The problem of bike covers is that they often form condensation on the inside of the cover, every morning that dew forms, or the inside of the cover is warmer than the outside.
That makes a nice humid, wet environment that promotes corrosion.
This is largely the same reason that sealed bags often contain dessicant, to absorb the remaining moisture inside the bags. So, don't think a cheap tarp is going retard the corrosion on your bike, it often traps water and accelerates the corrosion, unless you can circulate warm dry air inside it.