Conversion Coatings Based on Rare Earth Nitrates and Chlorides for Corrosion Protection of Aluminum Alloy 7075-T6 Available to Purchase

Conversion coatings are expected to provide high corrosion protection and to be environmentally friendly at the same time. The aim of this study was to investigate rare earth (cerium and lanthanum) nitrate and chloride conversion coatings deposited on aluminum alloy AA7075-T6. Individual Ce and La nitrate and chloride coatings and their mixtures (Ce(NO₃)₃:La(NO₃)₃ and CeCl₃:LaCl₃) in the ratio 1:3 and 3:1 were studied. The properties of the coatings were investigated as a function of conversion bath parameters: conversion time (from 1 min to 12 h) and conversion temperature (room temperature and 60°C) at constant salt concentration of 0.01 M in the presence of 0.13 M hydrogen peroxide. The morphology and composition of the conversion coatings were analyzed using scanning electron microscopy and energy dispersive x-ray spectroscopy. The corrosion properties of uncoated and coated AA7075-T6 substrates were studied in 0.1 M NaCl using electrochemical potentiodynamic polarization method and salt spray testing in 5 wt% NaCl at 35°C. Adhesion of the conversion coatings was tested using a standard procedure. Mixing of Ce and La rare earth salts offers a beneficial protection in corrosion environment indicating their synergistic effect. The optimal corrosion performance (up to 72 h in the salt spray chamber) was achieved for chloride salts in ratio Ce:La = 3:1 prepared at room temperature.