A protective oxide layer (ceramic) has formed on the metal surface.

You can see the chalky white layer on aluminum, but the passivated layer is invisible on stainless steel.

The passivated layer that forms on stainless steel is chrome oxide (Cr2O3).

When the metal becomes passivated, the corrosion rate slows down to less than 0.002 inches per year (0.05 mm/ year)