(a) Eight Factors Influencing Corrosion Rate:
- Salinity: Higher salinity in seawater leads to increased conductivity and promotes faster ionic movement, accelerating corrosion.
- Temperature: Warmer water generally increases corrosion rates due to higher ionic mobility and chemical reaction kinetics.
- Dissolved Oxygen: Oxygen acts as a cathodic reactant in the corrosion process, and higher dissolved oxygen levels in seawater intensify corrosion.
- pH Level: Acidic (lower pH) environments accelerate corrosion, while alkaline (higher pH) environments can offer some protection.
- Biological Activity: Marine organisms like barnacles and bacteria can create micro-environments that promote localized corrosion.
- Water Flow Rate: Higher water flow can remove corrosion products and expose fresh metal to the corrosive environment, increasing the rate.
- Metal Properties: Different metals have varying corrosion resistance based on their inherent properties and electrochemical potentials.
- Stress and Surface Condition: Stresses and rough surfaces can act as initiation points for corrosion and accelerate the process.
(b) Galvanic Corrosion Explained:
Galvanic corrosion occurs when two dissimilar metals are in electrical contact and immersed in a conductive electrolyte, like seawater. Here’s the process:
- Potential Difference: The metals have different electrochemical potentials, creating a voltage difference between them.
- Anode & Cathode Formation: The more active metal (lower potential) becomes the anode, where metal atoms ionise and release electrons.
- Electron Flow: The released electrons flow through the conducting path (seawater) to the less active metal (higher potential), the cathode.
- Cathodic Reaction: Electrons react with oxygen and water at the cathode, forming hydroxyl ions.
- Metal Ion Movement: The metal ions from the anode migrate through the electrolyte towards the cathode.
- Corrosion & Precipitation: At the cathode, the metal ions combine with hydroxyl ions to form insoluble metal hydroxides, which appear as corrosion products.
This cycle continues, with the anode corroding faster than it would alone, while the cathode is protected.
(c) Two Major Factors Influencing Severity of Galvanic Corrosion:
- Potential Difference: The greater the difference in electrochemical potential between the two metals, the faster and more severe the galvanic corrosion.
- Conductivity of the Electrolyte: Increased conductivity of the electrolyte (e.g., saltier seawater) facilitates higher electron flow and intensifies the corrosion process.
Understanding these factors and processes is crucial for mitigating marine corrosion. Using materials with similar potentials, applying protective coatings, and employing cathodic protection are some approaches used to combat this damaging phenomenon in marine environments.