What is Galvanic Corrosion?
The scientific definition of galvanic corrosion describes it as an electrochemical process in which two different metals or alloys come in electrical contact with each other in the presence of an electrolyte under the conditions of a corrosive environment, which leads to relative corrosion of both the materials in contact.
Rusting of iron is the most common example of galvanic corrosion, in which the core steel of the iron sheet is attacked and the protective coating of zinc is destroyed. Zinc being less noble is susceptible to the galvanic attack and once fully exterminated; it can cause the base metal to corrode very quickly. On the other hand, tin being more noble than the core steel makes it harder to break, but when it does, the underlying steel suffers the worst.
The mechanism behind galvanic corrosion
The process of galvanic corrosion is only possible when two different metals and alloys having different electrode potentials come in contact with each other. The less noble metals form the anode, while nobler metals take the cathode position. The difference in electrode potential accelerates the attack on the anode, which later gets dissolved into an electrolyte and deposits are collected on the cathode, which in this case, is a metal.
The electrolytes trigger the migration of ion from the anode to the cathode, which causes the anode metal to corrode quickly while inhibiting the effect of corrosion on the cathode metal. Availability of electrolyte ensures that the ions keep migrating in order to allow the galvanic corrosion to take place.
Even though it’s not a desirable process, it has a few applications. In primary batteries, carbon-zinc cells are intentionally provided to facilitate the preferential corrosion of zinc which produces electrical voltage. Another application is the preservation of buried structures by cathodic method, in which the anode materials undergoes corrosion to inhibit the corroding effect on the cathode metal.
How to prevent galvanic corrosion
The best way to avoid the contact between two metals and alloys is to carry electrical insulation of the same. You can use non-conductive materials such as glass, rubber or plastic in between the metals of different electrode potentials. For piping with aluminum rods, one can use a roll of plastic pipe to coat the interior.
Electroplating techniques can be employed, in which the nobler metals such as silver, gold or nickel is used to form the anode. 316 stainless steel studs can be protected by galvanizing with zinc, which is used as a sacrificial anode.
Metals having similar electrode potentials or at least near to each other are less likely to form a galvanic couple. Utilizing similar metals for all sorts of productions is the simplest way to match the electropotentials.
Galvanic corrosion cannot take place in the absence of an electrolyte. You can employ greasing or oil-painting of metals to eliminate the chances of contact with an electrolyte. In case it’s not feasible to provide coating to both the metals, coat the one which is more noble so that when the corrosion takes place; there will be a smaller anode area suffering the maximum effect of corrosion.
