Factors influencing the deposition of Ce-based conversion coatings, Part II: The role of localised reactions

In Part I it was demonstrated that the deposition of Ce-based conversion coatings onto the matrix of AA2024-T3 was well modeled by titrations of the Ce-coating solution with added Al³⁺ ions. In Part II the coating composition over the surface was imaged with Raman spectroscopy and X-ray elemental mapping. Multivariate analysis was used to interpret variance in the X-ray elemental maps. The coating composition was found to be determined by the underlying microstructure of the alloy. The microstructure is dominated by the distribution of Cu resulting from enrichment as a consequence of anodic etching of the matrix and dealloying of intermetallic particles, principally Al₂CuMg during coating. A model is presented that proposes two mechanisms for the deposition of the coating. For the matrix, coating deposition can be explained on the basis of a pH rise in the adjacent solution analogous to precipitation from solution during titration as described in Part I. For the rest of the coating, the mechanism of deposition is dictated by the heterogeneous surface of AA2024-T3.     

Cerium-based conversion coatings on aluminium alloys: a process review

Abstract

Rare earths are among the most promising options for replacing chromate conversion coatings on aluminium. In nearly three decades of research, several hundred papers have been published in the area, the bulk of which has never been reviewed. This paper reviews the literature on rare earth coating processes, with particular emphasis on those based on cerium. It is concluded that several process areas are poorly understood, some of which are critical to further progress in the field. These include the development of industrially suitable pretreatments, technologies for coating non-aerospace alloys and seals to enhance corrosion performance and paint adhesion.     

Ageing effects in the growth of chromate conversion coatings on aluminium

Conversion coatings have been formed in two stages on sputtering-deposited aluminium using a chromate/fluoride bath. The first stage, common to all specimens, was conversion treatment for 1 min to produce a coating of thickness of about 70 nm, with associated thinning of the aluminium substrate. Further treatment was then carried out for 13 min, either immediately or with intervening ageing in humid air, water or laboratory air. Notably, ageing in laboratory air for 1 h was sufficient to prevent significant growth of new coating material upon re-immersion in the coating bath. In contrast, ageing in humid air or water allowed additional thickening of the coating, although with a reduced growth rate. The behaviour appears to be related to loss of free or weakly bound water from the coatings in laboratory air, with the composition and structure of the coating being modified such that transport of reactant and product species of the coating process is impeded.     

Chromate conversion coatings on aluminium: influences of alloying

The growth kinetics of chromate/fluoride conversion coatings are examined for 99.99% aluminium, and Al-2.3at.%Cu, Al-1.9at.%Au and Al-20at.%Au alloys. The thickening of coatings and thinning of substrates, the latter deposited by magnetron sputtering, are determined by transmission electron microscopy. The results reveal consumption of substrates during the coating process, to the maximum immersion time of 24 min. Initial relatively rapid thinning in the period to 6 min is followed by slower, approximately constant thinning at about 9.3, 6.2, 6.0 and 5.4 nm min⁻¹ for the 99.99%Al, Al-2.3at.%Cu, Al-1.9at.%Au and Al-20at.%Au alloys respectively. The ratio of the number of chromium atoms in the conversion coating, determined by Rutherford backscattering spectroscopy, to the number of oxidized substrate atoms is about 3, indicating a low efficiency of coating growth. Alloying decreases the coating thickness, as well as metal consumption. Alloying elements initially enrich beneath the coating, in alloy layers of a few nanometres thickness. Later, copper may be oxidized, and then enter the coating, whereas gold is occluded as nanoparticles. The gold nanoparticles act as markers to indicate formation of coating material at the metal/coating interface. Sudden loss of coating on the alloys, after about 6-9 min, occurs soon after entry of alloying element species into the coating. The loss may be related to the presence of nanoparticles of gold and copper near the alloy/coating interface, the latter possibly forming by reduction of copper species, that disrupt the bonding of the coating either directly though their presence or through their action as local cathodes.     

The characterization and corrosion resistance of cerium chemical conversion coatings for 304 stainless steel

A golden yellow-colored cerium conversion coating was obtained on 304 stainless steel surface by immersing the steel into a solution containing cerium (III), KMnO₄ and sulfuric acid. The corrosion resistance of the coatings was evaluated by electrochemical methods, potentiodynamic polarization experiments and electrochemical impedance spectrum. The experimental results indicated that the corrosion resistance for the conversion coated 304SS in 3.5% NaCl solution increased markedly. The corrosion potential of the treated steel increased to a more noble level, the pitting corrosion potential increased also, the passive potential range was enlarged markedly and the passive current density decreased about one order compared to that of the untreated steel. The cathodic and anodic reaction were both inhibited to some extent. The chemical state of the elements in the coatings was investigated by XPS. The cerium element was in the form of tetravalent state. And AES depth profile analysis suggested that the thickness of the conversion coatings was less than 66 nm. The mechanisms of coatings formation and corrosion resistance are discussed.     

Phosphate-permanganate conversion coatings on the AZ81 magnesium alloy: SEM, EIS and XPS studies

Conversion coatings on the magnesium alloy AZ81 were prepared using the phosphate-permanganate baths differing in composition. The corrosion behavior of the coated and uncoated alloys has been investigated by electrochemical impedance spectroscopy (EIS) and linear polarization methods. The choice of proper electric equivalent circuit (EEC) is discussed. The effect of temperature, bath composition and time of conversion as well as etching in acids before application on the corrosion resistance of the coated alloy has been evaluated. The best corrosion resistance was obtained for the samples coated in the bath containing 25 g KMnO₄, 150 g Na₂HPO₄ and 50 ml H₃PO₄ in 1 dcm³, applied at 80 °C. Differences in the morphology and composition of coated surfaces were investigated by the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques and correlated with the corrosion resistance of the samples.     

An evaluation of the corrosion resistance and adhesion properties of an epoxy-nanocomposite on a hot-dip galvanized steel (HDG) treated by different kinds of conversion coatings

Hot-dip galvanized steel (HDG) samples were chemically treated by Cr(III), Cr(VI), Cr(III)-Co(II) and Cr(III)-Ni(II) conversion coatings. Epoxy nanocomposites were prepared using 2 wt.%, 3.5 wt.%, 5 wt.% and 6.5 wt.% nano-ZnO. Electrochemical impedance spectroscopy (EIS), pull-off adhesion tester and scanning electron microscope (SEM) were utilized to evaluate epoxy coatings properties on the surface of pre-treated HDG samples. Results showed that addition of nano-ZnO particles (specially 3.5 wt.%) can significantly improve the corrosion resistance of the epoxy coating on HDG. Decrease of contact angle (φ) and increase of surface roughness (Ra) of the pre-treated HDG samples were obtained. Decrease of the value of φ was more pronounced when the Cr(III) pre-treated samples were post-treated by Cr(III)-Co(II) and Cr(III)-Ni(II) conversion coatings (CCs). The dry and wet adhesion strengths of the epoxy coating to HDG were significantly increased after the surface treatment of the samples. Increase of the adhesion strength and decrease of the adhesion loss were more pronounced on Cr(III)-Co(II) and Cr(III)-Ni(II) post-treated samples. The corrosion resistance of epoxy coating was also increased on the surface of pre-treated HDG samples. Increase of the corrosion resistance of the Cr(III) pre-treated HDG samples was more pronounced on the samples which were post-treated by Co(II) and Ni(II).     

Corrosion in artificial defects. I: Development of corrosion

Artificial defects (slots) were milled through a polyurethane topcoat and chromate-inhibited epoxy polyamide primer to the underlying aluminium alloy 2024-T3. The slots were then exposed to neutral salt spray (NSS) for up to 16 days. Prior to and after exposure, the slots were examined using scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDS) and Raman spectroscopy. The milling process generated features on the surface not seen on polished surfaces, including smearing of the matrix alloy and fragmentation of Cu-Fe-Mn-Al intermetallic particles. It was found that the smears and S-phase particles acted as sites for the initiation of corrosion, which eventually developed more generally across the surface.     

The nanostructure, wear and corrosion performance of arc-evaporated CrBxNy nanocomposite coatings

The composition, nanostructure, tribological and corrosion behaviour of reactive arc evaporated CrB_xN_y coatings have been studied and compared to CrN. The CrB_xN_y coatings were deposited on a commercial Oerlikon Balzers RCS coating system employing 80:20 Cr:B targets. To vary the composition, the nitrogen fraction was adjusted (N₂ fraction = N₂/(Ar + N₂)) and a moderate bias voltage of − 20 V was applied during coating growth. The coating composition and nanostructure was determined using time-of-flight elastic recoil detection analysis (TOF-ERDA), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Ball-on-disc dry sliding wear tests were conducted using an alumina ball counterface both at room temperature and at 500 °C with the relative humidity controlled at 20%. Potentiodynamic corrosion tests were undertaken in 3.5% NaCl aqueous solution. The wear tracks were examined using optical profilometry and scanning electron microscopy (SEM); the surface composition inside and outside of the wear tracks were investigated using Raman spectroscopy and XPS. All coatings exhibit nanocomposite structures and phase compositions which are in fair agreement with those expected from the equilibrium phase diagram. The lowest wear rate at room temperature and 500 °C was found for CrB_0.14N_1.14, which was shown to exhibit the highest hardness and possesses a nanocomposite nc-CrN/a-BN structure. CrB_0.12N_0.84 coatings showed the lowest passive current density in potentiodynamic corrosion tests.     

The effect of post-treatment time and temperature on cerium-based conversion coatings on Al 2024-T3

Corrosion performance, morphology, and electrochemical characteristics of cerium-based conversion coatings on Al 2024-T3 were examined as a function of phosphate post-treatment time and temperature. Corrosion resistance improved after post-treatment in 2.5 wt.% NH₄H₂PO₄ for times up to 10 min or temperatures up to 85 °C. Electrochemical impedance spectroscopy and polarization testing correlated to neutral salt spray corrosion performance. Hydrated cerium oxide and peroxide species present in the as-deposited coatings were transformed to CePO₄·H₂O for post-treatments at longer times and/or higher temperatures. Based on these results, processes active during post-treatment are kinetically dependent and strongly influenced by the post-treatment time and temperature.     

Influence of metallurgical states on the corrosion behaviour of Al–Zn PVD coatings in saline solution

The objective of this study is to define the corrosion behaviour of different Al–Zn coatings, deposited by magnetron sputtering. The coatings exhibiting the best corrosion resistance are then characterised during long immersion tests in neutral 5 wt.% NaCl solution.The results show that the corrosion behaviour is strongly dependent on the zinc content. The evolution of the degradation mechanism is also related to the microstructure of the alloys. These alloys present very interesting properties for steel protection. Nevertheless, the zinc content has to be well defined in order to avoid a high dissolution of the coating.     

The effect of Al content on the galvanic corrosion behaviour of coupled Ni/graphite and Ni–Al coatings

In this study, the corrosion behaviour of 75Ni/25graphite abradable coating and Ni–Al bonding coatings with different Al content were investigated with open circuit potential and polarization tests. The galvanic corrosion of the coupled Ni/graphite and Ni–Al coatings was studied by using a zero-resistance ammeter in 5 wt% NaCl solution. The experimental results showed that the corrosion resistance of the Ni–Al coatings decreased with increasing Al contents. In the galvanic corrosion test, the Ni–Al coatings with high Al contents (80Ni–20Al, 85Ni–15Al, and 90Ni–10Al) were the anodic element of the couples, while, the 95Ni–5Al acted as the cathode element in the couple.     

The performance of hydrogenated and non-hydrogenated diamond-like carbon tool coatings during the dry drilling of 319 Al

Aluminium alloys, though widely used in the automotive industry, are difficult to machine, particularly by drilling and tapping without the use of metal removal fluids, because of aluminium's strong tendency to adhere to the cutting tool. Tribological tests have revealed that carbon-based tool coatings, such as diamond-like carbon (DLC), promise an improved performance due to their low friction and adhesion. However, the tribological performance of DLC coatings depends on both their hydrogen content and the testing environments. Hence the experimental approach taken in this study was designed to understand the cutting performance of hydrogenated DLC (H-DLC) and non-hydrogenated DLC (NH-DLC) tool coatings during the dry drilling of a 319 Al (Al–6%Si) alloy. An experimental drilling station was built to measure torque and thrust force changes using a cutting speed of 2500 rpm and a feed rate of 0.25 mm/rev. The cutting performance was assessed by measuring the torques and thrust forces generated during the drilling of the first 150 holes or by drill failure—depending on which occurred first. The results indicated that superior cutting performance was achieved, in both torque and thrust force responses, using DLC-coated drills rather than uncoated high-speed steel (HSS) drills. The uncoated HSS drills failed after drilling only 49 holes as a result of excessive aluminium adhesion. At least 150 holes could be drilled using the DLC-coated drills, and both the torque and thrust forces generated during drilling were lower than those with uncoated HSS drills. In addition, a smaller proportion of holes exhibited abrupt increases in torque (at the end of the drilling cycle) during drilling with the DLC-coated drills. Scanning electron microscopy (SEM) investigations showed that the H-DLC drill flutes displayed minimal aluminium clogging—resulting in lower torque. H-DLC coating also diminished metal transfer and buildup edge formation on the drill's flank face and cutting edge. Thus, torque and thrust force measurements, supported by metallographic data, indicated that H-DLC-coated drills provided better dry drilling performance than NH-DLC.     

The use of macroscopic modelling of intermetallic phases in aluminium alloys in the study of ferricyanide accelerated chromate conversion coatings

Intermetallic (IM) second phases of FeAl₃, Cu₂FeAl₇, and CuAl₂, were coupled to aluminium for the macroscopic study of the deposition of chromate conversion coatings. Characterisation of the coating deposition was done using X-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, Auger electron spectroscopy, scanning electron microscopy with X-ray analysis, and Fourier transform infrared spectroscopy. The coatings covering the IM phases were only one-tenth the thickness of the matrix, and contained higher levels of F, Al, and O. Cr, O, Fe, and N, indicative of a chromate conversion coating, were detected over the matrix. Over IM phases, decomposition of [Fe(CN)₆]3−/4−, and fluoride ion attack were found to be responsible for reduced rates of deposition.     

The effects of immersion time on morphology and electrochemical properties of the Cr(III)-based conversion coatings on zinc coated steel surface

The main purpose of this paper is to develop a dynamic and non-destructive method to quantify and correlate the microstructure changes of the Cr(III) layer by electrochemical techniques. The open circuit potential (OCP) analysis reveals the nucleation growth mechanisms of the Cr(III) layer and the dissolution phenomena of Zn. In addition, the effects of immersion time to the corrosion behavior of Cr(III)-based conversion coatings (TCCCs) on electrogalvanized steel were studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution. Furthermore, surface morphology of the Cr(III) coatings under different immersion times was examined using both a scanning electron microscope and an atomic force microscope.From the potentiodynamic polarization experiment, the corrosion current density (I_corr) of the specimen with immersion time of 60 s was found appreciably small, representing the inheritance of the best anticorrosion performance. Additionally, the corrosion resistance of the Cr(III)-coating for the specimens obtained between 30 s and 60 s is two order higher than those of the untreated specimen from the EIS experiments. Results show that the quality of Cr(III)-based conversion coatings was strongly influenced by the immersion time of Cr(III) solution. And the optimal immersion time is recommended in the range of 30–60 s.     

The role of microstructure in the high temperature oxidation mechanism of Cr3C2-NiCr composite coatings

Composites of Cr₃C₂-NiCr provide superior oxidation resistance to WC-Co composites, which has seen them applied extensively to components subjected to combined high temperature erosion and oxidation. This work characterises the variation in oxidation mechanism of thermally sprayed Cr₃C₂-NiCr composites at 700 °C and 850 °C as a function of heat treatment. Carbide dissolution during spraying increased the Ni alloy Cr concentration, minimising the formation of Ni oxides during oxidation. Compressive growth stresses resulted in ballooning of the oxide over the carbide grains. Carbide nucleation with heat treatment reduced the Ni alloy Cr concentration. The oxidation mechanism of the composite coating changed from being Cr based to that observed for NiCr alloys.     

Electrochemical behaviour of cermet coatings with a bond coat on Al7075: Pseudopassivity, localized corrosion and galvanic effect considerations in a saline environment

The behaviour of an HVOF WC-17Co/Ni-5Al coating on Al7075 in aqueous NaCl is investigated. The coating susceptibility to localized corrosion depended on the potential of polarization reversal. A two-stage pseudopassivity was observed for WC-17Co: At low overpotentials, oxidation occurred in the binder leading to surface films composed of anhydrous Co- and W-oxides. At high overpotentials, oxidation extended to the carbide phase leading to the formation of hydrated WO₃ films. Ni-5Al notably reduced the galvanic effect between WC-17Co and Al7075, whereas it hindered corrosion propagation into the substrate. The coating showed a high corrosion resistance during salt spraying for 49 days.     

Electrodeposition of nanocrystalline Co-W coatings from citrate electrolytes under controlled hydrodynamic conditions part 3: The micro- and macrodistribution of the deposition rates,the structure,and the mechanical properties

Using a Hull cell with a rotating cylindrical electrode (RCE), the effect of the hydrodynamic conditions on the deposition rate and its micro- and macrodistribution and the composition and microhardness of the cobalt-tungsten coatings deposited from a citrate electrolyte containing cobalt sulphate (0.2 mol/l) and sodium tungstate (0.2 mol/l) at pH = 6.7 and the solution temperature of 60°C in the Re numbers range of 0–3000 is examined. It is shown that the hydrodynamic deposition conditions have no effect whatsoever on the surface roughness, but they slightly influence the electrolyte’s throwing power. The correlation between the intensity of the hydrodynamic flows, the deposition rate (due to the current efficiency’s increase), and the composition of the coatings (along with their distribution over the surface) is established. It is shown that certain modes of the process at a high deposition rate make it possible to reach a high uniformity of the coating’s composition with a high tungsten content, which results in obtaining the maximum surface microhardness.