Cathodic Protection System

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  • Topic: Anode, Battery, Corrosion
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  • Published : September 17, 2011
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K.C. LTD.
MARINE IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM

ICCP DOCUMENT REV(E) : 14/04/06

NOTES : a) Do not attempt to service or readjust the system operating levels without first reading and understanding this Manual. b) The system operates at low d.c. voltage levels and may be severely damaged by high voltage test equipment such as a 500V Megger. Read the relevant section in this manual before carrying out any tests. c) There is no need of protective current as far as ship is well enclosed by a good hull coating (newbuildings or just repaired ships) or the temporary sacrificial anodes around ship’s bow and stern hull area are still alive, which means our system dissipates no current under fully automatic control realizing ship is well protected from corrosion. Then you may see protective current when ship's hull coating gets thinner and thinner or partly damaged. d) Should any information be required which is not covered by this manual, please contact K.C. LTD. immediately. (Address on cover)

CAUTION Customers are recommended for the purchase of genuine parts from us. Imitated parts make the system get fatally damaged.

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K.C. LTD.
1.0 INTRODUCTION:
1.0.1 PRINCIPLES OF CORROSION AND CATHODIC PROTECTION:

ICCP DOCUMENT REV(E) : 14/04/06

Metallic corrosion is an electro-chemical reaction in which the metal combines with a non metal, such as 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. These potentials can be tabulated and form the electro-chemical series. A more practical approach is the determination of the tendency of certain metals to corrode in a particular electrolyte, such as sea water. This is termed the galvanic series of which the following table is an abridged form. Active or Anodic Magnesium Zinc Mild Steel Wrought Iron Cast Iron Ni-Resist 18.8.3% Molybdenum SS, Type 316 (Active) Lead Tin Manganese Bronze Naval Brass Aluminium Bronze Copper 70 Copper 30 Nickel Nickel (Passive) Monel, 70% Nickel-30% Copper 18.8.3% Molybdenum SS, Type 316 (Passive) Noble or Cathodic Note Some metals and alloys have two positions in the series, marked Active and Passive; the active position is equivalent to the position if 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. For example, type 316 stainless steel in sea water is more likely to be passive than type 304 and is therefore generally preferred for immersed marine applications.

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K.C. LTD.

ICCP DOCUMENT REV(E) : 14/04/06

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. This simple cell may be represented as:

Figure 1.1 - Simple Corrosion Cell 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. In addition there are long line type cells that occur on pipelines that pass through aggressive low resistivity solis. These sections form anodic areas and corrode in preference to cathodic areas in less aggressive higher resistivity...
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