  We establish a logarithmic stability estimate for the problem of detecting
corrosion by a single electric measurement. We give a proof based on an
adaptation of the method initiated in \cite{BCJ} for solving the inverse
problem of recovering the surface impedance of an obstacle from the scattering
amplitude. The key idea consists in estimating accurately a lower bound of the
$L^2$-norm, locally at the boundary, of the solution of the boundary value
problem used in modeling the problem of detection corrosion by an electric
measurement
