September 2010 Archives

Crevice Corrosion occurs with steel and other metals, as well as with passive surface layer forming metals such as stainless steel, aluminum alloys, and titanium alloys.

Compounded metal structures contain numerous clearances (crevices) where similar and dissimilar metal members and materials are jointed, as well as under scales and corrosion byproducts and coatings.

The crevices in discussion here are unlike the ones in typical discussions but are very small, in order of 1/100mm scale. Even into these small crevices, water and other liquids will intrude. Once such liquids intrude into these clearances, they would stay there since there would be little exchange with the outside and the oxygen concentrations become low. This oxygen concentration differentials cause formation of differential aeration batteries, and the crevices low in oxygen concentration will corrode. [Fig.1] illustrates an example of crevice formation on a metal plate with a material in contact.

Anodic reaction in crevices, and cathodic reaction on the external surfaces will progress. The metal ions generated by the anodic reaction will become of corrosion byproduct. With crevice corrosion of stainless steel equipment, potential difference between the equipment body and crevices may be as high as 0.2V (200mV) or more.

As a reference, corrosion potential of metals and alloys in sea water is shown in [Table 1]. The metals in the upper positions are more susceptible to corrode than the ones in the lower positions. This hierarchy is called Corrosion Potential Series.

These potentials are referenced by comparative SCE (saturated calomel electrode). The stainless steel (passive) is with a passive surface layer, and stainless steel (active) is without the passive surface layer.

[Table 1.] Corrosion Potential of Metals in Sea Water (V)

As the term suggests, a corrosion with more depth relative to the opening size is called Pitting Corrosion. This occurs on carbon steel, low-alloy steel, copper alloy, stainless steel, and aluminum alloy, but well known examples are the ones occurring on metals with highly corrosion resistant passive surface layers such as aluminum and stainless steel alloys.

The Pitting Corrosion is a local corrosion advancing into the metal from the surface in a pit like form, and can occur sporadically or numerously. The unaffected area of the passive surface layer will retain its original shine.

[Fig.1] is a sectional schematic of pitting corrosion on stainless steel. Stainless steel exhibits noble potential among many metals due to its passive surface layer, as explained previously. Suppose that a portion of the passive layer is destroyed by chloride ions. That portion will become of a negative pole because of non-noble potential and the surrounding passive layer becomes the positive pole, and a battery will form. The positive pole area is far larger than the negative pole area, and the corrosion advances.

The Pitting Corrosion advances as depicted above. The corroding current flows into water from the negative pole where the passive layer has been damaged toward the surrounding positive pole. The reason for the electrical current flowing through the water is not because the electrons are moving, but the water dissolved ions are moving. In a battery scheme, negative ions migrate to a negative pole, and positive ions migrate to a positive pole to form an electrical current flow.

Stainless steel sustains Pitting Corrosion when Chloride Ions (CL-) exist in water, and the Chloride Ions promote the electrical current flow within the environment. The Chloride Ions are negative ions and raise the chlorine concentration of the pitted areas, then accumulate in the pits. The pH of the pitted areas become low and metal dissolution is accelerated. This is further accelerated by Hydrogen Ions formed by hydrolysis of metal ions from dissolved stainless steel as the corrosion progresses.
Since the accumulation of Hydrogen Ions H+ and Chloride Ions Cl- occurs inside the pits of progressing corrosion, they do not escape out of the pits into the environment. Thus, the Pitting Corrosion sill sustain themselves endlessly in locations where they once started.

Differential Aeration Corrosion is a type of corrosion phenomenon occurring with oxygen concentration differentials in water where a differential aeration battery environment is formed. As an experiment example shown in [Fig.1], a glass vessel divided with an earthenware barrier wall is filled with 3% salt water, and a pair of polished steel plates with a conductor connected to each are submerged. Using glass tubes, one steel plate is exposed to air and the other steel plate is exposed to nitrogen gas. After some time, the steel plate exposed to the nitrogen gas will corrode. The earthenware barrier wall is to keep the liquid in each compartment separated but to allow for electrical conduction. This corrosion occurs due to a differential aeration battery formation where the air exposed steel plate assumes a reduction reaction of dissolved oxygen as shown below, and the nitrogen exposed steel plate dissolves.

As an example, let us examine a case of corrosion of a stake driven into water. With passing time, a local corrosion just below the water line will be evident.

This is a case of Differential Aeration Corrosion as shown in [Fig.2]. The water around the stake at the water line is slightly raised due to water surface tension. The water layer is thin at this immediate area and receives rich oxygen from the air in comparison to the water below the water line. This creates a differential aeration battery condition.

Another example is old water plumbing made of cast iron pipes with rust bumps. When these rust bumps are removed from the surface, advanced corrosion can be seen on the cast iron pipe surface below the bumps.

This is also caused by the differential aeration battery effect. The rust bumps are surrounded by rich dissolved oxygen but the surface below the bumps are relatively isolated from the oxygen, causing the formation of a differential aeration battery, and the corrosion advances under the rust bumps.