Corrosion is a redox process by which metals are oxidized by oxygen in the presence of moisture. The corrosion of iron may be characterized by the half-reaction: EQU Fe.fwdarw.Fe.sup.2+ +2e.sup.-.
The two electrons so produced may flow to a point which acts as a cathode where oxygen is reduced to OH.sup.- as follows: EQU 1/2O.sub.2 +H.sub.2 O+2e.sup.- .fwdarw.2OH.
Adding the two half-reactions: ##EQU1##
The ferrous hydroxide, Fe(OH).sub.2, may then be oxidized to ferric hydroxide, FeO(OH), which may lose water to form ferric oxide, Fe.sub.2 O.sub.3.
One method of inhibiting the corrosion of steel is galvanization, wherein the steel is coated with elemental zinc, a more active metal. Zinc, being a better reducing agent than iron, protects the iron by reversing the reaction for the formation of Fe.sup.2+ : ##EQU2## Hence, the zinc coating corrodes sacrificially but the structural iron in contact with the zinc is unaffected as long as the zinc lasts. Small gaps in the zinc exposing the underlying iron are protected by the galvanic action of the zinc. Where the corrosion of zinc leaves large gaps (or where defects otherwise exist in the galvanic coating) the corrosion of iron can occur normally.
One prior art method of evaluating corrosion resistance is to perform accelerated corrosion testing such as a salt fog or salt spray test wherein the component is placed in a corrosive environment having predefined conditions and using predetermined procedures. One drawback of such tests is that they are generally time consuming, often requiring weeks or months to complete. Another drawback to the prior art salt fog tests is that they do not take into account fatigue or cyclic deformation of the metals. It has been found in accordance with the present invention that corrosion is accelerated at points undergoing mechanical fatigue.
One method of predicting cable life under fatigue conditions might be to subject cable to repeated fatigue stresses or strains until the cable fails due to fatigue/corrosion interaction, i.e., until the cable breaks. Such testing may be performed under conditions at which corrosion is accelerated, such as conditions similar to salt fog conditions. However, testing until failure would be a very time consuming process. Therefore, it would be highly desirable to provide an apparatus and method for testing cable that may be used to accurately predict cable life, but that can be performed in a period of time significantly shorter than the time required to test until cable failure.