Pitting Corrosion

Pitting corrosion is a localized form of corrosion that creates small but deep cavities in metal surfaces, often due to chloride exposure, stagnant fluids, or material defects. Effective solutions involve both repair techniques and preventive strategies:

Repairing Pitting Corrosion:

• Mechanically remove pits via grinding, polishing, or blasting to eliminate affected material.
• Fill or resurface deeper pits with welding or metal-filled epoxies when structural integrity is compromised.
• Reapply protective coatings such as chromium, nickel, epoxies, or paints to restore corrosion protection.

Preventing Future Pitting:

• Use corrosion-resistant alloys with high PREN values, such as 316 or duplex stainless steels, instead of lower-grade 304 stainless.
• Design components to eliminate crevices, promote drainage, and reduce fluid stagnation.
• Apply barrier coatings or corrosion inhibitors in high-risk environments (marine, food processing, chemical washdowns).
• Schedule regular inspections and flush out salt or chemical residues to reduce localized attack.

These steps are especially important in heat exchangers, pumps, tanks, and stainless piping systems, where pitting can lead to failure even if general corrosion appears minimal.

Pitting corrosion is primarily a wet corrosion process. It occurs in the presence of an electrolyte, typically water containing chloride ions or other aggressive anions. The localized breakdown of the passive protective layer on the metal surface leads to the formation of small pits or cavities.

Key Points:

• Wet Process: Requires the presence of a liquid electrolyte, such as seawater, de-icing salts, or acidic solutions.
• Localized Attack: Pitting corrosion is highly localized, creating deep, narrow pits that can penetrate the metal.
• Environment: Common in marine environments, chemical processing industries, and areas with high humidity or salt exposure.

Understanding that pitting corrosion is a wet process helps in designing appropriate preventive measures, such as proper drainage, application of protective coatings, and use of corrosion-resistant materials.

Pitting corrosion is a localized form of corrosion that results in small, but deep, holes or pits on a metal surface. Here are the primary causes:

1. Chlorides: Presence of chloride ions, commonly found in saltwater and de-icing salts, can break down the passive oxide layer on metals, leading to pitting.
2. Metallurgical Factors: Inhomogeneities in the metal, such as inclusions or second-phase particles, can act as initiation sites for pitting.
3. Localized Chemical Conditions: Areas with stagnant solutions, crevices, or deposits can create localized environments that promote pitting.
4. Temperature: Higher temperatures accelerate the rate of pitting corrosion by increasing the chemical activity and diffusion rates.
5. pH Levels: Low pH (acidic conditions) can destabilize the protective oxide layer, making the metal more susceptible to pitting.

Prevention:

• Material Selection: Use corrosion-resistant alloys, such as stainless steel or titanium, especially those with higher molybdenum content.
• Protective Coatings: Apply coatings or surface treatments to protect the metal surface from aggressive environments.
• Environmental Control: Avoid stagnant conditions and ensure proper drainage and cleaning to remove corrosive agents.
• Cathodic Protection: Employ cathodic protection systems to prevent pitting in susceptible environments.

By addressing these causes and implementing preventive strategies, the risk of pitting corrosion can be significantly reduced, ensuring the longevity and reliability of metal components.

Crevice corrosion is a localized form of metal degradation that occurs in confined spaces where access to oxygen is limited, such as under gaskets, seals, or overlapping metal joints. This corrosion type occurs when stagnant moisture or solutions become trapped, creating an environment fostering aggressive attack on metal surfaces.

The process begins with depletion of oxygen levels in the crevice, disrupting the protective oxide layer that naturally forms on many metals. This leads to an imbalance in electrochemical reactions, causing the trapped area to become highly acidic and corrode more rapidly than exposed surfaces.

Crevice corrosion is seen between two painted metal sheets.

Key Characteristics of Crevice Corrosion:

• Common Metals Affected: Stainless steel, aluminum, and other alloys prone to localized corrosion.
• Triggering Conditions: Presence of a narrow gap, stagnant fluid, and low oxygen levels.
• Industries Impacted: Marine, aerospace, and chemical processing, where moisture or aggressive environments are common.

Preventing Corrosion:

• Use corrosion-resistant coatings like thin dense chrome (TDC) to protect surfaces.
• Minimize crevices in design by welding or using continuous seals.
• Regularly inspect and clean equipment to remove trapped debris or moisture.

For more details on preventing this corrosion and protecting metal components, visit our dedicated pitting and crevice corrosion guide.

Rust is a type of oxidative corrosion, specifically affecting iron and its alloys. When iron reacts with oxygen and water, it forms iron oxide, commonly known as rust. This process can be classified under uniform corrosion because it usually affects the metal surface uniformly, but it can also lead to more localized forms if the conditions vary across the surface.