The Nature, Cause and Effect of Porosity in Electrodeposits: A Microscopic Examination of Nickel-Chromium Coatings after Atmospheric Corrosion
This paper is one of seven reports constituting AES Research Project #13, dealing with the nature, cause and effect of porosity in electrodeposits, performed at the then-National Bureau of Standards, in Gaithersburg, Maryland. It was one of several seminal articles produced through AES Research during the height of the automotive/decorative segment of the surface finishing industry in the 1950s and 60s.
#automotive #research #plating
Donald W. Ernst and Fielding Ogburn
National Bureau of Standards
[now National Institute for Standards and Technology (NIST)]
Gaithersburg, Maryland, USA
Editor’s Note: Originally published as Plating, 48 (5), 491-497 (1961) and as AES Research Report, Serial #47, this paper is one of seven reports constituting AES Research Project #13, dealing with the nature, cause and effect of porosity in electrodeposits, performed at the then-National Bureau of Standards, in Gaithersburg, Maryland. It was one of several seminal articles produced through AES Research during the height of the automotive/decorative segment of the surface finishing industry in the 1950s and 60s. A printable PDF version of this report is available by clicking HERE.
Nickel-chromium coatings on steel, which had been subjected to atmospheric corrosion, were examined microscopically. With chromium, bright nickel was perforated more by pitting corrosion than Watts nickel, but the latter contained many more micropits. Pits which did not extend to the basis metal were similar in appearance regardless of type of nickel or atmosphere. Cracks appeared in chromium plated bright nickel after pits had formed in the nickel. Observations of galvanic cells in the laboratory showed that the polarity of the nickel-steel couple changes with variations in the electrolyte. These changes can explain differences in pit shapes and in rust exudation associated with sea coast and industrial atmospheres. Also the polarity of nickel-chromium couples was found to change unpredictably in several electrolytes. The chromium was usually found to be the anode. A nickel-bright nickel cell was found to be more active than any nickel-chromium cell.
Investigations have been made of the atmospheric corrosion of nickel-plated steel with and without chromium. In most of these investigations, the nature and extent of the corrosion have been determined solely by visual observations, with the unaided eye, of the exposed surfaces of the coatings. These investigations have produced valid and useful information. To understand the nature of the corrosion process, however, one needs to have more information than is obtainable from visual appearance. For example, information is needed about the microscopic nature of the corroded surfaces of the metal, the history of the defects which ultimately lead to rust spots, and the galvanic effects which are of significance.
An important purpose of this project was to explore some new approaches to the investigation of the corrosion of nickel-plated steel. The first part of this final report of Project 13 covers the microscopic examination of a number of steel panels plated with nickel, or chromium and nickel, which had been exposed outdoors. The second part of the paper covers the microscopic examination of a few panels before and during outdoor exposure. A third part deals with a few exploratory experiments related to the galvanic corrosion of couples of iron, nickel, and chromium.
Examination of specimens after outdoor corrosion
The specimens* which were examined consisted of one-half of standard 4 × 6 inch (10 × 15 cm) steel corrosion test panels, each plated with one of the following coatings:
|WU||Watts nickel, unbuffed|
|WB||Watts nickel, buffed|
|WB, Cr||Watts nickel, buffed, with chromium|
|A||Bright nickel A|
|A, Cr||Bright nickel A with chromium|
|B||Bright nickel B|
|B, Cr||Bright nickel B with chromium|
|TT||Double-layer nickel (semi-bright and bright)|
|TT, Cr||Double-layer nickel with chromium|
Each specimen was plated with about 38 μm (1½ mils) of nickel. The chromium was the usual decorative chromium, about 3 μm (0.01 mil) thick. One set of these specimens had been exposed at Kure Beach, North Carolina, a rural sea coast atmosphere, 250 meters (800 feet) from the ocean, and a second set had been exposed at Bayonne, New Jersey, an industrial atmosphere. Both sets had been exposed for about nine months. There is no record available of the progress of corrosion of these specimens insofar as a detailed microscopic examination and classification of the corrosion pattern is concerned.
The surface of each coating was cleaned and carefully examined microscopically. Cross sections and parallel sections were also examined. Uniform surface attack or etching of the surface appears under the microscope as a multitude of tiny pits randomly distributed. This general surface etching seems to be more pronounced at Bayonne than at Kure Beach. There was, however, no apparent difference in the extent of the surface attack on the different types of nickel such as has been reported from visual observations.
Superimposed on the uniformly attacked surface are the corrosion pits, (including rust spots, which are deep pits filled with rust) which appear as relatively large dark spots or areas (surface view) or as relatively wide and deep cavities (cross section view). These pits occurred principally in two forms as single isolated pits with vertical sides and as clusters of shallow pits. Both types of pits formed at both exposure sites. Examples of each are shown in Figs. 1 and 2 which are photographs of specimens described in the next section of this paper.