Some Practical and Theoretical Considerations of Corrosion and the Role of Chromium - The 12th William Blum Lecture
This paper is a re-publication of the 12th William Blum Lecture, presented at the 58th AES Annual Convention in Buffalo, New York, on June 14, 1971. The corrosion of plated coatings under varying environmental conditions, including the effect of polarity, porosity, combination of plate, as well as thickness and other factors on the durability of the plated product is covered.
Jesse E. Stareck
Recipient of the 1970 William Blum
AES Scientific Achievement Award
Editor’s Note: Originally published as Plating, 59 (4), 303-308 (1971), this paper is a re-publication of the 12th William Blum Lecture, presented at the 58th AES Annual Convention in Buffalo, New York, on June 14, 1971. A printable PDF version is available by clicking HERE.
The corrosion of plated coatings under varying environmental conditions is discussed. The effect of polarity, porosity, combination of plate, as well as thickness and other factors on the durability of the plated product is described and touched on more fundamentally. Less well understood is the undermining effect that often occurs during plating, particularly of such metals as magnesium, aluminum, zinc and even steel. Some theoretical aspects are developed and presented.
I wish to take this opportunity to thank the Society I for this signal honor. I know you had my research team in mind, and much credit must go to them and to my loyal friends and associates in the plating industry, who over the years have contributed so much that made this occasion possible.
Also, I think it fitting at this time to mention my first contact with Dr. Blum. It was back in the depression days of 1933, almost 40 years ago. I sought his advice in connection with color plating, which I came upon in the process of working out my doctoral thesis.
If I may digress a bit here, this is how it happened. After considerable searching for a suitable topic, I finally concluded that an alkaline divalent copper plating bath should make an interesting subject for a thesis. A readily available complexing agent at the time was Fehling's solution (an alkaline copper tartrate reagent). I ran some polarograms on it and kept getting an unexpected break in the cathode curve at low voltages. In trying to determine the cause, I examined the cathode film from time to time. To my great surprise a different color was obtained every time I looked at it.
As it turned out, the colored deposit was cuprous oxide, plating out at the cathode just like a metal. Needless to say, my attention was diverted from the copper project to one of color plating. It was not until later, in 1935, that a suitable alkaline complexing agent was finally found for the copper project, namely, pyrophosphate. This time, in reverse, it was an outgrowth of the color plating (Electrocolor) research carried out for United Chromium.
The initial color plating investigation, however, proved to be a fascinating topic for a thesis. As the research phase neared completion, it was in an effort to find out whether color plating might be novel or useful that my professor at the University of Kansas, the late Dr. Robert Taft, suggested that I write to Dr. Blum at the National Bureau of Standards to seek his advice and counsel. I still have his kind reply.
Over the years we had many other associations, particularly in connection with the various corrosion projects that were being carried out by the Bureau of Standards, the AES and ASTM. In following these programs, I became interested in the unexplained anomalies that often showed up at the various corrosion sites.
I should like to review at this time some of the more interesting situations that came to light, and some of their interpretations and applications, particularly with respect to composite electrodeposited coatings. Before doing this, however, it might be instructive to review briefly some of the pertinent factors that have been established with respect to the corrosion behavior of electrodeposited coatings.
It has long been recognized, for example, that the position of the plated metal in the galvanic series is an important factor in determining protection. Figure 1 shows the relative positions of the more common metals and alloys used by the plating industry, and some of the cathodic displacements that result from natural passivation in the atmosphere. The scale, however, is not changed appreciably in the salt spray, inasmuch as the passivating agent (oxygen) is still the same. The salt mainly increases the cell conductivity and shortens the time to corrosion failure.