Due to its excellent corrosion resistance, stainless steel is one of the most widely used alloys in many industries, including petroleum, petrochemicals, food processing, pharmaceuticals, and shipbuilding. However, even stainless steel can be susceptible to pitting corrosion under certain conditions . Unlike uniform corrosion, which damages the metal surface evenly, pitting corrosion results in the formation of small spots or holes on the surface. While these spots or holes may sometimes be invisible, they can actually lead to sudden and serious equipment failures .
Definition of a thorn
Pitting corrosion is a form of localized electrochemical corrosion that begins with small depressions on the metal surface and gradually penetrates deeper. This phenomenon typically occurs in environments containing chloride ions (Cl⁻), such as seawater, salt solutions, and also in humid industrial environments. In stainless steel, pitting corrosion is directly related to the destruction of the chromium oxide layer (Cr₂O₃) , the natural protective layer on the steel surface.
Stainless steel drilling mechanism
Under normal conditions, stainless steel is coated with a very thin, resistant, and firmly adhering layer of chromium oxide, which protects the metal from direct contact with oxygen and corrosive substances. However, if this layer is damaged by chloride ions, surface defects, or mechanical stress , a small portion of the metal surface is exposed to the corrosive environment.
The firing process typically consists of the following steps:
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Catalytic effect:
Chloride ions penetrate the passivation layer and destroy it at certain points, such as in impurity zones or at grain boundaries. -
Corrosion progress:
The internal environment of the cavity becomes more acidic, with a higher chloride concentration, which accelerates the corrosion reaction. -
Stability and self-acceleration:
Unique conditions arise within the cavity, making the restoration of the passivation layer virtually impossible. Therefore, the corrosion reaction progresses self-acceleratingly and continuously increases the cavity depth. -
Crack propagation:
When the hole depth reaches a certain level, leading to a reduction in the effective cross-sectional area of the metal or to a stress concentration, crack propagation and sudden failure can occur.
Factors influencing pitting corrosion
Pitting corrosion depends on many factors, including environmental conditions, alloy composition, and operating conditions:
1. The presence of chloride ions
The most important factor for pitting corrosion is the presence of chloride ions . These ions are found in seawater, industrial salts, and even human sweat. The higher the chloride ion concentration, the greater the risk of damage to the passivation layer.
2. Temperature
High temperatures accelerate the corrosion reaction and destabilize the chromium oxide layer. Therefore, stainless steel is more susceptible to pitting corrosion at high temperatures.
3. Chemical composition of the steel
Molybdenum (Mo) plays a crucial role in increasing resistance to pitting corrosion. Molybdenum-containing stainless steels such as types 316 and 904L
exhibit significantly higher resistance to pitting corrosion compared to stainless steel type 304. Furthermore, the addition of nitrogen (N) and chromium (Cr) can further improve the stability of the passivation layer.
4. The pH value of the environment
An acidic environment (low pH value) creates conditions for rapid cave growth, as the acidity level inside the cave is usually high locally.
5. Surface finish
Clean, polished surfaces are more resistant to pitting corrosion. Scratches, foreign metal particles, or contaminants can cause pitting corrosion.
Fossil Corrosion Resistance Index (PREN)
To compare the corrosion resistance of different stainless steel grades, the PREN value (corrosion equivalent value) is used, which is calculated according to the following formula:
)PREN=\
The higher the PREN value, the greater the steel’s resistance to pitting corrosion. For example:
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Stainless steel 304 has a PREN value of approximately 18-20.
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Stainless steel type 316 has a PREN value of approximately 24–26.
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Duplex steel 2205 has a PREN value of approximately 35-40.
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Superduplex steel with a PREN value greater than 40
Methods for preventing the erosion of fossils
Preventing fossil corrosion depends on a combination of three factors: selecting the right alloys, a solid design, and controlling environmental conditions.
1. Selecting the right alloys
For chloride-containing environments, molybdenum- or nitrogen-treated stainless steel is recommended. Duplex stainless steels of grades 316L or 2205 are ideally suited for applications in the marine or chemical industries .
2. Landscaping and terrace paving
Chemical passivation (with nitric or citric acid) restores and strengthens the passivation layer on steel surfaces. Mechanical polishing can also reduce the risk of pitting corrosion.
3. Environmental control
The likelihood of pitting corrosion can be reduced by reducing chloride concentrations, adjusting the pH value, and avoiding high temperatures.
4. Rational technical design
To prevent pitting corrosion, avoid dead corners, sharp edges, and areas with salt deposits when designing your equipment . Also, avoid contact between dissimilar metals to prevent the formation of galvanic cells.
5. Cathodic protection
In certain environments, such as underwater pipelines, the corrosion process can be suppressed by the use of sacrificial anodes or cathodic current control.
Methods for the detection and assessment of pitting corrosion
Early detection of focal corrosion in industrial plants is crucial. Common methods include:
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Visual and microscopic examination: observation of pores on the metal surface.
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Corrosion Potential Test (EPIT): Measures the voltage at which corrosion begins.
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Salt spray test: used to assess resistance to chlorides.
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Electrochemical testing and surface analysis (SEM, EDS): Investigation of the chemical composition of corroded areas.
The importance of industrial application and protection
In industries such as oil and gas, food processing, medical technology, energy, and shipbuilding, pitting corrosion can lead to leaks, contamination, production downtime, and increased maintenance costs. For example, even a small hole in a steel heat exchanger or seawater pipe can cause a leak and thus a system failure.
In conclusion
Pitting corrosion of stainless steel is among the most serious and insidious types of corrosion. Although the surface may appear intact, the internal structure is often severely damaged. For engineers and specialists in all industries, it is therefore essential to understand the mechanisms of its formation, the contributing factors, and the methods of prevention . The correct alloy selection, proper maintenance, environmental protection measures, and regular inspections can significantly reduce the risk of this type of corrosion and extend the service life of equipment.