In situ pH measurement during the formation of conversion coatings on an aluminum alloy (AA2024)

105

The formation of a trivalent chromium conversion coating on AA2024-T351 aluminum alloy has been studied using electron microscopy, scanning Kelvin probe force microscopy, ion beam analysis and X-ray photoelectron spectroscopy (XPS). The coating contained oxide, hydroxide, fluoride and sulfate species, and consisted of two main layers: a zirconium-and chromium-rich outer layer and a thinner, aluminum-rich, inner layer. XPS indicated that zirconium and chromium are mainly present as ZrO 2 , Cr(OH) 3 , Cr 2 (SO 4 ) 3 and CrF 3 . In addition, a thin aluminum-rich layer, probably composed of hydrated alumina, occurred transiently at the coating surface. The coating above cathodic second phase particles was usually thicker than that above the matrix due to the locally increased alkalinity. In the early stages of treatment, the thickest coating formed above the S-phase particles. Localized corrosion and copper enrichment of the matrix occurred at the coating base. The localized corrosion is possibly related to the observed accumulation of fluoride ions at the inner layer and the enrichment of copper in the alloy. The thickness of the coating above the alloy matrix was significantly less than that of a coating formed on high purity aluminum. AA2024-T3 aluminum alloy is widely used in the aerospace industry due to its high strength to weight ratio and damage tolerance. However, the presence of copper as a primary alloying element leads to an increased susceptibility to localized corrosion, especially pitting corrosion.2,3 Hence, the alloy requires protective treatments to provide the required corrosion resistance for many applications. Such treatments often involve the use of chromate conversion coatings. However, the toxicity and high disposal costs of Cr(VI) wastes necessitate the development of eco-friendly alternatives. 4,5 One promising candidate treatment uses a trivalent chromium bath, generally consisting of zirconium hexafluoride and trivalent chromium salts. 6,7 The coating formation involves the dissolution of the native oxide on the aluminum surface due to the acidic, fluoride-containing solution, 8 and the subsequent pH-driven deposition of zirconium and chromium species. The increase in interfacial pH is promoted by the cathodic reactions, i.e. hydrogen evolution and/or oxygen reduction. 9Previous work has indicated that the resultant coating consists of two layers. 7,9,10 In one study, it was proposed that a first, dense layer is formed by hydrolysis of Cr(OH) x (3-x)+ and Zr(OH) x (4-x)+ ions that retards oxidation of the alloy and reduction of protons; a second layer then forms by preferential deposition of zirconium hydroxide at transient cathodic sites.10 Other studies revealed a coating composed of an outer, hydrated zirconium-rich layer that also contains chromium species and an inner, aluminum-rich layer. 7,9 Several investigations have been carried out to determine whether Cr(VI) species are formed in trivalent chromium conversion coatings. No Cr(VI) was detected by UV-visible spectroscopy in as-de...

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

Hashimoto

Walton

et al. 2015

105

“…This is due to the enhanced cathodic reaction at the particles, which generates a locally increased pH that promotes the deposition of the Zr-and Cr-rich coating material. 7,20 Figures 6 displays transmission electron micrographs of crosssections of the coatings at locations on the aluminum surface remote from copper-rich particles after conversion treatment for (a-c) 120 s 3 (a, d) and solutions containing 0.05 g/L (b, e) and 0.5 g/L (c, f) copper sulfate. The crosssections reveal a chromium-and zirconium-rich outer layer above a thin aluminum-rich layer that contains oxygen and fluorine species.…”

Section: Resultsmentioning

confidence: 99%

Influence of Copper on Trivalent Chromium Conversion Coating Formation on Aluminum

Miao

Wang

et al. 2017

In the present work, modified copper-containing trivalent chromium conversion (TCC) coating processes for aluminum were investigated. The copper addition to the TCC bath was made for the purposes of reducing the generation of hydrogen peroxide and chromium (VI) species during the coating growth. The morphologies and compositions of the coatings were examined using highresolution electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy. UV photometric measurements were employed to determine the amount of hydrogen peroxide in the TCC solution. The resultant coatings contained zirconium oxides, chromium hydroxide and fluorides and sulfate constituents, as well as copper oxides and copper-rich deposits that were preferred cathodic sites for oxygen reduction. Of most significance, no chromium (VI) species were detected in the coatings by Raman spectra. It is suggested that this results from reduced generation of hydrogen peroxide, as disclosed by photometric measurements, at the cathodic copper-rich particles, due to favoring of the four electron oxygen reduction reaction. Trivalent chromium conversion (TCC) coatings are promising ecofriendly alternatives to chromate conversion coatings due to the lower toxicity of trivalent chromium species compared with hexavalent chromium.1 The TCC coating solution can be regarded as a modified Zr-based conversion coating solution with addition of small amounts of trivalent chromium salts.2,3 Our previous work used scanning electron microscopy and atomic force microscopy to reveal cracking and spalling of the coating formed on superpure aluminum, especially after a prolonged conversion treatment. 4 This was associated with the fast kinetics of aluminum dissolution due to the attack by fluorine ions in the reaction solution and the stress in the coating. 5 In contrast, the TCC coating formed on AA2024 alloy suppressed the oxygen reduction reaction, due to the physical barrier created by the Zr-/Cr-rich coatings, 1,6,7 and the coating displayed improved adhesion, although the coating was significantly thinner than that formed on superpure aluminum.With respect to the effect of copper alloying, George et al. 8 investigated binary alloys containing 1, 5, and 25 at.% Cu prepared by magnetron sputtering and revealed a decrease in the coating growth rate of zirconium-based coatings with increase of the Cu/Al ratio. This was suggested to be due to the copper species present in the coating impeding the cation transport to the coating base. Furthermore, a layer of corrosion products formed at the coating base. Cerezo et al.9-11 modified the Zr-based conversion coating solution by adding a small amount of copper salts (30-50 ppm). The copper components, which had a high deposition tendency, formed copper and/or copper oxide agglomerates on the substrate, which created local alkalinity that supported the coating formation. As a consequence, the coating thickness on the multi-metal substrate (AA6014, cold rolled steel and hot dip galvanized steel) was increased.One conc...

“…The removal of the oxide layer exposes the metal surface where cathodic reactions take place. Oxygen reduction reaction, and possibly hydrogen discharge, which counterbalance aluminum oxidation, consume protons and causes a pH increase at the interface between the metallic substrate and the solution, as measured by Li et al 15 The high pH favors the precipitation of ions present in the solution, forming a hydrated layer of zirconium and chromium oxide. The thickness of the TCP layer is usually of the order of 100 nm, depending on the nature of the aluminum alloy, the concentration of the solution and the immersion time.…”

mentioning

confidence: 98%

Role of Post-Treatment in Improved Corrosion Behavior of Trivalent Chromium Protection (TCP) Coating Deposited on Aluminum Alloy 2024-T3

Ely

Światowska

Seyeux

et al. 2017

The effects of post-treatment applied on Trivalent Chromium Protection (TCP) coatings deposited on aluminum alloy 2024-T3 was studied by electrochemical and surface analytical techniques: X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The aim of the post-treatment was to improve the corrosion resistance of the TCP coating. To understand the influence of the post-treatment, using a bath containing H 2 O 2 and a lanthanum salt as an inhibitor, our approach was to study the effect of these two components separately. It was found that the improved corrosion protection provided by the post-treated TCP was related to a synergetic effect of the two components. XPS and ToF-SIMS analyses showed that (i) the thickness of the TCP coating is not modified by the post-treatment, (ii) lanthanum is present on the surface and in the bulk of the TCP and (iii) is found in very small amount only on the extreme surface. The total concentration of Cr(VI) (oxide/hydroxide) was estimated to be below 0.1 wt% in the post-treated TCP layer. It was concluded that lanthanum plays a significant role in improving the coating density and homogeneity and has a beneficial effect on the TCP coating cracking as observed by Scanning Electron Aluminum alloys are used for aerospace applications for their low weight and good mechanical properties.1-3 To improve the corrosion properties of aluminum alloys and to provide good paint adhesion, the first layer of coating applied on the aluminum alloy surface used to be a chromate conversion coating (CCC).4-6 However, hexavalent chromium used in the CCC coatings is highly toxic, 4,7 its use is very much restricted in Europe 8 and the United States 9 and will be forbidden by 2024 by the European Community with respect to the Registration, Evaluation, Authorization and Restriction of Chemicals (REACh).8 Therefore, new conversion coatings have been developed, and one of the most promising solutions is the Trivalent Chromium Protection coating called also as the Trivalent Chrome Process (TCP).10-14 TCP conversion coatings are formed by immersion in a solution containing Zr 4+ , Cr 3+ and F − ions, at a pH between 3.8 and 4.0. As it has already been widely discussed, the coating formation mechanism occurs by multiple chemical steps. 10,11,13,[15][16][17] The initial step involves dissolution of the aluminum oxide layer by fluorides. The removal of the oxide layer exposes the metal surface where cathodic reactions take place. Oxygen reduction reaction, and possibly hydrogen discharge, which counterbalance aluminum oxidation, consume protons and causes a pH increase at the interface between the metallic substrate and the solution, as measured by Li et al. 15 The high pH favors the precipitation of ions present in the solution, forming a hydrated layer of zirconium and chromium oxide. The thickness of the TCP layer is usually of the order of 100 nm, depending on the nature of the aluminum alloy, the concentration of the solution and the immersion time.11,12,18 ...