  This report is dedicated to the construction and analysis of so-called
Generalized Impedance Boundary Conditions (GIBCs) used in electromagnetic
scattering problems from imperfect conductors as higher order approximations of
a perfect conductor condition. We consider here the 3-D case with Maxwell
equations in a harmonic regime. The construction of GIBCs is based on a scaled
asymptotic expansion with respect to the skin depth. The asymptotic expansion
is theoretically justified at any order and we give explicit expressions till
the third order. These expressions are used to derive the GIBCs. The associated
boundary value problem is analyzed and error estimates are obtained in terms of
the skin depth.
