Crevice Corrosion

Crevice corrosion (concentration cell corrosion) is the most common type of corrosion. This type of corrosion is found in many old aircraft that have not been properly repaired.

revice corrosion, is an aggressive corrosion that occurs locally in crevice and other protected areas of metals exposed to corrosive liquid.

Annealed duplex stainless steel groove corrosion

The mechanism ofcrevice corrosion is schematically shown in the figure below.

Crevice corrosion mechanism

Crevice corrosion occurs in protected areas where a small amount of static corrosive liquid between two surfaces, such as sub-paints that have lost their adherence, or joints that have not been sealed.

Oxygen molecules have little solubility in stagnant fluid. As a result, when the fluid inside the crevice remains stagnant; It becomes empty from protects the atmosphere and oxygen. The lower amount of oxygen in the crevice helps to form the anodic zone on the metal surface. The metal surface exposed to air in contact with trapped moisture forms a cathode.

Over positively charged ions are formed in the crevice, causing the stagnant solution to become acidic. To compensate for the excess of positive ions, the chloride ions in the liquid migrate into the crevice, it causes a small, localized galvanic cell.

Because crevice corrosion occurs only when the level of oxygen in the corrosive fluid inside the crevice is different from the surface outside the crevice, it is also called oxygen concentration cell. The most effective way to eliminate crevice corrosion is to keep water away from joints and tight space.

Crevice corrosion is a type of localized corrosion that results from the presence of a crevice. This type of corrosion occurs in closed areas where fluid flow is limited. Such as under sealing in valves and under gaskets in valves and flanges.

As soon as an electrochemical reaction occurs in this limited volume, the composition of the existing liquid changes. So, the dissolution potential becomes more electronegative and the surface inside the cavity becomes more anodic than the rest of the structure.

In the slit, aluminum is oxidized according to the following reaction:

While at the edge of the gap, oxygen decreases:

While the metal in the recess is being corroded, oxygen dissolved in the liquid will be consumed. Therefrom the geometry of the crevice limits penetration, oxygen is discharged into the cavity and Al^{3 +} ions is too much. Accumulation of Al³⁺ ions causes the adsorption of chloride ions.

As a result, hydrolysis of aluminum chloride begins:

due to the small volume of the gap, its environment quickly becomes acidic and the pH decreases by 2-3 degrees. Unlike some alloys, such as stainless steel, aluminum exhibits a rather low susceptibility to Crevice corrosion.

When a bolted or riveted assembly that has been exposed to a liquid such as seawater for a long time is dismantled, the crack appears that crevices are usually sealed by corrosion products of aluminum. This is certainly one of the reasons why aluminum has a rather low susceptibility to Crevice corrosion.

However, it is recommended that as far as possible, irregular or intermittent welding beads be avoided in reservoirs where deposits can accumulate.

Crevice corrosion associated with galvanic corrosion may develop under steel washers (ordinary or stainless) used for mounting aluminum wall panels exposed to the seashore.

Therefore, assemblies should be installed with extreme caution in such an aggressive environment. To prevent the formation of areas that retain moisture, which causes galvanic and crevice corrosion.

Crevice corrosion of stainless steels has a similar mechanism to pitting corrosion.

The start-up phase begins with the creation of suitable gap geometry. Crevices are formed by special manufacturing processes, including the following:

• Riveted seams
• Incompletely fused welds
• Interference
• O-rings
• Gasketed joints
• Paints marking components

The solution in the crevice becomes acidic as described above. It also locally anodizes the metal surface compared to the surrounding metal.

The onset of the gap largely depends on the geometry of existing gap. Slits with smaller diameters shorten the start time.

The crevice corrosion of copper alloys is essentially similar to that of stainless steel. But a concentration cell starts from a metal ion instead of an oxygen concentration cell.

The copper in the crevice corroded, forming Cu⁺ ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu²⁺ ions.

These ions are highly oxidizing and form the cathodic agent. They also being reduced to Cu⁺ ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system.

Crevice corrosion is more dangerous in carbon steel and alloy steels. Therefore, it is a common practice to overlay alloy Inconel 625 on carbon steel materials and crevices and gaps such as seat pockets and sealing parts as well as the flange faces with crevices such as ring-type joint flanges.

The figure below shows the 309LMO stainless steel cover on the flange surface of a carbon steel body with a valve applied to prevent crevice corrosion.