Electrochemical Study of Corrosion Behavior of Rare Earth Based Chemical Conversion Coating on Aerospace Aluminum Alloy

Abstract

The anticorrosion properties of cerium and praseodymium based chemical conversion coatings deposited on AA6061-T6 were evaluated in NaCl 0.01 M aqueous solution at room temperature using Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP). Coatings microstructure and morphology were observed by Scanning Electron Microscopy (SEM). Coatings compositions were characterized by Energy Dispersive X-Ray Spectroscopy (EDX). The obtained results showed that: 1) Cerium sulphate based coatings possess superior resistance to localized corrosion in comparison to the praseodymium nitrate based ones; 2) One minute of immersion shows good anticorrosive properties of the Ce 2 (SO 4 ) 3 based coatings; 3) Post-deposition phosphate treatment notably improves the anticorrosive behavior and 4) Hydrogen peroxide addition to the cerium based formulation accelerates the coating formation.

Cerium-based conversion coatings on aluminium alloys: a process review

“…Other workers have used abrasion or polishing before chemical pretreatments such as NaOH 23,29,104 or acid pickling steps 98,119,128,173,176 or both, 135 but, with the possible exception of repair processes, abrasion is suitable only for small samples and cannot reasonably be expected to form the basis of industrial applications, despite claims to the contrary. 169…”

“…153 Conde et al, by contrast, massively increased the amount of chloride in an attempt to accelerate coating formation on AA 8090, 102 although the coating rate remained slow (requiring up to 30 min), and the process failed completely on AA 2024 and AA 7075. 102 Sulphate is poorer at coating than chloride, 173 being slightly inhibitive to the coating process. 177 Nitrate, however, is strongly inhibitive to the coating reaction 114,177 and gives slow coating rates.…”

“…195,196 The mechanism was reiterated in later papers 59,127,128 and was confirmed, or simply repeated, by many later researchers. 9,15,19,21,23,33,41,42,53,58,59,80,91,92,113,114,119,121,139,143,144,157,159,160,162,168,170,173,174,198,256,[260][261][262][263][264][265] In fact, the mechanism accepted today is essentially the same as that proposed by Hinton et al in 1985, 52 although it may have had an antecedent in the work of Da Fonte et al [266][267][268] It may also be noted that, while all the rare earths will precipitate given a high enough pH, only two of them, cerium and europium, have oxidation couples [Ce(III)-(IV) and Eu(II)-(III)], which are stable in aqueous solut...…”

Searching for chromate replacements using natural language processing and machine learning algorithms

“…The past three decades have seen much research focus on alternatives for chromates. Some of the most widely explored alternatives that have demonstrated promising performance approaching that of chromate-containing inhibitors include rare-earth-based inhibitors 11 and rare-earth coatings, vanadate-based coatings 12 that are currently utilised in aerospace systems 13,14 , lithium-containing coatings 15 , organic coatings, nanocomposites, phosphate coatings 16 and metal-rich primers 17 . Undoubtedly, the search for chromate alternates remains a very timely topic and a puzzle that is yet to be solved in the field.…”

Searching for chromate replacements using natural language processing and machine learning algorithms

“…The past three decades have seen much research focus on alternatives for chromates. Some of the most widely explored alternatives that have demonstrated promising performance approaching that of chromate-containing inhibitors include rare-earth-based inhibitors [10] and rare-earth coatings, vanadate-based coatings [11] that are currently utilised in aerospace systems [12,13], lithium-containing coatings [14] organic coatings, nanocomposites, phosphate coatings [15] and metal rich primers [16]. Undoubtedly, the search for chromate alternates remains a very timely topic, and a puzzle that is yet to be solved in the field.…”