Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy

Abstract

In this paper, a chromate free conversion coating based on titanium/zirconium salts and aminotrimethylene phosphonic acid (ATMP) was prepared on 7A52 aluminum alloy. The morphology and composition of conversion coating were investigated by test measurements of SEM、EDS、XRD and FT-IR. The adhesion strength between aluminum matrix and subsequent epoxy primer was tested directly. The roughness and wettability of conversion coating were also detected to indirectly characterize the adhesion strength from another side. The electrochemical tests and neutral salt spray tests were also conducted to measure the corrosion resistance of the prepared conversion coating. Results reveal that a uniform and dense conversion coating consisted by metal oxides and other salts is obtained on the surface of 7A52 aluminum alloy. Significantly, the adhesion strength between matrix and epoxy primer increases from 7.97 MPa to 16.23 MPa, which dues to the increase of surface roughness and the improvement of wettability. In addition, the neutral salt spray test time of the conversion coating reaches as long as 120 h. Three orders of magnitudes decrease of current density proves the excellent corrosion resistance of conversion coating, which is also confirmed by electrochemical impedance spectroscopy test. This dues to the barrier property of conversion coating, preventing the corrosive solution to have a direct contact with aluminum matrix. Anyhow, both the adhesion strength and corrosion resistance are greatly reinforced by the existence of conversion coating.