Tuberculation corrosion


What causes it?

  • Ferrous hydroxide Fe(OH)2 will form on steel and cast iron when placed in service in oxygenated water near neutral pH and at or slightly higher than room temperature.
  • This corrosion product will shield the metal surface underneath from the oxygenated water. Causing the oxygen concentration to decrease underneath the corrosion product layer or rust.
  • Beneath the rust layer more reduced forms of oxide are present, such as Fe3O4
  • A black layer of magnetite is formed between corrosion product layers. The metal is attack beneath these deposits.
  • Oxygen migration through the corrosion product is reduced as more products accumulate.
  • An oxygen concentration cell is developed in the region below the corrosion product layer. Producing the knob like, mound particles we call tubercles.

Where does it occur?

  • Ductile iron pumps and piping in municipal water systems is a favorite target

What can you do about it?

  • Concrete lined, ductile iron pipe has been a standard in these industries for many years.
  • Sand blasting and replacing pump components is a temporary solution at best.
  • Coating the pump’s wetted surfaces with ceramic epoxies has worked very well in recent years and also has the advantage of increasing the pump’s efficiency.
  • These low cost, brushed on epoxies can be easily applied in the pump shop after the wetted parts, including the impeller, have been sand blasted