Pitting corrosion
The process of pit nucleation is initiated by the depassivation of the protective oxide layer isolating the metal substrate from the aggressive solution.
The depassivation of the protective oxide layer is the less properly understood step in pitting corrosion and its very local and random appearance probably its most enigmatic characteristic.
The chemical conditions prevailing in the solution and the nature of the metal, or the alloy composition, are also important factors to take into consideration.
Anions with weak or strong ligand properties such as chloride (Cl−) and thiosulfate (S2O2−3) respectively can complex the metallic cations (Men+) present in the protective oxide layer and so contribute to its local dissolution.
Finally, according to the point-defect model elaborated by Digby Macdonald, the migration of crystal defects inside the oxide layer could explain its random localized disappearance.
[3][4][5] The main interest of the point-defect model is to explain the stochastic character of the pitting corrosion process.
The more common explanation for pitting corrosion is that it is an autocatalytic process driven by the random formation of small electrochemical cells with separate anodic and cathodic zones.
The random local breakdown of the protective oxide layer and the subsequent oxidation of the underlying metal in the anodic zones result in the local formation of a pit where acid conditions are maintained by the spatial separation of the cathodic and anodic half-reactions.
In the case of pitting corrosion of iron, or carbon steel, by atmospheric oxygen dissolved in acidic water (pH < 7) in contact with the metal exposed surface, the reactions respectively occurring at the anode and cathode zones can be written as follows: Acidic conditions favor the redox reaction according to Le Chatelier principle because the H+ ions added to the reagents side displace the reaction equilibrium to the right and also increase the solubility of the released Fe2+ cations.
However, the solubility of Fe(OH)2 (Fe2+) is relatively high (~ 100 times that of Fe3+), but strongly decreases when pH increases because of common ion effect with the OH−.
The formation of anodic and cathodic zones creates an electrochemical cell (i.e., a small electric battery) at the surface of the affected metal.
The localized production of positive metal cations (Men+, Fe2+ in the example here above) in the pit (oxidation: anode) gives a local excess of positive charges which attract the negative ions (e.g., the highly mobile chloride anions Cl−) from the surrounding electrolyte to maintain the electroneutrality of the ion species in aqueous solution in the pit.
[7] In the case of metallic iron, or steel, the process can be schematized as follows:[8] Under basic conditions, such as under the alkaline conditions prevailing in concrete, the hydrolysis reaction directly consumes hydroxides ions (OH−) while releasing chloride ions: So, when chloride ions present in solution enter in contact with the steel surface, they react with Fe2+ of the passive layer protecting the steel surface and form an iron–chloride complex.
This kind of corrosion is often difficult to detect and so is extremely insidious, as it causes little loss of material with the small effect on its surface, while it damages the deep structures of the metal.
The high electromobility of both anions could also be one of the many factors explaining their harmful impact for pitting corrosion when compared with other much less damaging ion species such as SO2−4 and NO−3.
Pitting corrosion is defined by localized attack, ranging from microns to millimeters in diameter, in an otherwise passive surface and only occurs for specific alloy and environmental combinations.
In environments where the pH value is lower than 10, carbon steel does not form a passivating oxide film and the addition of chloride results in uniform attack over the entire surface.
Thiosulfates are a concern for corrosion in many industries handling sulfur-derived compounds: sulfide ores processing, oil wells and pipelines transporting soured oils, kraft paper production plants, photographic industry, methionine and lysine factories.
[14] Moreover, in the case of steel and stainless steel, reducing conditions are conducive to the dissolution of the protective oxide layer (dense γ-Fe2O3) because Fe2+ is much more soluble than Fe3+, and so reducing conditions contribute to the breakdown of the protective oxide layer (initiation, nucleation of the pit).
Reductants exert thus an antagonist effect with respect to the oxidants (chromate, nitrite) used as corrosion inhibitors to induce steel repassivation via the formation of a dense γ-Fe2O3 protective layer.
Pitting corrosion can thus occur both under oxidizing and reducing conditions and can be aggravated in poorly oxygenated waters by differential aeration, or by drying/wetting cycles.
Critical infrastructures and metallic components with very long service life may be susceptible to pitting corrosion: for example the metallic canisters and overpacks aimed to contain vitrified high-level radioactive waste (HLW) and spent nuclear fuel and to confine them in a water-tight enveloppe for several tenths of thousands years in deep geologic repositories.
In the specific case of steel, the Fe2+ cation being a relatively soluble species, it contributes to favor the dissolution of the oxide layer which so loses its passivity.
This approach is also at the basis of the chromate conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys.
[15]: p.1265 [16] As hexavalent chromate is a known carcinogen, its aqueous effluents can no longer be freely discharged into the environment and its maximum concentration acceptable in water is very low.
[17][18][19] Under the basic conditions prevailing in concrete pore water nitrite converts the relatively soluble Fe2+ ions into the much less soluble Fe3+ ions, and so protects the carbon-steel reinforcement bars by forming a new and denser layer of γ-Fe2O3 as follows: Corrosion inhibitors, when present in sufficient amount, can provide protection against pitting.
One example is the explosion in Guadalajara, Mexico, on 22 April 1992, when gasoline fumes accumulated in sewers destroyed kilometers of streets.
Fume hoods are of particular concern, as the material constitution of their ductwork must suit the primary effluent(s) intended for exhaust.
[25] If the chosen vent material is unsuitable for the primary effluent(s), consequent pitting corrosion will prevent the fume hood from effectively containing harmful airborne particles.
"Effects of sulfur compounds on the pitting behavior of type 304 stainless steel in near-neutral chloride solutions".
inside the pit) and cathodic zone ( O 2 reduced into OH −
elsewhere outside the pit) developing on a metal immersed into an aqueous solution containing dissolved oxygen. Here, the pH conditions are neutral or alkaline (presence of OH −
ions in solution). The transport of ions occurs into the aqueous solution while electrons are transported from the anode to the cathode via the base metal ( electrical conductor ).
