cathodic protection part 1

8/12/2019 Cathodic Protection Part 1

1/31

CorrOcea

customer

have

technologic

edg

1

COURSE IN CP INSPECTION

METHODS

FORCORROCEAN

Part I Cathodic Protection

Part II

CP Inspection Methods


2/31

CorrOcea

customer

have

technologic

edg

2

COURSE IN CP INSPECTION

METHODS

Part I

Cathodic Protection


3/31

CorrOcea

customer

have

technologic

edg

3

Offshore Corrosion

Corrosion:

Based on the Latin word

corrodere = to gnaw


4/31

CorrOcea

customer

have

technologic

edg

4

Wet corrosion in an electrolyte

containing oxygen


5/31

CorrOcea

customer

have

technologic

edg

5

Electrode Potentials


6/31

CorrOcea

customer

have

technologic

edg

6

CATHODIC PROTECTION

PRINCIPLE:

The material to be protected is supplied with

an external cathodic current

The electrochemical potential of the protected material is

moved in a negative direction to the immune area

The material is completely protected when it reaches the

Protection Potential


7/31

CorrOcea

customer

have

technologic

edg

7

Pourbaix Diagram


8/31

CorrOcea

customer

have

technologic

edg

8

ELEKTROCHEMICAL REACTIONS

Corrosion of FE:

a) Fe2+ + 2 e- = Fe

b) Fe3O4+ 8 H++ 8 e- = 3 Fe + 4 H2Oc) Fe3O4+ 8 H

++ 2 e- = 3 Fe 2+ + 4 H2O

d) Fe2O3+ 6 H++ 2 e- = 2 Fe 2+ + 3 H2O

e) O2+ 4 H++ 4 e- = 2 H2O

f) 2 H++ 2 e- = H2


9/31

CorrOcea

customer

have

technologic

edg

9

Applications of the Pourbaix

DiagramShows what reactions which can occur with

different pH and potential

Indication on the composition of the

corrosion/oxidation products

Shows the changes of the environment (pHand potential) which are nessesary to avoidcorrosion


10/31

CorrOcea

customer

have

technologic

edg

10

TECHNICAL SOLUTIONS

Sacrificial Anodes

Galvanic coupling to sacrificial anodesmade of Al-alloy or Zinc

Impressed Current Use of source for direct current (DC) and

none corroding anodes


11/31


12/31

CorrOcea

customer

have

technologic

edg

12

SACRIFICAL ANODE SYSTEMS

Advantages:

Robust system, reduced maintenance

Used on every platform on the Norwegian continental

shelf Disadvantages:

Limited driving voltage (0.25 V)

More anodes necessary for protection

More anodes necessary for securing long operating time

(Not suited for media with low conductivity, e.g. in soil)


13/31

CorrOcea

customer

have

technologic

edg

13


14/31

CorrOcea

customer

have

technologic

edg

14


15/31

CorrOcea

customer

have

technologic

edg

15

IMPRESSED CURRENTAdvantages:

High driving voltage (30 V) Few anodesreduced resistance

Disadvantages:

Vulnerable components

Need for regulation/control system

Risk of overprotection of highly charged materials

Coating damagescathodic accouplement

Need for/recommended protection shieldaround the anodes

Need for maintenance


16/31

CorrOcea

customer

have

technologic

edg

16

Example of Impressed Current

Installation


17/31

CorrOcea

customer

have

technologic

edg

17

Applications of Impressed

Current

Applied on steel in seawater or soil

Oil Platforms in steel and concrete

Subsea Pipelines

Hull

Quay structures and sheet pile curtains

Concrete bridges placed in seawater

Pipelines buried in soil Vessels/tanks buried in soil


18/31

CorrOcea

customer

have

technologic

edg

18

ELECTROCHEMICAL

POTENTIALS

St eel

Corrosion potential ca. -650 mV Ag/AgCl

Protected at ca. -800 mV Ag/AgCl

Al-anode and Zn-anode

Corrosion potential ca. -1050 mV Ag/AgCl


19/31

CorrOcea

customer

have

technologic

edg

19

CATHODIC PROTECTION

Anodic reactions: Zn = Zn2++ 2e-

Al = Al3++ 3e-

Cathodic reactions: 2 H2O = 4 H++ 4OH-

O2+ 4 H++ 4 e- = 2 H2O----------------------------------

O2+ 2 H2O + 4 e- = 4OH-

2 H++ 2 e- = H2 (g)

Anode and cathode reactions are always balanced, i.e congestion of

electrons does not exist


20/31

CorrOcea

customer

have

technologic

edg

20

CRITERIA FOR CATHODIC PROTECTION

Potential Criteria: maximum -800 mV, ref Ag/AgClminimum -1100 mV, ref Ag/AgCl

Demand for current: vary with O2in the electrolyte

solubility

flow velocity

temperature

construction geometry

geographical site


21/31

CorrOcea

customer

have

technologic

edg

21

Calcareous deposit reduces the demand for

current:

Calcareous deposits reduce the effective cathodic surface

area thereby lowering demand for current. The calcareous

deposit is formed when MgOH2and CaCO3salts precipitate

on the cathode (steel surface).

The following changes the composition and quality of the

calcareous layer:

current density

temperaturepressure

seawater quality

flow velocity


22/31

CorrOcea

customer

have

technologic

edg

22

CATHODIC PROTECTION

The most commonly used sacrifical anode materials are:

Al-Zn-In

Zn

Mg

Magnesium

relatively expensivelow capacity of current because of high selfcorrosion

may cause overprotection

short operating time

Often used where the electrolyte has low conductivity


23/31

CorrOcea

customer

have

technologic

edg

23

Zinc:classical anode material

low driving voltage (230 mV)

low capacity of current results in high weight of anodes

(780 A/kg)

temperature limits < 40 Co

Often used on subsea piplines and constructions buried in mud

Aluminium:has to be alloyed otherwise it is passive

high capacity of current (2500 Ah/kg)

long operating time saves weight

high driving voltage

Al-Zn-In anodes most commonly used offshore


24/31

CorrOcea

customer

have

technologic

edg

24

PRACTICAL CP DESIGN

where will the construction be placed?what kind of environmental parameters should be taken

into account (temp.,res.)

areas to protect

operating lifetime

what kind of design standards should be used (DnV,NORSOK, NACE)

what demand for current is expected

will the construction be protected by coating, if so, what

kind of coating

degradation mechanisms for coating (Coating Breakdown)

possible current drainage to e.g. wells, poles, other

structure

influence from other structures, pipelines etc.


25/31

CorrOcea

customer

have

technologic

edg

25

DEMAND FOR CURRENT

INITIAL DEMAND FOR CURRENT:Demand for current to polarize the structure down to a safeprotection potential ( -800 mV) and build a good calcareous

deposit.

AVERAGE CURRENT:Demand for current to maintain a safe protection potential afterpolarization of the structure. Used to calculate necessary anode

weight.

FINAL DEMAND FOR CURRENT:

Demand for current to repolarize the structure after a possiblebreakdown/damage of the calcareous deposit (after winter storms).

It also gives the demand for current at the end of the operating

lifetime.

Ill t ti f d i t d it


26/31

CorrOcea

customer

have

technologic

edg

26

Illustration of design current density

Currentde

nsity(mA/m2

)

Time

Initial current density

Final (peak) current density

Average current

density


27/31

CorrOcea

customer

have

technologic

edg

27

REQUIREMENT OF CURRENT FOR

PROTECTION

Bare steel in seawater:

100 - 200 mA/m2

Bare steel in soil:

10 - 20 mA/m2

Reinforced concrete:

1-3 mA/m2


28/31

CorrOcea

customer

have

technologic

edg

28

CATHODIC PROTECTION AND COATING

Reduces the requirement of current

Lowers anode weight

Easier to achieve good current distribution

and consequent protection of the entirestructure


29/31

CorrOcea

customer

have

technologic

edg

29

CALCULATE CATHODIC PROTECTION

Structure:- calculate area to protect (m2)

- calculate current requirement, I(A)

- calculate anode weight requirement, W (kg)

Anode data:- anode material

- anode type and dimensions

- calculate anode weight, Wa(kg)

- calculate anode resistance, Ra (ohm)

- calculate driving voltage, DE, (mV)

- calculate anode current output, Ia(A)


30/31

CorrOcea

customer

have

technologic

edg

30

CP DESIGN

Current requirement: I = i * Area

Anode weight requirement:W = I*L*8760

C * U

Anode current output: Ia = DE

Ra

Anode weight: Wa = Voluma*d


31/31

CorrOcea

customer

have

technologic

edg

31

REQUIREMENT OF ANODES

Calculate necessary number of anodes to meet the

current requirement (initial and final current):

N1 = I

Ia

Calculate necessary number of anodes to meet the

anode weight requirement for the total operating

lifetime:

N2 = W

Wa

cathodic protection part 1

Documents