Galvanic Corrosion

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Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is in contact with a different type of metal and both metals are in an electrolyte.

When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic cell is set up as different metals have different electrode potentials. The electrolyte provides a means for ion Electromigration 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.

Metals (including alloys) can be arranged in a galvanic series representing the potential they develop in a given electrolyte against a standard reference electrode. The relative position of two metals on such a series gives a good indication of which metal is more likely to corrode more quickly. However, other factors such as water aeration and flow rate can influence the process markedly.

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 (SSN-593) 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.)

Preventing galvanic corrosion There are several ways of reducing and preventing this form of corrosion. One way is to electrically insulate the two metals from each other. Unless they are in electrical contact, there can be no galvanic couple set up. This can be done using plastic or another electrical insulation to separate steel water pipes from copper-based fittings or by using a coat of grease to separate aluminium and steel parts. Use of absorbent washers that may retain fluid is often counter-productive.

Another way is to keep the metals dry and/or shielded from ionic compounds (salts, acids, bases), for example by painting or encasing the protected metal in plastic or epoxy.

It is also possible to choose metals that have similar potentials. The more closely matched the individual potentials, the lesser the potential difference and hence the lesser the galvanic current. Using the same metal for all construction is the most precise way of matching potentials.

Electroplating or other plating can also help. This tends to use more noble metals that resist corrosion better. Chrome plating, nickel, silver and gold can all be used.

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.

Finally, an electrical power supply may be connected to oppose the corrosive galvanic current. (see Cathodic protection#Impressed Current CP)

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 low galvanic potential) and a mild steel (a very active metal with high galvanic 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.

Factors that influence galvanic corrosion

Relative size of anode and cathode – As it is the anode that corrodes more quickly, the larger the anode in relation to the cathode, the lesser the corrosion. Conversely, a small anode and a large cathode will see the anode readily damaged. Painting and plating can alter the exposed areas.
Aeration of seawater – Poorly aerated water can affect stainless steels, moving them more towards the anodic end of a galvanic scale.
Degree of electrical contact – The greater the electrical contact, the easier for a galvanic current to flow.
Electrical resistance of electrolyte – Resistance in the electrolyte will decrease the current.
Range of individual potential difference – It is possible that different metals could overlap in their range of individual potential differences. This means that either of the metals could act as the anode or cathode depending upon the other conditions that affect the individual potentials.
Covering by bio-organisms – Slimes that build up on metals can affect the areas exposed as well as limiting flow rate, aeration, and altering pH.
Oxides – Some metals may be covered by a thin layer of oxide that is less reactive than the bare metal. Cleaning the metal can strip this oxide and thus increase reactivity.
Humidity – Can affect the electrolytic resistance and transport ions.
Temperature – Temperature can affect the rate resistance of metals to other chemicals. For example, higher temperatures tend to make steels less resistant to chlorides.
Type of electrolyte – Exposing one piece of metal to two different electrolytes (either different chemicals or concentrations) can cause a galvanic current to flow within the metal.

== Lasagna cell ==

A "lasagna cell" or "lasagna battery (electricity)" 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 Standard electrode potential (data page), so the solid aluminum turns into dissolved ions and the metal experiences galvanic corrosion.

References

The Straight Dope: why does ketchup dissolve aluminum?
PIRA physics lecture demonstration 5e40.25