Pitting corrosion near to the welded parts
Stainless Steel subjected to pitting corrosion
Pitting corrosion around a watch-case
The corrosion for pitting is a form that is part of the localized corrosion that can determine in a very short time the deterioration of the metallic part concerned through the formation of very deep cavities that can pierce the material while maintaining a form tapered or create very large caverns. This type of corrosion is considered among the most dangerous because, unlike the generalized corrosion, is not very visible to the naked eye on the material surface, it extends inside the material and the mechanical, optical and electrical characteristics designed for that particular activity decrease at an impressive rate.
Formation of caves by pitting
The phenomenon is characterized by the formation of localized anodic areas. These areas are much smaller than the area cathode which can be considered as the entire surface of the workpiece. This type is similar to corrosion galvanic contact because it creates the flow conditions of localized anodic currents of considerable density and hence a rapid piercing effect. The formation of pitting occurs only on particular types of metal materials subjected to certain conditions of corrosion. These metals are called “active-passive” behavior and include iron, nickel, aluminum, magnesium, zirconium, zinc, copper, tin, brass alloys and stainless steels. The corrosion process is activated when the material is present in solutions that contain specific ions (halides, perchlorates, etc.) such as sea water, very favorable to this type of corrosion environment. The pitting is generated through two different stages: the initiation and propagation / growth. The trigger occurs in correspondence of defects on the metal surface such as iron sulfide inclusions or absence / localized rupture of the passive layer that protects the surface from any corrosive attacks. This last statement can not be considered a general rule in that the trigger can also occur through the formation of adsorbed films that produce a slowing of generalized corrosion and determine the conditions for pitting corrosion process. The trigger can also happen on pure metals with a completely random distribution of the pit than the metallographic structure. The pit distribution depends on:
- Concentration and nature of the ions present in the solution;
- Nature of the cathodic process;
Visually you can see areas entirely without pit (cathodic areas) that feed the anodic reactions of the pit in development. After initiation, the pit is developed with autostimolante effect. The accretion process can give the pit different and unpredictable morphological aspects. The pit can follow the direction of gravity with a vertical growth, or tends to avoid very resistant surfaces mechanically (work-hardened).
From the kinetic point of view, the formation of pitting is represented by the following figure:
As you can see from the graph, once the pitting is triggered, the corrosion rate reaches very high values resulting in a deterioration of the workpiece in a short time. It can happen that some pit once triggered, do not continue in their penetrating action; They are extinguished because they don’t have the necessary conditions for their growth, such as the formation of another pit of greater activity that absorbs all the current supplied from the area surrounding the cathode. The corrosion rate for pitting is very much influenced by the reduction of metal ions with respect to the intervention of oxygen which is limited by the solubility in solution and by diffusive parameters. Mainly, the process of pitting is very influenced by:
- Concentration of specific ions (Cl-): The higher the concentration of these ions, the intensity of pitting becomes more marked. The saturated solutions of chlorine determine a decrease of the pitting process because the trigger points multiply rapidly, and then the corrosion assumes a generalized appearance with low penetration speed. In these conditions, the pitting process is also stopped following the reduction of the solubility of oxygen which does not sufficiently stimulates the anodic action of the chlorides that in this situation are at very high rates. The sea water which has a percentage of salt content equal to 3% represents the optimum conditions of corrosion pitting;
- pH of the solution;
- Corrosion products: are insoluble and can cause other processes localized corrosion (crevice corrosion);
- Presence of oxygen ions: their presence inhibit pitting corrosion;
- Interstices, surface roughness, geometric factors;
- Metallographic factors: martensitic and ferritic structures are more favorable to pitting corrosion than the austenitic structures that contain within them substances such as molybdenum, chromium, nickel which lower the susceptibility to corrosion pitting.
The propagation process is due to the establishment of a galvanic couple represented by the figure below.
Considering an active-passive behavior metal in a refreshed solution of sodium chloride (NaCl, sea water), the cathodic reaction takes place over the entire surface of the metal with the formation of OH- ions which make more cathodic surfaces outside the pit. The anodic reaction of metal dissolution occurs within the pit with formation of metal ions and development of diffusive phenomena which lead to a gradual enrichment of chlorine ions. The action of the chlorides ions and the increase of acidity (development of ions H+), ensure the state of activity of the metal within the cavity; simultaneously, the formation of corrosion products which, thanks to their higher specific weight, promote the corrosion process in the vertical direction by increasing the penetrating power of the process. The mechanisms of prevention from this form of corrosion can be of various types:
- Removal of chlorides and oxidants
- Use materials suitable environment aggression
- Avoid stagnant conditions
- Designing a proper water drainage
- Use of corrosion inhibitors
- Use suitable surface treatments