What Does Graphitic Corrosion Mean?
Graphitic corrosion is selective leaching of iron from gray cast iron, where iron particles are removed and graphite grains remain intact.
Graphitic corrosion is also known as selective leaching, dealloying, demetalification, parting and selective corrosion.
Corrosionpedia Explains Graphitic Corrosion
Graphitic corrosion forms when different metals and alloys have different electrochemical potentials or corrosion potentials in the same electrolyte. The potential difference between the alloying elements is the driving force for the preferential attack on the more active element in the alloy. Selective leaching takes place because graphite is cathodic to iron, and gray iron's structure develops an excellent galvanic cell.
Graphitic corrosion is favored by relatively mild environments such as:
- Soft waters, waters having a slightly acidic pH, waters containing low levels of hydrogen sulfide, and brackish and other high-conductivity waters
- Moist soils, especially those containing sulfates, will frequently produce graphitic corrosion of unprotected gray and nodular cast iron.
- Stray currents have also been identified as causes of graphitic corrosion in subterranean pipelines.
Graphitic corrosion can cause significant problems because, although no dimensional changes occur, the cast iron loses its strength and metallic properties. Thus, without detection, potentially dangerous situations may develop in pressure-containing applications.
Attack by graphitic corrosion is reduced by the following methods:
- Ductile cast iron is less prone to serious graphitic corrosion than gray iron, although it is not immune.
- White cast iron, which is essentially free of graphite, is immune to graphitic corrosion.
- Corrosion-resistant cast irons containing chromium, nickel, and silicon are also immune to graphitic corrosion.
- Raising water pH to neutral or slightly alkaline levels
- Use of inhibitors
- Avoidance of stagnant water conditions
Selective Leaching, Selective Corrosion, Demetalification, Deallyoing