Yildiz Technical University Chemical Engineering - Corrosion: Graphitic Corrosion

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Questions and Answers

What type of corrosion primarily occurs in buried cast iron pipes?

Intergranular corrosion

Galvanic corrosion

Graphitic corrosion (correct)

Pitting corrosion

How does graphitic corrosion affect pipes?

Enhance pipe strength

Improve pipe longevity

Prevent any damage to pipes

Weaken pipes and may cause explosions (correct)

What type of stress and environment combination leads to Stress Corrosion Cracking (SCC)?

Compressive stress and aggressive environment

Tensile stress and non-corrosive environment

Shear stress and dry environment

Tensile stress and aggressive (corrosive) environment (correct)

In Stress Corrosion Cracking, in what direction do cracks typically form?

Perpendicular to the tensile stress (D) Signup and view all the answers

What kind of stress can lead to Stress Corrosion Cracking?

Both externally applied and residual stress (A) Signup and view all the answers

What can residual stress in Stress Corrosion Cracking be a result of?

Phase changes due to cold-work (C) Signup and view all the answers

What effect does the penetration of air into the soil have on corrosion rate?

Increases the corrosion rate (B) Signup and view all the answers

How does the corrosion rate in soil change with increasing salt and water content?

Increases (D) Signup and view all the answers

What effect do bacteria have on the pH of soil?

Decrease the pH (D) Signup and view all the answers

In which type of soil do bacteria change values to make it more aggressive and corrosive?

Acidic soil (C) Signup and view all the answers

How does the corrosion rate in soil change as the temperature of the soil rises?

Decreases (D) Signup and view all the answers

What is the role of aerobic microorganisms in soil corrosion?

Reduce sulphur and sulphuric compounds to H2SO4 (D) Signup and view all the answers

What is the common form of metal oxidation?

O2 (D) Signup and view all the answers

At what thickness is the oxide film called Pilling-Bedworth Ratio?

Less than 100 nm (A) Signup and view all the answers

What does Pilling-Bedworth ratio indicate when it is less than 1?

Volume of oxide film is smaller than metal (B) Signup and view all the answers

Why is an oxide film with a Pilling-Bedworth ratio of 1 considered protective?

It has the same volume as the metal (C) Signup and view all the answers

What happens when the Pilling-Bedworth ratio is greater than 1?

The oxide film breaks and leaves the surface (D) Signup and view all the answers

Which gas can act as an oxidizing agent according to the text?

CO2 (A) Signup and view all the answers

Which metal has the highest corrosion rate in sea water?

Steel (C) Signup and view all the answers

Where does corrosion rate decrease due to the thickness of the water layer and weakness of O2 diffusion?

IIIrd zone (C) Signup and view all the answers

Which metal has the lowest corrosion rate in a rural atmosphere?

Copper (C) Signup and view all the answers

What is the primary factor affecting corrosion rate in moist regions?

Water layer thickness (B) Signup and view all the answers

What happens to the corrosion rate as the thickness of the water layer increases in underground corrosion?

Decreases (C) Signup and view all the answers

Which metal has the lowest corrosion rate in sea water?

Copper (A) Signup and view all the answers

What is Stress Corrosion Cracking (SCC)?

A type of corrosion that occurs due to the combined effect of tensile stress and a corrosive environment (A) Signup and view all the answers

What causes Fretting Corrosion?

Vibration and friction between two metal surfaces which are contacted and under load (C) Signup and view all the answers

What happens during the sliding movement in Fretting Corrosion?

The prongs that are amalgamated under high tension are scraped (C) Signup and view all the answers

What role do the formed oxide particles play in Fretting Corrosion?

They play a scraper role (D) Signup and view all the answers

How can Fretting Corrosion be prevented?

By reducing the load on the metal surfaces (D) Signup and view all the answers

What is the role of oxygen in Fretting Corrosion?

It oxidizes the scraped metal surfaces (B) Signup and view all the answers

Study Notes

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.

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Description

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.