acid solution, it will release hydrogen, which is called hydrogen evolution corrosion. Steel products usually contain carbon. In humid air, the steel surface will absorb water vapor and form a thin water film. Carbon dioxide dissolved in water film becomes electrolyte solution, which increases the number of hydrogen ions in water. This constitutes countless tiny primary cells with iron as the negative electrode, carbon as the positive electrode and acidic water film as the electrolyte solution.
The reason why many metals do not undergo hydrogen evolution corrosion in neutral solution is that the concentration of hydrogen ions in the solution is too low, the equilibrium potential of hydrogen is low, and the anode potential is higher than the equilibrium potential of hydrogen. However, when metals (magnesium and alloys) with larger negative potentials are selected as anodes, hydrogen evolution corrosion sometimes occurs, even in alkaline solutions, because their potentials are more negative than the equilibrium potential of hydrogen.