  We prove that there exists an interval of time which is uniform in the
vanishing viscosity limit and for which the Navier-Stokes equation with Navier
boundary condition has a strong solution. This solution is uniformly bounded in
a conormal Sobolev space and has only one normal derivative bounded in
$L^\infty$. This allows to get the vanishing viscosity limit to the
incompressible Euler system from a strong compactness argument.
