Some Engineering Aspects of Electrodeposition - The 6th William Blum Lecture
Mr. R.A.F. Hammond
Recipient of the 1963 William Blum
AES Scientific Achievement Award
Originally published as
Annual Technical Proceedings of the American Electroplater’s Society, 51, 9-20 (1964).
Editor’s Note: This paper is a re-publication of the 6th William Blum Lecture, presented at the 51st AES Annual Convention in St. Louis, Missouri, on June 15, 1964. A printable PDF version is available by clicking HERE.
The successful use of electrodeposits in engineering applications, including corrosion protection, depends not only on the properties of the coatings themselves but upon their influence on the mechanical properties of the basis metal. In this connection, three possible effects must be considered: (1) on the static tensile properties, (2) on the fatigue strength and (3) on the sustained-loading properties (hydrogen embrittlement). Unless suitable precautions are taken serious loss of fatigue and sustained-loading properties may result, particularly of high-strength steels, and the increasing use of such steels in the aircraft industry has highlighted this problem. As a result, much research effort has been concentrated on these subjects in recent years and the lecture reviews some salient features of current knowledge in this field.
Although electrodeposition has been practiced industrially for over a century, it is only in the last 50 years or so that the subject has been studied scientifically. In this development Dr. William Blum, in whose honor this series of lectures was inaugurated, was an outstanding pioneer.
As so frequently happens, war provided an early stimulus for electrodeposition research and, on consulting the very long list of Dr. Blum's published work, I found that, although his first two electrodeposition papers were on its application to the peaceful art of printing, his third was entitled "Military Applications of Electroplating" (1918).
World War I also provided the stimulus for the formation of the Electrodeposition Section of the Research Department, Woolwich (now Royal Armament Research and Development Establishment, Ministry of Defence, Army Department) in which I have been privileged to serve for most of my career. This arose directly from a British Army development about the year 1916 in which a plating shop for the reclamation of gun parts and other Services components was set-up behind the lines at the Le Havre Base-Workshops of the Royal Ordnance Corps. A parallel war-time development was initiated by the Royal Flying Corps (predecessor of the Royal Air Force).
These activities were, I believe, the very first examples of the application of thick electrodeposits for engineering purposes - for the first time the engineer had acquired a "putting-on tool."
However, success in this new field depended upon securing very high adhesion, freedom from defects and controlled mechanical properties of thick deposits (then usually copper, iron or nickel) - matters which up to that time had not received very much attention. Early work at Woolwich was therefore concentrated on these aspects. Much early work on the structure and properties of electrodeposits was also done by Dr. Blum at the National Bureau of Standards, Washington, D.C. and, somewhat later, the classic researches of Dr. Abner Brenner and his colleagues on chromium and nickel appeared, sponsored I believe by your Society under its Research Scheme. This work is of special importance in relation to the subject of my lecture.
These and other investigations have provided a great body of information on the properties of the deposits themselves. However, it is only more recently that attention has been concentrated on the influence of the deposits on the mechanical properties of the substrate material and this has arisen largely owing to the increasing use of electrodeposition for salvaging, hard-surfacing or corrosion protection by the aircraft industry where any failure could have serious consequences. Such failures are all too possible unless finishing techniques are used with discrimination, based upon sound knowledge of the methods involved. In the large majority of such applications, steel is the substrate material and, as the potentially harmful effects of pickling and plating processes tend to increase with the tensile strength of the steel, the importance of the subject has become even greater in recent years with the increased utilization of high and ultrahigh strength steels. For this reason, during the last 15 years or so, there has been a great concentration of effort to determine the effect of plating processes on the mechanical properties of the substrate.
It has been my aim in preparing this lecture to provide, in a compact form, an account of the salient features of current knowledge in this field, particularly in regard to coatings of chromium and nickel which are the deposits normally used for engineering applications, and to cadmium now so extensively used for corrosion protection.
In considering the possible effects of such processes, three aspects must be taken into account: (1) the effect on the static tensile properties, (2) the effect on fatigue strength and (3) the effect on sustained-loading properties due to hydrogen embrittlement. These will be discussed in turn.
STATIC TENSILE PROPERTIES
Electrodeposited chromium is a hard, brittle material which normally contains internal cracks. It cannot therefore be relied on to contribute strength to the component proportionately to the steel which it replaces. The coatings used are normally quite thin, however, and the quota of strength required of the chromium is insignificant. Nevertheless the possibility must be considered that the presence of the chromium may adversely affect the properties of the steel itself. This possibility was investigated by Logan1 who determined the effect of chromium plating on the mechanical properties of SAE 4130 steel of 187,000 psi. His trials included tensile, tensile impact, bending and crushing tests and he found that, although the tensile and yield strengths of plated specimens decreased as the plate thickness was increased from 1 to 15 mil, these properties did not fall below 91 per cent of the values of the unplated steel. The plastic deformation before fracture in the tensile tests became less as the plate thickness increased but baking at 200 or 440°C (392 or 824°F) for various times restored the ductility to a value approaching that of the unplated steel and it seems likely that hydrogen embrittlement of the steel was the cause of the reduced ductility.
It seems reasonable to conclude, therefore, that the presence of a chromium coating per se will not impair the tensile properties of a steel under any normal conditions of service.
The electrodeposition of nickel for repair or salvaging purposes is used much more extensively in the U.K. than in the U.S.A. In contrast to hard chromium plating, the deposits are frequently of very substantial thickness and, in exceptional cases, the cross-sectional area of the nickel coating may represent a sensible proportion of the whole. The effect of the nickel on the overall strength of the component could therefore be important, particularly in components with a low strength factor. Unlike chromium which usually contains cracks, nickel is a relatively strong material with a high degree of ductility so that a properly applied coating contributes very materially to the overall strength.
Tests to determine this effect quantitatively were made by Hothersall2 who used standard tensile test-pieces of steels of two strengths (Hardness 183 and 286 DPH respectively). These were machined undersize, plated with a surplus of nickel and machined back to size in such a manner that 20 per cent of the finished cross-section consisted of nickel. Two types of nickel (hard and soft) were included in the tests. The specimens were then tensile tested to fracture and the results are given in Table 1.
No significant effect on the ultimate tensile strength or on the ductility of the composite resulted from nickel plating, with the exception that the elongation was somewhat reduced by the harder nickel on the stronger steel.
Table 1 - Effect, upon tensile properties of steel, of replacing 20 per cent of the cross-sectional area with electrodeposited nickel (average results).2