Galvanic corrosion

Article

July 5, 2022

Galvanic corrosion is a phenomenon in which corrosion of one of the metals is promoted when different types of metals come into contact with each other and undergo electron conduction in a corrosive environment such as an electrolytic solution. be. It is caused by the formation of corrosive batteries between two metals and the environment. Also called galvanic corrosion. Although it cannot be said unconditionally due to the influence of various factors, basically, when galvanic corrosion occurs, corrosion is accelerated by the metal with the lower (base) natural potential in the environment. Ideally, dissimilar metals should not be brought into contact with each other as a countermeasure against galvanic corrosion, but in reality, there are many situations where dissimilar metals need to be contacted. Other measures include reducing the difference between the natural potentials of the two metals, making the surface area of ​​the noble metal relatively smaller than the surface area of ​​the base metal, and sandwiching an insulating material such as rubber or resin. And so on. When galvanic corrosion occurs between dissimilar metals, on the contrary, corrosion is suppressed in the metal with the higher natural potential (noble). By utilizing this, if a low-key material is intentionally added to the material to be protected from corrosion, the anticorrosion effect can be obtained. Such an anticorrosion method is known as a sacrificial anode method.

Basic mechanism

When two kinds of metals are brought into contact with each other in the electrolytic solution, a battery is formed between the two kinds of metals and the electrolytic solution, and an electric current flows. When iron and copper come into contact with each other in an aqueous solution, current flows in the order of "copper-> iron-> aqueous solution-> copper". At this time, the flowing current means the progress of corrosion, and the battery formed at the time of corrosion is called a corroded battery or the like. In the case of iron and copper, iron corrosion is promoted. That is, the corrosion caused by the contact with copper is added to the corrosion caused by the simple substance of iron with respect to the aqueous solution. Such a phenomenon in which corrosion of one metal accelerates when dissimilar metals come into contact with each other under an electrolytic solution is called galvanic corrosion. Galvanic corrosion occurs mainly near the contact area, and galvanic corrosion is regarded as a type of local corrosion. The current flowing through contact corrosion of dissimilar metals is also called galvanic current, and the two metals that make up contact corrosion of dissimilar metals are also called galvanic pairs. Contact corrosion of dissimilar metals is also called galvanic corrosion or galvanic corrosion. Although it is called dissimilar metal contact corrosion, materials such as carbon fiber reinforced plastics can also cause galvanic corrosion (dissimilar metal contact corrosion) by the same principle. Carbon fiber contained in carbon fiber reinforced plastic conducts electricity and causes galvanic corrosion. Galvanic corrosion is sometimes called galvanic corrosion, but in the field of corrosion engineering, the term galvanic corrosion refers only to stray current corrosion, and the term galvanic corrosion is incorrect to refer to galvanic corrosion. Which metal is corroded by galvanic corrosion is basically determined by the electrode potential when the metal is corroded by itself in the environment. Such an electrode potential when a metal is naturally corroded in a certain environment is called a natural potential or a corrosion potential. A metal with a higher natural potential than the other party is said to be noble, and a metal with a lower natural potential than the other party is said to be base. Galvanic corrosion occurs and corrosion is promoted by metals that are lower than their counterparts. The principle of galvanic corrosion of dissimilar metals is often explained by the difference in standard electrode potential (ionization tendency), which is inappropriate. The problem with galvanic corrosion of dissimilar metals is the difference in natural potential (corrosion potential). polarization