Critical Evaluation of International Cathodic Disbondment Test

Critical Evaluation of International Cathodic
Disbondment Test Methods
Benjamin T. A. Chang
(PolyLab LLC Houston, Texas)
Ole Øystein Knudsen
(SINTEF Materials and Chemistry, Norway)
Dennis Wong, Jiri Holub
(ShawCor Toronto, Ontario, Canada)
Amal Al-Borno
(Charter Coating Service (2000) Ltd. Calgary, Canada)
J. Alan Kehr
(3M Austin Center Austin, TX)
Pipe laying is often not conducted under the most ideal of conditions
pipe-to-soil
Voltage meter
With attached
Reference cell
• Cathodic Disbondment (CD) resistance is always on the top of
the coating property list required by end users.
• At least 22 International and national standard test methods
are available that evaluate the resistance to CD
• There is no universal agreement on which standard to use
• Test parameters vary over a wide range among the various
test standards
Review of Selected CDT Standard Methods
Voltage
[V]
Standard
Temperature
[°C]
Solution
[aqueous]
Duration
[days]
CSA Z-245
1.5 or 3.5 V
Selection
3 % NaCl
Selection
ASTM G8
1.5
RT
Triple salt
Selection
ASTM G42
1.5
Selection
Triple salt
30
ASTM G80
1.5
RT
Triple salt
60
ASTM G95
3.0
RT
3 % NaCl
90
ISO 15711
1.05
RT
Sea water
182
AS 3862
3 mA
Selection
3 % NaCl
Selection
NF A 49-711
1.5
Selection
3 % NaCl
Selection
Notes:
1. Selection means multiple choices.
2. Triple salt solution is 1%/1%/1% of NaCl/Na2CO3/Na2SO4.
CD Results on Coating α and Coating β
Disbondment [mm]
Standard
Current
[mA]
Temp.
[°C]
Duration
[days]
Coating α
Coating β
CSA Z-245
5-7
65
28
2-4
11 - 14
ASTM G8
20 - 25
RT
30
2
6
ASTM G42
40 - 55
65
30
4-5
8 - 14
ASTM G80
20 - 25
RT
60
2
6
ASTM G95
25 - 30
RT
90
5
15 - 25
ISO 15711
0.9 – 1.1
RT
182
7.5
13.5
AS 3862
3 mA
65
28
2-3
10 – 13.5
NF A 49-711
3-5
65
28
2.5 - 3
5-6
Notes:
1. The current draw corresponds to one sample being tested
2. The coatings were 800 to 900 μm
3. CD tests were run with two 2-component liquid epoxy materials
To develop a basis for a new international
standard for a cathodic disbondment test
method.
• NACE has formed a Technical Exchange Group, TEG
#349x to investigate the effect of different test
parameters on test results and document the results to
assist the development of a new international standard.
• This paper is a status report on the progress made by
the committee members of TEG #349X with the
objective states above .
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Oxygen Concentration in the Electrolyte
Electrolyte Type and Concentration
Applied Potential
Dry Film Thickness
Test Duration
Pre-treatment of the Substrate and Surface Profile
Effect of hypochlorite
Effect of Temperature
Specimen Configuration – Flat and Curved Steel
Holiday Size
Holiday Shape – Cone or Straight Hole
Specimen Orientation
Selection of Reference Electrode and Its Calibration
Oxygen concentration
1200
1000
Disbonded area [mm²]
Note:
• Dissolved oxygen
concentration in the
electrolyte affects the
disbonding rate
• Specifically the disbonding
rate increases with
dissolved oxygen content
• Nitrogen atmosphere
resulted in much reduced
disbondment
Oxygen
800
600
400
Air
200
Nitrogen
0
0
25
50
Hours
75
100
• Electrolyte solutions used in the CD test:
• synthetic seawater
• mixed salt solution
• NaCl solution
• The 3% by weight NaCl solution is considered the
most suitable electrolyte solution.
• The NaCl solution offers high conductivity and does
not form any calcareous deposit film on the metal
surface.
Formation of Calcareous Deposits
Current density (mA/cm²)
400
350
300
250
200
150
100
50
0
0
20
40
60
80
Days
Reduction in cathodic current density with time for steel samples exposed
in natural seawater polarized to -1050 mVSCE at 10°C
Effect of Cation
• The rate of disbonding depends on the type of cation in the
electrolyte. Disbonding was proportional to the molar
conductivity of the cations.
• The anions normally have little effect on the disbonding rate.
Disbonding rate is a Function of Applied Potentials
• The charge transport
through continuous
coatings was found to
increase with increasing
cathodic potential.
4.5
4.0
Disbonding rate (mm²/h)
• Linear relationship
between applied
potential and the
disbonding rate.
3.5
3.0
2.5
2.0
1.5
1.0
R2 = 0.7242
0.5
0.0
-1500
-1300
-1100
-900
Potential mV SCE
-700
-500
Dry Film Thickness
Disb. distance vs. coating α thickness
7
Disb. distance [mm]
6
5
4
3
2
1
0
0
200
400
600
800
1000
1200
1400
Coating thickness [μ]
• For thin film coatings the disbonding rate was reported to decrease
almost linearly as the DFT increased up to 100 µm
Dry Film Thickness
• Cathodic disbonding depends on transport of some
reactants through the coating film.
• Thicker film slows the permeation of reactive
species and the cathodic disbondment rate.
• The DFT of the test specimen should be clearly
recorded in the test report.
Test Duration
• Cathodic disbondment increases with test time.
• Disbonded area is proportional to test time.
• 12 weeks vs. 28 days : Longer test duration is highly
recommended because cathodic blistering is more
likely to be observable with the longer test
Pre-treatment of the Substrate and Surface Profile
• Surface profile and pre-treatment have significant impact on
cathodic disbondment.
– The disbonding rate was found to decrease with increasing
surface roughness
– Surface Pre-treatment: Phosphating the surface decreased the
disbonding significantly.
• The steel surface profile shall be specified by the coating
manufacturer and in the range of 2.0 to 4.0 mil (50-100 μm).
• The surface profile and cleanliness shall be clearly recorded in
the report.
Effect of hypochlorite
Anode Reaction:
2 ClCl2 + 2 eCathode Reactions:
2 H2O + 2 e2 OH½ O2 + H2O + 2e  2 OH
Hypochlorite Formation:
Cl2 + 2 OH-
+ H2
H2O + Cl- + ClO-
Not normally seen in Field because of separation of anode from
cathode (holidays in the pipeline coating)
Effect of hypochlorite
Effect of ClO- concentration on Rate of Delamination
6
5
Rate of Delam. [μm/day]
For Laboratory Testing:
• Anode Isolation
affects hypochlorite
concentration
• Refresh the
electrolyte solution
at least every 28
days
• For smaller cell
sizes, weekly
changing of
electrolyte solution
is necessary
4
3
Coating α
Coating β
2
1
0
0
1
2
3
4
5
Concentration [g/liter]
6
7
8
Effect of Temperature
• Cathodic disbondment increases with temperature
For Laboratory Testing
• Qualification Tests, maximum service temperature, RT and/or
65 ˚C to be optional
• Comparative coating performance, RT and/or 65 ˚C
• Testing >100 ˚C, the electrolyte shall be cooled down to 95 ˚C
Specimen Configuration – Flat and Curved Steel
The specimen
curvature had
no significant
effect on CD
suggesting that
flat or cylindrical
samples can be
used for CD
testing.
Coating
Material
Coating α
(800-900 μm)
Coating β
(800-900 μm)
Specimen
Diameter/Curvature
Cathodic
Disbondment
mm
inch
mm
Flat
Flat
3.0
114
4.5
3.0
60
2.2
3.1
20
0.8
2.9
Flat
Flat
11.0
114
4.5
11.0
60
2.2
12.0
20
0.8
10.0
Holiday Size
• Two holiday sizes, 1/8” (3
mm) and ¼” (6 mm) are in
common use for CD testing
• The holiday size made no
significant difference to the
CD test results
• For thicker DFT specimens,
the hydrogen gas bubbles
may be trapped at the
smaller size holiday
• It is recommended to use
only one holiday size of 6
mm for all coating
specimens for the CD
Coating
Material
Size
Inch (mm)
Current, mA
Cathodic
Disbondment
Initial
Final
mm
3.5
11.8
3.0
3.6
8.2
2.0
12.2
29.0
3.0
9.1
25.0
2.0
3.2
4.6
10.5
3.0
7.2
13.5
12.3
27.4
12.0
11.7
23.1
12.0
1/8 (3.2)
Coating
α
(800900μm)
1/8 (3.2)
¼(6.4)
¼(6.4)
1/8 (3.2)
Coating β
(800900μm)
1/8 (3.2)
¼(6.4)
¼(6.4)
CD test results (-1.5 V, 65°C, 28 days)
Holiday Shape – Cone or Straight Hole
• Two popular ways
to drill the holiday
using either flat
head end mill or
cone shape drill bit
Coating
Material
Coating α
• There was no
significant
difference between
the straight and
cone shape holiday
Coating β
Shape
Size
Current, mA
Cathodic
Disbondment
mm
Initial
Final
mm
Flat
3.2
6.7
10.2
2.0
Cone
3.2
7.0
10.7
3.5
Flat
6.4
20.6
26.0
4.0
Cone
6.4
11.9
20.4
4.0
Flat
3.2
4.0
5.7
17.0
Cone
3.2
6.4
11.0
15.0
Flat
6.4
15.3
29.1
17.0
Cone
6.4
15.1
22.6
13.0
CD test results (-1.5 V, 65°C, 28 days)
Specimen Orientation
There are generally two orientations that can be used
in the CD test; horizontal or vertical:
• There is no significant difference for the specimen
orientation as long as the hydrogen gas bubbles
escape freely and are not trapped at the holiday
Selection of Reference Electrode and Its Calibration
• Cu/CuSO4:
• Calomel:
• Ag/AgCl:
up to 57 ˚C
60 ˚C
up to 90 ˚C
Note that the accuracy of the electric potential measured by the
reference electrode shall be checked.
CONCLUSIONS
• The disbonding rate increases with dissolved oxygen content.
• The rate of disbonding depends on the type of cation in the
electrolyte.
• The anions normally have little effect on the disbonding rate.
• CD increases with decreased (more negative) potential.
• Thicker film slows the permeation of reactive species and the
CD rate.
• Disbonded area is proportional to test time.
• Surface profile and chemical treatment affects cathodic
disbonding rate.
• Hypochlorite formation in test attacks the coating.
CONCLUSIONS
• Cathodic disbondment increases with temperature.
• The specimen geometry has no impact on the cathodic
disbondment.
• The holiday size variation of 3 – 6 mm has no impact on
CD.
• Cone or straight shaped holiday has no influence on
cathodic disbondment.
• There is no significant difference for the specimen
orientation (flat or vertical)
• Ag/AgCl reference electrode is recommended
.
RECOMMENDATIONS
#
Test Parameter
1 Dissolved Oxygen
Concentration in the
Electrolyte
2 Electrolyte Type and
Concentration
3 Applied Potential
4 Coating Dry Film Thickness
5 Test Time
6 Surface Pre-treatment and
Surface Profile of Substrate
Test Condition
Equilibrium solubility of oxygen in the
electrolyte solution at the test temperature
3% by weight NaCl in DI water solution
-1.5 VDC for buried pipeline coating and -1.0
VDC for offshore / marine structures,
measured by Ag/AgCl reference electrode
DFT shall be specified by the coating
manufacturers for the specific field
application.
12 weeks at maximum service temperature
and 28 days (4 weeks) at room temperature
(optional)
The surface cleanliness and surface profile is
very important and must be recoded.
RECOMMENDATIONS
#
7
Test Parameter
Effect of hypochlorite
8
Test Temperature
9
Specimen Geometry
10
11
Holiday Size
Holiday Shape – Cone or
Straight Hole
Specimen Orientation
12
13
Reference Electrode and
Calibration
Test Condition
Anode Isolation
Changing the electrolyte solution
Maximum service temperature and room
temperature and/or 65 ˚C (optional).
Above 100 ˚C, the electrolyte temperature
shall be cooled to 95 ˚C.
Specimen can be either flat, curved panel or
tube.
6 mm
Cone shaped drill bit to prepare holiday
Specimen can lay flat horizontally with the
attached cell or hang vertically in the bath.
Cu/CuSO4, Calomel, or Ag/AgCl (latter
recommended) can all be used in their
allowable temperature range.
Acknowledgements
Thanks to members of NACE Technical Exchange
Group TEG#349X for their valuable discussions
on the evaluation of test parameters used in
cathodic disbondment test methods.