Metal Corrosion is the destruction of metals by direct chemical attack, and when used in connection with iron or steel it is commonly known as rusting. The dangerous effects of Corrosion may be the weakening of a metallic structure, as in bridges, boilers, or the introduction of metallic poisons into water and foodstuffs. Some metals, e.g. gold, silver and platinum, are not easily attacked and are thus useful where the maintenance of purity is important. Their expense forces the use of comparatively cheap but more easily corroded metals such as iron, zinc, tin and lead for many purposes.
Types of Metal Corrosion
There are many types of metal Corrosion but the most common is due to electrolytic action which is influenced by a large number of factors. Moist air contains water, oxygen, carbon dioxide and certain acids, and the effect of these, singly or together, can cause C. Iron is particularly susceptible to attack and will corrode in pure water. Rusting in air is promoted by an increase in relative humidity, which must be at least 60 per cent, and presence of solid particles and acid vapours such as sulphur dioxide and carbon dioxide, although above certain concentrations the latter will inhibit rusting. Rust is mainly hydrated ferric oxide. Fe2O3 . H2O, with some basic ferrous and ferric carbonates. The impurities in the iron and the iron itself form the poles of a large number of microscopical electrolytic cells in which iron is often the anode.
This enables ferrous ions to pass into solution and form insoluble ferrous hydroxide, which does not stifle the anode because the ferrous hydroxide decomposes in water to give a loose granular deposit of triferric tetroxide (Fea04) which is not protective. This has led to various methods for the protection of iron, such as alloying with a non-corrosive metal, e.g. chromium or nickel, in suitable proportions.
Another method is to form a film on the surface of the metal, but this film must be continuous and either be non-corrosive or must corrode preferentially. Such methods include galvanising, which is the formation of a zinc film, painting or subjecting the iron to the action of superheated steam to form an Fea04 film. This film is likely to crack owing to differencés in thermal expansion and contraction compared with that of iron.
A modern example of galvanic protection of iron is in the use of magnesium which is directly coupled by an electric cable to pipe lines; e.g. oil pipe lines in the Middle E., water pipe lines in England and Scotland, and gas mains in Texas. The magnesium, and zinc in the case of galvanising, becomes the anode since it is more active and higher in the electro- chemical series than iron, and so corrodes pre- ferentially.
This is known as sacrificial Corrosion. If however the iron is tin plated, the coating is relatively non-corrosive in air but is still porous and the pores cannot be closed. In moist air and salt solutions the tin is cathodic to the iron so that the iron dissolves and rusting occurs. In tinned iron containers containing fruit juices the tin is anodic to the iron. Hence the tin dissolves in the fruit juices, but tin compounds are relatively non-toxic and the amount dissolved over the usual passage of time prior to consumption of the contents is often much less than the maximum legal limit of dissolved tin.
To prevent the tin dissolving fruit and meat cans are often lacquered inside. Aluminium will corrode in moist air forming a partly protective oxide film, and use is made of this by artificially thickening this oxide film by ‘anodising’, i.e. by electrolysis in an acid solution using aluminium as the anode. The film so formed is very resistant to abrasion, very porous and adsorbent, and the immersion of the anodised aluminium in suit- able dyestuffs gives attractive finishes.
The porosity can be closed by boiling in water or in sodium silicate solution, but the material is still corroded by salt. Lead is rapidly tarnished in air, forming a superficial layer of the hydroxide and carbonate which tends to inhibit further attack. Air-free water does not attack lead, but water with dissolved air does dissolve lead slightly owing to the formation of the slightly soluble hydroxide which is poisonous, and it is important to note that the effects of lead poisoning are cumulative.
Copper is corroded by moist air, forming mainly oxide and sulphide films in industrial tns, a basic sulphate inland, and mainly basic chloride in coastal dists. The ease of formation of a sulphide film explains why copper which is to be used for electrical purposes and insulated with vulcanised rubber is first coated with tin. Copper also reacts with weak organic acids in the presence of air; e.g. it forms verdigris with acetic acid and may give rise to poisoning if vegetables are allowed to ferment in copper utensils.
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