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.
Pitting Corrosion
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.[1]
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).[2] 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.[2]
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. This creates a gradient of electrical potential and is responsible for the electromigration of aggressive anions into the pit.[6] For example, when a metal is exposed to an oxygenated aqueous solution containing sodium chloride (NaCl) as electrolyte, the pit acts as anode (metal oxidation) and the metal surface acts as cathode (oxygen reduction).
In the case of pitting corrosion of iron, or carbon steel, by atmospheric oxygen dissolved in acidic water (pH In the pit, the oxygen concentration is essentially zero and all of the cathodic oxygen reactions take place on the metal surface outside the pit. The pit is anodic (oxidation) and the locus of rapid dissolution of the metal.[9] The metal corrosion initiation is autocatalytic in nature however its propagation is not.
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 pits on the surface are often obscured by corrosion products. Pitting can be initiated by a small surface defect, being a scratch or a local change in the alloy composition (or local impurities, e.g. metallic sulfide inclusions such as MnS or NiS),[10][11] or a damage to the protective coating. Polished surfaces display a higher resistance to pitting.[citation needed]
Nitrite is also an oxidizing species and has been used as corrosion inhibitor since the 1950's.[15][16][17]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 Î-Fe
2O
3 as follows:
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.[18]
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 prevent pitting in firearm bores, most modern firearms have a bore lined with chromium.[citation needed]
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.[19]
Hesketh, J.; Dickinson, E. J. F.; Martin, M. L.; Hinds, G.; Turnbull, A. (2021-04-15). "Influence of H2S on the pitting corrosion of 316L stainless steel in oilfield brine". Corrosion Science. 182: 109265. doi:10.1016/j.corsci.2021.109265. ISSN 0010-938X. PMC 8276138. PMID 34267394.
Pitting corrosion affects metals and alloys such as steel, iron, aluminium and more. It is usually constrained to specific areas. It penetrates and attacks rapidly and is difficult to detect. It most commonly occurs where the passive coating layer is physically damaged or chemically attacked. This creates a weak point where water or corrosive solutions attack the substrate.
Adjacent materials will often appear unaffected. If left unchecked pitting corrosion can be devastating for roof systems or any metal structure. It occurs quickly and can easily be overlooked, which is why many consider it the most dangerous form of corrosion.
Pitting corrosion is a cavity, hole or pit that forms in a small area or point. The pits or holes are obscured by a small amount of corrosion product (rust) on the surface. When a cathodic reaction in a large area (coating) sustains an anodic reaction in a small area (exposed metal), a pit, cavity or small hole will form. Oxidation occurs in the metal even when there is no supply of oxygen.
High electron demand by the large cathode is put on the small anode, the result is intense pitting corrosion. It will be subtle and happen rapidly with very harmful effects. Only a small spot of rust is visible on the surface while damage happens deep in the metal structure below.
Pitting corrosion occurs when the cathode (damaged coating) is large and the anode (exposed metal) is small. Typically the surface protection layer or film becomes the cathode when it is damaged and cracked. A small area of metal is then exposed and becomes the anodic.
Pitting is vigorous when the solution on the metal surface contains chloride, hypochlorite or bromide ions. Other harmful solutions are those that contain fluorides and iodides. Sulphides and water are also known to enhance the pitting process.
Another example of pitting corrosion occurs when a metal is poorly maintained and exposed to water droplets and dust particles. The area below the droplet is insufficiency oxygenated while the surrounding areas are well oxygenated. This results in differential aeration corrosion where surrounding areas are cathodic and the small area below the droplets and dust particles become anodic. Electrons flow through the metal and are met by water and oxygen. Ions are formed and defuse together to produce rust. Pits, cracks and crevices develop in the metal as the rust is produced.
Pitting corrosion can appear in a variety shapes. The shape of the pit depends largely on the material affected and the direction of the grain within that material. Passive metals and alloys are most commonly affected, these include stainless steel and aluminium. However almost any metal or material susceptible to corrosion can be affected.
Trough pit shapes tend to be hemispherical, cup-shaped or irregularly shaped. Trough pitting corrosion occurs when the passive film (protective layer) is compromised and the metal wall is attacked forming narrow and deep troughs. Their flat walls expose the crystal structure of the metal. These can quickly perforate the thickness of the material, for example a roof sheet, truss or gutter component.
Sideway pits are covered with a semi-permeable membrane of corrosion product (rust) and appear in horizontal grain attack, undercutting and subsurface shapes. Sideway pitting corrosion can penetrate the metal very quickly. It is very difficult to detect because the surface of the metal will appear unaffected and free from corrosion. With only a few small spots of rust it may appear as if damage is very minor.
Electrochemical testing to measure pitting in any metal such as cyclic polarisation and potentiostatic tests are also an option. These are short term electrochemical tests that provide instant results.
An immersion test or weight loss method is another option. These tests take more time to run. They involve removing a metal sample and immersing in a solution. After a few days it can be removed so the corrosion rate can be calculated. You can observe the pits and pit depth under a microscope and make the necessary calculations.
It is bed to use recommended cleaning procedures to expose the pits fully and remove the corrosion products. Avoid using solutions that attack the base metal excessively. It is advisable during cleaning to periodically probe the pits with a pointed tool. This will enable you to determine the extent of any undercutting or subsurface corrosion. Vigorously scrubbing the pit with a wire brush will enlarge the openings sufficiently. Removing the corrosion products and undercut metal you will help to evaluate the extent of damage.
When the metal material is clean and free from debris carry out a final inspection of the damage. If you are happy the corrosion has been caught in time you may proceed with applying a base primer coating followed by one or two more top layers. If the damage is severe and the component is too weak it will need to be replaced.
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