Metals

What are the most common types of damage?

Metals are vulnerable to physical damage and to chemical deterioration.

Physical damage includes:

• dents;

• wear of machine parts through repeated operation;

• wear of metal surfaces through excess polishing; and

• scratches. On highly polished surfaces scratches can be distracting and mar the appearance of the object; and on finely machined surfaces, for example in machinery, scratches can adversely effect the operation of the machine.

Chemical deterioration in metals is called corrosion. Physical damage to metal objects can leave them vulnerable to corrosion. For example, a scratch in tin plate leads to corrosion of the base metal.

Corrosion

The tarnishing or corrosion of metals will be your major problem if you are responsible for the care of metal items.

Gold, silver and copper are the only metals that are stable enough to be found in their natural metallic state. All other metals are more stable when combined with other elements, to form oxides, sulphides, chlorides, carbonates or mixtures of these.

There is a natural tendency for most pure metals to return to their more stable, corroded states.

Because of this, metal objects need to be protected from environmental conditions and pollutants which encourage corrosion.

In some cases—as the metal corrodes—the oxide film that forms acts as an insulating barrier, which slows the rate of corrosion to an acceptable level. Copper and aluminium are two metals in which oxide coatings form protective layers—called passivating layers. When iron corrodes, however, it does not usually form a protective film. Iron continues to corrode until no metal is left, unless some other protective coating is applied to protect it from the elements.

It is important to note also that corrosion builds up on top of the metal surface, and is often much thicker than the original metal. You may find that when you come to treat an object there is little or no original metal remaining; and removing the corrosion products could mean ending up with a much reduced object or none at all.

Causes of corrosion

Moisture and oxygen are required for metals to corrode.

Chloride ions, which are found in common salt—sodium chloride—can speed up the corrosion rate, and are also capable of penetrating protective oxide layers.

Fats, oils and sweat can also enhance corrosion.

Acids formed when air pollutants combine with moisture will attack metals. Outdoor monuments and sculptures are particularly vulnerable to this type of attack.

Relatively unreactive metals, such as copper and silver, can suffer significant corrosion if sulphide-containing materials are in the same environment as the metal.

Dust absorbs moisture readily. So areas with a build-up of dust can have quite high local humidity—higher than the surrounding environment. Dust will absorb pollutants and other reactive materials as well.

Corrosion also occurs if dissimilar alloys and metals come into contact with each other. This type of corrosion is called galvanic corrosion.

Under these circumstances, the more reactive metal or alloy corrodes while the less reactive metal is protected. For example, if iron and copper were in direct physical contact in the presence of moisture and oxygen, then the iron would selectively corrode while simultaneously protecting the copper.

This problem can be overcome by avoiding direct contact between dissimilar metals. This will not be simple if the metals are components of the same object.

The oxide film which forms on particular metals acts as an insulating barrier or passivating layer barrier, slowing the rate of corrosion to an acceptable level. This type of passivation occurs with copper and aluminium.

When looking at the way in which artefacts have corroded and when deciding on management strategies, it is important to ask these basic questions: is it necessary to treat an object? or can the causes of deterioration be controlled?