Devicharan Chidambaram

Development of a technique to prevent radiation damage of chromate conversion coatings during X-ray photoelectron spectroscopic analysis

Photoreduction of hydrated sodium dichromate in the presence of carbon compounds has been studied by X-ray photoelectron spectroscopy (XPS). XPS results show that sodium dichromate also undergoes thermal breakdown during analysis. The photochemical and thermal reduction appears to be prevented by cooling with liquid nitrogen and using hydrocarbon-free vacuum pumping conditions. A model for the photoreduction of dichromate has been put forward based on an earlier photoreduction model developed by this group. Using the insight gained from this work, chromium spectra obtained from XPS of chromate conversion coatings (CCCs) on AA2024-T3 aluminum alloy have for the first time been fitted with six species of chromium compounds. The Cr 2p spectra have been critically examined for photoreduction of the hexavalent species: tetravalent and trivalent decomposition products have been identified. To explain the observed photochemical breakdown in CCCs even when performed under clean pumping conditions and liquid nitrogen cooling, its has been verified that the reduction is partly due to the cyanide species present in the CCCs. Cooling with liquid nitrogen prevents the adsorbed water on the coating from reacting with Cr(IV) to form trivalent species as the water molecules are immobilized and kinetics are slowed. As the Cr(VI)/Cr(III) ratio is critical to the performance of the CCC, and its accurate determination by XPS has not formerly been possible due to the photoreduction of Cr(VI) to Cr(III), we present a reliable method by which XPS can be used in characterization of chromate conversion coatings.

Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings

High-velocity oxy-fuel (HVOF) spraying is a versatile technique that can yield high-density coatings with porosities less than 1% by optimization of the process variables. Oxidation of the coating material can be greatly reduced by inert gas shrouding during the spraying process. This work evaluates the electrochemical behavior of HVOF-sprayed coatings aimed to have better or similar corrosion behavior as stainless steel AISI 316 in acidic medium. A total of eight different chromium-containing coatings with varying proportions of alloying elements such as Ni, Mo, Si, Fe, Co, W, B and C have been studied in hydrochloric acid medium. Porosity measurements, gravimetric analysis, open-circuit potential measurements, potentiodynamic polarization and optical microscopy have been performed. The electrochemical behavior of each coating has been compared with that of bulk AISI 316. The results indicate that HVOF-sprayed AISI 316 coating offers a lower corrosion resistance compared to bulk AISI 316. High nickel- and chromium-containing coatings appear to offer a corrosion resistance comparable to bulk AISI 316. However, results indicate that other alloying elements like molybdenum might be essential for obtaining higher corrosion protection.

Interactions of the Components of Chromate Conversion Coating with the Constituents of Aluminum Alloy AA2024-T3

Electrochemical techniques were employed to study the nature of individual interactions between the chemicals constituting a widely used commercial chromate conversion-coating treatment and the main constituents of AA2024-T3 alloy, namely, Al (99.999%), Al 2 Cu, Al-Al 2 Cu galvanic couple, and AA2024-T3 alloy. The samples were pretreated using chromate, ferricyanide, fluoride, tetrafluoroborate, and hexafluorozirconate, prior to electrochemical corrosion tests. The results were compared with untreated (control) samples. For the first time, the chemical interaction of aluminum with hexafluorozirconate has been studied. Maximum activation was observed in the case of hexafluorozirconate pretreatment, which also decreased the interfacial tension and increased surface wetting. The electrochemistry of the control and pretreated (except for hexafluorozirconate pretreatment) AA2024-T3 and Al 2 Cu were found to be similar. The cathodic electron transfer reaction rate for oxygen reduction was found to be enhanced by the presence of copper in the systems. The enhancement was proportional to the copper content. The cathodic reaction on all systems was inhibited by chromate. Aluminum was observed to undergo cathodic corrosion, leading to an enrichment of the surface copper content and subsequent enhancement of the cathodic electron transfer reaction.

The influence of acetone degreasing on the corrosion behavior of AA2024-T3

We have previously shown with X-ray photoelectron spectroscopy that acetone remaining on the surface of a copper strip after degreasing, can react slowly with water vapor under ambient light to form acetic acid and cause severe corrosion. The reaction was completely inhibited in darkness. This suggested that copper reacted photochemically with acetone and water. This paper shows that acetone degreasing of the AA2024-T3, as per ASTM E1078-97 cleaning protocol, has a similar effect on constituent copper-rich intermetallic particles. When an acetone-degreased alloy was exposed to a mist of sodium chloride solution under ambient light, the formation of acetic acid together with a layer of chloride solution resulted in severe pitting. Pitting was inhibited under conditions of darkness. The slow reaction of surface-absorbed acetone with water leading to the formation of acetic acid appeared to be prevented. There was also evidence for the redeposition of dissolved copper onto the alloy matrix under conditions that induced pitting.

A Duplex Mechanism-Based Model for the Interaction Between Chromate Ions and the Hydrated Oxide Film on Aluminum Alloys

In order to understand more clearly the nature of the anodic film formed on aluminum and its alloys, and the mechanism of inhibition of corrosion by chromates, a novel investigation has been made on the anodic film formed on AA1060 using X-ray photoelectron spectroscopy. The goal has been to elucidate whether the interaction of CrO 2- 4 on Al alloy surfaces is in certain aspects analogous to that of MoO 2- 4 on stainless steel surfaces and investigate whether a bipolar mechanism is applicable to aluminum surfaces. The aluminum hydroxide film was freshly formed on the metal surface and exposed to chromate and chloride solutions. Both chromate and chloride ions were observed to lead to a structural change in the hydrated aluminum anodic film consistent with deprotonation. Although both lead to deprotonation, exposure to chromate resulted in a structural transformation to an oxide layer associated with higher corrosion resistance and exposure to chloride lead to the formation of an oxyhydroxide film that is associated with a lower corrosion resistance. When the alloy was exposed to chromate prior to chloride exposure, the adsorbed chromate acted as a fixed negative charge substantially inhibiting chloride ingress. A reaction scheme, involving the formation of aluminum hydroxychlorides, has been proposed for deprotonation due to chloride. Deprotonation by chromate is explained for the first time in terms of bipolarity. Evidence suggests that two models may be operative on the surface of roughened AA106.

Photochemical breakdown of acetone on copper

X-ray photoelectron spectroscopy studies have shown that acetone, when used as a cleaning agent for copper in the presence of ambient light and water vapor, slowly reacts to form acetic acid. This reaction does not occur in the absence of light. This suggests that copper acts as a photocatalyst for the reaction between acetone and water vapor. The use of acetone for degreasing copper and copper containing alloys as recommended by ASTM E1078-97 is questioned in this paper.

Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings II. Composition and Electrochemical Behavior of the Chromate Conversion Coating

Rockwell Scientific Company, Surface and Electrochemical Processes Department, Thousand Oaks,California 91360, USAAluminum alloy AA2024-T3 samples, subjected to various pretreatments, were conversion coated using Alodine 1200S. The ratioof hexavalent chromium to total chromium in the chromate conversion coating ~CCC! was determined by X-ray absorptionnear-edge spectroscopy. The hexavalent chromium content of the CCC formed on AA2024-T3 varied with the method of pre-treatment. The coated surfaces were analyzed by X-ray photoelectron spectroscopy to determine the surface composition of thecoatings. Cr~VI! forms the major constituent ~46-74% of total chromium! on the surface. Protective passive films of aluminumoxide and aluminum phosphate inhibit the formation of CCCs. Corrosion behavior of the CCCs were studied using open-circuitpotential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy ~EIS!. Conversion coatedalloys exhibited up to 40-fold higher corrosion resistance when compared to bare pretreated alloys. EIS behavior was modeledusing a seven-element equivalent circuit. EIS results indicated industrially used bromate pretreatment to be best suited for CCCformation. Coupling the AA2024-T3 alloy with platinum during bromate pretreatment resulted in lowering the amount of copperintermetallics on the surface. This led to an increase in the corrosion resistance of the subsequently formed CCC by over an orderof magnitude as compared to CCCs formed following alternative pretreatments. Therefore, this simple modification of couplingthis alloy with platinum is recommended.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1806393# All rights reserved.Manuscript submitted December 1, 2003; revised manuscript received May 11, 2004. Available electronically October 27, 2004.

Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings I. Composition and Electrochemical Behavior of the Oxide Film

Aluminum alloy AA2024-T3 has been subjected to various pretreatments including those routinely carried out in the industry prior to conversion coating of the alloy. The surface chemistry of the samples after pretreatment was analyzed using variable angle X-ray photoelectron spectroscopy (VAXPS). Electrochemical techniques were used to evaluate the corrosion behavior of the samples in quiescent 0.05 M NaCI solution. The results were compared with those obtained from an untreated sample used in as-received condition that acted as a control. The surface of the alloy was observed to be enriched in copper and magnesium after certain pretreatments. Composition of the surface oxide film as well as the electrochemical behavior was found to vary significantly depending on the pretreatment. In all cases, the overlayer formed on the alloy was less than the sampling depth of the XPS (∼90 A). Pretreatment of the alloy using Alconox® was found to result in the formation of a highly protective aluminum oxide (Al 2 O 3 ) surface film. The corrosion current of the samples was observed to be directly proportional to the aluminum oxyhydroxide content of the film, while the polarization resistance of the sample was inversely proportional to the surface copper content.

Spectroscopic Elucidation of the Repassivation of Active Sites on Aluminum by Chromate Conversion Coating

It has long been hypothesized and recently shown that hexavalent chromium ions in the chromate conversion coating (CCC) migrate to active sites. This repassivation behavior of chromate conversion coatings leads to protection from corrosion resulting from mechanical damage. The nature of this repassivation has been explored here using Raman spectroscopy and synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. Raman spectroscopic analysis of a mechanically damaged area on the CCC supports the theory of chromium ion migration and formation of an Al(III)-Cr(VI) complex at the active sites. Results of an investigation performed using XANES corroborate the formation of the complex at active sites such as pits/valleys/scratches even under other chromating conditions. This study therefore provides unambiguous evidence for the theory of migration of chromates.

Synchrotron Radiation Based Grazing Angle Infrared Spectroscopy of Chromate Conversion Coatings Formed on Aluminum Alloys

For the first time, synchrotron radiation based Fourier transform infrared spectroscopy using a grazing angle objective has been performed on a chromate conversion coating (CCC) formed on an aluminum alloy. Synchrotron radiation (SR) based grazing angle infrared spectroscopy (GAIRS) has been compared with the most commonly performed near-normal reflection-absorption infrared spectroscopy (RAIRS). Our results indicate that the use of grazing angle incidence increases the sensitivity to the surface species. The SR-GAIRS technique was observed to have nearly an order of magnitude higher sensitivity to chromate compared to the SR-RAIRS technique. Significant amounts of the Cr-O bonds associated with the hexavalent chromium seem to be oriented in a direction normal to the surface. Thickness of the CCC was calculated using theoretical expressions. Calculated thickness agrees with reported values, thereby validating the use of SR-GAIRS as a tool for rapid, nondestructive thickness measurements. Hence, synchrotron radiation based grazing angle infrared spectroscopy can prove to be a highly effective tool to study the local chemistry on surfaces under ambient conditions.