(a) With reference to fretting corrosion:
(i) explain the process;(3)
(ii) state a common cause;(1)
(iii) state how it is normally detected.(1)
(b) With reference to pitting corrosion:
(i) explain the term pitting corrosion;(1)
(ii) state TWO common causes;(2)
(iii) explain why it is considered to be dangerous.(2)
Fretting Corrosion
Fretting corrosion is a type of degradation that occurs at the interface of two surfaces in contact with each other under load and subjected to oscillatory movement. This movement, even if microscopic, causes wear and tear on the surfaces, creating debris and promoting corrosion.
1. What is Fretting Corrosion? – Nye Lubricants
Key factors in fretting corrosion:
- Relative motion: Small oscillatory movement between the surfaces. 1. (PDF) Solutions for fretting corrosion – ResearchGate www.researchgate.net
- Load: Pressure between the surfaces. 1. Fretting Corrosion: Bearing & Mechanism – Vaia www.vaia.com
- Corrosive environment: Presence of oxygen or moisture accelerates the process.
Effects of fretting corrosion:
- Material loss: Wear debris is generated, reducing the material thickness.
- Fatigue crack initiation: Fretting corrosion can act as a starting point for fatigue cracks.
- Component failure: Severe fretting corrosion can lead to component failure. 1. Fretting and Its Insidious Effects, by EPI Inc. www.epi-eng.com
To prevent fretting corrosion, engineers often employ techniques such as:
Design modifications: Avoiding tight fits and reducing vibration can help mitigate fretting.
Surface treatments: Applying coatings or surface hardening to increase wear resistance. 1. Some Considerations on the Mitigation of Fretting Damage by the Application of Surface-Modification technologies | Request PDF – ResearchGate www.researchgate.net
Lubrication: Using appropriate lubricants to reduce friction and prevent wear. 1. Fretting Corrosion: Bearing & Mechanism | StudySmarter www.studysmarter.co.uk
The Fretting Corrosion Process
Fretting corrosion is a type of damage that occurs when two metal surfaces in contact experience relative motion under load. This oscillatory movement, even if microscopic, creates conditions conducive to wear and corrosion.
Process:
- Adhesion and Rupture: As the surfaces move relative to each other, microscopic welds (adhesion junctions) form between the asperities (surface irregularities) of the metals. These junctions subsequently rupture due to the continued motion, removing small particles of material.
- Debris Formation: The removed material forms abrasive debris, which is trapped between the surfaces, acting like sandpaper.
- Oxidation: Fresh metal surfaces are exposed due to the wear process, promoting oxidation and the formation of corrosion products.
- Abrasive Wear: The trapped debris accelerates wear, creating a vicious cycle of material removal and corrosion.
Key Factors:
- Amplitude of motion: The magnitude of the relative movement between the surfaces.
- Load: The pressure exerted between the surfaces.
- Frequency: The number of oscillations per unit time.
- Material properties: The hardness, ductility, and corrosion resistance of the materials involved.
- Environment: The presence of moisture, oxygen, and other corrosive substances accelerates the process.
The combination of mechanical wear and chemical attack leads to the characteristic damage associated with fretting corrosion, including surface pitting, crack formation, and material loss.
Common Causes of Fretting Corrosion
Fretting corrosion is often caused by a combination of factors:
Assembly Errors: Improper assembly or installation can create conditions conducive to fretting.
Relative Motion: Small oscillatory movements between components, such as vibration or thermal expansion. 1. Fretting – Wikipedia en.wikipedia.org
Load: Pressure between the components, which can be caused by clamping forces or weight.
Corrosive Environment: The presence of moisture, oxygen, or other corrosive substances accelerates the process.
Material Compatibility: Dissimilar materials in contact can increase the likelihood of fretting corrosion.
Design Flaws: Poorly designed components with tight fits or insufficient lubrication can contribute to fretting.
Detecting Fretting Corrosion
Fretting corrosion can be challenging to detect, especially in its early stages. However, there are several methods that can be employed:
Visual Inspection
- Discoloration: A change in surface color, often appearing as a dark or brown stain, can indicate fretting corrosion. 1. Fretting Corrosion – ONYX Insight onyxinsight.com
- Wear Debris: The presence of fine metal particles or debris between the mating surfaces is a clear sign.
- Pitting: Small pits or cavities on the surfaces can be evidence of fretting corrosion.
Non-Destructive Testing (NDT)
- Ultrasonic Testing: Can be used to detect changes in material properties caused by fretting corrosion.
- Eddy Current Testing: Can identify changes in electrical conductivity due to material loss. 1. Eddy Current Testing (ECT)- NDT | PPT – SlideShare www.slideshare.net
- Microscopy: Detailed examination of the affected surfaces using a microscope can reveal the characteristic features of fretting corrosion.
Other Methods
- Vibration Analysis: Increased vibration levels can sometimes indicate fretting corrosion.
- Acoustic Emission: Monitoring for acoustic emissions generated by the wear process can provide early warning.
It’s important to note that early detection is crucial for preventing the progression of fretting corrosion and its associated damage. Regular inspection and maintenance are essential for identifying and addressing the issue.
Pitting Corrosion
Pitting corrosion is a localized form of corrosion that results in the formation of small holes or pits in the metal. It’s often unpredictable and can lead to catastrophic failures, even with minimal overall metal loss.
How Pitting Corrosion Occurs:
- Breakdown of Protective Layer: A protective oxide film or coating on the metal surface can develop a small defect or breakdown. 1. What is Pitting Corrosion? – D&D Coatings www.ddcoatings.co.uk
- Localized Electrochemical Cell: The exposed metal at the defect becomes the anode, while the surrounding protected area acts as the cathode. 1. What is Pitting Corrosion? – D&D Coatings www.ddcoatings.co.uk
- Pit Formation: Anodic dissolution occurs at the defect, leading to the formation of a small pit. 1. Pitting corrosion – Wikipedia en.wikipedia.org
- Pit Growth: The pit deepens as the corrosion process continues, often with limited outward signs.
Factors Affecting Pitting Corrosion:
- Chloride Ions: The presence of chloride ions, often found in seawater, accelerates pitting corrosion. 1. Chloride-Induced Pitting Corrosion Of Austenitic Stainless Steel | Astro Pak astropak.com
- Oxygen Concentration: High oxygen levels can contribute to the formation and growth of pits.
- Metal Composition: Certain metals, such as stainless steels, are more susceptible to pitting corrosion. 1. Pitting corrosion – Wikipedia en.wikipedia.org
- Surface Condition: Scratches, abrasions, and other defects can act as initiation points for pits. 1. What is Pitting Corrosion? – D&D Coatings www.ddcoatings.co.uk
Preventing pitting corrosion involves careful material selection, proper surface preparation, and the use of protective coatings.
Common Causes of Pitting Corrosion in Marine Engineering
Pitting corrosion is a significant concern in marine engineering due to the aggressive nature of the seawater environment. The primary causes include:
1. Corrosion Behavior of Stainless Steel in Seawater in the Presence of Sulfide – MDPI
Environmental Factors
- Chloride Ions: The presence of chloride ions in seawater is a primary contributor to pitting corrosion. These ions disrupt the protective oxide layer on metals. 1. What is the cause of pitting corrossion? | 5 Answers from Research papers – Typeset.io typeset.io2. Corrosion Study on Duplex Stainless Steel UNS S31803 Subjected to Solutions Containing Chloride Ions – MDPI www.mdpi.com
- Oxygen Concentration: High levels of dissolved oxygen accelerate the corrosion process. 1. Coupling Effects of pH and Dissolved Oxygen on the Corrosion Behavior and Mechanism of X80 Steel in Acidic Soil Simulated Solution – MDPI www.mdpi.com
- Temperature: Elevated temperatures can increase the rate of pitting corrosion. 1. Effect of temperature on the pitting corrosion resistance of CrMn stainless steel in buffer solutions of pH 8 with different chloride concentrations. – ResearchGate www.researchgate.net
- pH Levels: Variations in pH can affect the stability of protective oxide films. 1. Pitting corrosion – Wikipedia en.wikipedia.org
Material Factors
- Metal Composition: Certain metals and alloys are more susceptible to pitting corrosion than others. Stainless steels, for example, can be susceptible if not properly selected or maintained. 1. Top 5 Causes of Pitting Corrosion – Heartland Repair & Coating www.heartlandrepaircoatings.com
- Surface Condition: Scratches, abrasions, or other surface imperfections can act as initiation points for pits.
- Residual Stresses: Internal stresses within the metal can accelerate pitting corrosion.
Operational Factors
Biofouling: The presence of marine organisms can create localized environments that promote pitting.
Stagnant Conditions: Areas with low water flow or stagnant conditions are more prone to pitting corrosion. 1. Identifying Types of Corrosion in Water Systems – Chem-Aqua www.chemaqua.com
The Dangers of Pitting Corrosion
Pitting corrosion is particularly dangerous due to its insidious nature:
- Hidden Damage: The majority of the damage occurs internally, often with minimal external signs. This makes it difficult to detect until the pit has penetrated deeply. 1. What is Pitting Corrosion? – D&D Coatings www.ddcoatings.co.uk
- Localized Weakness: The formation of a pit creates a stress concentration point, increasing the risk of fatigue failure or crack initiation. 1. Pitting Corrosion – AMPP www.ampp.org
- Catastrophic Failure: In critical components, a single pit can lead to catastrophic failure with potentially severe consequences. 1. Pitting Corrosion – AMPP www.ampp.org
- Economic Loss: Pitting corrosion can result in costly repairs or replacements. 1. Pitting Repairs for High-Temperature/High-Pressure Process Vessels – Materials Performance Magazine www.materialsperformance.com
Examples of industries where pitting corrosion is a significant concern include:
- Oil and gas: Pipelines and storage tanks
- Marine engineering: Hulls, propellers, and heat exchangers
- Chemical processing: Equipment exposed to corrosive fluids