The main causes of deterioration of metals include:
- exposure to conditions of high relative humidity;
- the presence of oxygen;
- exposure to salty conditions;
- reaction with pollutants;
- contact with other metals;
- stresses induced during manufacturing processes; and
- careless handling.
Moisture and oxygen are required for corrosion (oxidation) of metals to occur. Chloride ions, present in common salt (sodium chloride), may originate from human or animal contact (perspiration) or from airborne sea salts. They can speed up the corrosion rate and are capable of penetrating protective oxide layers on metals such as copper and aluminium to form metal chlorides. These latter materials do not form a coherent surface layer, effectively removing the corrosion resistance of the metal. Oils and sweat from people’s hands are potential agents of decay, with the organic acids and chloride ions transferred upon contact between skin and metal, capable of causing significant attack on metallic surfaces.
Any agent that affects protective films on metals may enhance corrosion of the underlying metal. Physical damage to painted or coated surfaces, or the uneven application of oil coatings for example, will alter the access of oxygen to the metal surfaces. This usually causes one part of the object to corrode at the expense of another.
Normally unreactive metals such as copper and silver can suffer significant corrosion if sulphide-containing species are in the same environment as the metal. Sulphide pollutants are usually associated with the breakdown of plant matter and the decomposition of sulphur-containing proteins such as wool. Common pollutants include hydrogen sulphide and carbonyl sulphide.
Inorganic acids such as hydrochloric acid derived from the decay of plastics such as polyvinyl chloride and nitric and sulphuric acids formed from air pollutants and moisture, will attack metals that are in the same storage environment or in the open air. Outdoor monuments and sculptures are particularly vulnerable to this type of attack.
Corrosion will also occur if dissimilar alloys and metals come into contact with each other. This type of corrosion is known as galvanic corrosion. In this situation the more reactive metal or alloy corrodes while the less reactive metal is protected. If for example iron and copper were in direct physical contact, then in the presence of moisture and oxygen, the iron would corrode preferentially while simultaneously protecting the copper from these agents of corrosion. A galvanic series, which lists metals in increasing order of reactivity is provided elsewhere (see Appendix 8). Many problems associated with galvanic corrosion can be overcome by avoiding direct contact between dissimilar metals.
Corrosion may be increased by stresses imparted to metals during their manufacture. Changes in the microstructure of metals, produced by processes such as hammering, drawing and rolling, usually encourage corrosion and in doing so reduce the service life of affected metals.
Handle metal objects carefully as it is easy to over-estimate their strength and toughness. Metal objects can be easily dented, bent or mis-shaped if handled inappropriately, resulting in unnecessary damage that may be difficult to repair.
It is important to remember these basic issues when examining corroded artefacts or when assessing storage environments. Removing chloride salts, restoring oxide coatings, repatinating metals with aesthetically pleasing patinas, coating metals with protective materials such as lacquers and paints and modifying environmental storage and display conditions are all effective tools in the conservation battle against corrosion.