Nickel and Chromium Plating

J. K. Dennis, T. E. Such

• Publisher: Woodhead Publishing
• Year: 1993
• Print ISBN: 9781855730816
• eBook ISBN: 9781845698638
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

During the last decade the engineering applications for nickel and chromium coatings have gained in importance. In this third edition the chapter dealing with engineering applications has been updated and expanded to include more information on electroforming and composite coatings, and engineering applications have been emphasised in the additions to the chapter on autocatalytic deposition of nickel. Additions have been made to the sections on pulse plating and use or rotating cathodes, and the section on trivalent chromium has been extended.

Chapter 1

Introduction and historical review

Electrodeposited metals are often an ideal means of providing a thin surface coating which has some property (or properties) superior to that of the substrate. It may, for example, be possible to employ a cheaper or stronger substrate than could otherwise be used and yet achieve good corrosion resistance by applying a suitable electrodeposited coating.

Electrodeposited nickel is typical of metals which can be included in the above category. It is often applied for decorative and protective purposes to cheap mild steel pressings and to die-cast zinc or aluminium alloy components. Die-casting is an economical means of mass-producing exact dimensional replicates of the original, but the alloys used are not suitable for service in a corrosive atmosphere without some form of protective coating. About 90% of the estimated 75 000t of nickel consumed during 1990 in the electroplating industries of non-communist countries was used in the form of thin, corrosion resistant and often decorative coatings on cheaply produced or strong substrates. This quantity excludes that consumed for plating in those countries which were part of the former USSR and also in China, as statistics are almost impossible to obtain. Most nickel coatings of this type are subsequently chromium plated to form the familiar composite nickel plus chromium system.

While nickel coatings may be applied solely for corrosion resistance where their inherent dullness is of no importance, the majority have to provide both decorative and protective functions. If a final bright appearance is required, dull deposits have to be polished to a high lustre before chromium plating and, since this polishing operation is very expensive, the major part of nickel plated for decorative applications has for many years been deposited in a fully-bright condition. Such coatings are obtained from solutions which contain organic chemicals in addition to the inorganic constituents. Not only can these additions modify the structure of the nickel deposit so that polishing is unnecessary, but many also have scratch-filling (the so-called ‘levelling’) properties which also eliminate or reduce the amount of polishing of the basis metal which is required. Variants of these solutions give bright, levelling deposits and form the vast majority of nickel baths now in industrial use.

However, it is not the authors’ intention to minimize the importance of the remaining uses of nickel electrodeposition. Indeed, these are of great and increasing economic importance. Often a comparatively thin nickel coating of which the weight and cost expressed as anode metal are quite small, may be used to repair a most expensive component which would otherwise have to be scrapped. Large engineering components which have involved much costly machining and heat-treatment, and have been damaged, worn or over-machined, perhaps only on small portions of their surface, can be salvaged by building up these specific portions with nickel to restore their original dimensions. If a large nickel thickness is necessary, this reclamation involves the deposition of more nickel than required, followed by machining to size. Obviously, the economics of these operations must be compared with those of fabricating an entirely new component.

The benefits conferred on the surface by thick or ‘heavy’ nickel plate are not only better corrosion protection, but also the greater abrasion resistance obtainable from certain types of nickel coatings. These advantages are now utilized on many new as well as reclaimed parts; this is discussed in Chapter 4. The superior wear-resistant properties of these nickel electrodeposits are often further enhanced by the deposition of fairly thick chromium coatings. This is the ‘hard’ chromium plate of the engineer.

Electroforming is the fabrication of articles entirely by electrodeposition. Nickel is a popular metal for this purpose since it can be plated in a ductile and low stressed form which has moderate hardness. Electroforms which have higher hardness and strength, both at ambient and elevated temperatures, can be produced from special solutions from which are deposited nickels whose structure is modified either by inorganic or organic chemicals present in the electroplating bath or by containing other metals as alloying ingredients. The working surface of the electroform may be chromium plated after removal from the mandrel. Sometimes the thick skin of the electroform is backed by even thicker copper plate, which although softer than nickel, can be deposited at a faster rate than can most types of nickel. Alternative and even faster means of applying a backing material to the electroformed nickel shell are by casting a low melting point alloy into it or by flame-spraying of a suitable alloy having a higher melting-point. Large electroformed ‘skins’ can be supported by filling with concrete. These are frequently used as tools for moulding large vehicle panels from plastics materials.

The functional uses of nickel whether as electroplated coatings or electroformed artefacts are growing steadily, as the advantages of this metal in its naturally electrodeposited form or as modified by various techniques become evident to design and production engineers. As described in Chapter 4, this versatility enables electroformed dies to be used for casting metals and glass, in addition to moulding plastics. The use of nickel for electroforming mesh is also a growth industry, for this mesh according to its hole dimensions and shapes can be used for a wide variety of applications.

The world market for electroformed nickel has increased substantially in the last two decades and appears likely to continue to grow. This increase has partly compensated for the decreased use of nickel electroplate in its traditional form as a bright coating. This decrease is partly due to the decline in world industrial output only too evident at the time of writing, but was occurring before the current trade recession commenced. The number and size of bright finished components on individual cars and domestic durables had been deliberately reduced as a policy. This policy was partly influenced by a wish to change to black or coloured finishes but also as a result of attempts to lower the costs of finishing. The combination of these factors has produced a serious decrease in the use of nickel electroplate for decorative coatings in Europe and North America. However, the total world usage of nickel for electroplating has increased during the last decade because of greater consumption in the developing countries of Asia.

It will be noted that for most decorative/corrosion protective purposes, many engineering uses and sometimes when electroformed, electrodeposited nickel is given a top coat of chromium. For this reason, it is impossible to dissociate chromium electrodeposition from that of nickel. Therefore, although the main purpose of this text is to discuss the deposition of nickel, the technology of chromium plating is discussed wherever it is relevant.

The chief emphasis is on electrodeposition of nickel, but Chapter 11 describes the autocatalytic plating of chemically reduced nickel (electroless nickel), as this is a valuable and increasingly used technique for applying uniform coatings to articles of complex shape, where the inherent limitations of the electrolytic process form an impossible barrier to obtaining a uniform coating of nickel over the whole surface. The properties of the nickel alloys deposited can also be advantageous. Accordingly, the industrial use of electroless nickel, especially the nickel/phosphorus alloy, has grown steadily during the last decade because of its unique properties.

The science of electroplating is about 180 years old. Nickel was first deposited 150 years ago and chromium some 20 years later, but chromium has only been used as an overlay in decorative nickel plus chromium coatings since the late 1920s. However, the great advances in nickel technology have been achieved only in the last 70 years, and these and the early pioneering work form a fascinating historical introduction to the present processes used in industry.

History of nickel plating

The electrodeposition of nickel was first described in 1837. G. Bird¹ electrolysed solutions of nickel chloride or sulphate for some hours and so obtained a crust of metallic nickel on a platinum electrode. In 1840, the first patent for commercial nickel plating was granted to J. Shore² of England who specified a solution of nickel nitrate. Soon afterwards a number of investigators published the results of their experiments. A. Smee of England (1841), Ruolz of France (1843) and Bottger of Germany (1843) were the first of these. Ruolz used nickel chloride or nitrate but Bottger’s was the first publication to mention an electrolyte solution based on ‘acid ammonium sulphate’; this bath, with variations, was the one mainly used in commerce for the next 70 years. However, G. Gore (1855) seems to have been the first to publish³ details of the neutral nickel ammonium sulphate bath (nickel ammonium sulphate is colloquially known as double nickel salts). Becquerel also published the process in 1862, using a concentration of 70–80g/l, although it is possible he used an excess of ammonia. Therefore, undue credit has perhaps been given to Dr I. Adams of the USA for developing this process, although he did apparently use it in the laboratory at Harvard University in 1858–1860⁴. However, he certainly appears to have been the first to commercialize it by plating gas burner tips in 1866⁵.

In 1868, W.H. Remington of Boston also commenced the deposition of nickel on a commercial scale using a nickel ammonium chloride solution but ran into difficulties, probably due to the use of an excess of ammonia in the bath. He was the first to des...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Preface to the first edition
• Preface to the second edition
• Preface to the third edition
• Chapter 1: Introduction and historical review
• Chapter 2: Metallurgical aspects of electrodeposition
• Chapter 3: Electroplating baths and anodes used for industrial nickel deposition
• Chapter 4: Engineering applications
• Chapter 5: Bright nickel electroplating
• Chapter 6: Control and purification of nickel electroplating solutions
• Chapter 7: Physical and mechanical properties of electrodeposits and methods of determination
• Chapter 8: Chromium plating
• Chapter 9: Thickness and corrosion testing of nickel plus chromium coatings
• Chapter 10: Decorative nickel plus chromium coating combinations
• Chapter 11: Autocatalytic deposition of nickel
• Chapter 12: Electroplating onto plastics
• Chapter 13: Deposition of nickel alloys
• Chapter 14: Plating on difficult-to-plate metals
• Chapter 15: High-speed plating
• Index