Язвенная коррозия: механизмы и особенности развития

Form of insidious localized corrosion in which a pit develops at the anode site

Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation reaction) while an unknown but potentially vast area becomes cathodic (reduction reaction), leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions. Another term arises, pitting factor, which is defined as the ratio of the depth of the deepest pit (resulting due to corrosion) to the average penetration, which can be calculated based on the weight loss.

According to Frankel (1998) who performed a review on pitting corrosion, it develops in three successive steps: (1) initiation (or nucleation) by breakdown of the passive film protecting the metal surface from oxidation, (2) growth of metastable pits (growing up to the micron scale and then repassivating), and (3) the growth of larger and stable pits. The evolution of the pit density (number of pits per surface area) as a function of time follows a sigmoid curve with the characteristic shape of a logistic function curve, or a hyperbolic tangent. Guo et al. (2018), after a statistical analysis of hundreds of individual pits observed on carbon steel surfaces at the nano to micro scales, distinguish three stages of pitting corrosion: induction, propagation, and saturation. The main interest of the point defect model is to explain the stochastic character of the pitting corrosion process.

In order to maintain the solution electroneutrality inside the pit populated by cations released by oxidation in the anodic zone (e. g., in case of steel), anions need to migrate inside the narrow pit. It is worth to notice that the electromobilities of thiosulfate and chloride anions are the highest after these of and ions in aqueous solution. Moreover, the molar conductivity of thiosulfate ions is even higher than that of chloride ions because they are twice negatively charged (weak base reluctant to accept a proton). In capillary electrophoresis, thiosulfate moves faster than chloride and eluates before this latter. 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 |SO4(2 ) and |NO3 .

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. Thus, this type of corrosion typically occurs in alloys that are protected by a tenacious (passivating) oxide film such as stainless steels, nickel alloys, aluminum alloys in environments that contain an aggressive species such as chlorides (Cl–) or thiosulfates (S2O32–). In contrast, alloy/environment combinations where the passive film is not very protective usually will not produce pitting corrosion. A good example of the importance of alloy/environment combinations is carbon steel. 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. However, at pH greater than 10 (alkaline) the oxide is protective and the addition of chloride results in pitting corrosion. Besides chlorides, other anions implicated in pitting include thiosulfates (S2O32−), fluorides and iodides. Stagnant water conditions with low concentrations of dissolved oxygen also favor pitting. Thiosulfates are particularly aggressive species and are formed by partial oxidation of pyrite (|FeS2, a ferrous disulfide), or partial sulfate reduction by microorganisms, a. o. by sulfate reducing bacteria (SRB). 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.

Although in the aforementioned example, oxic conditions were always considered with the reduction of dissolved in the cathodic zones, pitting corrosion may also occur under anoxic, or reducing, conditions. Indeed, the very harmful reduced species of sulfur (, , , , , S0 and ) can only subsist under reducing conditions. Moreover, in the case of steel and stainless steel, reducing conditions are conducive to the dissolution of the protective oxide layer (dense γ ) because is much more soluble than , 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 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. Under strongly reducing conditions, in the absence of dissolved oxygen in water, or pore water of the ground, the electron acceptor (oxidizing agent) at the cathodic sites, where reduction occurs, can be the protons of water itself, the protons of hydrogen sulfide (|H2S), or in acidic conditions in case of severe pyrite oxidation in a former oxic atmosphere, dissolved ferric ions , known to be very potent oxidizers. The presence of harmful reduced species of sulfur and microbial activity feeding the sulfur cycle (sulfide oxidation possibly followed by bacterial sulfate reduction) have also to be taken into account. Strictly abiotic (i. e. inorganic) corrosion processes are generally slower under anoxic conditions than under oxic conditions, but the presence of bacteria and biofilms can aggravate the degradation conditions and causes unexpected problems. 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.

A single pit in a critical point can cause a great deal of damage. One example is the explosion in Guadalajara, Mexico on 22 April 1992, when gasoline fumes accumulated in sewers destroyed kilometers of streets. The vapors originated from a leak of gasoline through a single hole formed by corrosion between a steel gasoline pipe and a zinc plated water pipe. Firearms can also suffer from pitting, most notably in the bore of the barrel when corrosive ammunition is used and the barrel is not cleaned soon afterwards. Deformities in the bore caused by pitting can greatly reduce the firearm's accuracy. To reduce pitting in firearm bores, most modern firearms have a bore lined with chromium. Pitting corrosion can also help initiate stress corrosion cracking, as happened when a single eyebar on the Silver Bridge in West Virginia, United States failed and killed 46 people on the bridge in December 1967. In laboratories, pitting corrosion may damage equipment, reducing its performance or longevity. Fume hoods are of particular concern, as the material constitution of their ductwork must suit the primary effluent(s) intended for exhaust. 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.

Sulfur has long been known to contribute to damage. This is true for many materials such as metal corrosion, or concrete degradation. In King Lear, Shakespeare says:There’s hell, there’s darkness,    there is the sulphurous pit, Burning, scalding, stench, consumption;    fie, fie, fie!