Cathodic protection design

Cathodic protection 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

Cathodic protection shall be design with due to environment

condition, neighbouring structures and other activities

The general design procedure for both sacrificial anode and

impressed current systems is similar

Cathodic protection design

1. Initial considerations

Modifications to the structure to incorporate requirements are best

made at the early design and pre-construction phase of the

structure

For underground structures it may be necessary to visit the

proposed site, or for pipelines the proposed route, to obtain

additional information on low-resistivity areas, availability of

electric power, and the existence of stray dc current or other

possible interaction.

It is common practice for a survey to be made before design

This survey is often combined with a study to establish economic

justification for the recommended anti-corrosion proposal while the

principal data necessary for design (chemical and physical) are

also collected

If the structure already exists, measurement of existing structure-to-

soil potentials is essential to give valuable information as to which

areas are anodic and which are cathodic

If the new structure, the design of a cathodic protection system

should include the calculation of:

Current demand

Resistance to earth of the anodes

Quantity and location of anodes

Electrical supply requirements

Test and monitoring facilities

2. Drawings and material specifications

Engineering drawings:

• To establish the size and shape of the structure to be

protected in order to design an effective cathodic protection

system

Site drawings:

• To check all other metallic structures in the vicinity presence

may affect the operation of the system being designed

Material specifications:

• To establish material and surface conditions, particularly the

presence and quality of protective coatings.

3. Site survey

To establish the actual environmental condition

• Media characterizations

• water chemistry

• pH measurement

• soil chemistry

• Current requirement test

• Availability power supply

Water chemistry:

Samples of water should be analyzed for pH value, amount of

aggressive anions such as chloride, sulfate and other chemical

constituents, and resistivity value

Water containing such chemical constituents can affect the

current requirement necessary for protection

pH measurement:

To validate of the acidity or alkalinity of the media (soil or water)

Soil pH is measured of the pH value of water contained in the soil

pH range: 0 – 7 being acidic, 7 being neutral & 7 – 14 being

alkaline

Soil chemistry

• Soil resistivity

It is important used in the design of cathodic protection for

underground structures.

Soil resistivity closely related to soil corrosivity

Higher resistivity being associated with low corrosivity

• Chemical constituents

Chemical constituents such as sulfide, sulfate, chloride and

others should be analyzed prior to cathodic protection system

design

• pH measurement

• Made of Plexiglas

• Rounded corners for easy cleaning

• Current plates are made of stainless steel

• Potential pins made of brass and easily

removed

Soil box

4-Pin Soil Resistance Meter

Model 400 by NILSSON

Control Panel Features

Resistivity of soil or water by soil box

method using an earth resistivity

measuring set, such as Nilsson Model

400 Soil Resistance Meter.

Schematic diagram for media resistivity measurement

P1 P2

C1 C2

L

L

WDR

Notes: W, D, L in cm

R = resistance

= resistivity

P1 & P2 = potential connection

C1 & C2 = current connection

Schematic diagram soil box

Corrosivity ratings based on soil resistivity

Current requirement test

• The current requirement should be determined at the site

being to install cathodic protection system

• Need to install a temporary cathodic protection system

• The current supplied and the structure – to – electrolyte

potential results will be used to establish the required current

to protect the structure

Availability power supply

• Need AC power supply for ICCP

• Near AC power supply at site for cost saving (instead install

new ac power supply)

Temporary cathodic protection system for determining

current requirements

DC milliammeter Rectifier

Adjustable resistor

+ -

Temporary anode

Structure to be protected

Soil surface

v

DC voltmeter

Cu/CuSO4

4. Current density requirements

• It can be measured from a temporary cathodic protection

system

• The current densities shall be used for steel, stainless

steel and other metallic materials

• The total amount of current required is determined by

multiplying the required current density by the area of the

structure to be protected

• In the case of well coated structure, the amount of current

required can be two orders of magnitude less than the

current required of the same uncoated structure

5. Selection type of cathodic protection systems

• For selecting between SACP or ICCP systems is based on

feasibility and cost factors.

• Cost factors include operating, maintenance, and

appropriate replacement

• Feasibility factors for example the systems required small

stable current (normally consider protection by SACP) or

large current (protection by ICCP)

6. Sacrificial anode design

• Determination of the total current required either from

actual current requirement measurements or by multiplying

a typical current requirement by the surface area of the

structure to be protected

• Calculation of the individual anode current, Ia (A), required

to meet the current demand, Ic (A), is followed Ohm’s law

Ohm’s law

a

o

a

o

a

o

c

acR

E

R

EEINI

)(.

Where,

N = number of anodes

= the design protective potential 0.8V (relative to

Ag/AgCl/seawater reference electrode, accepted for carbon

and low-alloy steel

= the design closed circuit potential of the anode (V)

Ra = the anode resistance (Ohm)

o

cE

o

aE

Calculation anode resistance

The anode resistance, Ra (ohm), to be used shall be based on

the applicable formulas.

Anode type: Long slender stand-off (L 4r)

1

.4ln

..2 r

L

LRa

Note:

1- This equation is valid for anodes with minimum distance 0.30m from protection object.

However for anode-to-object distance less than 0.30m but minimum 0.15m the same

equation may be applied with a correction factor of 1.3.

2- For non-cylindrical anodes: where c(m) is the anode cross sectional periphery 2

cr

Where, is media resistivity (ohm cm)

L is length of anode (cm)

r is equivalent radius of anode (cm)

R is anode resistance (ohm)

Anode type: Short slender stand-off (L 4r)

22

21

2211

2ln

..2 L

r

L

r

L

r

r

L

LRa

Anode type: Plate anode (Long flush mounted hull or bracelet

anodes (L width, L thickness)

SRa

.2

Where, S is mean length of anode sides (cm)

SRa

.4

If the flat plate anodes are close to the structure

or painted on the lower face

2

baS

, where b 2a

Anode type: Plate anode (Short flush-mounted hull, bracelet and

other types)

ARa

315.0

Where, A is exposed area of the anode (cm2)

Calculation of total anode weight

Total required anode weight, mTA, (or mass) based on the average

total current demand, Ic, is calculated according to the following

equation:

a

cTA

CU

TIW

.

8760..

Where, T is lifetime (yr)

Ic is total current demand (A)

U is utilization factor for the anode

Ca is anode capacity (Ah/kg)

8760 is # hours/yr

Calculation of consumption of anode

hourAmpere

factornUtilizatioxweightGrossratenConsumptio

Calculation of number of anodes

Where, N is number of anode

Id is current demand (ampere)

T is design life (year)

Ca is anode capacity (Ah/kg)

Wa is anode weight (kg)

8760 is # hours/yr

pd IAI . Where, A is surface area of structure to be protected (m2)

Ip is current density required (ampere)

aa

d

CW

TIN

.

.8760.

Example: Calculating number of anodes for a buried steel

pipeline

1. If we assume the pipeline length to be 100 meters, and the

O.D. of the pipe to be 0.17 m. The area to be protected is the

outside area of the pipe. We will assume the pipeline is

uncoated, but coating will alter the calculations. The area is:

53.4 m2.

2. Tables of current density requirements have been found to be

in the range of 10 - 60 mA/m2. (F.W. Hewes, Cathodic

Protection Theory and Practice, V. Ashworth and C.J.L.

Booker, eds., Wiley (Horwood), Chichester, West Sussex, p.

226, 1986.) For our example we will assume a current density

requirement of 40mA/m2.

3. Current demand

mAmmAxmIAI pd 2136/404.53. 22

4. The output for zinc anodes is 810Ah/kg, and the efficiency is

normally taken as 90%. Thus, the useful output of zinc is

729 Ah/kg

5. Design life to be protected is 20years

6. Total anode weight:

kgAhx

yrhrsxyrsxA

CU

TIW

a

cTA

/8109.0

/876020136.2

.

8760..

kgWTA 514 for protecting 100m of pipeline

7. Every meter required:

anodes kgW

W TAa 14.5

100

514

100

8. Therefore, number of anodes required should satisfy both of

the following:

10014.5

514

a

TA

W

WN anodes

OR

10087.99 N anodes

yrAhxkg

yrsxyrhrxA

CW

TIN

aa

d

/72914.5

20/8760136.2

.

.8760.

NOTE: 100 ANODE REQUIRED FOR PROTECTING 100M

PIPELINE & EACH METRE SHALL BE INSTALLED ONE ANODE

WITH WEIGHT OF 5.14KG

7. Impressed current design

Three steps shall be taken in designing of impressed current

cathodic protection:

I) Total current

II) Total resistance

III) Voltage and rectifier

I. Total current

Same as for sacrificial anode cathodic protection system

Determination of current requirement from the actual

current measurement or by multiplying a current by the

surface area of the structure to be protected

II. Total resistance

The major factor in the determination of the total circuit

resistance is the anode-to-electrolyte resistance

It is also known as "ground bed resistance," and this is

often the highest resistance in the impressed current

cathodic protection system circuit

III. Voltage and rectifier

Using the total circuit resistance and the current required,

the appropriate voltage for the rectifier is then calculated as

below:

RIE

Where,

E is required voltage

I is required current

R is total circuit resistance

Equivalent circuit

The total circuit resistance is:

R = Rc+ + Rc- + Rs + RE + RA

Rc+ + Rc- is The resistance of the positive and negative cables will

be dependent on the length and cross sectional area of

the conductor

RS : The resistance of structures such as platforms may be ignored.

RE : The cathode to electrolyte resistance may be calculated

using ohms law:

I

ER

E is the change of the structure-to-electrolyte potential to achieve

cathodic protection (usually 1/3 to 1 Volt) and I is the total

current requirement in amperes.

RA : The anode-to-electrolyte resistance will be dependent on the

shape, number, and spacing of the anodes used, and the

electrolyte resistance