Marine Corrosion & Cathodic Protection
Today much of our infrastructure built upon steel components is seriously threatened by a destructive and persistent enemy: Corrosion.
This undesirable process is initiated by the formation of corrosion cell which has three essential components : an anode and a cathode coupled in an electrically conductive solution ( electrolyte).The corrosion of bare steel occurs by an electrolytic mechanism requiring the presence of water and oxygen. The overall reaction may be expressed as : 4 Fe + 3O2 + 2H2O →2 FeO3 H2O (rust).
This is the result of two separate reactions , one taking place at anodic areas and producing electrons , another at cathodic areas and consuming electrons.
Anode 4Fe → 4 Fe++ + 8e
Cathode 2O2 + 4 H2O + 8e → 8OH
The ferrous and hydroxyl ions produced by these reactions then react to form ferrous hydroxide , which in the presence of oxygen is oxidized to hydrated ferric oxide , or rust.
4 Fe ++ + 8 OH → 4 Fe ( OH) 2
4 Fe (OH2) + O2 → 2Fe2O3 H2O + 2 H2O
Accordingly in the rusting of iron there is a flow of electrons in the metal from the anodic to the cathodic areas and a corresponding corrosion current in the electrolyte carried by the movement of ions in solution. It has been demonstrated that the amount of current that is flowing is equivalent to the amount of metal corroded, in accordance with Faraday’s law. Corrosion always develops at the anode where current leaves the metal and enters the electrolyte , whilst a protective effect occurs at the cathode. Thus if the whole metal made sufficiently cathodic , corrosion will not occur.
This is the basic principle of cathodic protection – the practice of using electrochemical reactions to prevent the corrosion of steel structures.
The essential factor in cathodic protection is to ensure that the unwanted anodic reactions are suppressed by the application of an opposing current.
This opposing current can be achieved by either of two techniques:
1. Sacrificial Anodes – By corroding away a more reactive metal.
Anodes based on alloys of Zinc , Aluminum and occasionally Magnesium are attached to the structure and corrode in preference to the protected metal. Consequently these anodes require renewal at routine intervals.
2. Impressed Current – A protective current is impressed on the structure through semi-inert anodes.
This undesirable process is initiated by the formation of corrosion cell which has three essential components : an anode and a cathode coupled in an electrically conductive solution ( electrolyte).The corrosion of bare steel occurs by an electrolytic mechanism requiring the presence of water and oxygen. The overall reaction may be expressed as : 4 Fe + 3O2 + 2H2O →2 FeO3 H2O (rust).
This is the result of two separate reactions , one taking place at anodic areas and producing electrons , another at cathodic areas and consuming electrons.
Anode 4Fe → 4 Fe++ + 8e
Cathode 2O2 + 4 H2O + 8e → 8OH
The ferrous and hydroxyl ions produced by these reactions then react to form ferrous hydroxide , which in the presence of oxygen is oxidized to hydrated ferric oxide , or rust.
4 Fe ++ + 8 OH → 4 Fe ( OH) 2
4 Fe (OH2) + O2 → 2Fe2O3 H2O + 2 H2O
Accordingly in the rusting of iron there is a flow of electrons in the metal from the anodic to the cathodic areas and a corresponding corrosion current in the electrolyte carried by the movement of ions in solution. It has been demonstrated that the amount of current that is flowing is equivalent to the amount of metal corroded, in accordance with Faraday’s law. Corrosion always develops at the anode where current leaves the metal and enters the electrolyte , whilst a protective effect occurs at the cathode. Thus if the whole metal made sufficiently cathodic , corrosion will not occur.
This is the basic principle of cathodic protection – the practice of using electrochemical reactions to prevent the corrosion of steel structures.
The essential factor in cathodic protection is to ensure that the unwanted anodic reactions are suppressed by the application of an opposing current.
This opposing current can be achieved by either of two techniques:
1. Sacrificial Anodes – By corroding away a more reactive metal.
Anodes based on alloys of Zinc , Aluminum and occasionally Magnesium are attached to the structure and corrode in preference to the protected metal. Consequently these anodes require renewal at routine intervals.
2. Impressed Current – A protective current is impressed on the structure through semi-inert anodes.
