CATHODIC PROTECTION FOR STEEL PILES

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CATHODIC PROTECTION OF STEEL PILES : 

CATHODIC PROTECTION OF STEEL PILES M. Shoaib Sarwar 2003/II MS-CIV-STR-06

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What is CATHODIC PROTECTION A secondary protective measure against corrosion of metal surface. Used against corrosive actions on the steel, underground or underwater- like steel pipe lines, storage tanks, steel pier piles, ships, offshore oil platforms, steel sheet piles, etc. It works on Electrochemical Principle. First Time used in 1820 by Mr. Humphrey Davy. He used it against the corrosion of copper sheeting been used as cladding the hulls of navel vessels. Most rapid and extensive development has been made in 1945 by USA, in Oil & Gas Industry.

CORROSION : 

CORROSION Corrosion is defined as a Destructive Attack on materials through Chemical and Electrochemical Reactions. Corrosion of Metals is an Electrochemical Reaction, where the metal or metal alloy, combines with oxygen to form less energy-rich stable compositions. All Metals have a tendency to return back to stable conditions. This Tendency is greater for Least Noble Metals. These Metals are called ACTIVE and have the lower or more Negative Potential.

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MORE ACTIVE Magnesium -1,48V Zinc -1,03V Aluminum 35-H -0,79V Wrought Iron and Carbon Steel -0,61V Cast Iron -0,61V Stainless Steel Type 430 AISI (17% Cr) -0,57V Stainless Steel Type 304 AISI (18% Cr,18% Ni) -0,53V LESS ACTIVE Rolled Brass -0,40V Copper -0,36V Aluminum Brass -0,32V Nickel -0,2OV Titanium -0,15V Silver -0,13V

ELECTROCHEMICAL REACTION : 

ELECTROCHEMICAL REACTION When Two Metals are Electrically Connected with each other, in an electrolyte (e.g. Sea water, Soils, etc.), electrons flow from more active metal to least active, due to the difference in electric potential. More Active Metal is called ANODE i.e. it supplies current (Electrons), Corrosion takes place here. Least Active Metal is called CATHODE i.e. it receives current (Electrons). Oxygen is reduced here.

SALIENT FEATURES OF CORROSION OF STEEL PILES & STEEL SHEET PILES : 

SALIENT FEATURES OF CORROSION OF STEEL PILES & STEEL SHEET PILES Corrosion is expressed by thickness reduction as a function of time (mm / year). Corrosion can be of various types as, Soil Corrosion, Galvanic Corrosion and Leakage Corrosion. In case of Steel Piles and Steel Sheet Piles, soil corrosion is most important. Soil corrosion is caused by diffusion of oxygen from soil to steel surface. Ground water acts as a diffusion barrier so, corrosion is most significant in unsaturated zone. Corrosion decreases with depth on piles and steel sheet piles, due to reduced oxygen contents. However in flowing waters, corrosion can be significant at greater depths too. Sea water causes more corrosion than fresh water due to more salt contents.

MEASURES AGAINST CORROSION : 

MEASURES AGAINST CORROSION Corrosion Protection Paints. P.E Cover (In Case of Tube Piles only) Zinc Coating (Galvanizing) Electro-chemical (Cathodic Protection) Cement or Concrete Cover

PRINCIPLE OF CATHODIC PROTECTION : 

PRINCIPLE OF CATHODIC PROTECTION The basic principle of Cathodic Protection is to make the steel surface as a CATHODE instead of an ANODE. This can be achieved in TWO Ways :- 1. SACRIFICIAL ANODES. 2. IMPRESSED CURRENT SYSTEM.

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SACRIFICIAL ANODES. Connecting a Less Noble (More Active) metal to the steel surface, hence flow of Electrons (Current) is from that attached metal to Steel. i.e. the attached metal will act as an ANODE and steel surface as CATHODE. Corrosion will take place at Anode. And Steel Surface is protected from corrosion. The attached metals corrode and are called Sacrificial Anodes, these can be replaced periodically. Usually sacrificial anodes are Zinc, Aluminum and Magnesium or their alloys.

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SACRIFICIAL ANODES Sacrificial Anodes are welded on each and every pile. This method works for each pile independently and has no effect on other piles in the area.

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Corroded Samples of Heavy Weight Aluminum ANODES

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Aluminum Anodes mounted on a Steel Structure.

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IMPRESSED CURRENT SYSTEM Impressed-Current System employ inert (Zero or Low Dissolution) anodes and use an external source of DC Power to impress a current onto the cathode (metal) surface. The metal to be protected is coupled to the negative pole of a DIRECT CURRENT Source. Where as positive pole is coupled to an auxiliary anode. This system is adopted because for larger structures or many number of piles, sacrificial anodes can not economically deliver enough current to provide complete protection. In this method, a TRU (Transformer Rectifier Unit) is installed and connected with cables to the “bounded” piles (-ve) and to an external anode (+ve). In this method all the piles in the area needs to be bounded / connected with each other. If installed incorrectly to the group of piles, could become an anode by itself and lead to an even faster corrosion process.

DATA REQUIRED TO DESIGN A C.P. SYSTEM : 

DATA REQUIRED TO DESIGN A C.P. SYSTEM Type of Piling Number and Length of Piling Depth of Piling Type and Specification of Pile Coating Method of Bounding of Piling General Layout Drawings of Area

REFERENCES : 

REFERENCES BAB Project. Abu Dhabi New Compressor Station-7 Project. Bahrain Project Specification And Method Statements of DOLPHIN Project. Qatar Web Sites:- Imbema pacific indonesia. Geoforum Pile Info. POLCOR Protection. Hyper Physics Corrosion Wikkipedia Encyclopedia

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