| Galvanic Corrosion -
Historical: One of the earliest documented incidences of
serious galvanic corrosion occurred in the eighteenth century on the Royal Navy's frigate,
H.M.S. Alarm. This vessel had its wooden hull covered by copper sheathing, which was in
turn attached by iron nails.
One of the purposes of this copper covering was to limit marine
biofouling, plaguing many materials immersed in sea water. Reduced speed and
maneuverability of ships are consequences of such growths. Copper generally acts as a
biocide, limiting such fouling.
Not surprisingly, the iron fasteners in contact with the copper were
subject to rapid galvanic corrosion (refer to galvanic series)
and led to detachment of some sheathing. For a more details refer to the account by
Trethewey and Chamberlain referenced below. They describe an interesting insulating
feature that probably prevented more widespread damage. The small anode (iron) : cathode
(copper) area ratio was obviously particularly unfavorable to the iron, if the galvanic
current density is considered (corrosion rate is proportional to current density).
| The Dreaded Teredo Worm...... Another
reason for applying copper sheathing over wooden hulls was the risk of attack on wood by
the Teredo worm (shipworm). This wood boring marine organism is well known and still
causes damage to submerged wooden structures. The ancient Roman navy is reported to have
used lead cladding on its ships to minimize this type of perforation.
From Polished Oak to Plastic......
In their book "Corrosion for Science and Engineering", K.R.
Trethewey and J. Chamberlain use the above galvanic corrosion example to show that many
corrosion lessons are forgotten and later repeated (see also the
corrosion cycle). They mention fundamentally similar reports falling on a polished oak
desktop of the Admiralty in 1763 and much later on the plastic desktop of the
corresponding modern Ministry.
An Unflattering Cartoon.....
Early in his military career, a distinguished British Admiral was
reportedly evicted from H.M.S. Alarm by the ship's captain, for his complicity in
producing an unflattering cartoon of the captain's mistress. |
References/Literature:
K.R. Trethewey and J. Chamberlain: Corrosion for Science and Engineering,
Second Edition, Longman, 1995.
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