TUBERCULATION CORROSION T025-1
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 pumpвЂ™s wetted surfaces with ceramic epoxies has worked very well in recent years and also has the advantage of increasing the pumpвЂ™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