D. H. Ross

Surface pretreatments of aluminium for electroplating

Abstract The effects of concentration and temperature of sodium hydroxide and nitric acid + hydrofluoric acid pretreatment solutions on the morphology of aluminium surfaces have been studied. SEM/EDS observations indicated that NaOH solutions produced etch concavities on the surface, but failed to etch second-phase particles, namely FeAl3. The acid solutions, however, succeeded in preferentially removing these particles, while they did not change the surface roughness of the aluminium. In an attempt to improve the coverage rate of zinc alloy films, chemically deposited prior to nickel electroplating, a new pretreatment sequence has been designed. It consists of a double-step nitric + hydrofluoric acid solution immersion, with NaOH immersion treatment between the two acid immersion steps. The results obtained with this new sequence indicate that the first acid step removes most of the biggest FeAl3 particles, as well as the previous oxide layer, while the alkaline step generates a micro-roughness on the surface whereby a net of shallow cellular cavities, as in a honeycomb structure, forms. The last acid treatment preferentially etches the small FeAl3 particles and the most protuberant inhomogeneities of the surface. This new pretreatment sequence proved to be more effective than those previously reported in the literature, in terms of producing a good surface on which zinc alloy films could develop. The adhesion of nickel electrodeposits to such a surface reached values one order of magnitude higher than those for conventional two-step sequences. Interpretation of these results is based on the improved chemical homogeneity of the surface resulting from the removal of second-phase particles and on the small (ca. 1 μm in diameter) and uniform dimensions of the cells produced by the etch treatment. This effectively leads to a good surface coverage during the zincate treatment, as intially intended.

A model for the mechanism of nickel fluoride cold sealing of anodized aluminium

The sealing of porous anodic oxide films on aluminium by the use of nickel fluoride solutions at ambient temperatures has been proposed as an alternative to hydrothermal sealing methods, on energy grounds. The process of cold sealing shows some promise and a mechanism is proposed as to how it may operate. As fluoride ions enter the pores, a place exchange mechanism occurs causing a shift in the local pH which is sufficient to cause nickel ions to precipitate. The nickel hydroxide blocks the pore mouths, effectively sealing the film. A slow step then occurs whereby water from the atmosphere diffuses into the film, effectively hydrating the film and causing general pore blocking. This slow step explains the ageing effect seen with such films. Eventually a film is produced which is effectively sealed.

An EXAFS investigation of molybdate-based conversion coatings

The composition of molybdate-based conversion coatings on zinc–nickel alloy electrodeposits was examined by extended X-ray absorption fine structure (EXAFS). The absorption spectra from appropriate standards showed distinct differences between the Mo(IV) and Mo(VI) oxidation states. A pre-edge peak was apparent for the various molybdate species and molybdenum trioxide, with the energy and intensity dependent on the coordination of the molybdenum. A shift in the molybdenum K-edge spectra was also evident between Mo(IV) and Mo(VI) compounds. A qualitative analysis revealed that both Mo(IV) and Mo(VI) species were present within the molybdate-based conversion coating, with a significant prevalence of hexavalent molybdenum.

A study of modified alloy zincate pretreatments for plating on aluminium PART II: Elemental distribution in the film formed during immersion treatments

SummaryThe distribution of the various elements present in films obtained by a modified alloy zincate immersion treatment on aluminium was studied using electron probe microanalysis, energy dispersive analysis on a scanning electron microscope and Auger electron spectroscopy. As expected, zinc was the main component of the film; however appreciable concentrations of copper were found towards the film/substrate interface. Another unexpected result was the variation in spatial distribution of some elements.

A study of modified alloy zincate pretreatments for plating on aluminium Part I—Electron microscope observation of the immersion formed film on aluminium substrates

SummaryAluminium is a metal which is difficult to plate, due to the presence of aluminium oxide which prevents metal to metal bonding. A commercial alloy zincate treatment formed the basis of this work, the main objective of which was to study the morphology and development of the immersion formed film. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Quantitative image Analysis, were used to examine the process. It has been possible to follow the way in which the film nucleates and grows, to cover the substrate, and elucidate some reasons for the better performance generally claimed in the literature for the double immersion technique.

A contribution to the understanding of a.c. anodizing of aluminium

Commercial aluminium has been anodized in 10% v/v sulphuric acid using alternating current. At 20 V no significant differences in colour and thickness between specimens anodized at the neutral and live terminals were detected. At higher voltages the oxide on the neutral specimens was always more yellow and thinner than that on the live specimens. The colouration was attributed to the reduction of sulphate to sulphide during the cathodic half-cycle, no other sulphur-containing species being detected by XPS. EPMA has shown that the total sulphur distribution across the film was practically uniform and independent of the operating conditions, except in thin films, where it was greatest at the free surface. With additions of ferric sulphate to the solution the reduction to sulphide was inhibited and the differences between live and neutral specimens were avoided. The total amount of sulphur remained unchanged at approximately 5% weight and no iron was incorporated in the film.

Optimization of the nickel fluoride cold sealing process

SUMMARYThis paper investigates the effect of varying sealing parameters, in order to optimize the conditions used to achieve a good seal quality for nickel fluoride cold sealed anodized aluminium. The effect of time, temperature and solution pH on seal quality was studied by British/ISO standards tests such as phosphoric acid/chromic acid, dye absorption and admittance. The quantity of nickel absorbed in the pores of anodic films as a function of solution parameters was measured by Atomic Absorption Spectroscopy (AAS), in order to find out the relationship between seal quality tests and nickel absorption.Although all the sealing parameters were found to have an effect on sealing quality and quantity of nickel absorbed in the pores, the major effect resulted from temperature which was found to be critical. The best results were found to be at a temperature of ≤ 25°C for a 20μm film. The quantity of nickel absorbed in the pores as shown by AAS, tends to correlate with the results of the seal quality tests.T...

Effect of post treatment on nickel fluoride cold sealed finishes

SUMMARYThe performance of accelerated aged (post-treatment) nickel fluoride cold sealed finishes is compared with those of room temperature aged and conventional hydrothermal sealed finishes. It was found that accelerated ageing at temperatures of 60 and 80°C for 5–30 minutes in deionized water will improve the results of sealing quality tests of cold sealed anodized finishes, however it does not satisfy all the sealing quality tests applied to conventional hydrothermal sealed films. The results of acid dissolution and dye spot tests are acceptable for post-treated cold sealed films, and in general even better results than conventional hydrothermal sealing can be obtained with various metal salts used as post-treatment. Nevertheless, the admittance test was found to be difficult to satisfy and further ageing of 1–14 days was necessary to pass it. Such a problem was overcome by dual sealing, in which the cold sealed film was treated for a very short time (0.5min/μm) in boiling deionized water, resulting in...

The Effect of Molybdate Ions on A.C. Sulphuric Acid Anodized 6063 Aluminium Alloys

6063 aluminium alloys were AC anodized in molybdate containing and molybdate free sulphuric acid solutions. The total impedance of the oxides produced in molybdate containing solutions decreased faster in 0.01 N HCl and after 8 days, following the dissolution of oxide film in 0.01 N HCl, the pits produced on the aluminium substrate surfaces were shallower and larger.

Characteristics of thin anodic oxide films on aluminium

A key characteristic property of thin anodic oxide films is the apparent film density which shows significant variation from the conventionally quoted value for thick films. Knowledge of its value is important for the calibration of the films thickness and the process efficiency. Data from the literature has been critically evaluated and compared with new results for films <4 μm thick produced under high rate conditions, i.e., high current density and short times