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 surface is made sufficiently
cathodic, corrosion will not occur. This is the basic principle of Cathodic Protection.
In marine structures, such corrosion cells may result from the use of dissimilar metals. Usually, however, localised anodic and cathodic areas arise on the surface of the same metal through
differences in the metal itself, variations in protective films or changes in the electrolyte. ie: aeration, temperature and salinity.
A technique for preventing corrosion which is the most important of all methods used to control corrosion of marine structures, including ships’ hulls. 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 (base) metal. Anodes based on alloys of Zinc, Aluminium 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. The system converts the ship’s a.c. supply into a controlled low voltage d.c. output, which is then delivered onto the metal surface by long life anodes attached to, but insulated from the hull structure. Various factors affect the amount of current required, therefore to ensure the correct level of protection is accurately controlled it is necessary to measure the potential of the steel against a
known and reliable reference cell.
This potential is monitored by reference electrodes mounted on the underwater hull surface. The number of electrodes and their locations is carefully selected in conjunction with the anode configuration and hull geometry. Solid state circuitry within an automatic control unit compares the
reference potential against a desired and pre-set optimum. Any difference between these will induce a resultant error signal, which is electronically conditioned to provide suitable regulation of the d.c. power supply to the anodes.
cathodic, corrosion will not occur. This is the basic principle of Cathodic Protection.
In marine structures, such corrosion cells may result from the use of dissimilar metals. Usually, however, localised anodic and cathodic areas arise on the surface of the same metal through
differences in the metal itself, variations in protective films or changes in the electrolyte. ie: aeration, temperature and salinity.
A technique for preventing corrosion which is the most important of all methods used to control corrosion of marine structures, including ships’ hulls. 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 (base) metal. Anodes based on alloys of Zinc, Aluminium 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. The system converts the ship’s a.c. supply into a controlled low voltage d.c. output, which is then delivered onto the metal surface by long life anodes attached to, but insulated from the hull structure. Various factors affect the amount of current required, therefore to ensure the correct level of protection is accurately controlled it is necessary to measure the potential of the steel against a
known and reliable reference cell.
This potential is monitored by reference electrodes mounted on the underwater hull surface. The number of electrodes and their locations is carefully selected in conjunction with the anode configuration and hull geometry. Solid state circuitry within an automatic control unit compares the
reference potential against a desired and pre-set optimum. Any difference between these will induce a resultant error signal, which is electronically conditioned to provide suitable regulation of the d.c. power supply to the anodes.
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