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)

Critical Evaluation of International Cathodic Disbondment Test Methods

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

Notes: 1. Selection means multiple choices. 2. Triple salt solution is 1%/1%/1% of NaCl/Na2CO3/Na2SO4.

Standard Voltage

[V] 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

CD Results on Coating α and Coating β

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

Standard Current

[mA] Temp.

[°C] Duration

[days]

Disbondment [mm]

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

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. Oxygen Concentration in the Electrolyte

2. Electrolyte Type and Concentration

3. Applied Potential

4. Dry Film Thickness

5. Test Duration

6. Pre-treatment of the Substrate and Surface Profile

7. Effect of hypochlorite

8. Effect of Temperature

9. Specimen Configuration – Flat and Curved Steel

10. Holiday Size

11. Holiday Shape – Cone or Straight Hole

12. Specimen Orientation

13. Selection of Reference Electrode and Its Calibration

Oxygen concentration

0

200

400

600

800

1000

1200

0 25 50 75 100

Hours

Dis

bo

nd

ed

are

a [

mm

²] Oxygen

Air

Nitrogen

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

• 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

0

50

100

150

200

250

300

350

400

0 20 40 60 80

Days

Cu

rren

t d

en

sit

y (

mA

/cm

²)

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

• Linear relationship between applied potential and the disbonding rate.

• The charge transport through continuous coatings was found to increase with increasing cathodic potential.

R2 = 0.7242

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-1500 -1300 -1100 -900 -700 -500

Potential mV SCE

Dis

bo

nd

ing

rate

(m

m²/

h)

Dry Film Thickness

• For thin film coatings the disbonding rate was reported to decrease almost linearly as the DFT increased up to 100 µm

Disb. distance vs. coating α thickness

0

1

2

3

4

5

6

7

0 200 400 600 800 1000 1200 1400

Coating thickness [μ]

Dis

b. d

ista

nc

e [

mm

]

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 Cl- Cl2 + 2 e-

Cathode Reactions:

2 H2O + 2 e- 2 OH- + H2

½ O2 + H2O + 2e 2 OH

Hypochlorite Formation:

Cl2 + 2 OH- H2O + Cl- + ClO-

Not normally seen in Field because of separation of anode from cathode (holidays in the pipeline coating)

Effect of ClO- concentration on Rate of Delamination

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7 8

Concentration [g/liter]

Rate

of

Dela

m. [μ

m/d

ay]

Coating α

Coating β

Effect of hypochlorite

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

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

Specimen Diameter/Curvature

Cathodic Disbondment

mm inch mm

Coating α (800-900 μm)

Flat Flat 3.0

114 4.5 3.0

60 2.2 3.1

20 0.8 2.9

Coating β (800-900 μm)

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 Current, mA Cathodic

Disbondment

Inch (mm) Initial Final mm

Coating α

(800-900μm)

1/8 (3.2) 3.5 11.8 3.0

1/8 (3.2) 3.6 8.2 2.0

¼(6.4) 12.2 29.0 3.0

¼(6.4) 9.1 25.0 2.0

Coating β (800-

900μm)

1/8 (3.2) 3.2 4.6 10.5

1/8 (3.2) 3.0 7.2 13.5

¼(6.4) 12.3 27.4 12.0

¼(6.4) 11.7 23.1 12.0

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

• There was no significant difference between the straight and cone shape holiday

Coating Material

Shape Size Current, mA

Cathodic Disbondment

mm Initial Final mm

Coating α

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

Coating β

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: up to 57 ˚C

• Calomel: 60 ˚C

• Ag/AgCl: 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

# Test Parameter Test Condition

1 Dissolved Oxygen Concentration in the Electrolyte

Equilibrium solubility of oxygen in the electrolyte solution at the test temperature

2 Electrolyte Type and Concentration

3% by weight NaCl in DI water solution

3 Applied Potential -1.5 VDC for buried pipeline coating and -1.0 VDC for offshore / marine structures, measured by Ag/AgCl reference electrode

4 Coating Dry Film Thickness DFT shall be specified by the coating manufacturers for the specific field application.

5 Test Time 12 weeks at maximum service temperature and 28 days (4 weeks) at room temperature (optional)

6 Surface Pre-treatment and Surface Profile of Substrate

The surface cleanliness and surface profile is very important and must be recoded.

.

RECOMMENDATIONS

# Test Parameter Test Condition

7 Effect of hypochlorite Anode Isolation Changing the electrolyte solution

8 Test Temperature Maximum service temperature and room temperature and/or 65 ˚C (optional). Above 100 ˚C, the electrolyte temperature shall be cooled to 95 ˚C.

9 Specimen Geometry Specimen can be either flat, curved panel or tube.

10 Holiday Size 6 mm

11 Holiday Shape – Cone or Straight Hole

Cone shaped drill bit to prepare holiday

12 Specimen Orientation Specimen can lay flat horizontally with the attached cell or hang vertically in the bath.

13 Reference Electrode and Calibration

Cu/CuSO4, Calomel, or Ag/AgCl (latter recommended) can all be used in their allowable temperature range.

RECOMMENDATIONS

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.