Research and Decorative Coatings - The 19th William Blum Lecture
Burton B. Knapp
Recipient of the 1977 William Blum AES Scientific Achievement Award
Editor's Note: Originally published as Plating & Surface Finishing, 65 (8), 24-30 (1978), this article is a re-publication of the 19th William Blum Lecture, presented at the 65th AES Annual Convention in Washington, DC, on June 26, 1978. A printable PDF version is available by clicking HERE.
In selecting a subject for this lecture, I found it was difficult to deal with any phase of electroplating without encountering some contribution that Dr. Blum had made. Having known Dr. Blum personally, I was impressed with one thing and that was his concern for the success of the American Electroplaters' Society. A part of this concern was for harmony between the practical plater and the research man. This is well expressed in the first William Blum Lecture in which he concluded that "education in its broadest sense and application is the most important problem that confronts the plating industry." With this thought in mind, I thought that the subject of decorative coatings discussed from the practical and theoretical aspects would be apropos for this lecture.
History shows that bright finishes appealed to man in very early time. The Egyptian tombs have revealed many articles that were decorative in some way, partly as a result of their being smooth, bright and reflective. Of course, many of these items were solid gold and because of the nobility of the metal, they retained their bright finish. Today, the use of a solid noble metal would be prohibitive both from the standpoint of cost as well as availability. The result therefore has been to resort to plated coatings to conserve metal and reduce costs. However, there is still the demand for the durable, bright, decorative finish that we have come to associate with quality.
Plated nickel coatings that were first used for their decorative appearance were dull as-plated and required buffing to bring up a lustrous finish. In the atmosphere, frequent cleaning and polishing were required to maintain the original appearance. The discovery that bright chromium could be plated over the buffed nickel provided a permanence to the finish. This combination of nickel and chromium performed well because the thick nickel layer provided good corrosion resistance and protection of the basis metal and the chromium coating provided a surface that was resistant to staining. Even though this was a big step forward at that time, the metal finishing industry was well aware that there was need for improvement. The objective of their research centered on a better performing and less expensive system.
The development of an improved system followed several lines of attack. Progress was made both from an empirical as well as a theoretical approach and in many instances the two were complimentary. At the outset, progress was hampered by the lack of a theory to account for a mechanism for the deterioration of a decorative nickel-chromium coating. It was also hampered by the lack of a reliable accelerated performance test to appraise and thus determine quickly the influence of a variable being studied. A third factor was a system for rating the performance of a plated coating so that objective comparisons could be made between two systems. All three of these problems have been dealt with, and the AES through its Research Program made a major contribution in the development of accelerated tests, the Corrodkote and CASS tests. The results of research on these problems have expedited the development decorative coatings to the point they are today.
At first, static exposure of plated panels met the need for information on composition, thickness and other properties of the coating. One of the first comprehensive programs of this type was carried out under the direction of Dr. Blum at the National Bureau of Standards in cooperation with AES and ASTM. In this program in 1932, SAE 1010 steel was used for the basis material and Watts nickel was buffed and chromium plated. The conclusions from this exposure program were follows:1
- Thickness of nickel was the most important factor.
- Copper under the nickel was detrimental only with thin nickel coatings.
- Chromium coating resisted tarnish but added little to protective value.
- Pretreatment or plating conditions had no marked effect.
In a later program2 in 1936, zinc-base die castings and brass were included with several types of steel. Buffed Watts nickel was again the principal coating but a few bright nickels available at that time were included. The conclusions arrived at from this work were as follows:
- Thickness of nickel was the most important factor on all three basis materials.
- Copper added little to protective value on steel or zinc.
- Type of nickel had no large effect but there was more variability with the bright nickels.
- Optimum thickness of chromium appeared to be in the range of 0.25 to 0.75 μm (10-30 microinches) above which cracking occurred.
The variables studied in these programs and the results obtained raised many questions to be answered. Among the major ones were the cause of pitting and corrosion, effect of basis metal, effect of type of nickel, effect of copper under nickel and the effect of the chromium layer.
Nature of corrosion
Corrosion of coatings manifested itself by severe pitting and general staining of the surface. In some cases the pitting was so severe that the surface took on the appearance of a "nutmeg grater" and has been classified as crater rusting in the new system of rating. At this stage of deterioration the surface could no longer be cleaned to restore the lustrous appearance. Examination of these pits in cross section, shown schematically in Fig. 1, established that corrosion had penetrated to the steel basis metal and that the expansive pressure of the corrosion products had raised the coating around the periphery of the pit. The exposure results indicated that this type of failure was more pronounced in salt-containing atmospheres and in the presence of a copper undercoat.