1. coat the metal with paint
2. Electroplating is method to coat surfaces to prevent corrosion. Zn is one of the most popular method to prevent corrosion on steel. Ni/Cu is a common plating metals for corrosion protection.
3. Anodic protection also used to take advantage of the I-V curve of the protected parts to apply potential in the range of the passive current. However, this technique can be applied to few materials, such as Cr, Al, with precise control of the applied potential.