Graphitic corrosion is a process that may happen in equipment made of iron, particularly grey cast iron, but also ductile cast iron. Graphitic corrosion can lead to unexpected catastrophic failure of the affected part because the cast iron can lose its strength without a visible warning such as a change in size, shape, or appearance.
Cast irons subject to graphitic corrosion have a metal alloy structure formed from iron and graphite (a form of carbon). In graphitic corrosion, the iron corrodes preferentially to the graphite. The iron oxide corrosion product can leach from the structure, leaving a mostly graphite structure that has little of its original strength.
Graphitic corrosion is usually a form of galvanic corrosion that takes place in wet or moist environments where water acts as an electrolyte and allows a weak electrical current created by dissimilarities in the iron and graphite to remove the iron from the more cathodic graphite. Physical changes include the structure becoming weak, becoming very much lighter than the original cast iron, and losing its magnetic qualities.
Graphitic corrosion is often found in cast iron piping used for water mains and sewage systems. Such piping is usually buried in damp soil where the corrosion is unnoticed until there is a catastrophic leak. Graphitic corrosion is also common in fire sprinkler valves, steam system heat exchangers, and marine exhaust systems.
The presence of graphitic corrosion can be difficult to determine before a failure. Rust bleeding from the part, corrosion that extends beneath the normal surface corrosion present on cast iron, and a loss of attraction to a magnet are symptoms indicative of galvanic corrosion.
Initial field confirmation of a failure resulting from graphitic corrosion can be made by checking for a gross weight loss, loss of attraction to a magnet, and most importantly a loss of strength that can result in additional fractures by hand pressure or tapping with a hammer. The remaining graphitic structure can also leave marks similar to that of a pencil when rubbed on the hands or paper.
John Phillips, senior consulting engineer at Warren, has more than 30 years of crane and heavy equipment experience and more than 20 years of experience in forensic engineering. A licensed professional engineer in South Carolina, North Carolina, Georgia, Louisiana and Ohio, he’s NCEES registered both as a model engineer and with The United States Council for International Engineering Practice, USCIEP. John has designed crane systems, supervised installation, tested and certified lifting equipment even serving as a project engineer for maintenance and certification of nuclear weapon lifting and handling systems. He is a certified fire and explosion investigator and fire and explosion investigator instructor by the National Association of Fire Investigators. John is a member of the American Society of Materials and American Society of Testing and Materials, as well as a voting member of ASTM Ships & Marine Forensic Sciences, Forensic Engineering, and Performance of Buildings committees.