Beneficial Aspects of the Environmental Instability of Materials
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MRS BULLETIN/SEPTEMBER 1993
pipe decreased heat transport to the water inside the copper pipe. However, as the old English proverb says, "One man's meat is another man's poison." Thus, for those applications where in-situ production of thermally or electrically insulating barrier layers on metal substrates is desirable, exposure to environments which optimize production of that barrier layer is the objective, rather than something to avoid. A classical example of this practice is embodied
DC Power Source
Counter Electrode
Metal
Oxide Film Working Electrode
Figure 1. Schematic of an amortization circuit.
in those procedures where barrier layers are forced to grow on exposed metal surfaces—commonly called "anodization." Anodization—Forced Growth of Oxidation Products Aluminum is unstable in air saturated with neutral water, tending to corrode according to the reactions: 2A1 + 3H 2 O = A12O3 + 6H + + 6e~
(anodic)
O 2 + 2H 2 O + 4 e ' = 4 O H -
(1)
(cathodic) (2)
If the aluminum oxide reaction product provides resistance to either or both of these two contributing electrochemical reactions, the rate at which aluminum reacts can be reduced in proportion to the resistance of the barrier layer. Carrying this one step further, one should be able to decrease corrosion rates to infinitesimally low values by producing disproportionately thick deposits of barrier oxide, analogous to decreasing heat transport by adding thicker layers of insulation. This process is most commonly accomplished using electrochemical methods, typically with impressed current or with applied voltage. Anodization of metals involves in-situ growth of electrochemical oxidation products on surfaces of metals.13'4 These reaction products are produced by forcing current through an electrical circuit—from the metal to be anodized (working electrode) through a suitable electrolyte and into an auxiliary or counter electrode (Figure 1). By convention, current is considered as positive charges moving from high to low potential. As such, current may be carried by cations or electron holes moving in the direction of the potential gradient and/or by anions or electrons moving in the opposite direction. Whenever the electrochemical potential of the metal is conducive to insoluble oxidation product formation (i.e., a particular metal oxide is thermodynamically stable or metal ions are concentrated enough to precipitate as a hydroxide or other salt), the rate at which the anodic film grows is determined by the resistance to current flow through the electric circuit. On aluminum, the nature of anodic films formed depends on the composition of the electrolyte used.2 In boric acid, borate, phosphate, and several organic acid salts, anodic films of exceptional resistivity are produced, and these deposits are suitable for use in electrolytic capacitors. In sulfuric or oxalic acids, however, anodization products are of lower resistivity
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Beneficial Aspects of the Environmental Instability of Materials
Metal
Anodized Film "Pores"
Oxide
Figure 2. Schemat
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