Formation mechanism of phosphate conversion film on Mg–8.8Li alloy

A novel phosphate conversion film was obtained on Mg–8.8Li alloy. The film formation mechanism was studied by electrochemical measurements, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. The deposition of conversion film contained three stages. Stage I: loose surface film consisting of oxides and hydroxides was dissolved to expose the bare substrate. Stage II: β phase was preferentially dissolved, accompanying with the deposition of conversion film, and then α phase was deposited with conversion film. Stage III: conversion film was gradually improved until the dynamic balance of film dissolution and formation was established.