What Does Graphitization Mean?
Graphitization refers to electrolytic corrosion most often seen in iron or low-alloy steel in which the metallic components are converted into corrosion products. Gray iron, or gray cast iron as it is also called, is most susceptible to graphitization. The process of graphitization separates the alloy, creating free carbon (also known as graphite), which clumps together. Graphitization occurs when the alloy is exposed to high temperatures (at least 800°F/ 426°C) for a prolonged period of time. The result is a substantial reduction of localized strength in the affected alloy.
Trenchlesspedia Explains Graphitization
When susceptible alloys, especially cast iron, are exposed to excessive heat, metallurgical degradation in the steel’s microstructure occurs. During graphitization, carbon (graphite) migrates to grain boundaries, forming microscopic carbon protuberances. Because of the properties of graphite, such as low ductility and low resistance to thermal or mechanical fatigue, the carbon nodules have an embrittling effect on the steel. Unfortunately, the early stages of graphitization are not easily identified as the affected pipe is left relatively unchanged. The weakened pipes, however, have a much lower threshold to external factors like fluctuations in pressure, temperature and ground movement.
To form cast irons such as gray iron, ductile iron and compacted graphite iron, primary graphitization is expected to occur during solidification.
Although the two terms are sometimes used interchangeably, graphitization should not be confused with graphitic corrosion. Graphitization forms a localized collection of carbon, whereas graphitic corrosion selectively leaches the iron from the alloy, leaving only graphite in its place.