This is a process in which a solid, and especially a metal solid, is eaten and changed by chemical action as in the case of the oxidisation of iron. Corrosion includes rust and therefore the separate exclusion of rust is unnecessary. Corrosion is also included in the phrase inherent vice, but, again, it does no harm to make it clear to the assured that such processes are excluded.

In the case of Bettigole v. American Employers Insurance Co. [1991] SO Mass App 272, 567 NE 2d 1259, an all risks policy contained an exclusion against loss caused by corrosion. Structural damage to a parking deck was caused when chloride ions were brought onto the deck by de-icing salts on automobiles. The chloride ions gradually filtered through the concrete and attacked steel reinforcing bars. The damage was caused by corrosion and it was held that the term was not limited to wearing away by natural means but included corrosion resulting from the conduct of the insured and also corrosion created by the actions of third parties.

Again, however, it is important to look at the proximate cause of the loss. For example, in the US case of Adams-Arapahoe Joint School District v. Continental Insurance Co. 891 F 2d 772 (10th Cir 1989), an all risks policy covered a school district against “all risks and direct physical loss” but excluded loss caused by rust or corrosion and inherent or latent defects unless such loss resulted from a peril not excluded in the policy. A portion of the roof collapsed due to corrosion which resulted from defective design or construction. The defective design and construction was not known at the time the policy was issued and was held not to be an inherent or latent defect but a fortuitous loss and was a covered risk. Therefore, the proximate cause of the loss was held to be not the corrosion but the fact of the design fault in that the corrosion would never have occurred if the roof had not been designed defectively. It is likely that an English court would have found that the design fault was excluded as a latent defect.