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Pitting corrosion attacks a metallic body in a very localized manner. This highly focused corrosion creates pits or cavities in the surface of an anode. Very often an engineering material can be deeply penetrated by corrosion pits while its surface remains close to free from any corrosion. Corrosion products can hide such pits in an apparently devious way, making them appear to be much less severe than they actually are. When considering pitting corrosion one must remember that although certain pits may not cause structural failure due to deformation or fracture, they may cause a conduit or pressure vessel to fail by creating leaks through the artifact's cross-section.
This corrosion is especially common in aluminum alloys or stainless steels that are exposed halide solutions, or in slightly acidic solutions that contain chloride. A metal's chance for forming pits increases with temperature in chloride solutions containing strong oxidizers. Often the resistance to pitting is characterized by the minimum temperature that is needed to cause it. In the presence of chlorides, dissolved oxygen can easily cause pitting; this will reduce with higher temperature as less oxygen will be able to dissolve.
The maximum pit depth is usually used to classify pits. A way of quantifying the extent of pitting corrosion is to measure the maximum penetration (p) by microscopy and, indirectly, the average specimen penetration (d) via weight loss in the sample. The ratio of these two numbers, p/d, is called the pitting factor. If this factor is 1, then the corrosion is totally uniform. For very small pit depths, the pitting factor takes on values so large that it becomes difficult to work with.
Although a specific mechanism for neither (1) pit nucleation nor (2) pit growth is agreed upon, a general description of the pitting process can be found in concentration cell phenomena.
This model relates pit nucleation or initiation as the result of a localized loss of passivity in the metal. "Breakdown" of the passive film, according to one believed mechanism, results from strong complexing agents in solution (i.e. Cl-) interacting with this film chemically, catalytically dissolving the passive layer until the metal surface is exposed.
In another proposed initiation mechanism an anion such as chloride diffuses through the oxide film to form metal chloride species at the boundary between the metal and passive layer. Since the molar volume of metal chloride is greater than that of metal oxide, the accumulation of the chloride species causes mechanical stresses, which eventually rupture the thin film. Thus, a direct dissolution of the anode (resulting in pit growth) is possible; creating a localized anode.
Pit nucleation generally occurs at sites that are capable of depassivation. Some such sites are sulfide inclusions, regions that are depleted of the cathode reactant required for passivity, and grain boundaries.
The formation of local active regions that result from electrochemical processes occur at a pitting potential. A large increase in current density occurs at this potential, resulting in local active regions where the metal is normally passivated. This potential is correctly interpreted as the minimum potential achievable above which (in the case of ferrous metals) stable pits are formed. The term ââ‚¬Ëœstable pitsââ‚¬â„¢ is suggested by experimental data: variation or "noise" in measured current density near the pitting potential indicates that pit nucleation also occurs below the pitting potential in the form of metastable pits. This type of initiation site does not result in failure of the material and often vanishes quickly upon nucleation.
In order for the pit to grow once initiation is established, a supporting cathode is required. To understand this, it is instructive to note the chemistry of the anode. Both metal cations and H+ ions are produced at the anode, through dissolution and cation hydrolysis, respectively. This concentration of positive species results in anions (i.e. Cl-) diffusing into the pit, which further increases the aggressive anion and H+ concentration. Dissolution of the anode is then increased and a pit begins to form. The process outlined above is termed autocatalytic or self-propagating. As a result of anodic dissolution electrons are liberated and are necessarily consumed by a cathodic reaction (such as the reduction of O2). Thus, the cathode permits pit growth in that the anode reaction located in the pit is polarized. The exterior surface of the pit remains passive (with the reduction of O2 further raising pH) and is thus the cathode.
Anode: M = Mn+ + ne-
Cathode: O2 + 2H2O + 4e- = 4OH-
The nucleation and growth stages are then the result of a concentration cell effect. As noted previously localized accumulation of the metal chloride and H+ anolytes create active regions on the metal. The survival of a pit is thus dependent on its retention capability of the metal cations, which permit a proper diffusion barrier concentration of acid chloride species.
To understand the nucleation and growth processes for a specific system, it is insightful to consider pitting corrosion of stainless steel in an oxygenated sodium chloride solution. The system will be examined in three areas: (1) pit interior, (2) pit perimeter ("mouth"), (3) pit exterior.
(1) Through one proposed initiation mechanism with the aggressive anion Cl- localized activity is established on the alloy surface at a sulfide inclusion. This is considered stage 1 in pit development. It is important to note that in the presence of Cl-, stainless steel shows a dramatic increase in current density at the pitting potential; thus the alloy is considered locally active.
The anodic reaction of interest is the oxidation reaction: Fe = Fe2+ + 2e-. The Fe2+ ion is then hydrolyzed in the following reaction: Fe2+ + H2O = FeOH+ + H+.
(2) The reaction products magnetite and rust form a semi-permeable membrane over the pit.
2FeOH2+ + Fe2+ + 2H2O = 6H+ + Fe3O4, magnetite
Fe(OH)2+ + OH- = H2O + FeOOH, rust
This membrane acts as a diffusion barrier, retaining a concentration of Cl- and H+ ions sufficient for pit growth. The resulting occluded cell facilitates transformation of metastable pit growth (stage 2) into stable pit growth (stage 3).
Without the magnetite cover during stage 2, repassivation of the metal would occur due to destruction of the acid chloride concentration cell. The transition from stage 2 to 3 occurs when the product of the pit growth current density and the pit radius reaches a critical value (3 mA/cm for stainless steels). In the stable growth phase a membrane is not needed, as the pit depth is a sufficient diffusion barrier.
(3) The surrounding alloy remains passivated through reduction of dissolved oxygen and rust in that the pH of the bulk solution is increased: O2 + 2H2O + 4e- = 4OH-
and 3FeOOH + e- = Fe3O4 + H2O + OH-.
Thus, the supporting cathode induces anodic polarization. The result of this autocatalytic process is a macro-scale pit.
Using potentiostatic or dynamic measurements it was determined that a passivating metal (alloy) undergoes hysteresis. That is, as the potential for a particular metal (alloy) was reduced below the pitting potential an active path was followed by the anodic polarization curve. The potential of this new path at the passive current density is called the protection potential. This electrochemical hysteresis has three interesting results:
1) below the protection potential, pit growth is not observed; 2) in the region between the protection potential and the pitting potential growth is observed, but not nucleation; 3) when the potential is greater than the pitting potential, pit growth and nucleation are observed.
Prevention of Pitting Corrosion:
General prevention of pitting corrosion involves the following concerns: (1) decreasing the aggressiveness of the environment, (2) Increasing the resistance of the material, (3) eliminating points of stagnation in the solution, and (4) reducing the amount of scratching done to the passivated layer.
Reducing the amount of oxidizers such as chlorides can decrease solution aggressiveness; regulating the temperature, and the acidity of the environment can also slow pitting corrosion. By cleaning the material and removing deposits, the chance of pitting due to imperfections in the passivated layer is greatly reduced.
Corrosion resistant materials often have a higher alloy content. In ferrous materials, the addition of nickel, chromium, and molybdenum are generally beneficial. At temperatures below 70 degrees Celsius, the addition of titanium can reduce the chances of pitting. Also, carbon and sulfur are usually detrimental to the resistance of the material. Corrosion inhibitors can be added, but insufficient quantities will cause fewer but deeper pits, thus promoting the corrosion. Lastly, cathodic protection can be used, but due to its expensive nature, it should be considered as a secondary protection.
An alloy with higher alloy content is selected according to the following formula:
PI = Cr + 3.3 (Mo) + X (N) where PI is pitting index and
x = 0 for ferritic stainless steels
x = 16 for duplex (austenitic/ferritic) stainless steels
x = 30 for austenitic stainless steels
Stagnant points in a medium can sometimes be eliminated with proper hydrodynamic design, but measures as simple as increasing the flow rate will help raise the overall solution velocity.
Scratching can be reduced by fairly obvious methods, such as removal of sediment from a flowing fluid.
Field Use of Prevention and Case Study:
Generally, case studies on pitting corrosion will be made when leaking is first observed. This often makes the methods of further prevention of pitting quite limited. The situation rarely facilitates pitting corrosion prevention anyway, since most environments cannot be easily controlled for acidity, temperature, or other promoting factors for corrosion.
The following describes the actions taken when leaks were found in underground radioactive waste storage tanks at the United States Department of Energy's Hanford Waste Management Site in Richland, WA.
The Hanford Site has 177 underground waste tanks that store approximately 253 million liters of radioactive waste from 50 years of plutonium production. Of these tanks, 149 were constructed of a single-shell ASTM A537-Class 1 stainless steel. These tanks contain liquid, sludge, and solid waste in an environment with high pH (pH>12). They contain sodium nitrate, sodium hydroxide, sodium nitrite, and minor radioactive constituents. When leaks started to appear in these tanks shortly after the introduction of nitride-based wastes, a study was launched to remedy the problem. After a corrosion probe was constructed and analyzed, it was found that large raw water additions (over 7000 gallons) were shown to induce pitting in the electrodes. After this phenomenon was observed, a system for corrosion monitoring was installed and a corrosion inhibitor addition project was launched in the 1998 fiscal year.
Pitting Corrosion in Aviation:
The following table, from http://www.corrosion-doctors.org/Journal-2000/No3/No3-table-1.htm, lists data from aircraft accidents in which pitting corrosion was at least partially responsible.
Location of Failure
Aero Com-mander 680
Lower Spar Cap
Corrosion pitting, improper approach
Substantial damage to plane
Fatal and serious, loss of plane
Loss of plane, no injuries
Engine, compressor assembly disk
Loss of plane, no injuries
Damage to plane, no injuries
Damage to plane, no injuries
Trailing-edge Flap (TEF) Out-board Hinge Lug
Corrosion pitting, fatigue
Loss of TEF