The figure shown below is a potentiodynamic polarization curve of a metallic material in a solution. Answer the following questions.

(1) When we discuss the corrosion resistance of the metallic material based on the polarization curve, the current density presented in the perpendicular axis indicates the rate of electrochemical reactions. What does the potential in the horizontal axis indicate or imply?

(2) The polarization curve was measured by using an Ag/AgCl reference electrode. Explain the shape of the polarization curve when one measures a polarization curve of the same metallic material under the same conditions by using a saturated calomel electrode and answer the rationale of your explanation.

(3) You have to carefully select the sweep rate of potential when you measure polarization curve. Explain the reason.

(4) The polarization curve presents a typical behavior, that is, active, passive and transpassive states are found in the curve. In the passive state, the current density is strongly suppressed, compared to that in the active state. Explain why the current density in the passive state is suppressed.