The most effective method to mitigate corrosion on the external surface of a buried steel pipeline is to apply a protective coating that is supplemented by cathodic protection (CP). Ideally, these two systems should work together so that the CP system would provide corrosion protection to the pipe if the coating becomes
disbonded or has defects that allow electrolyte to contact the pipe’s steel surface. The researchers note, however, that the success of this approach greatly depends on the nature of coating failures. Corrosion protection to the pipe with coating failures can only be achieved if sufficient CP current is able to reach the exposed pipe steel. For instance, if adequate CP current is able to penetrate a coating
disbondment, then corrosion related damages may be prevented. Otherwise, the exposed pipe steel can be susceptible to corrosion and environmentally assisted cracking.
The extent of CP penetration into coating failures is associated with many factors, such as the dimensions of the coating disbondment, electrolyte solution composition and conductivity, and coating type. It has been observed that CP is less effective at reaching the pipe surface as the disbondment becomes tighter (i.e., as the height of the gap between the coating and steel substrate decreases). Also, CP penetrates further into a coating disbondment in a solution with higher electrical conductivity. Research has shown that CP penetration into disbonded areas was only a relatively short distance—no more than 100 mm in ground water with conductivities in the range of 0.5 to 5.0 mS/cm (equivalent to a resistivity range of 200 to 2,000 Ω∙m). Ground water containing calcium ions (Ca2+), magnesium ions (Mg2+), and carbonate ions (CO3-2)can affect the environment inside the disbondment and CP penetration by forming mineral deposits. These deposit scales formed on the top of the opening to the disbondment may act as barriers to CP penetration and alkalinity migration.
In this study, the researchers focused on examining the effect of different pipeline coatings, namely a field-applied two-part epoxy, fusion-bonded epoxy (FBE), and high-performance powder coating (HPPC), on the penetration of CP into a circular-shaped coating disbondment. The intention was to investigate the effect of soil in ground water, as well as the influence of the coating disbondment shape on CP penetration to the underlying pipe surface.
Two pipeline coatings were used in this study: a 350-μm thick FBE coating, and a 1.5-mm thick HPPC that consisted of an FBE inner layer, a chemically modified polyethylene (PE) adhesive layer, and a medium density PE (MDPE) outer layer. Two sizes of steel panels (70 mm by 170 mm by material thickness and 100 mm by 170 mm by material thickness) were cut for the study from a 1,067-mm diameter, X100 steel grade pipe section Test solutions used were an NS4 solution (a synthetic groundwater solution that simulates ground water found around pipelines with failures due to near-neutral-pH stress corrosion cracking [SCC]) and a mixture of one part NS4 and two parts clay soil. The detection probes were comprised of a home-made antimony/antimony oxide (Sb/Sb2O3) pH electrode and a silver/silver chloride (Ag/AgCl) (0.1 M potassium chloride [KCl] in 4% agar) reference electrode.
A 130-mm diameter circular-shaped coating disbondment with a holiday in the center of a 170 by 170 mm steel panel was used in the test with the FBE coating and soil slurry. Four embedded detection probes were located at 10, 20, 35, and 50 mm from the holiday. The 160 by 20 by 1 mm tunnel-shaped disbondment had a holiday at one end and was fabricated with an FBE or HPPC coating on a 100 by 170 mm steel panel. Five embedded detection probes were located at 10, 30, 60, 100, and 140 mm from the holiday.
The CP potential and current were applied and monitored with a multi-channel potentiostat. All daily pH and potential measurements inside the disbondment areas were performed manually using a digit precision multimeter. The potential values at each location inside the coating disbondments were measured against the Ag/AgCl reference electrode embedded in that location. Three CP potentials of –776, –926, and –1,126 mV vs. a saturated calomel electrode (SCE) (equivalent to –850 mV, –1,000 mV and –1,200 mV vs. a copper/copper sulfate [Cu/CuSO4], were chosen to mimic the situations where the pipe system was under standard CP protection, modest CP overprotection, and aggressive CP overprotection, respectively. Additionally, CP was interrupted for ~15 min on a regular basis to monitor the open circuit potential of the steel panel at the holiday and locations with the detection probes. All the tests were conducted at room temperature.
The researchers reported several conclusions. They found that, in general, CP penetration was dependent on CP level, the environment inside the disbondment, and the distance from the holiday. CP penetration increased with increasing CP level, and decreased with increasing pH and distance from the holiday.
For a the circular-shaped FBE coating disbondment, the presence of clay soil in the solution acted as a physical barrier that prevented hydroxide (OH–) generated inside the disbondment to diffuse out. This led to a very alkaline environment (pH 14) around the holiday. Therefore, the presence of soil would cause an inhibiting effect to CP penetration.
The majority of the CP current was consumed at the holiday and its vicinity, and only a very small portion of CP current could reach beyond 10 mm into the FBE and HPPC coating disbondments, even at a CP level of -1,126 mV vs. an SCE. Therefore, CP current could reach deeper into the narrow tunnel-shaped disbondment than into a circular-shaped disbondment with a similar holiday and disbondment gap size.
The tunnel-shaped coating disbondment was subject to hydrogen accumulation inside the disbondment. The trapped hydrogen gas could effectively block the CP current from penetrating the disbondment. Therefore, the critical parameters required for the occurrence of near-neutral-pH SCC could be realized inside the tunnel-shaped HPPC coating disbondment under elevated CP levels of -926 mV and -1,126 mV vs. an SCE due to the presence of trapped hydrogen gas.
More detailed information on the study results can be found in CORROSION 2018 paper no. 11023, “Effect of Soil and Disbondment Configuration on CP Penetration into Coating Disbondment,” by L. Yan, J.-P. Gravel, and M. Arafin.