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.
COBISS.SI-ID: 30577447
The effectiveness of inhibition by cerium(III) chloride and acetate salts as corrosion inhibitors has been investigated for aluminum alloy AA7075-T6 in 0.1 M NaCl. The electrochemical and corrosion characteristics have been studied, using linear polarization and potentiodynamic polarization, in NaCl with and without the addition of 3 mM cerium salts. The effectiveness of inhibition was studied as a function of immersion time. Both salts act as typical cathodic inhibitors. However, on prolonged immersion () 24 h) a distinctive passive region is established in the presence of acetate. A long-term immersion test of 2 months confirms the high degree of surface protection, with no signs of corrosion products. Scanning electron microscope images of surfaces after immersion for 24 h in NaCl and in inhibiting cerium salts show remarkable differences between samples. While in NaCl the samples were subject to pitting and intergranular corrosion, they were protected by the presence of cerium salts. Depending on the type of salt, cerium precipitation was observed predominantly on intermetallic particles or throughout the alloy matrix, as shown by compositional mapping. Cerium acetate is established as an effective inhibitor for aluminum alloy. Moreover, it enables the repassivation of the surface defects. The ability of repassivation was not observed in the presence of cerium chloride.
COBISS.SI-ID: 29450535
The corrosion protection of pure aluminium and its alloys AA2024-T3 and AA7075-T6 has been studied in 0.1 M NaCl with and without the addition of Ce(III) chloride, Ce(III) nitrate, Ce(III) acetate and Ce(IV) sulphate. The study of Ce(III) acetate has been scarce but is here shown to be the most effective of the cerium salts studied, especially for AA7075-T6. The inhibition effectiveness of cerium salts is: Ce(III) acetate ) Ce(III) chloride ) Ce(III) nitrate for the metals studied. Ce(IV) sulphate does not inhibit corrosion. Electrochemical potentiodynamic measurements were used to investigate the mechanism of inhibition by cerium salts. Immersion tests in 0.1 M NaCl, with and without the addition of Ce(III) chloride and Ce(III) acetate, were performed to determine the effects of the inhibitors on the extent of corrosion damage at the surface of all three metals. For pure aluminium the inhibitory effectiveness was 78.5%, 82.6% and 84.0% in the presence of 3 mM Ce(III) nitrate, chloride and acetate, respectively. For AA7075-T6 the values of 92.1%, 99.1% and 99.6% were obtained. The inhibitory effectiveness of cerium salts is shown to depend on a variety of factors: type of substrate, type of anion and salt concentration
COBISS.SI-ID: 29113639
The effects of mechanical and chemical pre-treatments of AA7075-T6 and AA2024-T3 aluminium alloys were studied in terms of changes of morphology, composition and wettability of the surface, and of the related corrosion properties in sodium chloride solution. The mechanical treatments tested include water grinding and non-water diamond polishing. The latter preserved the content of Mg at the AA7075-T6 surface. The effect of mechanical treatment was less pronounced on AA2024-T3. The chemical treatments included etching with sodium hydroxide and with a commercial cleaner and desmutting in nitric acid. The sample pre-treatment affects the polarization curves in sodium chloride.
COBISS.SI-ID: 30464295
Aluminum alloy AA7075-T6 is most commonly used in the aircraft and automotive industries. Due to the presence of intermetallic particles, AA7075-T6 is susceptible to localized corrosion in chloride solutions. In the present work multilayer hybrid sol-gel coatings, based on silane precursors 3-glycidyloxypropyl(trimethoxysilane) (GPTMS) and tetraethoxysilane (TEOS), were used to protect AA7075-T6 from corrosion. To enrich the barrier properties of the coating, SiO2 nanoparticles were added to the GPTMS/TEOS sol-gel solution. Inhibition and a self-healing effect were achieved by doping Ce(NO3)3 in the coating. A multi-layer system was applied on the substrate composed of a first layer doped with Ce(NO3)3 and the second, undoped sol-gel layer. The addition of cerium increases the lifespan of hybrid sol-gel coating and has a role in self-healing if it is locked within the first layer of the multilayer coating. The self-healing effect was confirmed by immersion of unscribed and scribed coated substrates in 0.1 mol/L NaCl by using the immersion test, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy with chemical analysis.
COBISS.SI-ID: 31262759