I.e the book says once an iron object is retrieved from a site it is stored in a NaOH 2%w/v solution. The reason is that "the metal surface is passivated". What exactly does this mean and how does it occur?
Iron artefacts are normally stored in highly alkaline solutions (pH>10) so that it prevents further corrosion. This is because the chloride ions present in the iron hydroxy chlorides (e.g. Fe(OH)Cl) can react with water to form HCl and thus a basic medium would be most appropriate as it prevents this from occurring and stabilises the artefact. This basic medium also allows for iron to undergo passivation (i.e. iron will form oxide layers which prevent further corrosion), which is beneficial until a proper treatment can be used.