Source: Canadian Conservation Institute (CCI) Notes 9/14
Many metal objects in museum collections will have already corroded due to exposure to moisture before becoming a part of the collection. The extent to which this happens will vary depending on several factors that include the type of metal, the amount and type of previous corrosion, the presence of surface contamination like dirt or salts and the relative humidity (RH) of the surrounding air. Not all of these factors can be controlled, such as the type of metal in the object or the amount or type of corrosion that has occurred in the past. However, RH, the key factor, can be reduced for museum objects made only of metal (for more information, consult CCI Note 9/2 Storage of Metals).
Generally, only a part of a collection is made up of metal objects; of those, only a few will be at risk of ongoing corrosion. As a result, few institutions can provide a collection space specifically tailored for the dry storage of metal objects. Nevertheless, there is a solution for smaller metal objects that consists of a plastic box and a dry moisture sorbent. The idea is to establish dry conditions as a microenvironment in a container that will slow down or halt active corrosion initiated or accelerated by humidity in the air. Whatever system is chosen, it should be designed for the storage of metals over many years with no need for frequent maintenance. This type of storage is often not suitable for non-metallic materials.