What is Graphitic Corrosion and How to Prevent Water Main Breaks

What is Graphitic Corrosion and How to Prevent Water Main Breaks

When it comes to managing the health and longevity of your water mains, corrosion is one of the biggest obstacles to overcome. Graphitic corrosion can be particularly hard to avoid, and if left undetected, can lead to costly damages, if not total failures of your pipeline. Learn more about graphitic corrosion, as well as how to inspect for it, below.

What is Graphitic Corrosion?

Graphitic corrosion refers to the deterioration of grey cast iron pipes, in which the metallic constituents are leached or turned into corrosion products, leaving the graphite intact and exposed. Graphitic corrosion is sometimes incorrectly called graphitization, which is the decomposition of carbide into graphite as the result of exposure to extreme heat.

What Causes Graphitic Corrosion?

As with most forms of exposure, graphitic corrosion is typically the byproduct of prolonged exposure to minerals and compounds found in soils and in water transported by lines. It is commonly found in mains that are exposed to salt water, acidic mine water, dilute acids, and soils; especially those that contain chlorides from deicing salt or sulphate reducing bacteria.

How Does it Occur

Graphitic corrosion will typically manifest in one of three ways:

  1. Solely affecting the surface

In certain cases, only the surface of the pipe will graphitize, leaving a thin layer on top. This may not directly harm the pipe, and in fact can occasionally add a protective element.

2. A plug may develop in the pipe wall

A graphite plug may form if the corrosion extends through the wall. While the plug may not immediately affect the operation of the line, it poses a significant risk. Plugs leave the area vulnerable to pressure surges and water hammers, which can cause the area to burst, leaving behind a large hole.

3. The entire pipe may experience graphitic corrosion

In more extreme cases, the entire pipe may be subject to corrosion. As pipes that have been graphitized are weaker, they are prone to experiencing circumferential breaks due to a heavy earth load, or washout. This is often what leads to water main breaks.

How To Identify Graphitic Corrosion?

Since graphitic corrosion tends to occur on water mains buried underground, visual inspection is not an easy option to determine if your pipes have been affected. As stated above, the chemicals found in the surrounding soil often attack the outside of cast iron or ductile pipe. In the process of leaching iron from the metal, a graphite matrix that looks normal is often left behind, but in reality, it can be chipped away easily.

NDT inspection is a dependable method of determining whether your pipes have been affected by graphitic corrosion. PICA frequently helps customers inspect their water mains and is able to identify vulnerable areas with ease. A few examples of previous cases include:

  • In 2016, PICA inspected a 12” diameter, Lime slurry pipeline and returned in 2018 to aid the customer in determining how far the corrosion had progressed from our initial assessment.
  • In 2018 PICA used a Bracelet Probe for bare pipe to inspect four excavations. On one site, the corrosion at the bottom of the pipe was holed through, even though the section had not started leaking yet (the pipeline was cement mortar lined). A repair clamp was installed over top of the hole to prevent further damage from spreading.

Regular inspection is the most reliable way to protect your water mains from falling victim to graphitic corrosion. To learn more about maintaining and caring for your pipes, as well as treating corrosion, contact our team today.