Example of crevice corrosion Crevices

corrosion of a S30400 stainless steel washer after 30 days in 0.5 FeCl3 + 0.05 M NaCl solution

Another example of crevicecorrosion

Stainless steel bolt used in seawater after 5 years of exposure.

Initiation of crevice corrosionCrevice corrosion is initiated by changes in local

chemistry within the crevice:– Depletion of inhibitor in the crevice– Depletion of oxygen in the crevice– A shift to acid conditions in the crevice– Build-up of aggressive ion species (e.g. chloride) in

the crevice

Popped rivets found on an external section of a maritime patrol aircraft fuselage.

Analysis of a crevice-corrosionrelated problem (popped rivets) Subsequent inspection revealed corrosion in the bonded

area of the skin plates. The cause was believed to be water ingress from a

global positioning system (GPS) antenna. It was believed that water penetrated through cracks in

the seal surrounding the GPS antenna. Once the water was in, crevice corrosion began and it

ultimately led to the loss of the bond between the two portions of the structure.

The loss of this bond allowed more water to seep in the structure and generalized the corrosion problem.

Analysis of a crevice-corrosionrelated problem (continued)

Underside of panel where severe corrosion was found

Analysis of a crevice-corrosionrelated problem (continued)

Close-up picture showing the severity of corrosion

Advanced crevice corrosion“pillowing”

Notice how the rivet heads appear to be lower than the surrounding skin surface.

Stage 0 of crevice corrosion•Consider the case of a passive metal corroding •Assume that the main cathodic species is dissolved oxygen •Crevice shown is not drawn to scale (it is much narrower and longer in reality)

•Initially, the dissolved oxygen content in the crevice is equal to the one outside. The corrosion rate is uniform.

Stage 1 of crevice corrosion•Due to the difficult access caused by the crevice geometry, oxygen is rapidly depleted in thecrevice by uniform corrosion.• The corrosion reactions now “separate”, i.e. metal dissolves more in the crevice (anode) and oxygen is reduced more on the outside metal surface (cathode).

•The large cathodic surface (Sc) vs. anodic surface (Sa) ratio (Sc/Sa) leads to an “acceleration” of the anodic (corrosion) reaction in the crevice.

Stage 2 of crevice corrosion•The metal ions produced by the anodic corrosion reaction form corrosion products (hydrolyze)giving off protons (acid) and.•The pH in a crevice can reach very acidic values, sometimes equivalent to pure acids.

•The acidification of the local environment produces a serious increase in the corrosion rate of most metals.•The corrosion products seal even further the crevice environment and the problem gets worse.

Galvanic corrosion The driving force for corrosion is a potential difference between

the different materials. In a bimetallic couple, the less noble material will become the anode of this corrosion cell and tend to corrode more compared with the uncoupled condition. The more noble material will act as the cathode in the corrosion cell and corrode less.

Example of galvanic corrosionStainless screw in contact with a cadmium plated steel washer