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Pitting Corrosion (Control Methods (Add inhibitor (Increase pH), Use…
Pitting Corrosion
Control Methods
Use materials that have high pitting resistance
Add inhibitor
Increase pH
Apply cathodic protection
Avoid using materials that have high pitting tendency.
Factor
Breakage or defects of passivation film
Presence of halogen ions
Stagnancy of electrolyte
Mechanism
Formation of Pit
Localized adsorption of aggressive ions.
Corrosion potential exceeds electrode potential of pitting.
Breakdown of passive film.
Pit growth if no repassivation occur (high corrosion rate of pit due to autocatalytic process).
Autocatalytic Process
Formation of small pit.
Dissolved oxygen hard to diffuse into the pit.
Small anodic area form inside the pit, large cathodic area form outside the pit.
Excess metallic postive ions accumulate inside the pit.
Aggressive negative ions migrate into the pit to balance the charge.
Metal ions diffuse outwards and hydrolyzed, H+ ions diffuse inside to replace metal ions and increase acidity of the pit.
Formation
Presence of aggressive ions create localized attack and forms holes in metal.