Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte. Conversely, a galvanic reaction is exploited in primary batteries to generate a voltage. A common example is the carbon-zinc cell where the zinc corrodes preferentially to produce a current. The lemon battery is another simple example of how dissimilar metals react to produce an electric current.
When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.
Even a single type of metal may corrode galvanically if the electrolyte varies in composition, forming a concentration cell.
A common example of galvanic corrosion is the rusting of corrugated iron sheet, which becomes widespread when the protective zinc coating is broken and the underlying steel is attacked. The zinc is attacked preferentially because it is less noble, but when consumed, rusting will occur in earnest. With a tin can, the opposite is true because the tin is more noble than the underlying steel, so when the coating is broken, the steel is attacked preferentially.
A rather more spectacular example occurred in the Statue of Liberty when regular maintenance in the 1990s showed that galvanic 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 of shellac between the two metals failed over a period of time and resulted in rusting of the iron supports. The renovation replaced 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 for what is considered a national US symbol.
An earlier example occurred in the Royal Navy frigate HMS Alarm. The wooden hull of the vessel had been sheathed in copper to prevent attack by barnacles. It was soon discovered that the sheathing had become detached from the hull in many places because the iron nails which had been used to fasten the copper to the timbers had been ‘much rotted’. Closer inspection revealed that some nails, which were less corroded, were insulated from the copper by brown paper which was trapped under the nail head. The copper had been delivered to the dockyard wrapped in the paper which was not removed before the sheets were nailed to the hull. The obvious conclusion therefore, and the one which was contained in a report to the Admiralty of 1763, was that iron should not be allowed direct contact with copper in a sea water environment if severe corrosion of the iron was to be avoided. Later ships were designed with this in mind. Not only was sea water a very good electrolyte owing to its high salt concentration, but attack of the nails was encouraged by their very small exposed area compared with that of the copper-sheathed hull.
Galvanic corrosion is of major interest to the marine industry. Galvanic series tables for seawater are commonplace due to the extensive use of metal in shipbuilding. It is possible that corrosion of silver brazing in a salt water pipe might have caused a failure that lead to the USS Thresher sinking with all men lost.
The common technique of cleaning silver by immersion of the silver and a piece of aluminium in a salt water bath (usually sodium bicarbonate) is an example of galvanic corrosion. (Care should be exercised for reasons such as this will strip silver oxide from the silver which may be there for decoration. Use on plated silver is inadvisable as this may introduce unwanted galvanic corrosion with the base metal.)
Cathodic protection uses one or more sacrificial anodes made of a metal which is more active than the protected metal. Metals commonly used for sacrificial anodes include zinc, magnesium, and aluminium. This is commonplace in water heaters. Failure to regularly replace sacrificial anodes in water heaters severely diminishes the life time of the tank. Water softeners tend to degrade these sacrificial anodes and tanks more quickly.
For example, consider a system is composed of 316 SS (a 300 series stainless steel; it is a very noble alloy meaning it is quite resistant to corrosion and has a high potential) and a mild steel (a very active metal with lower potential). The mild steel will corrode in the presence of an electrolyte such as salt water. If a sacrificial anode is used (such as a zinc alloy, aluminium alloy, or magnesium), these anodes will corrode, protecting the other metals. This is a common practice in the marine industry to protect ship equipment. Boats and vessels that are in salt water use either zinc alloy or aluminium alloy. If boats are only in fresh water, a magnesium alloy is used. Magnesium has one of the highest galvanic potentials of any metal. If it is used in a salt water application on a steel or aluminium hull boat, hydrogen bubbles will form under the paint, causing blistering and peeling.
A "lasagna cell" or "lasagna battery" is accidentally produced when salty food such as lasagna is stored in a steel baking pan and is covered with aluminum 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 aluminum.
This metal corrosion occurs because whenever two metal sheets composed of differing metals are placed into contact with an electrolyte, the two metals act as electrodes, and an electrolytic cell or battery is formed. In this case, the two terminals of the battery are connected together. Because the aluminum foil touches the steel, this battery is shorted out, a significant electric current appears, and rapid chemical reactions take place on the surfaces of the metal in contact with the electrolyte. In a steel/salt/aluminum battery, the aluminum is higher on the electrochemical series, so the solid aluminum turns into dissolved ions and the metal experiences galvanic corrosion.
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