With sodium molybdate and sodium twelve mixed solution as the membrane forming agent

composite film with Cr-free on the 6061 aluminum surface was prepared

and the forming process and microstructure of the composite conversion film were investigated

meanwhile

the formation mechanism of the conversion film was described.The results show that

with increasing in the film formation time

micro-hole in the conversion film surface is covered gradually with uniformly distributed micro-crack

and the width of micro-crack is increased increasingly.Composite conversion coating consists of amorphous oxides and hydroxides

such as MoO3

MoO（OH）2

CeO2

Ce（OH）4

Ce（OH）3

MnO2

Al（OH）3

Al2O3 and possible MoO2.