(a) Describe the problems associated with two dissimilar metals in contact in the presence of sea water.(4)
(b) Describe THREE different methods that may be used to reduce the problems described in part (a).(6)
Problems Associated with Dissimilar Metals in Seawater
When two different metals come into contact in a seawater environment, a phenomenon called galvanic corrosion can occur. This is an electrochemical process where one metal acts as an anode and corrodes preferentially, while the other metal acts as a cathode and is protected.
1. Understanding Galvanic Corrosion: Concepts, Causes, and Prevention
2. Galvanic Corrosion – Definition, Mechanism, Examples, Causes & Prevention with Videos
Key Problems:
- Accelerated Corrosion: The anodic metal corrodes at a faster rate than it would if it were alone in seawater. 1. Understanding Dissimilar Metal Corrosion – GHP www.ghp1.com
- Structural Integrity: Severe corrosion can compromise the structural integrity of the metal components involved.
- Economic Loss: Repair or replacement of corroded components can be costly. 1. Galvanic corrosion – Wikipedia en.wikipedia.org
- Safety Risks: In extreme cases, galvanic corrosion can lead to equipment failure, posing safety risks.
Factors Affecting Galvanic Corrosion:
- Metal Combination: The further apart the metals are in the galvanic series, the greater the potential for corrosion. 1. Galvanic series – Cathwell cathwell.com
- Electrolyte Conductivity: Seawater, with its high salt content, is an excellent conductor, accelerating the corrosion process. 1. Among distilled water, tap water, and ocean water, which will be the best electrical conductor and why? – BYJU’S byjus.com
- Surface Area Ratio: The ratio of the anode to cathode area affects the corrosion rate. A larger anode compared to the cathode can intensify corrosion. 1. Effect of Area Ratio on the Galvanic Corrosion of AZX611 Magnesium Alloy/A6N01 Aluminum Alloy Joint – J-Stage www.jstage.jst.go.jp
- Environmental Factors: Temperature, oxygen levels, and water flow can influence the rate of corrosion.
To mitigate the effects of galvanic corrosion, it’s essential to understand the galvanic series and take appropriate measures, such as using sacrificial anodes or insulating dissimilar metals.
Methods to Reduce Galvanic Corrosion
There are several effective methods to mitigate galvanic corrosion:
Material Selection
- Similar Metals: Using metals that are close together in the galvanic series can significantly reduce the corrosion rate. 1. Understanding Galvanic Corrosion: Concepts, Causes, and Prevention armoloy.com
- Noble Metals: Employing highly corrosion-resistant materials like titanium or stainless steel can minimize the risk.
Physical Separation
- Insulation: Placing non-conductive materials between dissimilar metals can interrupt the electrical connection. 1. Galvanic Corrosion: Avoidance and Risk Mitigation Explained – Unified Alloys www.unifiedalloys.com
- Distance: Increasing the distance between dissimilar metals can reduce the severity of galvanic corrosion.
Sacrificial Anodes
- Zinc or Magnesium: Attaching a more reactive metal (anode) to the protected metal (cathode) causes the anode to corrode preferentially, protecting the main structure. 1. Sacrificial Anode – Chemistry LibreTexts chem.libretexts.org
Cathodic Protection
- Impressed Current: Using an external power source to apply a current to the structure, making it a cathode and preventing corrosion.
Coatings and Inhibitors
- Protective Coatings: Applying coatings like paint or epoxy can act as a barrier between the metal and the electrolyte. 1. Transport of Electrolyte in Organic Coatings on Metal – IntechOpen www.intechopen.com
- Corrosion Inhibitors: Adding chemicals to the seawater can slow down the corrosion process.
By implementing these methods, the risk of galvanic corrosion can be significantly reduced, extending the lifespan of marine structures and equipment.