Elsevier

Applied Surface Science

Volume 239, Issue 1, 15 December 2004, Pages 45-59
Applied Surface Science

Microstructural effects on the initiation of zinc phosphate coatings on 2024-T3 aluminum alloy

https://doi.org/10.1016/j.apsusc.2004.04.038Get rights and content

Abstract

The initiation of coatings deposited on to 2024-T3 aluminum alloy from supersaturated zinc phosphating solutions has been studied using scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The alloy microstructure, especially associated with the second-phase particles, strongly affects the formation stages of the coating process, where etching of the substrate has a significant role. At the start, zinc phosphate (ZPO) crystals form on the Al–Cu–Mg second-phase particles, rather than on the matrix or on the Al–Cu–Fe–Mn particles, with the initial nucleation appearing at interfaces between Al–Cu–Mg particles and the matrix. In contrast, the formation of the ZPO coating is delayed on the cathodic Al–Cu–Fe–Mn particles, compared to those of the Al–Cu–Mg composition. When the coating process is completed, the whole sample surface is covered with ZPO although its thickness varies at the different micro-regions.

Introduction

The increasing involvement of aluminum alloys in industrial applications has stimulated a growing interest in improving their corrosion protection properties. Conventional pre-treatments depend on chromate-based conversion coatings which provide the best level of underfilm corrosion resistance, as well as excellent paint adhesion [1], [2]. Nevertheless, recent concerns about the carcinogenic nature of Cr(VI) solutions are encouraging new research to find effective and safer replacements [3], [4]. One promising approach depends on the zinc phosphate conversion coatings that were developed originally for iron and steel [5], [6], [7], [8], [9], [10]. A number of phosphating processes are available through patents [11], [12], [13], [14], [15], [16], [17], and various studies have reported on the effect of changing solution parameters, such as composition, pH, temperature, time of coating, as well as the influence of prior treatments that may result in surface enrichment of the alloying elements [18], [19], [20], [21], [22], [23], [24]. Only a few investigations have been reported that relate to the coating mechanism [25], [26], [27], and at this time it must be concluded that the mechanistic details are insufficiently known to provide a basis for the design of new processes.

The zinc phosphating of metals depends on the precipitation of Zn3(PO4)2·4H2O from supersaturated solutions, but the initiation of the phosphating process appears important to the nature of the final coating, including its effectiveness against corrosion. This in turn depends on the size and packing of the crystallites formed on the metal surface. The initially-deposited crystallites provide nucleation sites for the subsequent coating, and in general the smaller the size of the initial crystals, and the higher their coverage, the better for obtaining a more effective coating. The first part of the phosphating process is governed by electrochemical reactions occurring at local micro-anodes and micro-cathodes on the heterogeneous metal surface [6], [7], [28]. In an acidic phosphating solution, most of a metal surface exhibits anodic behavior (i.e. the metal oxidizes and dissolves), while the cathodic areas are confined to heterogeneities with distinctive physical or electrochemical properties [28]. Hydrogen evolution occurs at the micro-cathodes, which is followed by a local increase in pH that initiates precipitation of the insoluble tertiary phosphate [6], [7], [28].

Early work by Cheever [29] showed that phosphate coatings on iron and steel substrates primarily initiate at cathodic grain boundaries, and more recently it has been reported that the base-metal microstructure in iron and steel also affects the thickness and local properties of phosphate coatings [30]. We are not aware of analogous work having been done on aluminum alloys, and accordingly the present work undertakes an initial exploration in this area. The 2024-T3 aluminum alloy is used. This alloy has copper as the major alloying element, and although its presence significantly improves mechanical properties for aerospace applications, it also greatly reduces the corrosion resistance [1], [31], [32]. The added elements (e.g. Cu, Mg) distribute differently within the material. Besides the basic alloy matrix (solid solution), independent intermetallic compounds (second-phase particles) precipitate during the solidification and thermomechanical processing [1], [31], [32], [33], and the alloying elements (e.g. Cu) often occur in considerably greater concentrations in these particles than in the alloy matrix. A consequence is that the type, size and distribution of the second-phase particles markedly influence corrosion processes [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], and a recent study reported that these particles could also affect the nature of organosilane adsorption applied for protective purposes [44].

The objective of the present study is to explore the effect of the different microstructural regions, that occur at a surface of a 2024-Al alloy, on the initiation of zinc phosphate coatings. The prime characterization techniques used are scanning electron microscopy (SEM), to assess coating morphology, and scanning Auger microscopy (SAM) to probe chemical compositions at local micro-regions; X-ray photoelectron spectroscopy (XPS) is also used to give more general information on chemical composition over broader regions of the coated surfaces.

Section snippets

Experimental

Samples of 2024-T3 aluminum alloy (1 cm × 1 cm × 0.12 cm) were mechanically polished with alumina paper up to 1200 grit; in addition, some samples were further polished with diamond paste to 1 μm roughness, and such samples are referred to below as being “mirror polished”. These treatments were followed by ultrasonic cleaning in acetone and methanol, and drying in air. Zinc phosphate (ZPO) coatings were formed by immersing the polished alloy samples in Gardobond R 2600 (Oakite) phosphating solution

Preliminary observations

Fig. 1 shows SEM micrographs of ZPO coatings formed on mechanically-polished (1200 grit) 2024-T3 alloy samples after immersing in the coating solution at 50 °C for different times (3 s–1 min). The alloy surface is rough with polishing lines clearly detectable, and it is observed that the coating coverage increases as the immersion time increases to 1 min, although no change in coverage is apparent with a further increase to 3 min. For this coating time, the SEM image indicates that the crystallites

Discussion

This work shows that the formation of a zinc phosphate conversion coating on a mechanically-polished 2024-T3 aluminum alloy is strongly affected by alloy microstructure, especially the presence and distribution of second-phase particles. When the 2024-Al alloy is polished, intermetallic compounds involving the elemental combinations Al–Cu–Mg and Al–Cu–Fe–Mn become exposed to create an electrochemically heterogeneous surface. Different processes occur at these micro-regions on contact with the

Concluding remarks

This research attempts to strengthen an underdeveloped area in materials science, namely that associated with how chemically different microstructural regions of an alloy surface react when exposed to a common environment. The work emphasizes the initiation of zinc phosphate conversion coatings deposited on to a 2024-T3 aluminum alloy sample, and the following observations have been made on coated samples through study with various surface analysis techniques:

  • 1.

    The 2024-Al alloy microstructure

Acknowledgements

We are grateful for the support provided for this research by the Natural Sciences and Engineering Research Council of Canada.

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