2.2 Degradation, dissolution and corrosion
A variety of common terms are used to describe the ways in which structural materials can be attacked by environments and although they do have specific connotations, they are frequently used as blanket terms for material deterioration. I shall attempt to define them in a more specific way, namely:
Degradation: loss of strength of non-metals such as wood, rope or textile.
Dissolution: removal of material in solution owing to the attacking medium.
Corrosion: attack of metallic materials.
Suggest appropriate terms for the following phenomena:
(a) Rusting is a form of corrosion where the iron of the roof is converted to iron oxide, or red-brown rust.
(b) Removal of limestone by rain water is a kind of dissolution process, where the calcium carbonate goes into solution.
(c) Rot is a form of degradation of the cellulose fibres in wood by natural organisms like fungi (such as dry or wet rot).
When real products are examined in detail, one is forced to examine the many specific mechanisms by which they can deteriorate. Rusting, rotting and dissolution are very common in practical experience simply because of the widespread use of steel, limestone and timber in structures.
However, in order to study these (and other) mechanisms, we need to apply more rigorous analyses. The point of study is to design ways of eliminating deterioration, or at least (if attack is inevitable, as it often is in practice) ways of controlling and reducing the rate of attack. Most structures need to have a protracted life, not only to justify the expense of their erection, but also to protect the users. One of the unfortunate features of structural deterioration is the insidious way in which attack can occur, often hidden from view, and proceeding at a rate that can result in sudden and catastrophic failure of a safety-critical component. We shall be examining some examples later in this unit.