3.4 Degradation of metals and alloys
3.4.2 Corrosion of metals and alloys in aqueous environments
Metals and alloys 125
3.4.2 Corrosion of metals and alloys in aqueous
Materials for engineering 126
the lowest content of dissolved oxygen and acts as an anode; Fe2+ ions thus pass into solution (i.e. the metal will corrode) in this region, equation [3.13]
and electrons pass up the specimen and are discharged near the surface, where OH– ions form in the electrolyte by equation [3.14].
Diffusion occurs in the liquid over a period of time and the Fe2+ ions and the OH– ions interact to form an iron hydroxide (rust). Two points are significant here: firstly, there is the geometrical fact that the metal goes into solution in one region, electrons are discharged in another region (that where oxygen is most readily available) and the corrosion product is formed in a third place.
Since the corrosion product does not form at the site where the metal is dissolving, there can be no stifling of the attack (as in the case of dry corrosion, when a protective film progressively reduces the rate of oxidation). The second point to emphasize is the importance of the gradient in oxygen concentration in the electrolyte: this phenomenon of differential aeration is the origin of the EMF of the corrosion ‘cell’ and is, in practice, a very common source of corrosion in iron and steel. This is the reason why corrosion is often concentrated in crevices in structures, the regions of minimum oxygen availability forming anodes. Conversely, if oxygen is totally excluded from the system, electrons cannot be discharged by the process of equation [3.14]
and so corrosion ceases. This is why steel wrecks immersed in very deep seawater (e.g. the Titanic) survive for long periods, whereas those on or near the seashore quickly disintegrate by corrosion since they are so well aerated.
If two different metals are in electrical contact in an electrolyte, a galvanic EMF may exist and corrosion of the more reactive metal can proceed preferentially, even in the absence of oxygen. An example of this would be a galvanic couple between Al and Fe in an electrolyte,where the Al would form the anode and be preferentially dissolved. The relative areas of anode and cathode in contact with the electrolyte are important here, most dangerous being a situation where the area of the anode is much smaller than that of the cathode leading to a high local intensity of anodic attack. For example, it would be unwise to fasten a steel plate with aluminium bolts or rivets since, under corrosive conditions rapid attack of the bolts will occur.
Galvanic corrosion can be employed deliberately to reduce the corrosion of metals such as iron by coupling them to more reactive ones (such as Zn or Al) which corrode ‘sacrificially’. In terms of their susceptibility to galvanic attack in seawater, metals and alloys have been empirically grouped as follows:
Galvanic series for alloys immersed in seawater Titanium alloys
Nickel alloys Stainless steels Silver alloys
Metals and alloys 127 Copper alloys
Lead and tin alloys Cast irons
Structural steels Cadmium Zinc alloys Aluminium alloys Magnesium alloys
Galvanic effects are negligible between alloys in the same groups, but become increasingly more pronounced with alloys that are widely separated in the table. Galvanic corrosion may be remedied by preventing electrical contact between the two metals by means of bushes or washers, for example.
Controlling corrosion arising from differential aeration may be more complicated and there are various means of effecting this.
Soluble inhibitors. In closed systems (e.g. when the corroding liquid is recirculated), the liquid may be treated with soluble inhibitors, which are of two main categories: the first is a reagent that removes oxygen from the solution, and the second is one that leads to the formation of a passive film on the surface of the metal, thus stifling attack.
Cathodic protection. An example of this process is sacrificial protection referred to earlier, whereby the metal to be protected is connected electrically to a more reactive metal in the galvanic series. Galvanizing steel with a layer of zinc works in this way, as do slabs of Zn, Al or Mg that are attached at intervals to buried steel pipelines or to marine structures. An alternative method of obtaining cathodic protection is to use an impressed current from a suitable dc source: the steel to be protected is connected to the negative terminal and an inert metal anode is placed nearby.
Paints and lacquers. The main aim of these surface coatings is to exclude water and air from the metal surface. In many cases, the exclusion is not total and a paint layer may be regarded as introducing a large ionic resistance into the corrosion cell, thus reducing the corrosion current and hence the rate of attack. Some pigments used (such as red lead, Pb3O4) may act as inhibitors, while others (such as primers containing metallic zinc powder) are essentially sacrificial pigments.
Weathering steels are structural steels in which the resistance to atmospheric corrosion has been improved by the addition of small amounts of elements such as copper, phosphorus, silicon and chromium. These steels rust at a lower rate than plain carbon steels and, under favourable climatic conditions, they can develop a relatively stable layer of hydrated iron oxide which retards further attack. This can provide cost savings by eliminating the
Materials for engineering 128
initial painting operation and subsequent maintenance work. They have not been used extensively in the UK because frequent rain inhibits the formation of the stable oxide layer and rusting continues (athough it is at a reduced rate). Long dry summer periods are desirable in order to develop the adherent oxide layer.