To understand pitting it is essential to understand the mechanism of standard corrosion. Every metal is metallurgically bonded. This means not all the electrons are associated with a particular metal core. The electrons are able to move within the metall.
The oxygen which is dissolved in the water, attracts electrons. As the drawing power of the oxygen is stronger as of the metal (in this example iron), electrons are wrested away from the iron.
The Oxygen, the electrons and water molecules react to hydroxides ions.
O2 + 4e– + 2H20 ⇌ 4(OH)–
Due to the missing electrons the iron gets positively charged. In order to get electrical neutral again, it repels Fe2+ions into the water. In this way material is lost.
No discoloration anywhere due to this process but the ions will react within the water. The hydroxide ions built by the oxygen and the electrons will react with the Fe2+ ions to form Iron-(II)-Hydroxide.
4(OH)- + 2Fe2+ ⇌ 2Fe(OH)2
Iron-(II)-hydroxide is not stable, and will convert with water and oxygen to Iron-(III)-hydroxide
4Fe(OH)2 + O2 +2H2O ⇌ 4Fe(OH)3
Fe(OH)3 will precipicate, split off water and transform into a mixture of Fe0, Fe2O3 and settle on the surface as brownish rust.
We see an increase in iron content and maybe discoloration. Countermeasures can be taken at a very early stage.
When it comes to pitting, the previously described basic reactions within the water are the same. Pitting is a self-accelerating corrosion on a tiny spot. It starts with some local corrosion building a small pit in the pipe. This might start due to damage in passivation layers (in the case of passivation layer building metals like copper or stainlesss steel), concentration differences, material faults or out of constructive reasons. Comparing the ratio water volume/surrounding pipe surface for inside and ouside the pit we will find a smaller ratio for inside the pit. More surface means more electrons moving into the water so more oxygen is consumed inside the pit. This leads to a difference in oxygen concentration in- and outside the pit. The force on the electrons is now bigger on the outer surface as inside the pit. Electrons start to move to the outer surface. The outer surface becomes negatively charged, the inside face will become positively charged.
Fe2+ + 2H2O ⇌ Fe(OH)2 + 2H+
Fe2+ + Cl– + H2O ⇌ Fe(OH)2 + HCl
HCl ⇌ Cl– + H+
Fe(OH)2 settles. The increase in H+ concentration lowers the ph value within the pit, accelerating the material loss even more. Ph values of bellow 3 have been measured. Depending on temperature and concentrations pitting might spread with a speed of up to one inch/year.