Active corrosion protection
When it comes to active corrosion protection, experts differentiate between two variants.
Corrosion inhibitors’ task is to significantly reduce the rate of corrosion of the metal to be protected. The mechanism for doing this can be physical or chemical in nature. Depending on reaction, corrosive agents such as oxygen and chloride can be chemically bound by sodium sulphite and hydrazine and thus taken out of the corrosion system. Other inhibitors, called adsorption inhibitors, are used in crude oil processing. They are organic substances, such as imidazolines or quinolines, that through the way they work prevent corrosion by hydrochloric acid or hydrogen sulphide. Pickling inhibitors on the other hand reduce ferrous decomposition and thus hydrogen embrittlement when descaling and removing rust from steel.
Cathodic corrosion protection (CCP)
Use is made here of the principle of metals differing electrochemical potentials. The less noble metal has a more electron-negative potential than the metal to be protected. In order to protect iron from oxidation in this way, zinc or zinc alloys are often used, providing cathodic corrosion protection.
The protective mechanism: In the event of any corrosive attack, zinc gets oxidised in place of the iron due to the former’s low electrochemical potential. The zinc positively ‘sacrifices’ itself, which is why experts also talk of a ‘sacrificial anode’. ‘White rust’ continues to occur until the zinc is used up. White rust consists predominantly of zinc hydroxide (Zn(OH)2).
This is also the protective mechanism that zinc flakes use. Such flakes are surrounded by a hybrid bonding agent matrix, which both ensures the necessary electrical conductivity and also fulfils an additional physical barrier characteristic. This stops corrosive media from corroding the part. In this way a very high level of corrosion protection is achieved even by extremely thin layers. Protective coats made up of zinc flakes are between 5 and 15 µm thick. They are used for example in the steel-working industry and above all, in the automotive industry in order to protect all kinds of components.
Current from an external source
External current can also protect base metals from corrosive material removal. The external current compensates for the voltage drop of the galvanic element, made of metal and soil, resulting in only a negligible amount of metal removal. This method is often used for structures such as bridges, pipelines laid in the earth and containers such as hot water tanks made of steel.