training-presentation-on-cathodic-pritection_compress.pdf

Cathodic Protection Training
Ahmed Asad
Manager Corrosion Control Department
InspecTest Private Limited
ahmed.asad@inspectest.com

Speaker’s Introduction
 Mechatronics Engineering from NUST- 07 Batch
 NACE Senior Corrosion Technologist
 NACE CP level III
 NACE CP Level II
 M-Icorr Certified
NACE: National Association of Corrosion Engineers

Topics
 Corrosion, it’s Types and Consequences
 Methods to Control Corrosion
 Cathodic Protection (CP) System
 Types of CP System
 Cathodic Protection in Piping/Pipelines
 Cathodic Protection in ASTs
 Anodic Protection
 Surveys on Underground Pipeline

What is Corrosion?
“Gradual deterioration of a material by chemical
and / or electrochemical reaction with their
environment”

Example : Rusting of Apple
Oxidation: Loss of Electrons
Fe ---- Fe3+
Fe----- Fe203

Types of Corrosion
 Environmental Cracking
 Uniform Corrosion
 Galvanic Corrosion

Types of Corrosion
 Crevice Corrosion
 Pitting Corrosion
 Intergranular Corrosion

Types of Corrosion
 Selective Leaching
 Erosion Corrosion
 Stress-corrosion cracking

Types of Corrosion
 Microbiologically Influenced
Corrosion (MIC)
 Stray Current Corrosion
 AC Corrosion

Commonly Affected Structures
 Underground pipelines
 Above-ground storage tanks
 Underground tanks
 Vessels
 Oil platforms in steel & concrete
 Subsea pipelines
 Concrete bridges in seawater
 LPG bullets
 Oil Well Casings.

Consequences of Corrosion
When you rest, you rust.
Rush Before it’s too late…….

Corrosion Cell
A corrosion cell is a condition on a metal surface
where flow of electric current occurs between two
metal surfaces
Corrosion Cell Components
 Anode
 Cathode
 Conductive Electrolyte
 Electrical Connection

Corrosion Reactions
Fe --- > Fe+2 + 2e-
Anodic Reaction
Cathodic Reaction O2 + 2H2O + 4e --- > 4OH–
2Fe(OH)2 + H2O + ½ O2 —> 2Fe(OH)2
Net Reaction
The final product is familiarly called as “Rust”

Polarization
 In the context of corrosion, polarization refers to
the potential shift away from open circuit
potential (free corroding potential) of a corroding
system

Polarization
 Anodic Polarization
Potential shift in “Positive" direction (Above Ecorr)
Anodic Polarization
Noble Potential
Electrode / Equilibrium Potential

Polarization
 Cathodic Polarization
Potential shift in “Negative" direction (Below Ecorr)
Cathodic Polarization
Active Potential
Electrode / Equilibrium Potential

Polarization
 Activation Polarization
 Concentration Polarization
 Resistance Polarization
Activation Polarization
Concentration Polarization
Resistance Polarization

Polarization Reduces ΔV Along the Structure

Corrosion Control Techniques
 Material Selection
 Environmental Control
 Inhibitors
 Protective Coatings
 Cathodic Protection

Cathodic Protection (CP)
 A technique used to control the corrosion of a
metal surface by making it the cathode of an
electrochemical cell
 Most commonly used technique
 CP + Coatings = No Corrosion
 Microscopic Cells on a Pipeline ---> P.D.

Cathodic Protection – How it Works…?
 Formation of Anodic & Cathodic regions on a pipe due to
various microscopic sites having different potentials
 External Direct Current (DC) is made to flow from an
anode to cathode (pipe)
 It shifts cathodic sites (having +Ve Potential) to the anodic
potential (-Ve Potential), thus achieving equilibrium*
 Potential difference is minimized reducing current flow
 P.D. = 0 ---- No Corrosion
*Potential shifts towards
Open Circuit Potential

Prevents Corrosion by converting all of the anodic (active)
sites on the metal surface to cathodic(passive) sites by
supplying electrical current (or free electrons) from an
alternate source

How to Verify that U/G Structure is Protected?
 Potential measurement through a reference electrode such
as Cu-CuSO4 R.E.
 -850 mV Vs Cu-CuSO4 R.E. as per NACE International
criteria for effective CP

Why -850 mV…?
 For any metal, it’s energy level is highest when it is new or “bright & shiny.”
 Laboratory measurements of new steel samples in many different soil
environments have indicated the most active corrosion potential is between
−800 mV and −850 mV both referenced to a saturated CSE
 Thus, polarizing a steel structure to −850 mV Vs CSE would satisfy the
true criterion

Galvanic Vs ICCP
Galvanic Anode CP System Impressed Current CP System

Types of Cathodic Protection
Galvanic (Sacrificial) CP Impressed Current CP
– Typically cheaper shorter life
– Does not require an external
power supply
– Current distribution can be a
significant problem leading to
gaps in protection
– Unforgiving in its design because
it has no “volume” switch
– Must have electrical isolation
between the piping and plant
ground over the life of the plant
– Typically a little more expensive
– Has an external power supply
– When designed using linear
anodes, assures thorough
current distribution
– Much longer operating life than a
galvanic system
– Can be designed to work
regardless of electrical isolation
between the piping and plant
ground
– Easy to maintain and test

Power Sources For
ICCP System
 Transformer – Rectifier
Unit (TR Unit)
 Solar Panel
 Thermo-Electric Generators
(TEGs)

Cathodic Protection in Pipeline/Piping
 Cross-country Line --> Less Complicated
--> Better Current Attenuation
--> Less Number of Anodes
--> Easy Installation
 Plant Piping --> Much Complicated
--> Challenging Construction
Environment
--> Limited Above Ground
Access

training-presentation-on-cathodic-pritection_compress.pdf

Remote anode beds are used to project current
over a wide area just as stadium lighting is used to
distribute light on a football field.
Cathodic Protection Ground Beds
 Remote Anode Bed

 Typical system consists of
one or more wells with
depth ranging from 50 m to
100+ m
 Active area is bottom
section
 Typically requires a
significant amperage output
due to common grounding
system
 Interference, soil variances and achieving good coverage
complicates design effort
 Can be pre-assembled for ease of installation
 Requires vent gas pipe
 Will not work through bedrock
 Deep Anode

As with spot lights around a house, discreet anodes
must be properly spaced to assure thorough current
distribution. Areas that do not receive sufficient current
could be subject to corrosion, just as some areas of the
house may be covered in shadows.
 Discreet Anode Systems

If coverage
is not
thorough,
some areas
may be
unprotected
Anode
Grounding Grid
Gas Line
 Distributed Anodes

Distributed anodes can be:
 Galvanic
Typically Magnesium type
 Impressed Current
MMO tubes or canister anodes,
High Silicon Cast Iron or Graphite.

Rectifier
Anode
Simplified design
Ensures complete
coverage
Easy to install
Minimizes stray
current and
interference issues
Lower output
current required
due to proximity of
anode to structure
Grounding Grid
Gas Line
 Linear Anodes

Unlike discreet anode systems which rely on the designer to
properly locate the anodes to assure that there is thorough
current distribution, the linear anode system assures current
distribution.
The analogy would be fiber optic lighting
 Linear Anodes - Advantages

INSULATED
COPPER CABLE
ANODE TO
CABLE JOINT
FABRIC
INSULATED
COPPER CABLE
POROUS
FABRIC
BRAIDING
1-1/2”
DIAMETER
LENGTH UP TO
1,500 FEET
COMPACTED
COKE BACKFILL
MIXED METAL
OXIDE ANODE
COLOR CODED
BRAIDING
 Linear Anodes Construction

LINEAR-ANODE TYPE APPLICATION
Linear-Anode Along pipelines, beneath above ground
storage tanks (ASTs) and congested areas
SandAnode Beneath above ground storage tanks
(ASTs) with very little clearance
SS Braided
Anode
Horizontal directional drilling (HDD)
applications
Internal-
Anode
Pipe internal surface protection.
Braid-Anode Special applications, water wells and
industrial environments
HDPE-Anode Marine environments, concrete and
special applications
Types of Linear Anodes

Cathodic Protection of Tanks
 External Corrosion (Bottom Plate)
 Internal Corrosion
 CP is critical for tanks service
 Reduces corrosion thus extends service life

Sr. No. Tank Services Dia. (m) Height
(m)
Tanks Weight
Ton
Tank Construction
Cost (USD)
CP Cost
(USD)
% of CP
cost
1 Crude/ FW/ PMG 10 14.5 50 577,275 20,000 3.46%
2 Crude/ FW/ PMG 20 14.5 160 1,744,080 23,000 1.32%
3 Crude/ FW/ PMG 30 14.5 270 2,827,035 29,000 1.03%
4 Crude/ FW/ PMG 40 14.5 431 3,586,136 35,000 0.98%
5 Crude/ FW/ PMG 50 14.5 620 5,158,710 39,000 0.76%
Cost of CP System for Tanks

 External Protection
 MMO Grid System
 Concentric Rings System
 Internal Protection
 Galvanic Anodes attached to tank’s body (Internally)
 Galvanic Anodes Suspended into Tanks

 Anode Grid System
 MMO Grid system consists of MMO grid anode,
conductor bars and power feed systems.
 In use for more than 20 years
 Field fabricated and welded
 Field fabricated system relying on installation
 Key connections at the power feed are field installed
 Contractor to properly weld anode and Conductor bar
 Bare MMO may cause issues with oxygen generation

Anode Grid System
Reference Cell
MMO Ribbon
Anode
Conductor Bar

Concentric Ring System - Linear Anodes
1. The installation of custom-
manufactured Tank Ring
Linear-Anode System is fast
and easy
2. Lay out anode rings per the
drawing provided
3. Run the anode lead wires to
the junction box or rectifier
without splicing or connecting
to a header cable
4. Place reference electrodes
where designated
5. Run all cables to rectifier or
junction box

Concentric Ring System - Linear Anodes

Field View of Linear Anode System

Aging Tank Bottoms
• Soil Side Corrosion.
• Hazard of Tank Leakage in Plant and
Refinery.
• High cost of tank bottom replacement.
• Solution: Linear Anodes.

Tank Retrofit Applications
 Installing cathodic protection on existing
tanks can be done in a variety of ways
depending on the application
 Is there a secondary liner?
 What is the tank diameter?
 Is there access around the tank for
directional drilling?
 What other structures are in the area?

Retrofit Tank Project
Notes:
The anodes lead wires will be on one side of the tank (opposite side where the anode
is pulled from. Cables will be then be routed to the nearest existing positive header
cable where a connection will be made at a test station.

Existing Tank Bottom - CP
AJB AJB
100
Existing
Tank Bottom
10
20
20
20
20
10
Anode
Tail Cable
Linear Anode
4
2
1
3
5

 Internal Protection

 Internal Protection – Bottom Plate

 Internal Protection – Tank’s Shell

Anodic Protection (AP)
 A technique to control the corrosion of a metal
surface by making it the anode of an
electrochemical cell
 Controlling the electrode potential in a zone
where the metal is passive
 Used for internal protection of tanks containing
acids (Generally H2SO4)
 Applicable to only those metals which show
active – passive behaviour
 Low operating cost but high installation cost
 Feasibility can be predicted in laboratory
and the design is easier

Integrity Surveys for Pipeline / Piping
Under CP
 Close Interval Potential Survey (CIPS)
 CIPS results in graphs of voltages
between two variable potentials
 The on and off measurements are not as
required by science
 The traces do not show wave forms but
point to point lines between separate
voltages

CIPS - Manual Noting of Voltages

CIPS - Pipeline Location and Data Logger

The First Recorded CIPS in the World

What CIPS Graph Actually Shows…?

 Direct Current Voltage Gradient (DCVG)
 A technique used to asses the effectiveness of
corrosion protection on buried steel structures
 It help locate Coatings faults and highlights
deficiencies in Cathodic Protection (CP) strategies.
 The direction of voltage gradients caused by CP
currents (which can be measured using a voltmeter)
identifies the location of coating fault
 Plotting the direction of voltage gradients around a
fault, the type and nature of faults may be deduced.

 Direct Current Voltage Gradient (DCVG )

 Direct Current Voltage Gradient (DCVG )
A stacked graph of DCVG calculated gradients
shows a defect (circled in black)

Questions / Comments / Suggestions

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