Non-uniform strain on the surface of magnesium alloys has been found out to affect the formation of a hydroxide layer, which shows corrosion resistance. There is possibility to control the hydroxide layer by applying of variable non-uniform strain to the magnesium alloy surface prior to the layer forming processes. To apply non-uniform strain to AZ91D high pressure die casting magnesium alloy, a sodium bicarbonate peening process was carried out under the following conditions; projection pressure 0.6 MPa, distance between nozzle and material 150 mm, duration 10 seconds. After peening specimens were evaluated with the surface properties, surface and depth residual stress distribution, and non-uniform strain distribution. Peening forms processing layer on surface the depth was 10 to 20 micrometers. In addition, many twins were formed inside the material when using the larger grain size of projectile. It was found out that nou-uniform strain, and compressive residual stress around -130 MPa could be formed by regulating the grain size difference of the sodium bicarbonate projectile.
