Comments by "SeanBZA" (@SeanBZA) on " Фабрика Монстр" channel.

Etching as well the areas of steel that have the highest internal tension stress will dissolve faster into the acid, so those areas will etch faster, while the areas that are the most compressed will, due to the atoms being forced slightly closer together, be slower to dissolve relative to the stressed areas. Also any grain boundaries in the crystals will allow the liquid to penetrate, deepening those boundaries, and exposing the darker harder metal oxides there to see.
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