A metal in contact with an electrolyte will ionize slightly until reached the equilibrium, e.g.
The metal will then possess a certain electrical charge. Different metals ionize to different extents and will possess different values of electrical charge at equilibrium. The electrical charge or potential difference between a metal electrode and a standard hydrogen electrode is called the “standard electrode potential” for the metal. In general, metals with negative values of electrode potential are more active than metals with positive electrode potentials.
Galvanic cell: anode and cathode
Corrosion of most metals is of the galvanic type. A galvanic cell is set up when two metals, of different electrode potentials, are in contact with a same electrolyte and are also electrically connected external to the electrolyte. An example is copper and zinc. Copper with the higher electrode potential series will become the “cathode” and zinc having the lower electrode potential will become the “anode”. In the circuit external to the electrolyte, electrons will flow from the anode to the cathode. The anode will tend to ionize continuously with this removal of electrons. The galvanic cell becomes a “corrosion cell” where the anode is being corroded away in the electrolyte.
In a carrions osion cell, it is always the anode that is corroded away. That is the metal will go into the solution or electrolyte:
In some cases, (OH)- ions will be attracted to the anode and the metal ions react with (OH)- ions to form oxide, hydroxide or salt of the metal. A common example is the rusting of iron in water, which contains dissolved oxygen. The compound, ferric hydroxide, Fe(OH)3, is “rust”:
Fe^(2+)+1/2 O_2+H_2 O→Fe^(2+)+2OH^-→Fe〖(OH)〗_2
2Fe〖(OH)〗_2+1/2 O_2+H_2 O→2Fe〖(OH)〗_3
At the cathode, a number of possibilities will occur. Metal may be deposited (which is seldom in corrosion):
Hydrogen may be evolved:
Or in the presence of dissolved oxygen, hydroxyl ions may be formed:
Galvanic corrosion cells: common types
Corrosion cells may be set up in a number of ways. The main types are:
May be set up between two dissimilar metals
E.g. a steel pipe connected to copper plumbing, and a steel properller shaft in a brass bearing. In both cases, steel becomes the anode and corrodes away. Microscopic composition cells can be set up in a two-phase alloy when exposed to an electrolyte. In pearlite (steel), for instance, the ferrite is anodic with respect to the cementite. A pure metal or a single-phase alloy thus possesses better resistance to corrosion than an impure metal or a multi-phase alloy.
Metal atoms within a highly stressed region will have a higher tendency to ionize than their unstressed counterparts.
A stress cell may be established in a component where the stress distribution is uneven. In cold-worked component, residual stresses exist in the cold-worked portion and a stress cell may be set up between the cold-worked portion and the non-worked portion of the metal. The cold-worked portion is the anode subjected to corrosion. The grain boundary zone of a metal is under stresses and tends to be anodic with respect to the crystal grain. A coarse-grained metal is thus more corrosion resistance than a fine-grained sample of the same metal.
A metal in contact with a concentrated electrolyte solution will ionize to a smaller extent than when it is in contact with a dilute electrolyte. In situations like electrolyte flowing in metal pipes or ducts, the portion in contact with the dilute electrolyte will be anodic with respect to the portion in contact with the more concentrated solution.
Of widespread importance are oxidation-type concentration cells which can occur of there are variations in dissolved oxygen content throughout an electrolyte. For example, when a moist metal surface is exposed to air, corrosion has a tendency to occur in the region where there is a deficiency in dissolved oxygen content. The oxygen rich region becomes the cathode because oxygen is being reduced there:
The electrons are being removed from the oxygen-deficient region where the metal there becomes the anode and ionizes. Examples of oxygen-deficient areas are cracks or devices, or metal surfaces covered by dirt or other contaminations.
It is almost impossible to prevent corrosion completely but it is possible to delay or minimize corrosion. The commonest technique is the application of a protective coating to the metal surface so as to isolate the metal from contact with any electrolyte. Surface coatings can be paints, plastics or a layer of another metal or oxide coatings. Composition cells can be avoided by preventing two dissimilar metals from contact. Washers are used when copper plumbing is joined to steel pipes.
Aluminium, chrominum, nickel, tin and zinc are used for coating steels. The coating metal may either be anodic or cathodic with respect to the underlying steel. If the coating is cathodic such as tin on steel, any break on the coating will establish a galvanic cell with the anodic steel being corroded away. On the other hand, if zinc is used as the coating, the galvanic cell formed with a break on the coating will have the zinc as anode. While the zinc coating is corroded, the steel continues to be protected. This is the benefit of galvanizing of steel.
An oxide film can sometimes offer corrosion resistance to the underlying metal. Anodizing of aluminium is a common process that uses an electrical current to force the formation of a thick Al2O3 film on the aluminium. The oxide film has a better corrosion resistance and is itself tightly bounded and also bonds well to the underlying aluminium, stainless steels, formed by adding chromium to steel, is highly resistant to corrosion because the chromium readily oxidizes to form a protective layer of passive chrome-oxide film on the steel surface. However, under conditions where no oxygen is present, the oxide layer may break down and the steel is no longer protected.
Another method of corrosion control is cathodic protection. Galvanic cells are deliberately created where the metal to be protected is made the cathode and the anode is to be sacrificed. In galvanized steels, the zinc coating is the sacrificial anode. Zinc and magnesium are common sacrificial anodes used to protect ships’ hulls and buried steel pipes. An impressed current can also make the
Thursday, 15 January 2009