TESTS SHOW BARRIER COATINGS DO NOT BLOCK CATHODIC PROTECTION

Keith E. W. Coulson
NOVA Corp.
Alberta Calgary

Thomas J. Barlo
Northwestern University
Evanston, III.

Daniel P. Werner
Gas Research Institute
Chicago

Use of a barrier coating on a buried bare steel or corrosion-coated steel pipeline will not hamper performance of the line's cathodic-protection system.

That's the major conclusion of research funded by the American Gas Association's Pipeline Research Committee which studied the physical and electrochemical properties of the urethane foam and concrete barrier coatings.

The study showed that both types of coatings would absorb sufficient amounts of moisture to permit adequate cathodic protection of the underlying steel surface.

DUAL PROTECTION

To meet regulatory requirements, North American gas transmission pipelines are protected from external corrosion by a dual system of cathodic protection and anticorrosion coating.

As pipeline construction has entered more remote and difficult terrains, use of a select backfill, primarily compacted sand, has often been specified to protect the anticorrosion coating from damage during backfilling.

Increasing costs of transporting this material to more remote areas have made its .use prohibitive.

Recently, common practice has been to specify alternative barrier coatings, such as urethane foams or concrete, which are applied over the anticorrosion coating at the coating mill or at the construction site.

This approach, however, has raised concerns that the urethane foam and concrete barrier coatings may shield cathodic-protection currents from the pipeline and thus reduce the effectiveness of the cathodic-protection system in controlling external corrosion of the pipeline.

Evaluating the potential for such cathodic shielding requires consideration of several important parameters: barrier coating resistivity, soil resistivity, underlying steel polarization behavior, and steel corrosion rate.

AQUEOUS ELECTROLYTES

Experimental procedures for this study involved preparation of test specimens and aqueous electrolytes, monitoring of test specimens, and shielding calculations.

The research was conducted entirely in the laboratory with 0.5-in. OD SAE 1020 grit-blasted steel rods that were 8 in. long and covered over a 6-in. length with 1-in. barrier coating (Fig. 1).

The underlying steel surface was either bare, coated with fusion-bonded epoxy (FBE) entirely over its length, or FBE-coated with an intentional 0.5-in. wide band of bare steel at mid-length to simulate a holiday in the anticorrosion coating.

Five barrier materials were used in the study: 2 lb/cu ft closed-cell, urethane foam; 3 lb/cu ft closed-cell, urethane foam; two types of concrete coatings; and compacted sand.

Three conditions of the steel, anticorrosion coating, and barrier coating were selected for study:

Intact barrier coating on bare steel, with and without cathodic protection applied
Flawed (circumferential crack) barrier coating on FBE-coated steel with cathodic protection applied
Flawed barrier coating with a 0.5-in. wide bare band of metal used to simulate a holiday in the FBE-coated steel with cathodic protection applied.

The circumferential flaw was placed in the urethanefoam coating by a saw cut or by a scribe followed by a sharp blow to the scribe area for the concrete materials.

In all cases, the flaw was located 1.25 in. from the center of the specimen.

This placed the flaw about 1 in. from the edge of each holiday band in the FBE coating. The test specimens were configured to allow them to be polarized or to have their polarized potentials measured.

Fig. 2 shows the test cell and its ancillary parts in more detail.

The tests were conducted in four different aqueous electrolytes.

Three of the electrolytes were prepared by leaching soil obtained from along a pipeline right-of-way in distilled water.

The resulting leachate resistivities were 1,000 ohmcm, 10,000 ohm-cm, and 15,000 ohm-cm.

The fourth electrolyte, a simulated groundwater, was prepared by addition to distilled water of equal portions of three salts: sodium chloride, sodium sulfate, and sodium bicarbonate. This achieved an electrolyte resistivity of approximately 1,300 ohm-cm.

Once the assembly of the test specimens was completed, they were immersed in one of these four electrolytes. The electrolyte was contained in a 2 ft wide x 4 ft long x 3 ft high polyethylene tank filled with 150 gal of electrolyte, which provided a hydrostatic head of approximately 2.5 ft.

Each specimen was unpolarized and allowed to remain at the free-corrosion potential throughout the test period. Or it was cathodically polarized to a potential of - 850 mv (CU/CUSO4).

MONITORING

Once a test specimen was exposed to an electrolyte, several parameters were periodically measured, including the free-corrosion potential, the polarized potential, and the barrier-coating resistance.

All of the measurements were made with a specially designed bridge instrument that determined the polarized potential and coating resistance directly.

The barrier-coating resistivity was then calculated from the resistance and specimen geometry by the following formula:

Rx = pc In c + pe In b

-- -- -- --

2pL b 2pL a

where:

Rx = Barrier-coating resistance, ohms

Pc = Barrier-coating resistivity, ohm-cm

Pe = Electrolyte resistivity, ohm-cm

L = Barrier coating length, cm

a = Steel rod radius, 0.25 in.

b = Barrier-coating thickness + steel rod radius 1.25 in.

c = b + the radial distance to the point of measurement in the electrolyte, 2.5 in.

Once the specimens under test reached steady-state conditions in terms of polarized potential and barriercoating resistivity, polarization characteristics and general corrosion rates were measured.

The anodic and cathodic polarization behavior of dedicated specimens was determined with a Princeton Applied Research Model 273 Potentiostat/Galvanostat and SOFTCORR corrosion software.

This information permitted assessment of the corrosion rate at the free-corrosion potential as well as the polarization behavior.

Measuring the weight loss of the steel rod also yielded the general corrosion rates of dedicated specimens.

Measurements of pit depths on the steel surface provided pitting rates.

CALCULATIONS

Shielding of the various barrier coatings was characterized by quantitative comparisons among barrier-coating materials and between a specimen with a barrier material and a specimen with no barrier material present.

To provide a more practical characterization, laboratory data on the barrier and anticorrosion coating resistivity were used in a computer model of a pipeline to assess the effects on operations of an impressed current cathodic-protection system.

The computer model for the pipeline is a modified version of a code developed for AGA and Electric Power Research Institute (EPRI); the code is based upon an electrical analog of a pipeline.

The model assumed a 10 mile, 30-in. pipeline buried under 3 ft of soil with resistivities of either 1,000, 10,000, or 15,000 ohm-cm.

A remote-earth, 0.5-ohm groundbed was assumed at each end of the pipeline with a 1-y rectifier.

Other input parameters to the code included the barrier coating and anticorrosion coating properties and geometry.

The barrier coating was assumed to be 1 -in. thick over a bare pipe or an FBE-coated one to a thickness of 21 mils (0.021 in.). The output of the code was the pipe potential to remote earth and current density along the pipeline per unit of rectifier voltage.

From the polarization curves, the rectifier voltage to achieve a current density equivalent to that at 100 mv of polarization was determined. The rectifier voltages are conservative estimates because polarization effects were neglected.

RESISTIVITY

Steady-state resistivity was the primary basis for comparison among the various barrier coatings. Steady state is defined as a time beyond which the property being monitored no longer changes.

Since water absorption by the various barrier materials varies over time, it was important that the steady state be achieved with confidence.

Fig. 3 shows examples of the time dependency of the resistivity of the foam, concrete and sand barrier coatings. These three materials demonstrate the general time-dependence patterns of the barrier-coating resistivity.

On virtually all of the urethane-foam barrier coating specimens, approximately 23 weeks were required before potentials could be read for the steel and before coating resistances could be determined.

This time period was equated to the time for moisture to penetrate the 1-in. thickness of the foam and begin to reach the underlying steel surface. When it did so, resistivities climbed until they were of the order of 15 Mohm-cm.

Steady-state resistivities were reached in a period of approximately 200- 300 days.

As Fig. 3a also shows, relative steady-state resistivity was greatest for the FBE-coated steel, followed by the FBE-coated steel with a holiday, and then bare steel. This behavior pattern would be expected, based upon the relative resistance that the FBE coating represents.

Fig. 3b shows the time dependency of the concrete barrier-coating material. The concrete barrier materials absorbed moisture faster than the urethane-foam barrier coatings, allowing potentials and resistances to be measured almost immediately upon immersion.

As shown in Fig. 3b, the resistivity increased with time, eventually reaching steady-state conditions in 200-300 days.

For the bare steel specimen, the behavior was a bit more erratic. However, the relative order of magnitude of the resistivities for the concrete barrier coating specimens was in the range of 1520 kohm-cm, as compared to values of around 15 Mohm-cm for the urethane-foam barrier coating specimens.

Finally, Fig. 3c shows the time dependency of the compacted sand barrier resistivity. Again, steady state was achieved after 200-300 days of exposure.

Like the concrete coatings, potentials of the steel in the sand barrier materials could be measured shortly after immersion, and the steady-state resistivities were of the order of 20 kohm-cm.

RANGES

Overall, the resistivities can be categorized into one of two broad ranges.

For the foam barrier materials, the resistivities ranged from 1 to 30 Mohm-cm.

On the other hand, the concrete barrier materials, as well as sand, fell within the range approximately three orders of magnitude lower. For these specimens, resistivities ranged from approximately 10 to 50 kohm-cm.

The overall resistivities appeared unaffected by nominal levels of cathodic protection. The resistivities of the foam barrier materials appeared unaffected by the presence of the FBE on the underlying steel. This is attributed to the relatively poor immersion properties of the FBE used in the study.

Fig. 4 shows anodic and cathodic polarization scans conducted on bare steel specimens with a urethane foam, concrete, or sand barrier coating.

The polarization behavior of the urethane foam and sand specimen indicated active behavior throughout. This was in contrast to the indication of passive behavior in the anodic portion of the scan for the concrete barrier coating.

The passive behavior was characterized by a reduction in the current density at more positive potentials. This behavior, along with the fact that these specimens had a much more positive free-corrosion potential (- 450 my as compared to - 750 my for the foam barrier specimens) would suggest the presence of a local alkaline (high Ph) environment, one that could effectively reduce the corrosion rate of the steel.

Table 1 compares the corrosion rate of the steel, determined by weight loss, polarization resistance, and corrosion current, the last two obtained from the polarization scans, such as those in Fig. 4.

In general, the weight-loss determinations agree reasonably well with the corrosion rates determined from the electrochemical measurements.

Further, these data show that either foam or concrete barrier materials will provide an intrinsic reduction in the corrosion rate, by virtue of their presence on the steel, with the foam providing the greatest reduction in the corrosion rate, almost three orders of magnitude lower compared to bare steel without a barrier coating.

Overall, the resistivities of barrier materials can differ widely for several reasons. It appears, however, that the resistivity alone is not a good indicator of the potential for shielding. All of the barrier materials were capable of absorbing sufficient water to become electrically conductive.

Thus, in addition to resistivity, polarization behavior is an important consideration as well as the corrosion rate of the underlying steel when these barrier materials are being evaluated.

COMPUTER MODEL

In order to relate the laboratory results to the field, a computer model of a pipeline with a barrier coating was set up to assess the effects of the barrier coating on the operations of an impressed-current cathodic protection system.

The analysis was carried out for both a bare and an FBE-coated pipeline with a barrier coating present. Calculation of the necessary rectifier Voltages was based upon the current densities required to achieve at least 100 mv of polarization for each of the barrier materials.

Table 2 summarizes the results of those calculations for when the pipeline was bare and when it was FBE coated. The rectifier voltage was computed to obtain 100-mv polarization at the rectifier and at the electrical midpoint 5 miles away.

When the pipeline was bare, the rectifier voltages were quite high and resulted in significant overprotection at the rectifier location when the rectifier was adjusted to obtain the 100 mv of polarization at the electrical midpoint.

Although the voltages fell within the range of practicality, it was not expected that the situation of a barrier coating over bare steel would occur in practice.

The more likely situation was one in which the pipeline was coated with an anticorrosion coating and then covered by a barrier coating.

The calculation summarized in Table 2 for this situation indicates that very low rectifier voltages would be required to obtain the needed current density, even at the electrical midpoint 5 miles away.

Moreover, the voltage requirements were not significantly different when only the anticorrosion was present. Thus, the barrier coating materials appeared to have little or no effect on the resultant rectifier voltage required to achieve the protection level equivalent to 100 mv of polarization.

Based upon the results, it appeared that the barrier coating materials studied neither significantly shielded the pipeline from cathodic protection currents nor did they affect operations of the cathodic protection system.

INFERENCES

From the research conducted on these pipeline coatings, the following conclusions have been drawn:

Although there are tradeoffs in the physical and electrochemical properties of the barrier materials studied, those materials do not appear to shield cathodic-protection currents from the pipeline.
All of the barrier coatings will absorb sufficient water to become electrically conductive and allow cathodic protection to be achieved, including the urethane foams, which have low absorption properties.
The rectifier voltage required to protect an FBE coated pipeline with a barrier coating is comparable to the rectifier voltage required when there is no barrier coating present.
The resistivity of the barrier coating, the polarization behavior, and corrosion rate of the underlying steel surface are all important when the potential for shielding and protection ability of a barrier coating is considered.
A locally produced alkaline environment generated by the concrete barrier coatings produces some passivity of the unprotected steel that probably accounts for the reduced corrosion rates, while the foam probably lowers the corrosion rate by limiting access of moisture to the steel.

The results suggest that the steel under the urethane foams and concrete barrier materials can be cathodically protected with normal operating conditions in the field.

Although the resistivities of the urethane foams can be 1,000 times greater than that of the concrete barrier materials or sand, each of the barrier materials will provide an intrinsic reduction in the corrosion rate by limiting the access of the environment to the steel surface.