The localized corrosion occurs locally only on some parts of the metal surface and it can be due:
- Matching of materials of a different nature (corrosion galvanic);
- Environment not homogeneous, for example the difference in oxygen content or variations of potential;
- Presence of areas covered by deposits, cracks, fissures (crevice corrosion);
- Phenomena of turbulence, abrasion, cavitation, friction (fretting);
- Presence of substances that can lead to craters, ulcers, pinholes (pitting corrosion).
Figure 1 Galvanic corrosion of helix
Figure 2 Galvanic corrosion of hanger
This type of corrosion occurs when two or more metallic materials or in any case with electronic conductivity (graphite, conductive oxides or sulfides, etc;), are in contact with each other. During a galvanic corrosion the speed of damage increases, and it depends in large measure on their distance in the galvanic series (Figure 3) according to specific environment, the characteristics of the polarizability of the materials in that environment, the ratio between anodic areas and cathodic materials. In the latter case, if the anode area decreases compared to the cathodic area, the dissolution process becomes faster, resulting in a loss of material higher in the same time (in years). In the galvanic series, the more noble metal acts as a cathode and the less noble one as the anode, dissolving. It is possible to sort the metals according to the corrosion potential free (nobility practice in that environment) when they are in contact with a given electrolyte. The free corrosion potential is the potential assumed by the material when it is not electrically connected to the circuit but interacts freely with the environment in which it is located.
Figure 3 Galvanic series in seawater at 25°C
Generally, the galvanic corrosion is very dangerous when the materials are far apart each other in the galvanic series although the scale of the attack may depend to a greater extent by kinetic factors which then intervene in the process. The process of galvanic corrosion takes place even when the surface of a material contains impurities of another metal and is created a stack short-circuited, with passage into solution of the less noble metal (this also happens also between metals and oxides conductors; the case iron / flakes of magnetite is the most common). Suppose you have an impurity of iron on copper surface. Looking at the table of standard potentials (Figure 4), being ECu++ /Cu = 0,34 V and EFe++/Fe = -0,44V the less noble metal (Fe) sends in solution Fe2+ ions according to the half-reaction: Fe ⇒ Fe2+ + 2e–. The electrons migrate towards the material that is the most electropositive, Cu, where both Fe2+ both O2 in the air could be reduced. The reduction of O2 according to the half-reaction: O2 + 2H2O + 4 e– = 4 OH– has a standard potential equal to 0,40V; the oxidizing power is therefore much greater than that of the couple Fe2+ / Fe. On the cathode (Cu), there will be the production of hydroxyl ions OH– with possible precipitation of Fe(OH)2. This reaction catches Fe2+ ions from the solution, and facilitating the passage of new metal ions in solution.
To avoid the development of this process, it is possible to act:
- Eliminate dangerous metal contact where the difference of nobility is very high
- Electrical insulation
- Reduction cathodic areas / anodic areas
- Cathodic protection
Figure 5 Reducing the difference of nobility
Figure 6 Painting on Stainless Steel
Figure 7 Electric Insulation