chemical principles exemplified
ROBERT C. PLUMB Worcester P&ledank b h i Wwc.sl.r, M e u d m 01609
Durable Chrome Plating lllustroting principles o f electrochemistry and kinetics
Suggested by Roger L. Saur General Motors Technical Center Automotive engineers made a marked improvement in the corrosion resistance of chrome plated automohilc trim a few years ago by, of all things, decreasing the "quality" of the chromium laycr. This novel rcsult is simply cxplainod with principlrs of clcctrochemistry and the explanation may provide you, tho car owner, wit,h a. clue as to how to keep your car in good shape longer. A chromo-plated finish really relics on a rather thick laycr of nickcl for the corrosion protection. The thin laycr of chromium serves to liccp the nickcl from tarnishing in air and gives a bright, hard surface. The standard oxidation potentials for the metals arc
Although chromium has the greatest oxidation potential of t>hcthree, it is inert because of the very thin protcctive oxidc film which forms on the surfacc. The sites of corrosion arc the defects in the chromium layer-even if a perfectly continuous layer m r e produced by the mmufacturcr, normal wear and tear would soon gcnrratr dcfccts tnrough which corrosion would occur. In the 1B501s, bumpers of cars rusted badly, dcteriorating morc rapidly than the painted steel body. Elcct,roplatcrs then were doing the obvious-putting the most continuous thick chromium laycr possible on the trim. Then it was discovered that a chromium film with a lot of defects gave greatly improved corrosion protcction. Today bumpers are platcd with a microdiscontinuous chromium, and their lifetime is much longer-oft,cn exceeding that of the body of a car. Why should increasing the number of defects incrrasc the corrosion protcction? When the metal surface is wet, an electrolytic corrosion couple is formed. The half cells arc
-
anodic (at the defect) Ni NiP+ 2e-
+
-
cathodic (at Lhe chromium surface) 2e2Hf HIO
+
+
Edison's Chemical Meter &"a =
+0.25
lllustroting certoin principles of electrolysis and charge transport in metols ond electrolytes
=
+1.23
Contribution by Byron M . Vanderbilt Thomas A. Edison's invent,ion of the light bulb was widely publicized in t.he news media in Dcccmber of 1879 and by an open house a t his llenlo Park Lahoratory on New Year's Eve of that year. Thousands of
&OR
The greater availability of oxygen at the chromium surface cttusrs the cathodic reaction to occur there prrdomirinntly, rather than at the defect. The rate a t which thc ovcrall rraction takes place will be controlled 626
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Journal o f Chemiml Education
by the rate of the slo~vrststep in (tither of the half reactions. Thwc arr two steps in the cathodic rcaction which could be thr slov step-cith~r the rate of tunneling of electrons through the oxidc on the chmmium to an oxygcn species near the surface, or the rate of diffusion of dissolved oxygnn from the bulk solution to the surface. Thc anodic rcaction apparently is not kinetically limiting even if the arras of exposed nickel are quite small. As a rcsult of the elcctrolytic corrosion, t,he nickel beneath a defect will gradually dissolve, but it is bardy noticeable. H o w w x , when the niclcel becomes perforated, t.hc iron beneath is exposed and the corrosion becomes obvious as rust which stains a large area. Since t,hc rate of the corrosion reaction is detcrmi~wdby the cathodic rcaction, the electron flov from anodes to cathodic area is independent of the number of anodcs. If there arc few anodcs, holvcvcr, the nickel at thosr anodes mill be consumrd rapidly and prrforation will occur quicldy; while if thcrc arc a lot of anodes, proportionately less nickel will dissolve a t each on
