cathodic protection part 1
TRANSCRIPT
-
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
-
8/12/2019 Cathodic Protection Part 1
2/31
CorrOcea
customer
have
technologic
edg
2
COURSE IN CP INSPECTION
METHODS
Part I
Cathodic Protection
-
8/12/2019 Cathodic Protection Part 1
3/31
CorrOcea
customer
have
technologic
edg
3
Offshore Corrosion
Corrosion:
Based on the Latin word
corrodere = to gnaw
-
8/12/2019 Cathodic Protection Part 1
4/31
CorrOcea
customer
have
technologic
edg
4
Wet corrosion in an electrolyte
containing oxygen
-
8/12/2019 Cathodic Protection Part 1
5/31
CorrOcea
customer
have
technologic
edg
5
Electrode Potentials
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
7/31
CorrOcea
customer
have
technologic
edg
7
Pourbaix Diagram
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
11/31
-
8/12/2019 Cathodic Protection Part 1
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)
-
8/12/2019 Cathodic Protection Part 1
13/31
CorrOcea
customer
have
technologic
edg
13
-
8/12/2019 Cathodic Protection Part 1
14/31
CorrOcea
customer
have
technologic
edg
14
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
16/31
CorrOcea
customer
have
technologic
edg
16
Example of Impressed Current
Installation
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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.
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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)
-
8/12/2019 Cathodic Protection Part 1
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
-
8/12/2019 Cathodic Protection Part 1
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