TECHNOLOGY
Battelle Develops Better Chrome Plate Pore-free chromium plus two-layered nickel plate on auto parts of die cast zinc give built-in resistance to corrosion Coming: chrome plated auto parts with two built-in defenses against rapid corrosion. Key contributors to a longlived plating: pore-free, crack-free chromium plate over a special twolayered nickel plate. Zinc die castings chrome plated in this way can go at least two years—maybe as many as five—without "significant corrosion," say Dr. C. L. Faust and W. H. Safranek of Battelle Memorial Institute, developers of the plating process. Until now, a three-layer plated finish has been specified for auto parts of die cast zinc: 0.3 mil copper, 0.8 mil bright nickel, and 0.01 mil chromium. The copper and the nickel plates are expected to protect the zinc part,
while the chromium plate is intended only to provide brightness and to keep the nickel from tarnishing. Result: Unsightly white corrosion pits show up on chrome parts in three months or less on brand new cars. Concerned about this problem, the American Zinc Institute set up a research program at Battelle to track down the trouble and come up with higher quality plating on zinc die castings. Under this program, Battelle has evaluated several plating systems, is still carrying out studies. Battelle scientists found that not only was the thin top plate of chrome not protecting the nickel, but because of pores or cracks, it actually was con-
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tributing to pitting of the nickel plate. When exposed to air, the chromium is a cathode and the exposed nickel is an anode. In a moist salty environment, such as exists on streets during winter, the nickel corrodes and a pit forms under the chromium. The driving force of this electrolytic couple is many millivolts, say Battelle scientists, so that within a very few months the copper plate is exposed. At this point a new couple, nickelcopper, starts at the copper plate interface. The copper is anodic to nickel and rapidly corrodes away to expose the zinc. When this happens, white zinc corrosion products form in the pit and erupt, lifting the plate in blisters.
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OUTDOOR EXPOSURE. Battelle exposed these panels for nearly three years on its electrochemical engineering lab roof. Here, the institute's W. H. Safranek compares mirrorlike zinc die cast panel with corroded ones
Nickel the Stopper. Battelle scientists tackled the problem by working out a new way to deposit a crack-free, pore-free chromium plating. With no holes in the chromium, start of corrosion is much delayed. But corrosion does eventually get started—perhaps by a dirt particle which sets up a differential aeration cell effect, or by cracks in the chromium caused by gravel or cinders striking it. A ring-like pit forms in the chromium, with the chromium under the dirt particle acting as an anode (active) of the local cell and that at the fringe as a cathode (passive). Chromium dissolves away at the anode exposing nickel. Here is where the special two-layer nickel plate comes in. The layer next to the chromium is bright nickel. It is very fine grained and is likely to be layered. Beneath this lies a semibright, more ductile nickel which has a columnar structure. Brighteners containing sulfur are avoided in depositing the semibright layer of nickel. These two nickels have different electrode potentials—from 0.05 to 0.12 volt—in corroding media. Of the two layers, the innermost, or semibright nickel, should resist corrosion a little better. When pits develop in the chromium, the bright nickel layer is sacrificed.
SALT TEST. Fog from this corrosion test cabinet consists of an acidified salt solution. Battelle's H. R. Miller holds a conventionally-plated panel in his right hand; the other panel was plated with the institute's new process
Corrosion moves laterally instead of inwardly, stopping at the interface of the two nickel plates. Eventually, bits of chromium flake off. This results in a pit-like appearance in a year or longer, but these pits aren't as unsightly or as objectionable as the white eruptions previously seen. Lab specimens plated by the new process retain "perfect" appearance, according to Battelle, after exposure for 54 hours in the copper accelerated acetic acid salt spray corrosion test (CASS), and for four cycles of 20 hours each in Corrodekote (a synthetic "mud" made up of kaolin, a copper salt, and an iron salt smeared on the part to be exposed in a humidity cabinet). Present day plating is judged satisfactory if it withstands 18 hours in these tests. Correlation tests indicate that 18 hours equals about one winter for automobiles in Detroit, so quality of currently used plating can be judged overnight. However, the new plating is so good, says Battelle, that present accelerated corrosion tests are inadequate, and new, more severe tests will have to be devised. Workmanship Still Needed. The solution offered by Battelle isn't as simple as it may appear, Dr. Faust cautions. "There is no such thing as a nickel plate, a chromium plate, or a
copper plate." Technique and quality are still very important. For example, Dr. Faust points out, electrode potentials of nickel plates deposited under varying conditions can differ by as much as 240 millivolts. For best results, the potential between the two layers of nickel should differ by only a few millivolts (30 to 50). Further, the potential of the nickel plates should be as close as possible to that of passive chromium plate. Crack-free, pore-free chromium plate calls for changes in plating conditions and technique. First, Dr. Faust says, thickness of the plate must be increased from about 0.01 mil to at least 0.03 mil and preferably to 0.05 mil. Other changes needed include higher temperature in the plating bath (from around 110° F. to close to 130° F . ) , higher concentration of chromic acid, a higher ratio of chrome to sulfuric acid, and a current density of 250 to 350 amp. per sq. ft. to avoid a dull and possibly porous chromium. Further, cautions Dr. Faust, the nickel plate on which the chromium is deposited must be free of cracks, pores, or nodules, because such defects would lead to pores in the chromium plate. But, he says, properly applied plating on zinc die cast auto parts can remain gleaming even to the end of the auto's running days. JULY
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