Pitting Mechanism

Pitting often begins at a specific area of a passivated metal where there is a break in the passivation layer, which then acts as the anodic area, while the rest of the metal acts as the cathodic area. With a potential difference between the anode and cathode, extremely localized corrosion initiates, and since the surrounding area is passivated, the corrosion remains localized and causes pits to form in the metal. Moreover, since the anodic area is significantly smaller than the cathodic area, corrosion continues at a rapid pace.

A further danger of pitting is that corrosion in pits becomes self-sustaining by an autocatalytic process. Such a process involves the progression of pit growth by the dissolution of the metal near the bottom of the pit. It is thought that the environment is very acidic near the bottom of the pit, thus propagating the dissolution of the metal. The dissolution reaction, where electrons associated with a metal-metal bond are dispelled and a metal ion breaks away from the bulk material, works in conjunction with a cathodic reaction near the surface adjacent to the pit. The cathodic reaction supplies excess electrons to facilitate a reduction reaction by forming hydroxide ions from water molecules and diatomic oxygen. To maintain neutrality, anions (negative ions) from the electrolyte migrate into the pit where there is an excess of positive charge, and associate with the metal ions. Subsequently, this species is dissociated in water to form a metal hydroxide and an acid, which results in a reduction in the pH near the bottom of the pit. This means that there is an excess of positively charged hydrogen ions and anions, which stimulate and propagate further dissolution of the metal near the bottom of the pit. These reactions are shown in Equation 6, Equation 7, and Equation 8.

where
M – metal
M⁺ – metal ion
e^– – electron
I^– – anion (i.e. Cl^–)

Pitting corrosion is also very difficult to measure and predict, as there are usually numerous pits of varying depths and diameters, which do not form consistently under specified conditions. The holes that form from corrosive attack, however, tend to be greater in depth than in diameter. These pits typically form on the top-surface of a metal and proceed to deepen in the same direction as gravity. Thus, they do not usually form on surface planes that are parallel to the direction of gravity, but rather on those that are perpendicular to gravity. Moreover, pits do not tend to proceed away from the direction of gravity. Basically, they do not form on the bottom surface of a metal and proceed away from the direction of gravity. Initiation of the holes is a gradual and fairly long process, but once they are formed, the rate of growth of the pit increases significantly. Pitting usually occurs in static or low velocity fluid systems, since pitting corrosion will tend to decrease as fluid velocity increases. Pitting is often difficult to measure since the metal usually experiences minimal weight loss during the corrosion process. Also, pits can be filled in with corrosion products.