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Surface Finishing Tutorial

#056 Pitting Corrosion

Category : Corrosion - Corrosion Protection
September17, 2010

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.

[Fig.1] Pitting Corrosion Mechanism

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.