The Inhibitive Behavior of the Fluoride Ion and the Destruction of Its Protective Film in the Corrosion of Magnesium in Neutral Solutions

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Abstract
Fluoride ions exhibit an anomalous inhibitive ability against the corrosion of magnesium ; this may be attributed to the formation of a fine film of insoluble magnesium fluoride. This film conducts the cathodic current fairly well, but it conducts the anodic current not at all, as far as it is not destroyed. The cathode reaction through the magnesium fluoride film has a low apparent activation energy because of its electronic conduction, while, when magnesium is anodically polarized, the film is no longer electron-conductive and still permits to a lesser extent the diffusion of any other charged particles; consequently, this is a kind of rectifier in a solution containing potassium fluoride alone. If a solution contains chloride ions in addition to fluoride ions, the matter is quite different and fluoride ions rather promote the corrosion, like other halide ions. When magnesium, the surface of which has previously been covered with magnesium fluoride is anodically polarized in a solution containing only chloride ions or containing fluoride ions as well as chloride ions, the film soon begins to be destroyed at the initial stage, the bare area of high conductivity appears, and then the surface is gradually again covered with the bulky products, magnesium hydroxide, at the later stage. The destruction of the magnesium fluoride film may occur through a mechanical process regardless of the presence of fluoride ions.

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