Electrochemical corrosion is the most common type of corrosion. Electrochemical corrosion occurs when the metal is in contact with the surrounding electrolytically conductive medium. In this case, the reduction of the oxidizing component of the corrosive medium does not occur simultaneously with the ionization of the metal atoms and their rates depend on the electrode potential of the metal. The primary cause of electrochemical corrosion is the thermodynamic instability of metals in their environment. Rusting of pipelines, the upholstery of marine vessels, and various metal structures in the atmosphere are, and many more, examples of electrochemical corrosion.
Electrochemical corrosion includes such types of local damage as pitting, intergranular corrosion, and crevice corrosion. In addition, electrochemical corrosion processes occur in soil, atmosphere, and sea.
The mechanism of electrochemical corrosion can proceed in two ways:
1) Homogeneous mechanism of electrochemical corrosion:
- The surface layer of the meth. is regarded as homogeneous and homogeneous;
- Metal dissolution is caused by the thermodynamic possibility of cathodic or anodic acts;
- K and A sites migrate along the surface in time;
- the rate of electrochemical corrosion depends on the kinetic factor (time);
- The homogeneous surface can be considered as a limiting case, which can be realized in liquid metals as well.
2) Heterogeneous mechanism of electrochemical corrosion:
- In solid metals, the surface is nonhomogeneous because different atoms occupy different positions in the crystal lattice in the alloy;
- heterogeneity is observed in the presence of foreign inclusions in the alloy.
Electrochemical corrosion has some features: it is divided into two simultaneous processes (cathodic and anodic), which are kinetically dependent on each other; in some areas of the surface electrochemical corrosion can take a local character; dissolution of basic metal occurs precisely on the anodes.
The surface of any metal consists of a number of microelectrodes short-circuited through the metal itself. In contact with the corrosive environment the galvanic elements formed contribute to its electrochemical destruction.
The causes of local galvanic elements may be very different:
1) inhomogeneity of the alloy.
- Met. phase inhomogeneity, caused by the inhomogeneity of the alloy and the presence of micro- and macroinclusions;
- Non-uniformity of oxide films on the surface due to the presence of macro- and micropores, and non-uniform formation of secondary products of corrosion;
- Presence of crystal grain boundaries on the surface, dislocation out on the surface, anisotropy of crystals.
2) heterogeneity of the medium
- The area with limited access of the oxidant will be an anode in relation to the area with free access, which accelerates the electrochemical corrosion.
3) heterogeneity of physical conditions
- irradiation (irradiated area is an anode);
- External currents influence (place of stray current input - cathode, place of output - anode);
- temperature (in relation to cold areas, heated areas are anodes), etc.