A common example of galvanic corrosion occurs in
galvanized iron, a sheet of iron or steel covered with a zinc coating. Even when the protective
zinc coating is broken, the underlying
steel is not attacked. Instead, the zinc is corroded because it is less "noble". Only after it has been consumed can rusting of the base metal occur. By contrast, with a conventional
tin can, the opposite of a protective effect occurs: because the
tin is more noble than the underlying steel, when the tin coating is broken, the steel beneath is immediately attacked preferentially.
Statue of Liberty A spectacular example of galvanic corrosion occurred in the
Statue of Liberty when regular maintenance checks in the 1980s revealed that corrosion had taken place between the outer
copper skin and the
wrought iron support structure. Although the problem had been anticipated when the structure was built by
Gustave Eiffel to
Frédéric Bartholdi's design in the 1880s, the insulation layer of
shellac between the two metals had failed over time and resulted in rusting of the iron supports. An extensive renovation was carried out with replacement of the original insulation with
PTFE. The structure was far from unsafe owing to the large number of unaffected connections, but it was regarded as a precautionary measure to preserve a national symbol of the United States.
Royal Navy and HMS Alarm In 1681,
Samuel Pepys (then serving as
Admiralty Secretary) agreed to the removal of lead sheathing from English
Royal Navy vessels to prevent the mysterious disintegration of their rudder-irons and bolt-heads, though he confessed himself baffled as to the reason the lead caused the corrosion. The problem recurred when vessels were sheathed in copper to reduce marine weed accumulation and protect against
shipworm. In an experiment, the Royal Navy in 1761 had tried fitting the hull of the frigate
HMS Alarm with 12-ounce copper plating. Upon her return from a voyage to the West Indies, it was found that although the copper remained in fine condition and had indeed deterred shipworm, it had also become detached from the wooden hull in many places because the iron nails used during its installation "were found dissolved into a kind of rusty Paste". To the surprise of the inspection teams, however, some of the iron nails were virtually undamaged. Closer inspection revealed that water-resistant brown paper trapped under the nail head had inadvertently protected some of the nails: "Where this covering was perfect, the Iron was preserved from Injury". The copper sheathing had been delivered to the dockyard wrapped in the paper which was not always removed before the sheets were nailed to the hull. The conclusion therefore reported to the Admiralty in 1763 was that iron should not be allowed direct contact with copper in sea water.
US Navy littoral combat ship Independence Serious galvanic corrosion has been reported on the latest US Navy attack littoral combat vessel the
USS Independence caused by steel water jet propulsion systems attached to an aluminium hull. Without electrical isolation between the steel and aluminium, the aluminium hull acts as an anode to the stainless steel, resulting in aggressive galvanic corrosion.
Corroding lighting fixtures The unexpected fall in 2011 of a heavy light fixture from the ceiling of the
Big Dig vehicular tunnel in
Boston revealed that corrosion had weakened its support. Improper use of aluminium in contact with stainless steel had caused rapid corrosion in the presence of salt water. The electrochemical
potential difference between stainless steel and aluminium is in the range of 0.5 to 1.0V, depending on the exact alloys involved, and can cause considerable corrosion within months under unfavorable conditions. Thousands of failing lights would have to be replaced, at an estimated cost of $54 million.
Lasagna cell A "
lasagna cell" is accidentally produced when salty moist food such as lasagna or sauerkraut is stored in a steel baking pan and is covered with aluminium foil. After a few hours the foil develops small holes where it touches the lasagna, and the food surface becomes covered with small spots composed of corroded aluminium. In this example, the salty food (lasagna) is the electrolyte, the aluminium foil is the anode, and the steel pan is the cathode. If the aluminium foil touches the electrolyte only in small areas, the galvanic corrosion is concentrated, and corrosion can occur fairly rapidly. If the aluminium foil was not used with a dissimilar metal container, the reaction was probably a chemical one. It is possible for heavy concentrations of salt, vinegar or some other acidic compounds to cause the foil to disintegrate. The product of either of these reactions is an
aluminium salt. It does not harm the food, but any deposit may impart an undesired flavor and color.
Electrolytic cleaning The common technique of cleaning
silverware by immersion in a hot electrolytic bath with a piece of aluminium is an example of galvanic corrosion. Aluminium foil is preferred because of its much greater surface area than that of ingots, although if the foil has a "non-stick" face, this must be removed with steel wool first. The electrolytic bath is usually composed of water and
sodium bicarbonate, i.e., household baking soda. Silver darkens and corrodes in the presence of airborne sulfur molecules, and the copper in sterling silver corrodes under a variety of conditions. These layers of corrosion can be largely removed through the electrochemical reduction of silver sulfide molecules: the presence of aluminium (which is less noble than either silver or copper) in the bath of sodium bicarbonate strips the sulfur atoms off the silver sulfide and transfers them onto and thereby corrodes the piece of aluminium (a much more reactive metal), leaving elemental silver behind. No silver is lost in the process. == Prevention ==