In your material, all magnesium grains are surrounded by layers of magnesium oxide, so the cohesion of these grains is very weak and they crumble when polished. Black spots are fallen out grains of magnesium.

Since magnesium oxide reacts with water, grinding and polishing in water cannot be done (magnesium grains simply hang in the air after dissolving the oxide). It is clear that grinding and polishing in an organic liquid environment is very difficult, but there is no other alternative.

Hi

Mg alloy is highly sensitive to the etchant solution. I suggested that it is better to use nital 2% as an etchant solution. and just polish by 200,500, and 1000 paper, don't use diamond paste and lubricant spray anymore. it doesn't work. The point in this procedure is the enchantment time. It generally depends on your experience . I suggest using 10 seconds initially and if it wasn't enough, prologue it.

Dear Rawad Aljabr,

The black spots indicate the presence of porosity in your sample. This can't be removed by an etching process. This is made during the powder metallurgy route. You may achieve this by changing the metallurgy process or High-temperature annealing. These pores are created due to the lack of binding strength between the atoms, generally, it will happen in the casting process. Or otherwise, you can't remove these pores (Black spots).

Dear Rawad Aljabr

As Vadim Verlotski sir told, black spots indicate burnt phases. Magnesium readily react with oxygen and produce MgO. That can't be avoided completely even if vacuum furnace is used. MgO is not strongly bonded and removed easily from sample while grinding and polishing. That appears as black spot.