Impressed Current Cathodic Protection SystemImpressed Current Cathodic Protection System
The ship’s hull is corroding in sea water. Generally this is electro-chemical reaction in which the metal combines with an oxygen, to form a metal oxide or other compound. This depends upon the nature of the environment. Different metals have different tendencies to corrode, activity or potential.
Some metals and alloys have two positions in the series, marked Active and Passive.
The active position is when the corrosion is occurring and approaches the electro-chemical series position for the material. The passive position relates to a non-corroding situation where the material is protected by a self forming surface film.
If two metals are placed in an electrolyte (e.g. sea water or damp soil) and are in direct electrical contact, a current will pass through the electrolyte from the more active metal onto the least active metal.
The least active metal does not corrode and is termed the cathode. The more active metal, the anode, passes into solution and the flow of electrical current increases. This is a metal ion and electron transfer process i.e., it corrodes
The anodic and cathodic areas in a corrosion cell may be due to the electrical contact of two dissimilar metals, galvanic corrosion. Anodic and cathodic areas may be formed on a single metal surface as micro-cells for instance by rain drops on uncoated steel. Alternatively, they may be close but discrete cells found when accelerated corrosion occurs at uncoated anodic areas on a generally coated cathodic structure.
Large currents can occur at small anodic areas and lead to rapid corrosion of marine structures such as ship's internal tanks, external hull plates, sheet steel piling in harbours and tubular structures common in jetties and petrochemical drilling and production platforms.
Cathodic Protection is a system of preventing corrosion by forcing all surfaces of a structure to be cathodes by providing external anodes.
Sacrificial anode cathodic protection achieves corrosion prevention on a particular structure or component by forming galvanic cell where an additional anode of zinc, magnesium or aluminium corrodes in preference to the structure. The galvanic corrosion current (see simple cell before) available from this anode / electrolyte / structure combination should be sufficient to overcome the local surface corrosion currents on the structure until no current flows from anodic areas of the structure i.e the structure is entirely cathodic or under complete cathodic protection As indicated previously, a metal can be made cathodic by electrically connecting it to a more anodic metal within the electrolyte. The most commonly used anodic metals are alloys of aluminium, zinc and magnesium. Anodes of these metals corrode preferentialy, the corrosion current of the anode achieving cathodic protection of the structure to which they are connected.
The anodes deteriorate as an essential part of their function and they are therefore termed sacrificial
Introduction of ICCP
A metal also can be made cathodic by electrically connecting it to another metallic component in the same electrolyte through a source of direct electric current. The current flow from this metallic component must be sufficient to overcome the natural corrosion current. Thus we will direct the current flow to occur off the surface of added metallic component (anode), into the electrolyte and onto the metal (cathode).
All we need is to measure what the natural corrosion current is. So we add one more electrode – reference cell – completely passive metal. The potential difference between the hull and reference cell will form the natural corrosion current. So another electrode – anode - with a power source is introduced so that the current flow from this electrode is sufficient to overcome the natural corrosion current.
Because an external current source is employed, this type of protection is termed 'IMPRESSED CURRENT CATHODIC PROTECTION'.
A source of direct current is required, this is generally obtained from mains power units that contain a transformer and rectifier.
The magnitude of this current may be automatically controlled in response to a continuous monitor of the cathode / electrolyte potential or may be manually controlled after intermittent measurement.
The impressed current anode material is ideally non-consumed by the passage of current from it into the electrolyte, in practice the materials used are a compromise between this ideal and the cost and physical properties of available materials. Impressed current anodes are made from graphite, silicon iron, lead alloys some with platinum dielectrodes, platinised titanium or more exotic combinations such as platinum clad niobium. The selection of the correct anode material is critical in the formulation of an effective and economic cathodic protection scheme
Generally, for a given current demand, less impressed current anodes than sacrificial anodes are required for protection, as high anode currents are feasible.
Impressed current systems of cathodic protection are more sophisticated in design than sacrificial systems.
The tipycal elements of ICCP
Control Panel Anodes Reference cell
The interconnection is similar to given picture below:
Although modern hull coatings can provide some protection against corrosion they seldom offer a complete solution. For this reason, most operators choose to protect their vessels with a purpose designed impressed current cathodic protection system.
Using an arrangement of hull mounted anodes and reference cells connected to a control panel(s), the system produces a more powerful external current to suppress the natural electro-chemical activity on the wetted surface of the hull. This eliminates the formation of aggressive corrosion cells on the surface of plates and avoids the problems which can exist where dissimilar metals are introduced through welding or brought into proximity by other components such as propellers.
An essential feature of ICCP system is that they constantly monitor the electrical potential at the seawater/hull interface and carefully adjust the output to the anodes in relation to this.
Therefore, the system is much more effective and reliable.