Zinc Yellow in the Inhibition of Corrosion Fatigue of Steel in Sodium Chloride Solution

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Abstract
The corrosion fatigue of steel wire in sodium chloride solutions of concentrations ranging from 0.01M to 1.0M at room temperature was found to be slowed more by saturation with pigment zinc yellow than by addition of equivalent potassium chromate. The increase in the life of the steel sample ranges from 15% to 115%, depending upon the inhibitor and the stress range. Wire samples were tested in the Kenyon rotating‐wire arc fatigue machine. When the metal was active, the data formed a smooth curve, but when passive, a wide scatter of results was obtained, as a result of probability factors. Under these conditions repeated observations were required to provide reasonable precision of results. The life vs. stress curve does not flatten out, but continues to decrease with increasing life. On passivated short steel wire samples (mounted so as to adhere closely to true arc curvature), compact adherent mounds of corrosion product covering corrosion‐fatigue pits were comparatively large and far apart at low stresses and were smaller and much closer at high stresses. This is believed to indicate that the localized cathodic current density required to protect a corroding area increases with increased strain in the metal. With long wire samples (with which there is a slight departure from arc curvature and from equal stress distribution), corrosion‐fatigue occurs on the wire in spots distributed as above, but the attack develops more rapidly in areas of higher stress. Addition of chromate to sodium chloride solutions extends the wire life without causing occasional acceleration of attack, which it may cause under stressless conditions. Zinc ion increases wire life in either the presence or absence of chromate, due probably to its action as a polarizer of the corrosion cathode. Variation of pH from 6.5 to 7.6 in the potassium chromate‐inhibited solution has no marked effect. The relation between sodium chloride concentration and the sample life is shown by data obtained at three representative stresses and three sodium chloride concentrations uninhibited and with each inhibitor. Steel wire samples having a copper plate and others with a copper‐over‐zinc plate were tested to investigate the combined effect of plating and inhibitor. The attack was markedly accelerated by increasing temperature up to 50°C, apparently by increase in the function of chromate as a cathodic depolarizer. Exposure to radiation from a mercury vapor arc lamp affected wire life in inhibited solution, but not in uninhibited solution.

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