XPS Analysis of the Surface Chemistry and Interfacial Bonding of Barrier-Type Cr(VI)-Free Anodic Oxides

Abstract

In the transition to environmental friendly pretreatment of aerospace aluminum alloys, chromic acid anodizing (CAA) is being replaced by sulfuric acid (SAA), phosphoric acid (PAA) or phosphoric-sulfuric acid (PSA) anodizing. While generally the main concern is controlling the film morphology, such as the pore diameter, oxide-, and barrier layer thickness, little is known on how the anodic oxide chemistry affects the interactions at the interface upon adhesive bonding. To study the link between surface chemistry and interfacial bonding, featureless oxides were prepared by stopping the anodizing during the formation of the barrier layer. A model was developed to quantify the relative amounts of OH -, PO 4 3-and SO 4 2-by curve-fitting the XPS data.Calculations showed that almost 40% of the surface species in PAA oxide are phosphates (PO 4 3-), while about 15% are sulfates (SO 4 2 ) in SAA. When both anions were present in the electrolyte, phosphate incorporation was inhibited. Studies of the interaction between this set of Cr(VI)-free oxides and diethylenetriamine (DETA) -an amine curing-agent for epoxy resin, showed that all oxides interact with the nitrogen of DETA. However, larger ratios of Lewis-like acid-base bonding between the amine electron pair and the acidic hydroxyl on phosphate surface sites were observed.

Adhesive Bonding and Corrosion Performance Investigated as a Function of Aluminum Oxide Chemistry and Adhesives

“…Conversely, sulfur concentration is relatively constant in both measurements. These observations are in agreement with earlier in-depth study performed using Auger electron spectroscopy (AES) 7 . As oxide hydration is mostly dominant close to the surface, oxides other than SAA and PAA exhibit a higher hydroxyl fraction at θ =15° 42, 43 .…”

“…Currently, different electrolytes are typically mixed in the anodizing bath to create the conditions that will result in film features similar to CAA anodic oxides. Some examples include boric-sulfuric acid anodizing (BSA) 4 , phosphoric acid modified boric-sulfuric and anodizing 5 , tartaric-sulfuric acid anodizing (TSA) 6 and phosphoric and sulfuric acid (PSA) anodizing 7 . In this paper, we focus on the latter option being the prime Cr(VI)-free candidate for bonded components that will serve as part of the loadcarrying primary aircraft structure.…”

“…To exclude the contribution of mechanical interlocking, anodic oxide growth was stopped during the formation of the barrier layer, producing relatively thin and featureless oxides 7 . Since the hydroxyl fraction is expected to play a crucial role in bonding with aluminum oxides, the classic alkaline and boiling water pre-treatments that produce the highest amount of surface hydroxyls 36 are included for comparison.…”

Interface strength and degradation of adhesively bonded porous aluminum oxides

“…It was found that the density of hydroxyl groups at the surface of SAA oxide is larger than for PSA oxides (approximately double, depending on the anodizing conditions). 12 Therefore, higher peel strengths can be achieved with less surface area and performance differences can be related to changes in the oxide chemistry, as illustrated in Fig. 5d.…”

Particle Characterisation and Depletion of Li2CO3 Inhibitor in a Polyurethane Coating

“…Figure 5a-d all show an oxide at the interface, which is the anodised coating. S was detected in this layer, presumably due to the incorporation of SO 4 2− ions from the anodising process ( Figure 5c) [57].…”