Since inhibitors are used to minimize acid corrosion on metal, understanding the use of acid inhibitors requires a basic understanding of the nature of the corrosion process on metal.
The Basics: What is Corrosion?
Corrosion is essentially an electrochemical process. Any ferrous metal consists of iron, various other alloying elements (manganese, phosphorous, sulfur, carbon, silicon, copper, nickel, chromium, aluminum), and impurities such as sulphides, phosphides, etc.
The Mechanics: How Does Corrosion Occur?
Between any two constituents of steel there is a definite electro-potential difference. In an acid solution, this potential difference within the metal induces a flow of electrons to a developing cathode on the metal surface, with the formation of anodes where the metal is dissolved.
At the cathode, hydrogen is evolved from the reduction of hydrogen ions. After the acid dissolves the oxides and other deposits (see the reactions listed below) the acid completes the circuit on the metal surface. Hydrogen is evolved and metal is dissolved.
Some reactions for scale and deposit removal:
CaCO3 + 2 HCI —–> CaCI2 + C02 T + H20
FeO + 2 HCI ——–> Fe C12 + H20
Fe3O4 + 6 HCI + H2—->3 Fe C12 + 4 H20
An example of a corrosion cell on a metal surface:
The carbon in steel is present as iron carbide, Fe3C, or cementite. There is a potential between iron carbides and iron, in which the iron is the anode and the carbide is the cathode. The acid in this case attacks the iron, leaving the cathode untouched.
The oxidation reaction at the anode is:
Fe –> Fe2+ + 2e-
The reduction at the cathode is:
2H+ + 2e- H’ + H’–> H2t
If the amount of hydrogen being evolved is significant, the base metal can absorb the gas and change the physical properties of the alloys, causing hydrogen embrittlement.
Hydrogen embrittlement is the process by which metals such as steel become brittle and fracture due to the introduction and subsequent diffusion of hydrogen into the metal. This is often a result of accidental introduction of hydrogen during forming and finishing operations. This phenomenon was first described in 1875.