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

Abstract

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 ...

Protection of Aluminum Alloy 3003 in Sodium Chloride and Simulated Acid Rain Solutions by Commercial Conversion Coatings Containing Zr and Cr

“…Taking into account the sputtering rate of 0.35 nm/s, the thickness of conversion layer was estimated to 40 nm, and that of the interface layer to 50 nm. These results are in line with data published for similar TCP coatings [2,7,34,36]. Looking at the depth profiles, especially with normalized intensities, it seems that the layer exhibited not only biphasic structure (conversion layer/interface layer) but even a tri-layer structure.…”

“…The Cr 2p spectrum (Figure 17a) was deconvoluted using five component 2p 3/2 and 2p 1/2 doublets reflecting all compounds which may be formed in the conversion coating: Cr metal at 574.2 eV, Cr 2 O 3 at 575.3 eV, Cr(OH) 3 at 577.4 eV, Cr(VI) species at 579.7 eV and CrF 3 at 580.0 eV (values of E b cited relate to 2p 3/2 peak). TCP coatings were mainly composed of Cr(III) species, although the formation of Cr(VI) was also noted [6,24,29,34,35]. Since a strong fluorine peak indicated the formation of fluorides in addition to oxides (Table 8, Figure 15), the presence of CrF 3 should be taken into account as well.…”

Electrochemical Evaluation of Corrosion Resistance of Trivalent Chromate Conversion Coatings with Different Organic Additives

“…Thanyalux WANOTAYAN, 1) Yuttanant BOONYONGMANEERAT, 2) Joongjai PANPRANOT, 3) Eiji TADA 4) and Atsushi NISHIKATA 4) * trochemical impedance spectroscopy (EIS) is especially suitable for monitoring the degradation of galvanized steels exposed to accelerated corrosion tests and natural environments because it is a non-destructive method. The corrosion rate can be estimated from the charge transfer resistance that appears in the lower frequency region.…”

“…Electrogalvanized chromate steels are widely used for corrosion protection in various applications, including automotive, construction, and electrical. [1][2][3][4] To apply the coating, a steel substrate is submerged and electrochemically plated in a zinc bath and subsequently dipped in a chromiumbased passivation solution. The alkaline non-cyanide zinc plating solution and Cr 3 + -based passivation solution are environmentally-friendly chemicals that are available for these processes.…”

Chromate replacement: what does the future hold?

“…35,39,[44][45][46] Regardless of the recent findings on TCC chemistry, trivalent chromium-based formulations remain the most common Cr 6+ replacement to date for chemical conversion coatings on aluminum or zinc alloys and are currently commercialized by several coatings suppliers (such as Alodine T 5900 RTU from Henkel, 47 or Socosurf TCS 48 supplied by Socomore). This is often justified by the relative low toxicity of TCC formulations in regards to Cr 6+ , as the proportion of Cr 6+ present in the coating was purported to not exceed the 0.1 wt% set by REACH regulation, 49 although not yet met by all current industrial formulations. In addition, the trivalent chromium (Cr 3+ ) alternative still involves the need of chromium resources, extraction and disposal; and in light of recent studies, trivalent chromium may not be considered as a drop-in replacement to Cr 6+ -owing to some recent and emerging findings.…”