  We use direct numerical simulations of forced MHD turbulence with a forcing
function that produces two different signs of kinetic helicity in the upper and
lower parts of the domain. We show that the mean flux of magnetic helicity from
the small-scale field between the two parts of the domain can be described by a
Fickian diffusion law with a diffusion coefficient that is approximately
independent of the magnetic Reynolds number and about one third of the
estimated turbulent magnetic diffusivity. The data suggest that the turbulent
diffusive magnetic helicity flux can only be expected to alleviate catastrophic
quenching at Reynolds numbers of more than several thousands. We further
calculate the magnetic helicity density and its flux in the domain for three
different gauges. We consider the Weyl gauge, in which the electrostatic
potential vanishes, the pseudo-Lorenz gauge, where the speed of light is
replaced by the sound speed, and the `resistive gauge' in which the Laplacian
of the magnetic vector potential acts as resistive term. We find that, in the
statistically steady state, the time-averaged magnetic helicity density and the
magnetic helicity flux are the same in all three gauges.
