Iron bolts galvanically corroding.
Image credit: designbydx / Shutterstock.com
Galvanic corrosion may occur when two dissimilar metals are in contact with one another in the presence of an electrolyte creating an electronic pathway for the movement of electrons (such as water).
Similar to the process for producing electricity in batteries, the dissimilar metals will have different electrode potentials and so, when in contact, one metal will behave as an anode and the other as a cathode. The potential difference between the metals is the driving force behind the corrosive reaction.
The anodic metal will preferentially corrode at a rate determined by the magnitude of the potential difference between the metals and the surface areas of each metal.
An example of galvanic corrosion is a “lasagna cell”. This occurs when a piece of aluminum foil is used to cover the moist, salty lasagna cooked in a steel pan. Small holes will develop in the foil after only a few hours where it is in direct contact with the lasagna, and the food will have small pieces of corroded aluminum on its surface. Here, the salty food is the electrolyte, the aluminum foil is the anode and the steel pan the lasagna is cooked in is the cathode.
An example of a difference in the surface area being a big problem is if there is a large metal door bolted to a wall and the door is the cathodic metal and the small bolts are the anodic metal (in the presence of an electrolyte) then there is a far greater galvanic current, and so galvanic corrosion will occur at an accelerated rate.
Similarly, if using metals which have a large potential difference, galvanic corrosion will occur faster. This would all need to be in an environment where there is an electrolyte present that creates a conductive pathway for electron movement.
Using Table 1, we can see which metals are unsuitable to couple together. We can expect, using the same example as earlier, that if the door was made from gold, and the bolts from magnesium that the rate of corrosion (of the magnesium) would be incredibly high. If we swapped the materials around, making the bolts from gold and the door from magnesium, the anode would be far bigger, and so the rate of corrosion would be slightly slower.
Table 1. Anodic Index of metals. Data sourced from: Engineering Edge
Weather also has a large effect on the rate of galvanic corrosion. In “harsher” environments (typically outdoors; highly humid; salty environments) a potential difference of ~0.15V is sufficient enough to cause galvanic corrosion. In what is considered to be a “normal” environment (storage places; warehouses; humidity-controlled areas) there should not be a potential difference of more than ~0.25V, otherwise galvanic corrosion can occur.
In “controlled” environments (both temperature and humidity controlled) a difference of up to ~0.50V can be allowed, however close attention must be paid to avoid any small variations in either the temperature or the humidity as this can make a big difference and cause galvanic corrosion to initiate.
One of the most famous examples of galvanic corrosion is in the Statue of Liberty, New York. Built in 1886, the exterior of the statue was made from copper and the interior from cast iron. The large exterior skin was therefore cathodic and the interior a smaller anode, separated only by a thin asbestos skin impregnated with shellac, which eventually failed.
Statue of Liberty
Image credit: Joshua Haviv / Shutterstock.com
In 1984 the statue was shut down due to safety concerns as it was observed that some galvanic corrosion had occurred.
Upon removal of the paint surface on the copper skin, significant corrosion was found and it turned out that the torch famously held high in the air in the statue’s right hand had been leaking rain water into the structure.
The entire cast iron interior was removed and replaced with a low-carbon, corrosion resistant stainless steel.
There are many simple ways to prevent galvanic corrosion. The most effective way is ensuring that the two metals are not in contact, by electrically insulating them from one another.
No galvanic couple can occur as there is no electrical contact. This can be achieved using non-conductive materials between the metals in question, such as plastics or coatings.
Preventing the conductive path required for electron transport from forming is another effective way to protect metals from galvanic corrosion. If there is no electrolyte present then electrons cannot transfer between the metals.
This can be achieved through the use of oils, greases and other water-repellent compounds. Metals are often painted with a thick coat to prevent moisture from coming into contact with them, however if the paint chips or cracks exposing bare metal beneath, then corrosion may then occur.
Cathodic protection is often employed if it is too difficult to avoid the conditions where galvanic corrosion may occur, such as structures buried underground or under water. This is often done by using a sacrificial anode (where a more anodic material, often zinc, is attached to the structure which corrodes rather than the structure itself), but can also be done by applying a direct current (DC) electrical power supply which counteracts the galvanic current that causes corrosion.
Cathodic Protection – Galvanic/Sacrificial
Video sourced from: YouTube – CorrConnect’s Channel
Alessandro has a BEng (hons) in Material Science and Technology, specialising in Magnetic Materials, from the University of Birmingham. After graduating, he completed a brief spell working for an aerosol manufacturer and then pursued his love for skiing by becoming a Ski Rep in the Italian Dolomites for 5 months. Upon his return to the UK, Alessandro decided to use his knowledge of Material Science to secure a position within the Editorial Team at AZoNetwork. When not at work, Alessandro is often at Chill Factore, out on his road bike or watching Juventus win consecutive Italian league titles.
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Wich standard refers to this potential difference between metals in different climate?
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