A method for monitoring a buried metal drain under cathodic protection, comprising the steps consisting in: - measuring a first electrical current charged in a connection between the drain and a first coupon which is buried next to the drain, - measuring a first electrical voltage (U1 a.c.) between the first coupon and a first reference electrode which is buried next to the first coupon, characterized in that it comprises the steps consisting in: - determining an average direct current voltage (E1 d.c., ON) as a function of the first voltage (U1 a.c.), - polarizing a second coupon with said average direct current voltage (E1 d.c., ON), with respect to a second reference electrode, using a polarization circuit and an auxiliary electrode, the second reference electrode and the auxiliary electrode being buried next to the second coupon, - measuring a second current circulating in a branch of the connection toward the auxiliary electrode of the polarization circuit, and - storing for multiple times a first surface current density (J1 d.c.) corresponding to the first current and a second surface current density (J2 d.c.) corresponding to the second current and in that it further comprises an analyzing step consisting in: - determining a first average current density as a function of said first surface current densities (J1 d.c.) and a second average current density as a function of said second surface current densities (J2 d.c.), - determining a ratio (r) between on the one hand the difference between the first and the second average current density and on the other hand the second average current density, - estimating (E4-E8) a level of corrosion risk for the drain as a function of said ratio (r). - measuring a first electrical current charged in a connection between the drain and a first coupon which is buried next to the drain, - measuring a first electrical voltage (U1 a.c.) between the first coupon and a first reference electrode which is buried next to the first coupon, - determining an average direct current voltage (E1 d.c., ON) as a function of the first voltage (U1 a.c.), - polarizing a second coupon with said average direct current voltage (E1 d.c., ON), with respect to a second reference electrode, using a polarization circuit and an auxiliary electrode, the second reference electrode and the auxiliary electrode being buried next to the second coupon, - measuring a second current circulating in a branch of the connection toward the auxiliary electrode of the polarization circuit, and - storing for multiple times a first surface current density (J1 d.c.) corresponding to the first current and a second surface current density (J2 d.c.) corresponding to the second current and in that it further comprises an analyzing step consisting in: - determining a first average current density as a function of said first surface current densities (J1 d.c.) and a second average current density as a function of said second surface current densities (J2 d.c.), - determining a ratio (r) between on the one hand the difference between the first and the second average current density and on the other hand the second average current density, - estimating (E4-E8) a level of corrosion risk for the drain as a function of said ratio (r).