There is a small correction.....There is no polarisation concept in case of microwave specially when we are dealing with conductors....Its come when we have dielectric....and Al is a good conductor....The sparking is basically because of the penetration depth of Electromagnatic Waves.....Higher is the frequency lower is the penetration depth...so the charge remain on the surface of the conductor....There reaches a stage when charge start to discharge via discharge current with atmospheric gases....if you have vaccum in the microwave then you will not have any discharge and in that case it willbe a faraday cage....