Alternative corrosion protection pretreatments for aluminum alloys

Abstract

Aluminum alloys are extensively used in space programs for both structural and nonstructural applications. As is well known, the corrosion resistance of these alloys is quite limited and anticorrosion treatment is needed. Currently, the treatment most often used for improving the corrosion resistance of aluminum alloys is chromate (CrVI-based) conversion coatings (CCC). Due to the high environmental impact of these compounds, the REACH Regulation of the European Union decided to limit/restrict the use of hexavalent chromium. A sunset date of mid-2017 has been already set. For the time being, many chromatefree alternative products are available, but they have been found to be significantly inferior in terms of corrosion protection performances with respect to the chromate options. In this regard, there is a great need to develop high-performance hexavalent chromiumfree anticorrosion coatings with low environmental impact and conformity with the EU regulations. The European Space Agency (ESA) is currently involved in a frame contract with Instituto de Soldadura e Qualidade (ISQ) that is currently ongoing. The objective of this study is to evaluate the anticorrosion behavior of the different alternative pretreatments applied to the most used aluminum alloys. The outcome of this activity will be the identification and the optimization of the most promising anticorrosion pretreatment. Several commercial Cr(VI)-free processes were applied onto aluminum alloys (2024-T3 and 2024-T81) used in the construction of ESA spacecraft in order to investigate their anticorrosive properties compared with Alodine 1200, a widely used conventional CCC. One of these commercial processes presented good anticorrosion performance, even better than Alodine 1200, and these results are presented in this paper. Salt spray resistance is included in the testing program as well as metallurgical coating evaluations that include microscopy observations before and after testing and electrochemical impedance spectroscopy in order to evaluate surface and microstructural modifications due to degradation mechanisms. Further studies must be performed with this promising alternative, modifying application parameters in order to achieve improved corrosion protection efficiency.

Review—Conversion Coatings Based on Zirconium and/or Titanium

“…112 It was reported recently that PreCoat A32 coating consisting of Zr(IV), Cr(III) and fluorides achieved similar performance in a 168 h salt spray test as Alodine 1200 CCC, indicating that it is a promising alternative for aeronautic and aerospace industries. 128 Self-repair ability was studied in several reports. Vanadatecontaining Zr-based coatings on AA6063 promoted self-healing as indicated by EIS spectra in 3.5% NaCl.…”

A Molecular Approach for Mitigation of Aluminum Pitting based on Films of Zinc(II) and Gallium(III) Metallosurfactants

“…The propagation phase yields localized blisters of acidic pH that eventually erupt and expose the corrosion pit . The corrosion rate decreases over time, but perforation may occur, and chromium‐based coatings have been commonly used . However, concerns with the environmental impact of hexavalent chromium generated by these coatings impose steep restrictions to their use .…”

“…The corrosion rate decreases over time, but perforation may occur, and chromium‐based coatings have been commonly used . However, concerns with the environmental impact of hexavalent chromium generated by these coatings impose steep restrictions to their use . In searching for potential alternatives, we hypothesize that the presence of an ordered Langmuir–Blodgett (LB) multilayer of molecular metallosurfactants would act as a hydrophobic barrier that decreases access of water, dissolved chlorides and electrons to the Al 2 O 3 layer.…”

“…[15,16] However, concerns with the environmental impact of hexavalent chromium generated by these coatings imposes teep restrictionst ot heir use. [16] In searching for potential alternatives, we hypothesize that the presence of an ordered Langmuir-Blodgett (LB) multilayer of molecularm etallosurfactants would act as ah ydrophobic barriert hat decreases access of water, dissolved chlorides and electrons to the Al 2 O 3 layer.I nt his paperw ep robe the LB films of ab isphenolate zinc(II) complex [Zn II (L N2O2 )H 2 O] (1)a nd ab ulkier trisphenolateg allium(III) complex [Ga III (L N 2 O 3 )] (2)( Figure 1) as mitigators for pit corrosion when physisorbed onto 99.0 %a luminum substrates. The presence of hydrophobic tert-butyl groups installed to the phenolate groups along with hydrophilica lkoxyc hains installed to the phenylenediamineb ridge renderst hese speciesa mphiphilic, enabling the formation of ordered molecular films.…”

Trivalent chromium conversion coating on AA2024‐T3 used in aeronautical and aerospace industry

“…The aluminium alloy 2024‐T3 has been extensively used in the aeronautic industry due to its low density and good mechanical properties. As well known, the corrosion resistance of these alloys is quite limited, and anticorrosion treatment is needed . Indeed, the AA2024‐T3 is quite susceptible to localised corrosion, specially pitting and intergranular corrosion, due to the presence of intermetallic particles of cooper that provide preferred cathodic sites on the alloy surface …”