What are the differences between pitting corrosion and crevice corrosion of stainless steel pipes

The pitting and crevice corrosion of stainless steel pipes in marine environments have the following differences: 

Location of occurrence

Pitting corrosion: It usually occurs in areas on the surface of stainless steel pipes where there are defects in the passivation film or where chloride ions are locally concentrated. These areas may be scratches on the pipe surface, impurity spots, etc. The location is relatively random and can occur at any part of the pipe. 

Crevice corrosion: It mainly occurs at the connection points of pipes, at the gasket areas, near the weld seams, and where there is a deposit covering to form a crevice. These areas are due to the presence of narrow crevices, which allow electrolytes such as seawater to enter and create a special corrosive environment. 

Corrosion forms

Pitting corrosion: It appears as various-sized and varying-depth pits distributed on the surface of the stainless steel pipe. The pits are generally small and deep, with a relatively large ratio of depth to diameter. Some of the pits may penetrate into the interior of the pipe and even penetrate the pipe wall. 

Crevice corrosion: Occurs within the crevice. Its appearance may manifest as corrosion products accumulating around the crevice, with pits or ulcer-like corrosion on the metal surface within the crevice. The corrosion area is relatively concentrated near the crevice, unlike pitting corrosion which is more dispersed. 

Corrosion mechanism

Pitting corrosion: The abundant chloride ions in the ocean can destroy the passive film on the surface of stainless steel pipes. The local areas where the passive film is damaged form small anodes, while the areas with undamaged passive film act as cathodes, forming a corrosion cell and causing pitting corrosion. 

Crevice corrosion: This occurs when electrolytes such as seawater enter the crevice. Due to limited diffusion, a concentration difference battery is formed both inside and outside the crevice. The concentration of metal ions inside the crevice increases. To maintain electrical neutrality, anions such as chloride ions migrate in, causing the solution inside the crevice to become acidic, accelerating the dissolution of the metal, and thus leading to crevice corrosion. 

Development speed

Pitting corrosion: The development speed of pitting corrosion is relatively fast. Under suitable conditions, the corrosion holes may rapidly deepen within a short period of time. Especially in environments with high chloride ion concentration and oxidizing conditions, it is prone to cause local perforation of the pipe material, resulting in serious consequences such as leakage. 

Crevice corrosion: Its development rate is usually slightly slower than that of pitting corrosion. It requires a certain period of time to form a concentration difference and an acidic environment. However, once it occurs, it will gradually cause the metal in the crevice to be corroded continuously, eventually leading to the expansion of the crevice, loosening of the connection part or a decrease in the structural strength of the pipe material.