AME Unit 7 Marine, Galvanic and Stess Corrosion Quiz

Correct Answers:

A. Galvanic corrosion, where one metal corrodes preferentially due to differences in electrochemical potentials.
C. Formation of an electrolytic cell that accelerates corrosion of the less noble metal.

Explanation:

• A. Galvanic Corrosion: This is correct because when two dissimilar metals are in contact in seawater, they can form a galvanic cell. The metal with a lower electrochemical potential (anodic) corrodes faster than it would if it were not in contact with the other metal, while the metal with a higher electrochemical potential (cathodic) is protected.
• C. Formation of an Electrolytic Cell: This is correct because the contact of dissimilar metals in seawater can create an electrolytic cell that accelerates the corrosion of the less noble (anodic) metal due to the electrochemical reactions.

B. Increased Mechanical Strength Due to Protective Oxide Layers: This is incorrect because dissimilar metals in seawater do not typically result in increased mechanical strength; instead, the primary issue is increased corrosion.

D. Enhanced Durability of Both Metals Due to a Mutual Protective Effect: This is incorrect because dissimilar metals in contact generally lead to increased corrosion rather than enhanced durability. One metal corrodes faster, and there is no mutual protective effect.

E. Decreased Conductivity of Seawater, Which Reduces Corrosion Rates: This is incorrect because the presence of seawater actually facilitates corrosion due to its conductivity and ionic content. Seawater’s conductivity typically accelerates the corrosion process, rather than reducing it.

Correct Answers:

A. Using sacrificial anodes made of a more anodic metal to protect the less noble metal from corrosion.
B. Applying coatings or paints to isolate the metals and prevent direct contact.
D. Employing electrical insulation or non-conductive materials between the dissimilar metals to prevent galvanic corrosion.

Explanation:

• A. Using Sacrificial Anodes: This is correct because sacrificial anodes are made of a more anodic metal than the metals being protected. The sacrificial anodes corrode preferentially, protecting the less noble metal from galvanic corrosion.
• B. Applying Coatings or Paints: This is correct because coatings or paints can act as barriers, preventing direct contact between the dissimilar metals and thus reducing the risk of galvanic corrosion.
• D. Employing Electrical Insulation: This is correct because using electrical insulation or non-conductive materials between the dissimilar metals can prevent the formation of a galvanic cell, thereby reducing galvanic corrosion.

C. Increasing the Surface Area of the Less Noble Metal: This is incorrect because increasing the surface area of the less noble metal does not effectively reduce its corrosion rate and may actually increase the rate of corrosion by providing more surface area for the galvanic reaction.

E. Using Metals with Similar Electrochemical Potentials: This is correct in principle but does not directly address the issues of dissimilar metals already in contact. While using metals with similar electrochemical potentials can help prevent galvanic corrosion, the question specifically asks about methods to reduce problems with metals already in contact.

Explanation:

• A. Temperature of the Seawater: Higher temperatures accelerate corrosion by increasing reaction rates and reducing oxygen solubility.
• B. Presence of Dissolved Oxygen: More dissolved oxygen can enhance corrosion rates due to its role in electrochemical reactions.
• C. Type of Metal Used: Different metals have different corrosion rates based on their electrochemical properties.
• D. Salinity of the Seawater: Higher salinity increases electrical conductivity, which accelerates corrosion.
• F. Flow Rate of Seawater: Increased flow rates enhance the delivery of corrosive agents to the metal surface, increasing corrosion.
• G. Presence of Biological Organisms: Biofouling can create corrosive microenvironments and increase corrosion rates.
• H. Duration of Exposure: Longer exposure allows more time for corrosion to occur, increasing overall corrosion.

E. Presence of Protective Coatings or Paints: Incorrect because protective coatings or paints actually reduce corrosion rates.

I. Metal’s Color or Paint Scheme: Incorrect because the color or paint scheme of the metal does not influence the corrosion rate of the unprotected metal surface.

J. Amount of Sunlight Exposure: Incorrect because sunlight exposure does not directly affect the rate of corrosion in a marine environment.

Correct Answers:

A. The electrochemical potential difference between the dissimilar metals.
D. The presence and conductivity of the electrolyte.

Explanation:

• A. The Electrochemical Potential Difference: This is correct because a larger potential difference between the dissimilar metals increases the severity of galvanic corrosion. Metals with a greater difference in electrochemical potential will result in more pronounced corrosion of the anodic metal.
• D. The Presence and Conductivity of the Electrolyte: This is correct because the presence of an electrolyte (such as seawater) facilitates the flow of electric current between the metals. The higher the conductivity of the electrolyte, the more severe the galvanic corrosion.

B. The Color of the Metals: This is incorrect because the color of the metals does not influence galvanic corrosion. The key factors are the electrochemical properties and the environment.

C. The Temperature of the Surrounding Environment: While temperature can affect the rate of corrosion in general, it is not a direct factor in the severity of galvanic corrosion specifically. The key factors are the potential difference and the electrolyte conductivity.

E. The Thickness of the Metal Layers: This is incorrect because the thickness of the metal layers does not directly impact the severity of galvanic corrosion. The primary factors are the electrochemical potential difference and the presence of an electrolyte.

Correct Answer:

A. The formation of an electrolytic cell where two dissimilar metals in contact in the presence of an electrolyte leads to one metal corroding preferentially while the other is protected.

Explanation:

• A. Correct: Galvanic corrosion occurs when two dissimilar metals are in electrical contact within an electrolyte (such as seawater). The metal with a lower electrochemical potential (anodic) corrodes preferentially, while the metal with a higher electrochemical potential (cathodic) is protected. This creates a galvanic cell.
• B. Incorrect: This describes metal degradation due to high temperatures without an electrolyte, which is not galvanic corrosion. High-temperature corrosion is a different process.
• C. Incorrect: This describes general corrosion or rusting, which occurs due to chemical reactions with air and moisture, not specifically galvanic corrosion. It does not involve an electrolytic cell.
• D. Incorrect: This describes corrosion caused by the accumulation of salt crystals leading to mechanical stress, not galvanic corrosion. Galvanic corrosion involves electrochemical reactions rather than mechanical stress.

Correct Answer:

A. By providing a more anodic metal that corrodes preferentially, thereby protecting the hull fittings (which are less anodic) from corrosion.

Explanation:

• A. Correct: Sacrificial anodes are made of a more anodic metal compared to the hull fittings. When the anodes are in contact with the hull fittings and exposed to seawater, the sacrificial anodes corrode preferentially due to their higher electrochemical potential. This protects the hull fittings from corrosion by making them the cathodic part of the galvanic cell.
• B. Incorrect: Sacrificial anodes do not form a protective oxide layer. Instead, they provide a source of corrosion to protect other metals.
• C. Incorrect: While sacrificial anodes do affect the local electrochemical environment, their primary function is not to increase the electrical conductivity of the water but to corrode themselves to protect the hull fittings.
• D. Incorrect: Sacrificial anodes do not use coatings or physical shields. Their method of protection involves electrochemical reactions, not physical barriers.

Correct Answer:

C. Sacrificial anodes should be distributed along the hull surface and in contact with the water, as this allows them to create a protective galvanic cell that protects the entire hull including the fittings.

Explanation:

• C. Correct: Sacrificial anodes should be placed along the hull surface where they are exposed to the water. This placement ensures that the anodes can create a galvanic cell that protects not only the hull but also the hull fittings by reducing the rate of corrosion of all metal parts in contact with the water.
• A. Incorrect: Sacrificial anodes should not be fitted directly on the hull fittings themselves. The anodes are typically placed on the hull, not on specific fittings, to provide overall protection through a galvanic reaction.
• B. Incorrect: Sacrificial anodes need to be in contact with the water and close to the areas they are protecting. Placing them far from the hull fittings would not provide effective protection.
• D. Incorrect: Sacrificial anodes are not installed inside the hull fittings. They are mounted on the hull's surface to protect the entire hull and fittings through galvanic action.

Correct Answer:

A. An impressed current system uses an external power source to apply a direct current to the hull fittings, making the fittings the anode in the electrochemical cell, which prevents their corrosion.

Explanation:

• A. Correct: An impressed current system works by using an external power source (like a rectifier) to provide a continuous direct current to the hull fittings. This current forces the fittings to act as the anode, thereby preventing their corrosion by controlling the electrochemical reactions on the metal surface.
• B. Incorrect: This describes a natural galvanic or sacrificial anode system, not an impressed current system. Impressed current systems rely on external power rather than natural electrical potential differences.
• C. Incorrect: This describes a sacrificial anode system, not an impressed current system. Sacrificial anodes provide protection through galvanic corrosion, not impressed current.
• D. Incorrect: This describes the use of chemical coatings, which are not part of an impressed current system. Impressed current systems rely on an external electrical current rather than physical barriers.

Correct Answers:

A. Using corrosion-resistant materials or alloys for construction.
B. Installing sacrificial anodes in the cooling system.
C. Applying protective coatings to internal surfaces.
D. Regularly cleaning the cooling system to remove debris and biological growth.

Explanation:

• A. Using Corrosion-Resistant Materials or Alloys: Correct. Utilizing materials that are resistant to corrosion, such as stainless steel or specialized alloys, helps to prevent the onset of corrosion in seawater cooling systems.
• B. Installing Sacrificial Anodes: Correct. Sacrificial anodes are used to protect other metal parts of the cooling system from corrosion. They corrode preferentially due to their higher electrochemical potential.
• C. Applying Protective Coatings: Correct. Coatings can create a protective layer on internal surfaces, preventing direct contact with seawater and reducing corrosion.
• D. Regularly Cleaning the Cooling System: Correct. Cleaning the system helps remove debris and biological growth, which can cause localized corrosion and blockages.
• E. Increasing the Temperature of the Seawater: Incorrect. Higher temperatures can actually accelerate corrosion rates rather than minimize them. Maintaining moderate temperatures is usually preferable.
• F. Reducing the Flow Rate of Seawater: Incorrect. Reducing flow rate can lead to stagnation and increased risk of fouling and localized corrosion. Proper flow is necessary to keep the system clean and effective.

Correct Answer:

A. Stray currents from electrical sources in the marina create a galvanic cell with the boat’s metal hull, leading to accelerated corrosion and increased heat around dock fittings.

Explanation:

• A. Correct: Stray currents, often originating from electrical systems in the marina or neighboring boats, can flow through the water and create a galvanic cell with the boat's hull. This leads to accelerated corrosion (hot docking) on the boat’s metal parts that are in contact with the water, and the corrosion can cause localized heating around dock fittings.
• B. Incorrect: Stray currents do not directly heat the marina water to increase engine temperatures. The primary effect is corrosion rather than water heating.
• C. Incorrect: While stray currents can cause electrical interference, they do not primarily affect navigation systems or cause overheating of boat engines. The main issue is corrosion.
• D. Incorrect: Stray currents causing localized corrosion is correct, but the heating of hot spots and reduced efficiency in cooling systems is not a typical consequence of stray currents. The primary concern is accelerated corrosion.

Correct Answers:

A. Impressed current systems installed on marina docks.
B. Galvanic isolators installed on boats.

Explanation:

• A. Impressed Current Systems Installed on Marina Docks: Correct. Impressed current systems can be used to counteract stray currents by providing a controlled current to neutralize potential differences that cause corrosion. This helps prevent hot docking by reducing the amount of stray current that can affect boats.
• B. Galvanic Isolators Installed on Boats: Correct. Galvanic isolators prevent stray currents from flowing between a boat’s electrical system and the marina’s shore power system. They protect the boat’s metal parts from accelerated corrosion by breaking the galvanic path.
• C. Automatic Bilge Pumps: Incorrect. While automatic bilge pumps are useful for removing water from a boat’s bilge, they do not address stray currents or hot docking issues.
• D. Water Filters: Incorrect. Water filters are not related to preventing stray currents or hot docking. They are used for filtering out impurities from water, not for managing electrical issues.
• E. Anode Replacement Kits: Incorrect. Anode replacement kits are used for replacing sacrificial anodes, which are part of a galvanic protection system. However, they do not specifically address stray currents in the marina or prevent hot docking.

Correct Answer:

B. Fretting corrosion happens when two metal surfaces are subjected to repeated small oscillatory motions, leading to the wear of the protective oxide layer and localized corrosion.

Explanation:

• B. Correct: Fretting corrosion is characterized by the damage and corrosion that occurs when two metal surfaces experience repeated, small oscillatory movements. These movements can wear away the protective oxide layer, exposing fresh metal surfaces to the environment, which then leads to localized corrosion.
• A. Incorrect: While high temperatures can influence corrosion, fretting corrosion specifically involves mechanical movement between metal surfaces rather than just exposure to high temperatures.
• C. Incorrect: General corrosion in seawater is not the same as fretting corrosion. Fretting corrosion involves mechanical movement and wear, not just exposure to seawater.
• D. Incorrect: The application of high-pressure steam might lead to different types of corrosion or degradation, but fretting corrosion is related to mechanical movement between metal surfaces, not steam exposure.

Correct Answer:

B. Pitting corrosion is a form of localized corrosion that results in small, often deep pits or cavities on the metal surface.

Explanation:

• B. Correct: Pitting corrosion is characterized by localized attack on a metal surface, leading to the formation of small, deep pits or cavities. This type of corrosion is particularly dangerous because the pits can be much deeper than the surrounding surface, potentially leading to structural failure.
• A. Incorrect: Uniform thinning of a metal surface describes general corrosion, not pitting corrosion. Pitting corrosion is localized rather than uniform.
• C. Incorrect: High temperatures causing brittleness and cracking refer to thermal stress or heat damage, not pitting corrosion.
• D. Incorrect: Accumulation of salt deposits can contribute to corrosion but is not specifically descriptive of pitting corrosion, which is more about localized pit formation rather than general surface layer corrosion.

Correct Answer:

B. Repeated small oscillatory or vibratory movements between metal surfaces.

Explanation:

• B. Correct: Fretting corrosion commonly occurs due to repeated small oscillatory or vibratory movements between two metal surfaces. These movements cause wear of the protective oxide layer, leading to localized corrosion.
• A. Incorrect: While high humidity and temperature fluctuations can contribute to corrosion, they are not specific causes of fretting corrosion. Fretting corrosion is more directly related to mechanical movement between surfaces.
• C. Incorrect: Contact with acidic or alkaline solutions can lead to different types of corrosion, such as chemical or pitting corrosion, but fretting corrosion is caused by mechanical movement, not chemical exposure.
• D. Incorrect: Long-term submersion in seawater can lead to general corrosion or galvanic corrosion, but fretting corrosion specifically involves mechanical movement between metal surfaces rather than just being submerged in water.

Correct Answers:

A. Exposure to high levels of chloride ions, such as from seawater or salt.
B. Prolonged exposure to alkaline solutions or high pH environments.

Explanation:

• A. Correct: High levels of chloride ions are a well-known cause of pitting corrosion. Chlorides can disrupt the protective oxide layer on metals, leading to localized pit formation.
• B. Correct: Alkaline solutions or high pH environments can also contribute to pitting corrosion. These conditions can lead to localized breakdown of the protective layer on metal surfaces.
• C. Incorrect: Mechanical abrasion and physical wear generally cause surface damage and wear but are not primary causes of pitting corrosion. Pitting is more related to chemical environments rather than physical damage.
• D. Incorrect: Low temperatures and the formation of metal oxides are not directly related to pitting corrosion. Pitting typically involves localized breakdown of protective layers rather than issues caused by temperature alone.
• E. Incorrect: Contamination with sulfur compounds can lead to other forms of corrosion, such as sulfidation, but not typically pitting corrosion in the context given.

Correct Answer:

B. It causes localized damage, creating deep pits that can significantly weaken the metal structure and lead to unexpected failures.

Explanation:

• B. Correct: Pitting corrosion is dangerous because it leads to localized, often deep pits in the metal surface. These pits can penetrate deeply and create areas of weakness that can lead to sudden and catastrophic structural failures, making it difficult to predict or assess the overall integrity of the material.
• A. Incorrect: Uniform and gradual thinning is characteristic of general corrosion, not pitting corrosion. Pitting corrosion is localized rather than uniform.
• C. Incorrect: Slow and uniform oxidation describes general corrosion processes, not the localized pit formation seen in pitting corrosion.
• D. Incorrect: Pitting corrosion does not produce a protective layer. Instead, it results in localized breakdown of protective coatings or layers, leading to further corrosion.

Correct Answer:

C. By identifying wear patterns and discoloration in areas of contact between metal surfaces.

Explanation:

• C. Correct: Fretting corrosion is typically detected by inspecting the metal surfaces for wear patterns and discoloration where the surfaces come into contact. This includes signs of localized wear, surface damage, and discoloration caused by the removal of the protective oxide layer and subsequent corrosion.
• A. Incorrect: While visible rust and corrosion products may indicate general corrosion, fretting corrosion is more specifically characterized by wear and discoloration in areas of mechanical contact rather than just visible rust.
• B. Incorrect: Measuring electrical resistance changes is not a typical method for detecting fretting corrosion. Detection is generally based on physical inspection of wear and damage.
• D. Incorrect: Analyzing water samples for metal ion concentrations is more relevant for detecting general corrosion or contamination, not for detecting fretting corrosion, which is associated with mechanical wear and damage between metal surfaces.

Correct Answer:

B. Erosion damage results from the gradual wearing away of metal surfaces due to the flow of abrasive fluids or particles.

Explanation:

• B. Correct: Erosion damage is characterized by the gradual loss of material from metal surfaces due to the action of flowing abrasive fluids or particles. This is common in areas with high-velocity fluid flow or where solids are entrained in a liquid.
• A. Incorrect: High temperatures leading to oxidation and material loss describe thermal damage or oxidation, not erosion damage.
• C. Incorrect: Corrosion pits due to localized electrochemical reactions describe pitting corrosion, not erosion damage which involves abrasive wear.
• D. Incorrect: Exposure to corrosive chemicals leading to surface degradation describes chemical corrosion, not erosion damage which involves mechanical wear from abrasive particles or fluids.

Correct Answer:

A. Galvanic corrosion occurs when two different metals are in electrical contact and exposed to a common electrolyte, leading to corrosion of the more anodic metal.

Explanation:

• A. Correct: Galvanic corrosion occurs when two dissimilar metals are electrically connected and exposed to an electrolyte, causing the more anodic metal to corrode preferentially.
• B. Incorrect: High temperatures and oxidation describe different corrosion mechanisms, such as general corrosion or oxidation, rather than galvanic corrosion.
• C. Incorrect: Mechanical wear and localized corrosion are not related to galvanic corrosion. Galvanic corrosion specifically involves the interaction of dissimilar metals in an electrolyte.
• D. Incorrect: Chemical reactions with non-metallic substances describe other forms of corrosion, such as chemical or environmental corrosion, rather than galvanic corrosion which involves electrical contact between metals.

Correct Answer:

A. Atmospheric corrosion is the degradation of metals caused by exposure to environmental conditions such as rain, humidity, and temperature changes.

Explanation:

• A. Correct: Atmospheric corrosion refers to the degradation of metals due to exposure to various environmental factors including moisture, pollutants, temperature fluctuations, and other atmospheric conditions.
• B. Incorrect: High temperatures in industrial settings causing oxidation describes thermal or high-temperature oxidation, not atmospheric corrosion.
• C. Incorrect: Contact with acids and bases leading to chemical corrosion describes chemical corrosion rather than atmospheric corrosion, which involves environmental exposure.
• D. Incorrect: Electrical currents causing galvanic corrosion is a different process, not related to the environmental factors causing atmospheric corrosion.

Correct Answer:

B. Stress corrosion is a type of localized corrosion that occurs when metals are subjected to tensile stress in a corrosive environment, leading to cracking and failure.

Explanation:

• B. Correct: Stress corrosion occurs when a metal is under tensile stress and is simultaneously exposed to a corrosive environment. This combination can lead to the formation of cracks and eventual failure of the material due to the synergistic effects of stress and corrosion.
• A. Incorrect: Uniform corrosion is general corrosion over a surface, not specific to tensile stress or cracking.
• C. Incorrect: Pitting corrosion is localized but is not directly related to the presence of tensile stress in the way stress corrosion is.
• D. Incorrect: High temperatures causing thermal degradation describe thermal damage, not stress corrosion, which is specifically related to stress and corrosion combined.

Correct Answer:

B. Cavitation damage results from the formation and collapse of vapor bubbles in a liquid, causing shock waves that erode metal surfaces.

Explanation:

• B. Correct: Cavitation damage is caused by the formation and subsequent collapse of vapor bubbles in a liquid, leading to shock waves that erode and damage the metal surfaces. This typically occurs in pumps, propellers, and turbines.
• A. Incorrect: High temperatures causing brittleness and cracking describe thermal stress or heat damage, not cavitation damage.
• C. Incorrect: Continuous mechanical abrasion leading to wear describes abrasion or erosive wear, not cavitation damage which involves bubble collapse and shock waves.
• D. Incorrect: Exposure to acidic environments leading to chemical corrosion describes general corrosion, not cavitation damage which involves physical damage from shock waves.