This article was published in the journal Transactions of the Institute of Metal Finishing [© Maney Publishing / Institute of Metal Finishing] and is available at: http://www.ingentaconnect.com/content/maney/imf

Aluminium oxide films may be electrolytically treated in order to modify or enhance their physical characteristics, with electrocolouring being the common application. This research has shown that the incorporation of a metallic deposit into the porous oxide film can also improve the tribological properties of the film using methodology similar to electrocolouring. Performance of this coating system has been evaluated for two common aluminium alloys using standard mechanical tests, LIMA surface analysis and electron microscopy. Such films treated with In, Sn or In-Sn have shown improved abrasion resistance and reduced coefficient of friction compared to a 'standard' film making them especially suited to applications requiring low friction properties and good sliding wear resistance.

SUMMARYThe properties of anodic films produced by anodising aluminium in aqueous solutions of organic acids at room temperature at commercially interesting current densities were investigated with particular emphasis on the wear index, micro hardness, surface roughness and pore structure. Voltage time curves obtained using 10% w./v. solutions of the pure acids showed a rapid rise in voltage at commercially realistic current densities of 1–2 A dm−2 but this problem could be overcome by the addition of 1–2% v./v. sulphuric acid. The films obtained under these conditions using sulphosuccinic acid (SSA), sulphosalicylic (SCA) or sulphophthalic acids (SPHA) had properties closely resembling those obtained under hard anodising conditions in 10% sulphuric acid at 0–4°C. Films were not formed using para-toluenesulphonic acid (PTSA) and the use of maleic acid (MA) resulted in the formation of substantial quantities of fumaric acid. The oxide films exhibited a structure made up of densely packed columns suffused by narrow pores. The films fall into two categories. On the one hand films produced using sulphur containing organic acids have very narrow pores of less than 10 nm diameter resembling those produced by the conventional low temperature sulphuric acid process, and on the other hand films produced using maleic or citric acids exhibit much larger pores with diameters of the order of 20 nm. Attempts to incorporate PTFE particles from aqeous dispersions of this polymer into the films were unsuccessful due to their large size (~200 nm). Duplex films with a highly porous outer layer (~100 nm diameter) on top of a dense hard layer were produced and evidence is presented which shows that some of the smaller PTFE particles can enter the mouths of these pores.

The meaning of the word 'enhancement' is considered and discussed in terms of process efficiency and rate. The use of airless agitation in particular is emphasised to minimize power inefficiency. The use of rotating electrodes is also discussed and their use for three applications described: agitated reactors, dynamic Hull Cell sand production of multilayered electrodeposits.

The concept of etching is described and the characteristics of chemical etching solutions reviewed. Some typical processes are discussed and the opportunity to accelerate and improve selectivity of etching br means of anodic currents emphasised.

The new century opens with a large stock of available coating process technologies, among which are old (electroplating) and new (physical vapour deposition, PVD) techniques. While electroplating may have some disadvantages, its proven technology is of primary importance, which is demonstrated by a number of large, continuing applications. By contrast, the cost and operating disadvantages of PVD will ensure that it maintains a small specialist niche in the coating technology spectrum.

The significance and importance of solution conductivity for electrodeposition solutions and their efficient use are discussed. The dearth of reliable data is revealed and a collation of available data is provided for the commoner metals and solutions.

The importance of solution conductivity as a parameter in electroplating solution design is outlined and the basis of optimization defined. New data are presented for two zinc alloy solutions (Zn-Fe and Zn-Mn) and the commercial significance discussed.

An investigation has been undertaken to attempt to electrodeposit layered coatings of zinc. nickel and zinc-nickel alloy. A dual electrolyte approach has been applied with sequential layers being electrodeposited from the two baths. Coated steel substrates have been sectioned and examined using scanning electron microscopy and the layered nature of the deposits confirmed. Electrochemical and salt fog corrosion tests have been undertaken to ascertain the corrosion protection capabilities of various layered coatings. Preliminary data. particularly from the salt fog trials. suggests that some of the layered structures have considerably better protective qualities than similar thickness coatings of zinc and zinc-nickel alloy.

A study has been made of the zinc-iron alloy electrodeposition system, using chloride solutions, to establish the composition dependence upon the deposition parameters of current density and agitation achieved by rotation of cylindrical cathodes, It is shown that a deposit composition of approximately 10 to 80 percent iron in zinc can be consistently obtained, thereby enabling two layer deposits to be obtained from a single solution by varying these two parameters, Additions of ammonium chloride act not only as conductivity salts, but as deposit grain refiners.

Hexavalent chromium bath replacement is a high priority pollution prevention issue. The aim of this study was to develop corrosion protective, wear resistant and environmentally safe coatings as an alternative to hard chromium electroplated from the Cr(VI) bath. Cr, Ni-P, Ni-Mo, Ni-W-P and Mn-Zn coatings were electrodeposited from a modified conventional or newly developed bath. Cr was electrodeposited from both Cr(III) and Cr(VI) baths, Ni-P(5-12%) alloy from an acetate bath, Ni-Mo from both ammonium citrate (with stabilizer) and methane-sulphonic baths and Ni-W-P from an ammonium citrate bath. The electroplating parameters, chemical composition, structure, hardness, wear resistance of the above-mentioned coatings are discussed in detail.

A number of cleanliness assessment techniques related to quality surface preparation prior to electroplating, are in use but in general are comparative or subjective and not very sensitive. Electrochemical techniques are especially applicable to wet surfaces at intermediate stages of processing. One such technique, based on both two and three electrode polarisation, has been evaluated for silver, gold, copper, copper alloys, aluminium and iron alloy substrates and found to be easy to use without specialist competence reliable enough to give reproducible results and at the same time rapid and cost effective. Results are presented justifying this claim and demonstrating how it may be used as a quality measuring instrument.