|Galvanic corrosion processes- Environmental deterioration|
When two dissimilar metals are in contact, or in close proximity with a conducting fluid in between, an electrochemical cell can be formed that leads to the more reactive metal becoming an anode and the less reactive metal a cathode. This kind of corrosion is known as galvanic corrosion. It is not uncommon, since metals are often coated with others of different E0, and different metals are often in close contact with a common electrolyte.
One of the earliest examples of galvanic corrosion was recorded in the eighteenth century. The wooden hull of the Royal Navy frigate HMS Alarm (Figure 1) had been covered by copper sheathing, which was attached to the hull by iron nails.
One of the purposes of the copper sheath was to limit marine biofouling, which is known to plague many materials that are immersed in sea water. The growth of molluscs such as barnacles on the hulls of ships, which can then trap trailing seaweed, results in reduced speed and manoeuvrability. Copper limits fouling by inhibiting the attachment of molluscs.
The hull was covered in 1761, and the copper sheath was found to be detached two years after fitting, during which time the Alarm had visited the Caribbean and elsewhere. The iron nails were found mostly to have corroded. Some nails remained intact, however, where their brown paper wrapping had remained in place between the copper and the iron, a fortuitous event that prevented total detachment of the sheath. The iron nails in contact with the copper were subject to rapid galvanic corrosion that led to detachment of the sheathing. The small anode (iron nails) to cathode (copper sheet) area ratio favoured the loss of the iron, as the rate of corrosion is directly proportional to the current density (a measure of electron flow). In a sense, the nails acted as local electron concentrators, so attack was rapid. Where it was present, the brown paper insulated the nails and so there was insufficient electron flow to cause corrosion.
Polytetrafluoroethylene (PTFE) is a synthetic polymer that has an extremely low coefficient of friction and is very non-reactive.
The reason why marine environments are especially pernicious is the salt content of sea water. The presence of sodium and chloride ions increases the electrical conductivity compared with pure water, so galvanic or other cells formed between dissimilar metals react much faster.
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