Overview

The Basics of Pitting Corrosion

Pitting corrosion is a form of localized corrosion that occurs preferentially at small surface areas, leading to the formation of pits that are typically caused by an aggressive species in the operating environment, promoting the breakdown of the material’s passivity. In stainless steels, the increased likelihood of pitting corrosion is often caused by the presence of chlorides in a corrosive fluid. Aside from stainless steel, other passive metals like aluminum and titanium alloys are susceptible to pitting, particularly in small areas on the boundary regions away from the bulk material where the metal’s passivity is broken down. Depending on the material and type of component, pits can have several different morphologies.

Pitting corrosion can be defined as the selective attack on localized areas of a metal or alloy, resulting in the formation of small, often deep, cavities known as pits. These pits can lead to severe material loss, stress concentration, and eventually failure. Cavity corrosion and pit corrosion are terms often used synonymously to describe pitting corrosion. This metal failure mode can also occur by other forms of mechanically assisted degradation.

Various factors can influence the rate and severity of pitting in mechanical components. A few key variables include:

Environment: The chemical composition, temperature, and chloride ion concentration of the environment can affect the aggressiveness of the pitting process, influencing the extent of corrosion.
Passive Film Formation: The presence and stability of passive films on the material surface can affect the pitting process by providing a barrier to localized corrosion.
Exposure Time: The duration of exposure to the corrosive environment can impact the extent of pitting corrosion, as longer exposure times typically result in increased pit growth.
Material Properties: The composition, microstructure, and metallurgical history of the materials involved play a significant role in determining their susceptibility to pitting corrosion.

To mitigate the occurrence and severity of pitting in machined components, engineers can employ various strategies, including:

Material Modification: Enhance the material’s resistance to corrosion through protective surface coatings or heat treatments that improve the stability of passive films and reduce the material’s susceptibility to localized attack.
Inhibitors: Use chemical inhibitors that can be added to the environment to decrease the aggressiveness of the medium, slow down the electrochemical reactions, or promote the formation of stable passive films on the metal surface, reducing the rate of corrosion.
Proper Design: Design mechanical systems and components to minimize crevices, joints, and other minute areas where aggressive chemicals or deposits can accumulate, which can lead to localized corrosion.
Fluid Flow Control: Ensure appropriate fluid flow rates and minimize turbulence in piping systems, as high flow rates can lead to erosion-corrosion, which can exacerbate pitting.
Material Pairing: Avoid galvanic coupling with more anodic materials, which can accelerate the corrosion rate. Select appropriate material combinations that minimize galvanic corrosion effects.

Armoloy's Solution to Corrosion

Armoloy offers multiple metal surface treatments with varying levels of protection from the common causes of corrosion. Offering both broad-spectrum and industry-specific applications, our protective metallic coatings add significant value through increased performance and decreased revenue losses from unplanned maintenance and downtime.

Our protective coatings ensure a thin, precise coat that won’t impact production, but will improve surface hardness and prevent environmental defects. Beyond increasing wear life, Armoloy tailors our metallic coatings based on the specific requirements of your application and industry.

By gaining knowledge on the failure mechanism and implementing mitigation steps into the initial design process of industrial applications, engineers can effectively reduce the detrimental impact of corrosion on the performance, reliability, and lifespan of machined components.

Beyond the Lab: Metal Failures in Narrative Form

Other Metal Failure Modes

Other common metal failures include:

Corrosion can also result from, or be a precursor to, other potential metal failures

Eliminate pitting corrosion from your operations. Meet our group of curious, innovative engineers and learn how we can help improve your industry with science-based solutions.

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Pitting Corrosion