Pitting corrosion is a form of localized corrosion in which metal loss occurs in the form of holes with cross section small relative to the overall exposed surface. Most of the surface often suffers little or no metal loss. The penetration can be so great that the wall can be completely perforated resulting in leakage. Or, the penetration can stop at a certain depth or stop and then restart. For a component under tensile stress, pits can be initiation sites for cracks, which can then grow at a rapid rate, eventually ending in failure or breaking of the part.
Pitting corrosion does not need a well-defined surface heterogeneity to initiate. The sites of pitting, though, usually have microscopic (or smaller) heterogeneities in the passive layer on the surface (e.g. sulfide inclusions in stainless steels).
Pits come in a number of geometries. An example of more hemispherical pit is shown in this picture.
Additional examples are shown in these figures.
The first figure above shows that not all pits are hemispherical. Often pitting starts and stops, restarts, stops again, etc. Pits can undercut the surface so that the cross section observed at the surface can be far less than the cross section of the entire pit. Sometimes individual pits will be filled with corrosion products. One possible cause is the current density emanating from the pit base sometimes reaching values as high as 1 amp/cm2
. The metal salt that enters the immediate pit environment can reach saturation quickly. Further hydrolysis reactions can result in a fairly insoluble but fairly porous corrosion product. This product can depress the pit propagation rate but the metal that is lost remains lost.
General Mechanism Information
One distinct mechanism cannot be invoked to describe pitting on all alloys and in all environments. In fact, disagreement still exists as to the exact mechanism that causes pits to initiate and propagate. But, certain characteristics are common to most types of pitting.
- Most common pitting is associated with halide and halide containing ions such as chloride, bromide, and hypochlorites.
- A large cathode vs. small anode area relationship tends to exist. A large portion of the surface that is not attacked can act as the cathode and only a small region that is attacked can act as the anode.
- Ionic concentrations in the pit and in the bulk fluid are different. Ionic concentrations are much greater within the pit region.
- Hydrolysis reactions involving the metal-containing cations within the pit cause the acidity to increase, i.e. the pH to decrease significantly.
- Sometimes the hydrolysis reaction products can create an autocatalytic effect in which their presence accelerates pit propagation.
- Initiation can occur at discontinuities in the either the passive layer in the surface of the alloy or between the base metal and inclusions.
- Surfaces exposed to stagnant conditions (absence or diminished fluid motion) are often observed to pit more easily than the same surfaces exposed to fluid motion.
Pitting of Iron-Based and Nickel-Based Alloys
Many iron-based and nickel-based alloys rely on a fairly thin metal oxide surface region to impart corrosion resistance to the bulk material. This region or layer is often called a "passive layer" or "passive film". Alloys are not homogeneous. The surface region is not homogeneous. Commercial alloys contain numerous inclusions, second phases, and regions of composition-based heterogeneities. These regions are believed to provide initiation points for pitting in alloys. In addition, pitting can occur in homogeneous alloys depending on the presence of certain species in the environment, e.g. chloride ions.
The figure below depicts a propagating pit in an iron or nickel based alloy containing chromium in a chloride containing environment and a pH at which surface passivity is assured.
Once initiated, pits can propagate deeper into the alloy. The mechanism has the following characteristics:
- Oxygen reduction proceeds at the surface outside of the pit. The surface area for reduction is far greater than the pit surface area.
- The alloy consituents dissolve within the pit via oxidation. Chromium, nickel, and iron can all dissolve depending on the pH within the pit.
- The metal ions formed react with water (hydrolyze) to form hydroxides and to liberate hydrogen ions. This liberation of hydrogen ions decreases the pH which can fall to values as low as 2 even if the outside environment is at a pH of 7 to 9.
- The pH is maintained at a a low value at the bottom of the pit further accelerating dissolution.
- The counter ion, chloride, increases in concentration within the pit to maintain charge neutrality.
- A potential difference exists between the base of the pit and outside metal. This potential difference is maintained by the various electrochemical reactions.
- A porous metal hydroxide can form at the pit mouth. This solid, when it forms, allows electrolyte contact between the pit and the environment.
- Oxygen reduction at the alloy surface completes the electrochemical cell.
- The counter ion concentration (e.g. chloride ion) can actually increase the activity of the hydrogen ion so that the measured pH can reach zero.
- Sometimes, the metal ion concentration at the pit base can become so large that a metal salt layer precipitates at the bottom of the pit.
Pitting of Aluminum Alloys
A generalized picture of the propagation of a pit in aluminum in aerated solutions containing chloride ions is shown in the figure below. The pH of the environment is assumed to be in the range of about 5 to 9.
Though the exact mechanism of aluminum pit initiation depends on the alloy type, some general characteristics of the process can be summarized as follows.
- Microflaws exist in the aluminum oxide layers that provide passivity to the alloy. The surface is very likely a hydroxide form of aluminum.
- Chloride adsorption in the microflaws might aid in pit initiation.
- Alloys of aluminum contain intermetallic compounds can form dissimilar metal junctions at the surface. For example, 2000 series aluminum contain copper aluminide inclusions and 3000 series aluminum contain iron-aluminide inclusions. The copper aluminide inclusion may decompose to redeposit copper on the surface.
- A potential difference is created between the "unflawed" aluminum surface and that created by pre-existing flaws, chloride adsorption in flaws, or the intermetallic inclusions.
- As a result of the potential difference, oxygen reduction on the "unflawed" surface or on the intermetallic compound drives the anodic dissolution in the region of the flaw or on the aluminum surface adjacent to the intermetallic compound.
The result is a micropit. Some of the micropits repassivate. Some propagate to larger pits.
The above figure shows the process as the pit is propagating. The propagation process has the following characteristics.
- Aluminum dissolution proceeds within the pit especially at the bottom or base of the pit.
- Aluminum ions react with water (hydrolysis) to form aluminum hydroxide cations and hydrogen ions.
- The formation of hydrogen ions decreases the pH relative to the environment outside of the pit further accelerating the dissolution process.
- In a chloride environment, chloride ions increase in concentration to maintain charge neutrality. These ions can react with the aluminum hydroxide ions to form chloride containing adducts.
- Aluminum hydroxide can precipitate at the pit-environment boundary. Sometimes this hydroxide can cover the pit surface but maintain electrolyte contact between the pit and environment.
- Hydrogen ions can be reduced to form hydrogen gas bubbles.
- Oxygen reduction continues on the surface as the cathodic driver. In addition, copper from the copper-aluminide intermetallics at the surface can be reduced further driving the process.
In electrochemical terms, the "critical pitting potential" (sometimes called the "rupture potential") is an electrochemical characteristic that alloys relying on passivity share if they undergo pitting corrosion. This potential is the most negative potential above which pits can initiate and propagate. Assuming measurement artifacts are absent, the value of this potential provides an upper bound value. Control at higher (more anodic or more noble) potentials would destroy passivity and promote pitting. That is, if the corrosion potential is greater than (anodic with respect to) the pitting potential, pitting will initiate. These potentials are often estimated from Cyclic Potentiodynamic Polarization Scans.
The cyclic potentiodynamic polarization scan in the figure below shows the relative relationships among potentials.
In this particular case, the pitting potential lies about 200 mV anodic with respect to the corrosion potential. Once the potential is forced above that value, the increase in current signifies the rapid breakdown in passivity and the generation of pits. In this case, repassivation of the surface does not occur until the potential decreases to a point below or cathodic with respect to the original corrosion potential. Pits that were initiated when the potential rose above the pitting potential continued to grow until the potential fell cathodic with respect to the corrosion potential.
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