At first glance, pitting corrosion looks like a tiny localized type of electrochemical deterioration found on metallic surfaces that often goes unnoticed. However, it could be a dangerous form of deterioration from a material integrity point of view. On the surface it might appear to be confined to a tiny point or spot, whereas underneath it can often spread over large areas at various depths. This damage underneath the surface can cause significant loss of strength for metallic structures and components.

Onset and Severity of Pitting

Pitting corrosion normally occurs on the surfaces of passive or passivated metals such as steel alloys, stainless steels and aluminum. The onset of pitting corrosion can occur wherever the passive layer (oxide layer) is partially damaged and the damage is not automatically repaired by repassivation; this can cause significant perforations across the thickness of a metallic structure.

The characteristics of the passive film play a decisive role in the initiation of the pitting, the rate of pit growth and repassivation of the corroded surfaces. Because the pitting damage can spread in multiple directions, the pit's dimensions are found by metallography.

The severity of the pitting corrosion is dependent upon the pitting factor, which is the ratio of the depth of the deepest pit to the average pit depth (i.e., the average pit depth is found on the basis of the resulting weight loss due to the pits).

Factors that Influence Pitting Corrosion

Material defects such as inclusions, surface quality and corrosive chemicals such as chloride salts present in the environment are the main factors responsible for any initial pit formation. Metal oxide layers acting as passive films are easily attacked by the chlorides. Automatic healing or repassivation of the pit may begin only if a supply of oxygen is ensured through continuous aeration at the site of pit initiation. Repassivation is also facilitated by alloying elements such as molybdenum, vanadium and chromium present in the metal. (Learn more about passivation in Using Pickling and Passivation Chemical Treatments to Prevent Corrosion.)

Pitting can occur on an exposed metal substrate, where anodic reactions (oxidation) are constricted inside the particular pits and the balancing cathodic reactions (reducer reactions) occur in the adjacent areas. The presence of strong stagnant electrolytes as well as surfaces with imperfections can also contribute to pitting activity.

For a given metal the resistance to pitting corrosion is evaluated on the basis of its critical pitting temperature (CPT). For steel alloys and stainless steel, the critical pitting temperature is determined by the procedure specified in ASTM G48 E (applicable CPT procedure for stainless steel) and ASTM G48 C (applicable CPT procedure for nickel chromium based alloys).

Patterns of Pitting Corrosion

Pitting corrosion may be difficult to recognize with the naked eye as it may be covered with a layer of corrosion product. A coating defect can also produce pitting under the coating. (Be sure to download our Coating Failures and Defects Guide.) Holes or cavities that develop underneath the initial pit may take different patterns such as narrow, elliptical or round pits, grain attack pits (horizontal), subsurface sideway pits, vertical (grain attack) pits or wide-shallow pits.

Implications of Pitting Corrosion

Loss of wall thickness (i.e., wall loss) of structural members and pipes has important implications for various mechanical capabilities such as flexibility, tensile strength and compressive strength. An innocuous pit can become a stress raiser, cause initiation of failure due to stress corrosion cracking (SCC) or fatigue cracks, and even result in the catastrophic collapse of structures or ruptured piping with disastrous consequences. (For an example of a corrosion-induced pipeline failure, see INFOGRAPHIC: The El Paso Natural Gas Company Pipeline Explosion.)

Material Selection Considerations for Pitting Resistance

Apart from the critical pitting temperature, another important measure of pitting corrosion resistance is the pitting resistance equivalent number (PREN). This value is based on an empirical formula derived for each of the metal types.

The molybdenum (Mo), chromium (Cr), nitrogen (N) and tungsten (W) percentages by weight contained in the metal determine the PREN value. The generalized formula for PREN is:

PREN = %Cr + m(%Mo) + n(%N)

where m and n are multiplying factors for molybdenum and nitrogen respectively.

Most commonly for stainless steel, m is 3.3 and n is 16, so that:

PREN = %Cr + 3.3(%Mo) + 16(%N)

The formula is modified to consider the addition of tungsten along with molybdenum as shown below:

PREN = %Cr + 3.3(%Mo + 0.5{%W}) + 16(%N)

The PREN value is used to specify the requirement for resistance to pitting corrosion as well as crevice corrosion in a sodium chloride (NaCl) environment. The higher the PREN value, the higher the corrosion resistance to pitting and crevice corrosion.

Even when the material meets the specified PREN value, heat treatment and the manufacturing process can influence the final pitting corrosion resistance. In addition, the changes in the ferrite to austenite ratio in stainless steel composition will also influence pitting resistance.

According to ASTM standard G48 Method A, the pitting corrosion resistance is evaluated by a pitting test, which consists of immersing the steel and alloy specimen in a 6% solution of ferric chloride (FeCl3). The critical pitting temperature (CPT) is determined (via ASTM 48 Method C and E) by immersing the sample metal in a 1% solution of ferric chloride. The lowest temperature at which pitting begins in this solution is recorded as the CPT for the given composition of steel. Researchers have found that a relationship exists between the critical crevice corrosion temperature CPT as well as the PREN for chromium-nickel alloys as well as stainless steels.

Monitoring Methods

Various monitoring methods are used, three of which are discussed below.

Radiography

X-rays and gamma rays are used in radiography of the specimen. This technique develops images of pitting in inaccessible zones of the parts and components. Stringent radiation safety measures must be in place when using this method. The depth of flaws are determined by measuring the radiography densities. Most often pipeline pitting corrosion is detected by this method. (Discover more about pipeline pitting corrosion in the article Pitting Corrosion in Oil and Gas Wells and Pipelines.)

Acoustic emission (AE) technique and ultrasonic technique

In the ultrasonic thickness method, two probes are connected to the steel to be monitored, and are used to apply and receive the pulses. This technique evaluates changes in the thickness of the metal, which reflects indirectly on the corrosion rate.

In the case of the acoustic emission method, short ultrasonic waves pulses are sent through the metal to be monitored. The size and orientation of any pits are detected by the acoustic emission sensors.

Researchers have used the acoustic emission (AE) technique for detection and in situ monitoring of pitting corrosion of carbon steel in NaHCO3 + NaCl solutions. Simultaneously, the open circuit potential (OCP) monitoring method was also used. Results obtained from the AE technique were in line with results from the OCP monitoring method.

Researchers also acquired consistent results using the AE method to detect and monitor pitting corrosion in stainless steel samples in lab tests. A good correlation was found between AE signals and the occurrence of pitting corrosion activity.

Prevention of Pitting Corrosion

Preventive action to avoid pitting corrosion damage involves:

  1. Monitoring the chloride and sulfate concentrations
  2. Selecting materials with the appropriate pitting corrosion resistance as required for the service conditions

In addition, providing a corrosion-resistant coating or cathodic protection may be necessary in some cases.

In the case of austenitic stainless steel, the addition of at least 2% molybdenum can minimize the pitting corrosion rate in a NaCl solution. The addition of a passivation agent such as nitrate to the corrosive medium, wherever feasible, can also reduce the risk of pitting. Increasing the pH value of the corrosive medium can also reduce pitting damage.

In the case of aluminum, pitting corrosion is often seen as an aesthetic problem. However, in some instances it may affect the structural component performance, resulting in a premature failure. Chlorides and sulfates in the environment are the main culprits for this deterioration by pitting. Surface treatments such as coating or anodizing as well as keeping the aluminum surfaces clean can minimize the pitting corrosion damage.

In the case of structures, the chances of water accumulating may be avoided by choosing shapes and inclinations that avoid water pockets between the parts. Dust buildup can be minimized by increasing the corner radius for bends. Horizontal members may be designed to have an adequate slope to avoid water accumulation.

Conclusion

The failure risk that can be posed by the tiny pitting corrosion marks on metallic surfaces is very real and significant. These marks are a result of tiny breaks in the passive film that otherwise protects stainless steels,
aluminum and other alloy metals where corrosive chemicals have attacked the surface. Pitting corrosion can be prevented by choosing metals with appropriate pitting corrosion resistance and by protecting the surfaces with cathodic protection and protective coatings.