Critical Evaluation of International Cathodic Disbondment Test
Transcription
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.