March 2021
a) Galvanic Corrosion: Imagine a scenario where two dissimilar metals touch and are immersed in a conductive solution (electrolyte) like seawater. This seemingly innocent contact can trigger a treacherous process called galvanic corrosion. Here’s what happens:
- Dissimilar Potentials: Each metal has its own “electrical potential,” like a battery, but at different levels. The more active metal (lower potential) acts as the anode, while the less active one (higher potential) becomes the cathode.
- Electron Flow: Driven by the potential difference, electrons flow from the anode (active metal) through the electrolyte to the cathode (less active metal).
- Metal Dissolves: At the anode, metal atoms lose electrons and dissolve into the electrolyte as ions. This is where corrosion occurs, eating away at the anode metal.
- Cathodic Reaction: Electrons reaching the cathode react with the electrolyte (usually oxygen and water) to form harmless compounds like hydroxide ions.
This cycle continues, relentlessly corroding the anode while protecting the cathode. The severity depends on factors like the potential difference between the metals and the conductivity of the electrolyte. Avoiding direct contact between dissimilar metals, using sacrificial anodes (made of even more active metals), or applying protective coatings are some ways to combat galvanic corrosion.
(b) Cavitation Damage: Picture a fast-flowing fluid swirling around an object, creating bubbles due to pressure drops. When these bubbles collapse violently near a solid surface, the impact sends miniature shockwaves, like microscopic explosions. This phenomenon is called cavitation, and its destructive force can cause significant damage, aptly named cavitation damage.
The shockwaves erode the surface, creating pits, grooves, and rough patches. This type of damage is particularly common in propellers, turbine blades, and other components exposed to high-speed liquids. Materials with better tensile strength and resistance to fatigue are preferred to combat cavitation. Additionally, optimising fluid flow and reducing pressure fluctuations can help mitigate the problem.
(c) Erosion Damage: Erosion in the context of materials describes the gradual removal of surface material by the abrasive action of a fluid or solid particles. Imagine sandblasting against a metal surface – that’s essentially erosion damage. It can manifest as scratches, grooves, or even complete wear-through in severe cases.
Pipelines, pumps, valves, and other components exposed to fluid flow with solid particles, like slurries or sand-laden water, are particularly susceptible to erosion damage. Choosing abrasion-resistant materials, modifying designs to reduce flow velocity and particle impact, and implementing protective coatings are crucial to prevent this type of wear and tear.
(d) Stress Corrosion: Imagine a metal under constant stress, like a bridge bearing the weight of vehicles. This stress, combined with a specific corrosive environment, can lead to a type of localised attack called stress corrosion. It’s like a double whammy, where the stress weakens the metal and the corrosive environment exploits these weaknesses, leading to cracks and fractures.
Certain materials are more susceptible to stress corrosion in specific environments. For example, stainless steel under chloride stress in seawater is a classic example. Identifying potential stress factors and corrosive environments, choosing resistant materials, and employing stress-relieving techniques are essential to manage this type of damage.
(e) Atmospheric Corrosion: This is the most common type of corrosion, the one we see daily on everyday objects exposed to the elements. Rain, wind, sunlight, and pollutants in the air combine to form a constantly fluctuating corrosive environment. The process usually involves oxidation, where metal atoms react with oxygen in the air, forming oxides like rust on iron or patina on copper.
Atmospheric corrosion rates vary depending on the metal, the specific environment (coastal areas tend to be more corrosive), and protective measures like coatings or surface treatments. Regularly cleaning and maintaining surfaces, choosing corrosion-resistant materials, and applying protective coatings are effective strategies to minimise atmospheric corrosion.