The rare earth metal conversion coating process is recognized for its simple electrolyte composition, which is generally considered to be environmentally friendly. In this study, cerium conversion coatings were made in a cerium nitrate/hydrogen peroxide solution on AZ31 magnesium alloys after 20 s of pickling in hydrochloric acid (HCl) or hydrofluoric acid (HF) solutions. Results show that acid pickling in HF or HCl enhanced the adhesion and corrosion resistance of the cerium conversion coating. This enhancement is due to the inhibition of the blister formation, which presumably results from the rupture of the cerium oxide layer by hydrogen bubbles. Finally, the mechanism of HF and HCl acid pickling effect on the reaction rate and defect of the cerium conversion coating was discussed in detail. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.jes113585] All rights reserved.