Influence of post-treatment time of trivalent chromium protection coating on aluminium alloy 2024-T3 on improved corrosion resistance

Abstract

Low corrosion protection performances of Trivalent Chromium Process (TCP) coatings with reference to Chromium Conversion Coatings (CCC) deposited on aluminium alloys can be overcome by application of post-treatment processes. This work shows the effect of post-treatment bath (containing hydrogen peroxide and lanthanum salt) on the chemical composition, structure and the corrosion performances of TCP coating deposited on AA 2024-T3 aluminium alloy. Different times of post-treatment bath were applied on the TCP coating and the samples were analyzed by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), X-ray Photoelectron Spectroscopy (XPS) and Glow Discharge-Optical Emission Spectrometry (GD-OES). The ToF-SIMS and GD-OES depth profiles show that the post-treatment time has no influence on the thickness of the TCP coating and on its bi-layered structure. It is composed of an outer layer, rich in zirconium and chromium oxides and an inner layer rich in aluminium oxides and oxy-fluorides. 3D ToF-SIMS images reveal an enrichment of Cr and Zr-like species over the surface of intermetallic particles with reference to alloy matrix, whereas a homogenous distribution of La is observed. The analysis of in-depth distribution shows that La is present principally in the outer part of the TCP coating. The La concentration increases with increasing post-treatment time. The effect of post-treatment time on protection was evaluated by Electrochemical Impedance Spectroscopy (EIS) and Linear Sweep Voltammetry (LSV) in a 0.001 M NaCl + 0.1 M Na 2 SO 4 electrolyte. The increased posttreatment time enhances the cathodic inhibition against oxygen reduction. The post-treatment provides improved coating homogeneity and sealing properties.

Effect of surface preparation treatments on copper enrichment on 2024 aluminium alloy surface

“…This suggests an interfacial region rich in copper, due to the presence of Curich intermetallic particles located at the interface between metal and oxide. This region, that extends from 35 s to 85 s, corresponds to both the oxide and metallic substrate as the roughness of the sample is significant, as already discussed previously [4,32,33]. In this region, the CuOsignal shows a maximum of intensity at 45 s, whereas the maximum intensity of Cusignal is observed 5 s later (at 50 s).…”

Conversion coating distribution on rough substrates analyzed by combining surface analytical techniques

“…X-ray photoelectron spectroscopy (XPS) is widely used for nanometric oxides characterization because of its outmost ability to examine and quantify the surface chemical composition [18]. It can be used in parallel with depth profiling techniques such as glow discharge optical emission spectrometry (GD-OES) [19] and time-of-flight secondary ion mass spectrometry (ToF-SIMS) [20]. Although no common strategy was defined to perform this type of characterization, it appears that the coupling between ion beam sputtering and XPS offers a better understanding of the in-depth composition of the thin oxide layers [21].…”

Influence of ageing on the corrosion behaviour of 2024 aluminium alloy coated with a trivalent chromium conversion layer

“…Cuand Al -2 ion signals represent the metallic Cu and Al, respectively, and CuO -, AlO -2 and MgOrepresent the oxidised (oxide and/or hydroxide) Cu, Al and Mg, respectively. These ions have been used previously to characterise Al-Cu alloy surfaces [23,[55][56][57]. The intensity changes of different ions are shown, in a logarithmic scale to enhance the low intensity signals, as a function of sputtering time in s (bottom axis) and depth from the surface in nm (top axis).…”