Yildiz Technical University Chemical Engineering - Corrosion: Graphitic Corrosion

Types of Corrosion in Buried Pipes

• Graphitic corrosion occurs predominantly in buried cast iron pipes, leading to the formation of graphite instead of iron.
• Graphitic corrosion can weaken the structural integrity of pipes, creating holes and reducing flow capacity.

Stress Corrosion Cracking (SCC)

• SCC arises from a combination of tensile stress and a corrosive environment, significantly affecting metals like steel.
• Cracks in SCC typically develop in a direction parallel to the applied stress, making them often difficult to detect.
• Tensile stress, whether applied or residual, can instigate SCC.
• Residual stress in SCC can result from manufacturing processes, welding, or physical deformations.

Environmental Effects on Corrosion

• Penetration of air into soil can accelerate the corrosion rate by providing oxygen to oxidizing agents.
• Increasing salt and water content in soil generally elevates the corrosion rate due to enhanced conductivity and moisture retention.
• Bacteria can lower the pH of soil, creating a more acidic and corrosive environment, particularly in anaerobic conditions.
• In anaerobic, nutrient-rich soils, bacteria can create aggressive conditions that further exacerbate corrosion.

Temperature and Corrosion

• As soil temperature rises, corrosion rates typically increase due to higher electrochemical activity.
• Aerobic microorganisms in soil contribute to corrosion by producing aggressive metabolites, further lowering pH and enhancing reactions.

Metal Oxidation and Pilling-Bedworth Ratio

• Metal oxidation commonly manifests as the formation of an oxide film on metal surfaces.
• The Pilling-Bedworth ratio, which compares the volume of the oxide to the volume of the metal, is significant at a thickness less than 1.
• A Pilling-Bedworth ratio below 1 indicates non-protective oxide films that may lead to further corrosion.
• A ratio of exactly 1 signifies a potentially protective oxide layer, effectively preventing further oxidation.
• If the Pilling-Bedworth ratio exceeds 1, the oxide layer can spall, exposing the metal beneath and increasing corrosion risk.

Corrosion in Different Environments

• Oxygen acts as an oxidizing agent in many corrosion processes.
• The metal with the highest corrosion rate in seawater is typically magnesium.
• Corrosion rates decrease in deeper water layers due to increased thickness of the layer and limited oxygen diffusion.
• Aluminum usually has the lowest corrosion rate in a rural atmosphere, largely due to its protective oxide layer.
• Moist environments are primarily affected by moisture levels, which can accelerate corrosion processes.
• In underground corrosion, thicker water layers typically increase corrosion rates, creating a more conductive environment.

Fretting Corrosion

• Fretting corrosion is caused by mechanical wear due to micro-movements or vibrations between contact surfaces.
• During sliding movements in fretting corrosion, oxide films are disrupted, which can trigger localized corrosion attacks.
• Formed oxide particles in fretting corrosion act as abrasive contaminants, exacerbating wear and facilitating chemical reactions.
• Preventive measures for fretting corrosion include lubrication, material selection, and design modifications to reduce relative movement.
• Oxygen plays a critical role in fretting corrosion by promoting the oxidation of metal surfaces during disruption.

Learn about graphitic corrosion in buried cast iron pipes, its prevention methods, and the potential risks associated with this type of corrosion. Explore the causes of graphitic corrosion and how it can impact the structural integrity of pipes over time.