disbonded coatings.pdf

8/14/2019 Disbonded coatings.pdf

1/13

CORROSION ENGINEERING

804 CORROSIONOCTOBER 1994

Submitted for publication November 1993; in revised form, March 1994.* Department of Chemical Engineering, Clarkson University, Potsdam, NY,

13699-5705.

Cathodic Protection to MitigateExternal Corrosion of Underground Steel PipeBeneath Disbonded Coating

F. Gan, Z.-W. Sun, G. Sabde, and D.-T. Chin*

KEY WORDS: cathodic protection, coating holiday, crevicecorrosion, disbonded coating, high-resistivity soil, potentialdistribution, pulsed current

INTRODUCTION

Crevice corrosion beneath disbonded coating is acommon problem on underground steel pipelines.1-2

Underground gas and oil pipelines are protectedagainst corrosion by combined application ofcoatings and cathodic protection. Coatings usuallyare in the form of fusion-bounded epoxy orpolyethylene and polyvinyl chloride tapes with anadhesive backing, which are wrapped helically on thepipe surface.3Holidays are the pinholes and ruptureson a pipe coating. They are caused by defects in thecoating, coating misapplication, and mechanicaldamage during or after pipeline installation. Thedevelopment of a holiday leads to the loss ofadhesion between the pipe and coating in the

surrounding areas. Water in the soil then flowsthrough the holiday opening into the crevice betweenthe pipe and disbonded coating and initiatescorrosion on the pipe surface.

Field studies have shown that crevices beneathdisbonded coatings range from 5 mm to 125 mm indiam.3The corrosion within the crevice occurs in theform of elongated pits. Stress corrosion cracking candevelop if sufficient amounts of carbonates andbicarbonates are present in the soil water.4-5Cathodicprotection has not been able to mitigate crevicecorrosion properly beneath disbonded coatings,

especially on pipes buried in high-resistivity soil. This

ABSTRACT

A study was conducted to determine the feasibility of

cathodically protecting the steel surface beneath adisbonded coating with a holiday using an external powersource. A laboratory cell was used to simulate the fieldcondition of a steel pipe buried in a soil saturated with

ground water that had resistivity of 3,050 -cm to

4,400 -cm. The local pipe potential and solution pH withinthe crevice of the disbonded coating during cathodic

protection were measured over time. The relativeadvantages of using a pulsed current (PC) vs those of adirect current (DC) were examined. Results indicatedapplied cell voltage was an important factor for proper

protection of a coated steel pipe in high-resistivity soils. Alow cell voltage would not permit sufficient polarization ofthe steel surface within the crevice. A high voltage inducedexcessive hydrogen evolution at the crevice opening,

which blocked penetration of cathodic current into thecrevice. The optimal cell voltage was: (a) PC of afrequency of 1 Hz and a duty cycle 0.5 at a controlled

voltage setting (Von) = 15 V to 20 V and (b) DC atVon= 15 V to 20 V. Both DC and PC protected the steelpipe adequately in the test soil. The mechanism ofcathodic protection in the high-resistivity environment wasidentified as: (a) polarization to a corrosion immunity

potential for the steel surface in the vicinity of creviceopening, (b) oxygen depletion for the solution in the interiorof the crevice, and (c) a pH increase in the crevice andpassivation of the steel in the alkaline environment.

0010-9312/94/000187/$5.00+$0.50/0 1994, NACE International


2/13


805CORROSIONVol. 50, No. 10

is largely a result of the inability of imposed cathodicprotection systems to pass sufficient protectivecurrent to the corroding sites on the pipeline. Thedisbonded pipe coating acts as a shield against theflow of cathodic current to the pipe surface beneathit.6-7The cathodic current must flow through the

holiday opening into the crevice, and the excessohmic potential drop across the electrolyte within thecrevice reduces the true pipe-to-electrolyte polariza-tion potential from a protected value of 0.85 V vs acopper sulfate reference electrode (CSE) to a lessnegative value at which the cathodic protection failsto stop the corrosion process.

Toncre and Ahmad studied cathodic protectionagainst crevice corrosion of mild steel in seawater,saline water, and brackish water.4They simulated thecrevice that would be found under a disbondedpipeline coating and measured the potential alongthe crevice length. Their results showed cathodicprotection in low-resistivity solutions could beachieved if the potential at the crevice opening wasmore negative than 1.0 V vs a saturated calomelelectrode (SCE). Similar potential measurementswere carried out by Peterson and Lennox.8Theyfound it was not always necessary to pass currentinto the crevice to protect the metal surfacecathodically.8Corrosion could be mitigated when asufficiently negative potential was applied to thecrevice opening so that the concentration ofdissolved oxygen (O2) within the crevice wasreduced.

Berry,9Fletcher, et al.,10and Fessler, et al.,11

studied polarization of pipeline steel in a solutioncontaining 1 N sodium carbonate (Na2CO3) and 1 Nsodium bicarbonate (NaHCO3). Their data indicatedthe potential of a clean steel surface inside a crevicecould be maintained within the protected range bykeeping the potential at the holiday opening at anappropriate level. The steel samples that had a millscale needed a longer time to polarize and attained aless negative potential than clean steel samples.Using a segmented specimen, Fessler, et al.,showedthe protective current density decreased along the

crevice from the opening.11Proper protection couldbe achieved with a controlled holiday potential of

1.0 VSCEin the Na2CO3and NaHCO3solution. At amore negative controlled potential, hydrogen gas (H2)bubbles were produced at the holiday opening andcaused fluctuations in the current/potential measure-ment. Toncre found that, by extending the testingtime, proper polarization of the steel surface beneathdisbonded coatings could be achieved in a low-resistance solution for crevice thickness-to-depthratios up to 1:1,200.12

A field study on the effectiveness of cathodic

protection under disbonded coatings on a pipeline

was carried out by Orton in eastern Saudi Arabia.13

The soil environment consisted of rocks and desertsands. In a large area, the saline water table wasonly a few feet below ground. The water resistivityranged from 30 -cm to 200 -cm. The currentrequired to maintain the holiday potential at

0.85 VCSEand 1.0 VCSEwas almost the same,whereas at a 1.5 VCSEsetting, additional currentwas necessary because of H2evolution on the pipesurface. In most of Ortons experiments, steel waspolarized only within a short distance from thecrevice opening. McCoyreported the cathodic protec-tion of the Dampier-to-Perth natural gas pipeline inAustralia buried in a high-resistivity soil (50,000 -cmto 100,000 -cm).14Corrosion coupons wereinstalled along the pipeline to simulate disbondedcoating holidays and to determine the degree ofpolarization on the pipe by measuring couponpotential using a current interruption technique.When using a controlled potential of 1.1 VCSE, thepotential within the crevice was only 0.6 VCSE.Apparently, polarization within the crevice was notachieved. On a pipeline in Saudi Arabia, Toncremeasured the pipe-to-soil potential circumferentiallyaround pipe.12There was general corrosion on thepipe when the circumferential potential was

0.99 VCSE. Where the coating was disbondedextensively, pitting corrosion occurred on the pipesurface, and the circumferential potential was

0.5 VCSEto 0.6 VCSE.Results of the above studies indicate it is

possible to achieve cathodic protection of a metalsurface in a crevice containing a low-resistivityelectrolyte. However, in a high-resistivity environ-ment, it is difficult to achieve the 0.85 VCSEprotective polarization potential inside a crevice.At a high applied potential, H2evolution occurred atthe holiday opening, whereas at a low holidaypotential, cathodic protection was not achieved. Noexperimental study has been reported for the current/potential distribution within a crevice filled with ahigh-resistivity electrolyte. A detailed review of thecathodic protection of underground pipelines against

crevice corrosion was given by Sabde, et al.15Recently, pulsed current (PC) in the form of an

intermittent on-and-off current wave has been studiedfor the cathodic protection of oil well casings, waterheaters, and steel pipes.16-20Although no particularbenefit was identified, PC reduced energy consump-tion while maintaining the protection efficiencyafforded by the direct current (DC) cathodicprotection system. In electroplating, PC has beenreported to possess a larger microthrowing powerthan DC plating.21-22This raises the possibility thatPC might be able to penetrate deeper into the crevice

and provide a more uniform potential and current


3/13



distribution than DC when used in cathodic protectionagainst crevice corrosion.

In the present study, the effectiveness ofcathodic current using an impressed current tomitigate the crevice corrosion of a steel pipe beneatha disbonded coating holiday in high-resistivity soil

was investigated. The local pipe potential andsolution pH within the crevice were monitored as afunction of time. The relative merits of PC vs DCwere examined experimentally. To account for alarge difference in the behavior of local crevicepotential between the low-resistivity and high-resistivity solutions, a model crevice cell was used toexamine the effect of solution resistivity on cathodicprotection.

EXPERIMENTAL

A laboratory corrosion cell was used to simulatethe field condition of a steel pipe having a disbondedcoating with a holiday buried in a soil saturated withwater. The resistivity of the soil water was in therange 3,050 -cm to 4,400 -cm. The effect ofelectrolyte resistivity on cathodic protection andcrevice potential distribution was studied using amodel crevice cell filled with a dilute sodium chloride(NaCl) solution over a resistivity range of 158 -cmto 4,200 -cm.

Test PipeThe test pipe was a steel pipe (API Standard SL

Grade X65(1)) of 32.4-cm outer diameter (OD) and0.714 cm wall thickness. The pipe was sandblasted,machine smoothed, and cut into sections 10.2 cmlong. On each pipe section, four back-reference tapsand four pH monitoring holes were drilled through thepipe wall at the 12:00 (12 oclock; top), 3:00, 6:00(bottom), and 10:30 positions. Each reference tapwas fitted with a plastic tube on the inside surfaceof the pipe and was connected by flexible plastictubing to an external reference electrode compart-ment. The pH monitoring holes were sealed with arubber stopper, which permitted the insertion of a pH

microelectrode probe from the interior of the pipe formeasurement of the crevice solution pH.

Pipe Coating and Crevice ConfigurationThe exterior surface of the steel pipe was

polished manually using 600-grit sandpaper, cleanedwith detergent, degreased with acetone, and dried.The pipe then was wrapped circumferentially with anew polyethylene pipeline tape.(2)The tape was

1.3 mm thick by 10.2 cm wide and was fastened tothe pipe with two stainless steel pipe clamps. At bothends of the pipe, the space between the tape andexterior surface of the pipe was sealed using anadhesive strip 1 mm thick and 1.3 cm wide. Theoverlap sections of the tape on the pipe also weresealed with adhesive. In this way, a watertightcrevice 1 mm thick was formed circumferentiallybetween the exterior pipe surface and the poly-ethylene coating. A circular hole of 0.78 cm diam wascut through the tape at the 12:00 position to simulatethe holiday (or crevice opening). The ratio of the

crevice opening area to the total pipe surface withinthe crevice was 1:1,400 (i.e., 0.5 cm2of holiday areato 700 cm2of pipe area in the crevice). The crevicethickness-to-length ratio along the circumferentialdirection was 1:500.

Corrosion Cell and Test SoilFigure 1 shows the schematic of cell construc-

tion. The cell was a rectangular acrylic container48 cm wide by 36 cm deep by 50 cm high. The steelpipe with the disbonded polyethylene coating wassandwiched between two acrylic holding plates andwas mounted on one side of the cell wall. The pipe,

plates, and cell wall were bolted together using eighttitanium (Ti) bolts. The Ti bolts also served as theanode during the cathodic protection experiments.One holding plate served as a partition between thesoil in the cell and the interior of the test pipe. Theother holding plate, which faced the cell wall, had anopening to the atmosphere. The opening was usedfor connecting reference capillary tubes to thereference taps on the steel pipe and for making pHmeasurements during the test. A neoprene O-ringwas placed between each holding plate and the steelpipe to prevent soil water from leaking into the

interior of the test pipe. To simulate the temperatureof warm petroleum oil flowing in the pipe, an electricheat gun inserted through the opening was used toheat the air in the pipe to 50C 2C. The pipeinterior temperature was measured using a thermistorprobe and was controlled by a temperature controller.

In this way, two horizontal pipe test sectionswere mounted on each acrylic cell. The spacebetween the cell and coated pipe samples was filledwith soil taken from an excavation site on the Trans-Alaska Pipeline at mile post 160.5 in Atigun Valley,Alaska. The soil was mixed with deionized water in a

volume ratio of 1:1. The resulting soil water (3)had

(1) Steel composition: carbon (C), max. 0.16%; silicon (Si), max. 0.35%;manganese (Mn), max. 1.5%; phosphorus (P), max. 0.025%; sulfur (S),0.01% to 0.035%; copper (Cu), max. 0.2%; chromium (Cr), 0.3%;vanadium (V), max. 0.1%; aluminum (Al), max. 0.04%; nitrogen (N), max.0.008%; and iron (Fe), remainder.

(2) The electric resistance and capacitance of tape in the test soil water at50C, as measured by an alternating current (AC) impedance technique,were > 1011 and 3.0 pF/cm2, respectively. The water adsorption was1.2% of its weight after 80 days immersion in the soil water at 50C.

(3) Water composition in milligrams per liter: calcium ions (Ca 2+), 50;magnesium ions (Mg2+), 26; sodium ions (Na+), 6; chloride ions (Cl), 6;

bicarbonate ions (HCO3), 98; sulfate ions (SO42), 66; total solids, 302.


4/13



a pH of 6.4 to 8.4 and an electric resistivity of3,050 -cm to 4,400 -cm at a steady-state soiltemperature of 35C during the experiments. Air wasbubbled through the soil water continuously, and thewater was recirculated in the soil using an air-liftpump (Figure 1).

Electric Circuitand Cathodic Protection Procedures

The electric circuit for cathodic protectionexperiments is shown schematically in Figure 2. Apulse power supply or a DC power supply was usedto supply the controlled cell voltage between the Tianode and the steel pipe cathode. During theexperiment, the anode-to-cathode cell voltage wasmonitored with a digital multimeter (V1) and itswaveform was checked periodically on an oscillo-scope.

The protective cell current was determined bymeasuring the voltage drop across a standardresistor (R) with a second multimeter (V2). Thecurrent waveform was examined using the oscillo-scope. The SCE was used to determine the localcrevice potential at the 12:00, 3:00, 6:00, and 10:30positions on the test pipe. A high-input impedanceelectrometer (V3) was used for the potentialmeasurement.

Prior to filling the cell with the test soil, a leakagetest was carried out to ensure that the crevicebetween the exterior surface of the pipe anddisbonded coating was sealed properly and the soil

water could enter the crevice only through the holidayopening at the 12:00 position. Then, the cell wasfilled with the 1:1 soil-water mixture to cover the testpipe completely. The test began by connecting thepipe and Ti anode to the cathodic protection circuitand by switching on the pipe temperature controller.During the test, the local crevice potential wasmeasured daily, and the local crevice pH values weremeasured weekly with a combination pH microelec-trode. To avoid contamination of the crevice solutionby the reference electrode fluid, the SCE wasinserted into the pipes reference tap only during the

measurement, which took a few minutes. At the endof test, the crevice solution was collected, and itsfinal pH and electric resistivity were measured usinga pH meter and a conductivity meter. The cell thenwas disassembled, and the test pipe was stripped ofthe polyethylene tape and washed with water. Thepipe surface was examined for corrosion damagevisually, and its surface condition was recorded onvideotape.

Model Crevice Cell ExperimentThe electrolyte used in the model crevice cell

experiment was an aqueous NaCl solution at 25C.

FIGURE 1. Schematic of the corrosion cell used to test thecathodic protection of a steel pipe with a disbonded coating inwet soil.

The solution was prepared by dissolving anappropriate amount of analytical-grade NaCl indeionized water. Three NaCl concentrations wereused: 0.06 M, 0.006 M, and 0.002 M. The corre-

sponding electric resistivities of these solutions at

FIGURE 2. Electric circuit for cathodic protection and locations

of reference and pH measurement taps on the steel pipesample.


5/13



potential (i.e., the potential difference between thesteel rod and the reference electrode in the maincompartment) using a potentiostat. Direct and pulsedpotential were used. The pulsed potential wasgenerated by feeding a rectangular pulse signal froma pulse function generator to the potentiostat. A

multimeter was used to measure the cell currentduring the tests. The local crevice potentials weremeasured using a high-input impedance electrom-eter. The test lasted 7 h in the 0.06 M NaCl solutionand 70 h in the 0.002 M NaCl solution. At the end ofeach test, the solution pH was measured in the bulkelectrolyte as well as in the crevice electrolyte. Thesurface of the steel rod was examined visually forcorrosion damage.

RESULTS

Cathodic Protection of Steel Pipe in SoilThree sets of cathodic protection tests werecarried out. Each set tested four coated steel pipesamples buried in the test soil. Cathodic protectionwas carried out using DC and PC at controlledanode-to-cathode cell voltage settings of 10 V, 15 V,and 20 V. For the pulsed cathodic protection, the cellvoltage was pulsed between the open circuit and acontrolled voltage setting (Von) at a duty cycle of 0.5.Two pulse frequencies were used: 1 Hz and 10 Hz.The experiment lasted from 20 days in Test 1 to 45days in Test 3. Tests 1 and 2 included a coated pipeundergoing natural corrosion within the crevice of adisbonded coating holiday without any protectivemeasures. Table 1 lists the controlled cell voltage,the surface condition after the tests, the amount ofelectric charge passing through the cell (Q), and thetotal electric energy consumption during the test (E).The pH values of the bulk soil water and crevicesolutions at the 12:00, 3:00, 6:00, and 10:30positions at the end of the test also are listed. Theelectric resistivities of the bulk soil water and crevicesolution at the end of the tests are presented in thelast two columns of the table.

Pipe Surface Condition After TestThe condition of the pipe surface after the

cathodic protection tests was classified into one ofthe following categories:

Severe Corrosion The pipe surface rustedheavily, and the entire surface was covered with alayer of brownish corrosion products;

CorrosionThe pipe surface was covered witha thin rust layer, and a lustrous metal color wasvisible under the brownish coating;

Slight Corrosion The pipe surface wascovered with a light blackish film. A few patches of

light brownish rust were scattered around the

FIGURE 3. Model crevice cell for testing the effect of solutionresistivity on potential distribution within a crevice.

room temperature were 158 -cm, 1,440 -cm, and4,200 -cm.

Figure 3 shows the construction of the modelcrevice cell. It consisted of a horizontal glass tube10 cm long with an 11.8 mm inner diam (ID). Oneend of the glass tube was open to a large glassvessel, which was used to hold the bulk of the testelectrolyte. The other end of the glass tube wassealed with a rubber stopper. A cylindrical steel rodof 9.3 mm diam and having the same composition asthe steel pipe samples was inserted into the glasstube through a central hole on the rubber stopper to

simulate a model crevice. One end of the steel rodwas insulated from the test electrolyte with an epoxyglue and was placed at the open end of the glasstube to serve as the crevice (or holiday) opening. Theresulting model crevice had a crevice width of 1.3mm and a crevice width-to-depth ratio of 1:80. Themain electrolyte compartment contained 110 mL oftest electrolyte, a Ti anode, a SCE, and an airbubbling tube. The reference electrode was inside aLuggin capillary tube, whose sintered glass tip wasplaced near the crevice opening. This referenceelectrode in the main electrolyte compartment was

used to control the holiday potential of the steel rodduring the cathodic protection experiments. Threevertical reference taps were mounted on the glasstube 1.5 cm, 4.5 cm, and 7.4 cm from the creviceopening. A SCE was inserted into the reference tapsto measure the local crevice potential. The referencecompartments were separated from the crevicesolution with a sintered glass and were filled with thesame test electrolyte during the test. Air was bubbledinto the electrolyte in the main compartmentthroughout the test.

Cathodic protection of the steel rod within the

crevice was carried out by controlling the holiday


6/13



surface, and in some areas, the pipe surface wasbright and lustrous; and

Very Slight Corrosion Most of the pipe surfacewas bright and lustrous, with only a few smallpatches of light blackish film and brownish rust.

For the steel pipes undergoing the naturalcorrosion, severe corrosion was found in theneighborhood of the holiday opening. Heavybrownish rust covered almost the entire pipe surface.

For the steel pipes protected by cathodic protection,the surface condition after the tests depended on thecontrolled cell voltage settings. The most effectivesettings were PC of 1 Hz at Von= 20 V and DC at Von= 15 V, for which only very slight corrosion was foundon the pipe surface. The next most effective settingswere PC of 1 Hz at Von= 15 V and DC at Von= 20 V,for which slight corrosion occurred on the test pipes.The least effective settings were PC of 10 Hz atVon= 20 V and DC or PC of 1 Hz at Von= 10 V, forwhich the pipe surface after the tests was classifiedat a level between slight corrosion and corrosion. In

general, for the pipe samples protected by cathodicprotection, the pipe surface in the neighborhood ofthe holiday opening was grayish-silver and free ofrust. This could be considered as an area at whichthe cathodic current penetrated into the crevice andprotected the pipe at a sufficiently negative potential.The size of the area ranged from 2.5 cm to 13 cmdiam depending on the controlled cell voltage. Whitecalcareous deposits were beneath the holidayopenings. Farther downstream from the grayish-silverarea, some form of corrosion always took place, theextent of which depended on the cell voltage

settings.

Local Potential Within the Creviceof Disbonded Coating

For the pipe samples undergoing naturalcorrosion beneath the disbonded coating with aholiday, the pipe potential at different locationswithin the crevice all changed to a steady value of

0.76 VSCEto 0.78 VSCEafter 3 days of experiments.The steady-state potential value was more negative

than that of an uncoated pipe steel sample in the bulksoil (0.3 VSCEto 0.6 VSCE), probably because of thedepletion of dissolved O2 in the crevice.

For the cathodically protected pipe samples, thelocal crevice potential at the holiday opening at 12:00invariably became more negative than 1.0 VSCE.However, the local crevice potential at the 3:00 and6:00 positions all were more positive than 0.85 VSCE.Figure 4 shows the crevice potential-vs-time curvesfor the pipe protected at a DC cell voltage ofVon= 20 V. For this pipe there was no significantdifference between the local crevice potential at the

3:00 and 6:00 positions. The crevice potential atthese two positions reached a steady-state value of

0.83 VSCE. The pipe surface at these two locationswere protected technically in accordance with the

0.78 VSCEcathodic protection criterion set by NACEInternational.23

Figure 5 shows the crevice potential-vs-timecurves for the pipe protected at a PC of 1 Hz atVon= 20 V. For this pipe sample, there was a suddenshift of local crevice potential at the 3:00 and 6:00positions from 0.8 VSCEtoward a more positivevalue. The shift occurred after 250 h of testing. The

potential shift first occurred at the 3:00 position, and

FIGURE 4. Local crevice potential of a steel pipe protected by

a DC cell voltage of Von= 20 V. The pipe was wrapped in adisbonded polyethylene coating with a holiday at the 12:00position.

FIGURE 5. Local crevice potential of a steel pipe protected by

PC. The controlled cell voltage was pulsed between the opencircuit and a Vonof 20 V at a frequency of 1 Hz and a duty cycleof 0.5.


7/13



For the pipes protected by cathodicprotection, the crevice pH increased with time andreached a steady-state value after 2 weeks of test.

The rate of pH increase and the final pHvalues in the crevice increased with the cell voltage

settings. The highest pH measured was 12.3. The local crevice pH was the highest at the

holiday opening (i.e., at the 12:00 position). Thecrevice pH generally decreased with increasingdistance from the holiday opening.

Pulsed cell voltage seemed to give a moreuniform pH distribution than DC at the same Vonsetting.

With the exception of the steel pipe protectedby a PC of 10 Hz at Von= 20 V, there was less corro-sion on the steel pipe when the crevice pH at the12:00 and 10:30 position was in the 10 to 12 range.

The crevice solution for each pipe sample inTests 2 and 3 was collected at the end of the test,and its average resistivity at its steady-state tempera-ture of 43C was measured using a conductivity celland a conductivity bridge (Table 1). There was asubstantial decrease in the crevice solution resistivitywhen compared with the resistivity of the bulk soilwater. In some cases, the decrease was more thantenfold. The cathodically protected pipes had a lowercrevice solution resistivity than the pipe without anycathodic protection. The effectiveness of cathodicprotection seemed to correlate with the extent of

decrease in the resistivity of the crevice solution.

FIGURE 6. Crevice solution pH at the 6:00 position vs time for

a pipe without cathodic protection and for the pipes protectedby DC and PC at Von= 20 V. For comparison, the pH of the bulksoil water in two corrosion cells are shown also.

then at the 6:00 position within 24 h. The steady-state potentials were 0.6 VSCEat the 3:00 positionand 0.5 VSCEat the 6:00 position. The time of thepotential shift corresponded approximately to thetime of a crevice solution pH increase from 8 to 12 for

this sample (Figure 6). A similar potential shift at the3:00 and 6:00 positions also was observed for thepipe protected with a PC of 10 Hz at Von= 20 V.However, for this pipe the potential shift occurredafter a longer test time(600 h). The potential shiftagain corresponded to the time for pH to increase to12 (Figure 6). This phenomenon indicated the pipesurface may have become passivated under thealkaline condition, reducing the corrosion rate in thecrevice.

pH and Resistivity Changes

Inside the CreviceFigure 6 shows pH-vs-time curves at the 6:00

position for the four pipe samples in Test 2. Figure 7shows the local crevice solution pH of the four pipesat the end of Test 2 as a function of circumferentialtap location. For comparison, the pH values of thebulk soil water of two test cells also are given inFigure 6. They were nearly constant at 8.5 through-out the test duration. The characteristics of crevicepH changes could be summarized as follows:

For the natural corrosion pipe, the crevicesolution was slightly more acidic than the bulk soil

water, with a steady pH value of 7.2 to 7.4.

FIGURE 7. Local crevice solution pH at the end of Test 2 for a

pipe without cathodic protection and for the pipes protected byDC and PC at Von= 20 V.


8/13


9/13



crevice could not be polarized to a more negativepotential than 0.8 VSCEat any controlled potentialsetting within a test duration of 70 h. The crevicepotential distribution for this solution was in thereverse order from that for the 0.06 M solution, withthe least negative value occurring at the No. 1reference tap and the most negative value occurringat the No. 3 reference tap. This result indicated thecurrent did not penetrate into the crevice, and thesteel surface was not protected by the cathodic

current. The solution pH within the crevice after the

test was in the range 5.5 to 6.1, which was about thesame as the initial pH of the test solution. Corrosionoccurred 2 cm from the crevice opening. However,no corrosion was observed near the crevice entranceor at the sealed end of the crevice.

For the 0.006 M NaCl solution, which had a

moderately high resistivity of 1,440

-cm, thepotential distribution within the crevice exhibited atransition from the behavior in the 0.002 N NaCl(high-resistivity) solution to that in the 0.06 M NaCl(low-resistivity) solution. Figure 9 shows the localcrevice potential in the 0.006 M NaCl solution at acontrolled holiday potential of 1.28 VSCE. No H2evolution was observed in this run. During the initialperiod of this run, the potential distribution in thecrevice was in the reverse order, with the leastnegative potential occurring at the No. 1 referencetap and the most negative potential occurring at theNo. 3 reference tap. This behavior was similar to thatin the 0.002 M NaCl solution. As the cathodicprotection continued, the local crevice potentialgradually shifted toward the negative direction.However, the rate of potential shift was the highestat the No. 1 reference tap. This was followed by theNo. 2 and then by the No. 3 reference tap. After 16 hof test, the order of potential distribution waschanged to the normal order (the No. 1 referenceelectrode tap exhibited the most negative potentialand the No. 3 reference tap the least negative poten-tial [Figure 9]). This indicated that, in a moderatelydilute NaCl solution, an initial incubation period was

necessary for the current to penetrate into thecrevice. The crevice solution at the end of the run(22 h) had a pH of 10. The steel surface exhibitedonly minor corrosion 2 cm from the crevice opening.

Effect of Pulsed PotentialThe effect of rectangular pulsed potential on the

potential distribution in the model crevice cell wastested in the 0.006 M and 0.002 M NaCl solutions.The controlled holiday potential was pulsed between

1.03 VSCEand 1.53 VSCEat a frequency of 1 Hz anda duty cycle of 0.5. This gave a time-averaged

holiday potential of 1.28 VSCE. Figure 10 shows thecrevice potential-vs-time curves for a run in the0.006 M NaCl solution. Comparing Figure 10 toFigure 9, which shows the results for DC potentialcontrol at the same time-averaged potential, thepulsed potential improved the rate of potentialchange of steel in the crevice. The pulsed potentialeliminated the initial incubation period, and thepotential of the No. 1 reference tap shifted to theneighborhood of 0.8 VSCE immediately after thecurrent was switched on. A normal potentialdistribution in the crevice was observed, with the

most negative value occurring at the No. 1 reference

FIGURE 9. Crevice potential of a steel rod in the model crevice

cell containing 0.006 M NaCl at a controlled DC holidaypotential of 1.28 VSCE.

FIGURE 8. Time-averaged protective current density vs timefor the pipes protected by DC and PC at Von= 20 V.


10/13


11/13



Lennoxhave shown that it is not necessary to forcethe electric current into a crevice to protect itcathodically.8Sufficient cathodic protection can beachieved by applying an appropriate potential tosuppress the differential O2cell within the crevice.Cathodic protection effectively can stop the diffusion

of dissolved O2into the crevice by reducing allavailable O2at the pipe surface directly beneath thecrevice opening. Using a simple O2diffusion model,the steady-state local O2concentration, C(x), within acathodically protected crevice may be described bythe differential equation:

d

2C

dx2

k

DC = 0 (10)

at

x = 0, C = Cb; and at x =, dCdx

= 0 (11)

where x is the distance from the crevice opening, isthe crevice thickness, Cbis the O2concentration atthe crevice opening, D is the diffusivity of dissolvedO2, and k is a first-order reaction rate constant for thereduction of O2at the steel surface inside the crevice.The above equations may be integrated to give:

C(x)

Cb= exp

kd

D

1

2 x

(12)

Figure 11 shows the variation of O2concentrationinside the crevice for three values of a dimensionlessparameter, k/D. For a value of k/D 0.1, the localO2concentration inside the crevice is reduced to< 0.001 of the bulk O2concentration outside thecrevice at a distance of x = 25from the creviceopening. Since no O2 is transported into the crevice,the corrosion rate in the crevice eventually is reducedto a low level. This situation is illustrated schema-tically by the polarization curves in Figure 12, in

which A represents the anodic polarization curve ofsteel in a neutral crevice solution, C refers to the O2reduction curve at the initial stage of corrosion, andC refers to the O2reduction curve after the applica-tion of cathodic protection. The limiting current for Cis smaller than that for C because of a decrease ofO2concentration in the crevice. The point ofintersection between the anodic and cathodicpolarization curves gives a high initial corrosion rateof icorrand a low corrosion rate after the application ofcathodic protection, icorr. The corrosion potential inthe crevice is shifted from Ecorrto a more negative

value of Ecorrbecause of the depletion of O2.

reactions take place in the neighborhood of theholiday opening.

Corrosion of iron (Fe) and subsequenthydrolysis and oxidation of the corrosion product:

Fe = Fe2+ + 2e (7)

Fe 2+ + 2OH = Fe(OH)2 (8)

4Fe2+ + O2 + 10H2O = 4Fe(OH)3 + 8H+ (9)

The formation of Fe corrosion products were evidentfrom the blackish and rusty films on pipe samplesurfaces and the yellowish precipitates in the crevicesolutions collected at the end of experiments.

Mechanism of Cathodic ProtectionAgainst Crevice Corrosion

Past results have shown that cathodicpolarization of steel inside the crevice of a disbondedcoating can be achieved in low-resistivity solutions. Inthe present study, steel pipes with a disbonded coat-ing in a high-resistivity solution also were protectedcathodically, even though the electric current was notable to penetrate deep into the crevice.

Two possible mechanisms for the cathodicprotection of pipe surfaces within the crevice of adisbonded coating with a holiday were examined.

O2Depletion Mechanism In aerated neutraland alkaline environments, the corrosion of steel is

controlled by the O2 reduction reaction. Peterson and

FIGURE 11. O2concentration inside crevice as a function ofdistance from the crevice opening for three values of adimensionless parameter, k/D.


12/13



pH Increase and Passivation Mechanism TheO2 reduction reaction caused by cathodic protectiongenerates OHat the crevice opening as shown byEquation (1). Some of the OHdiffuses into theinterior of the crevice and causes an increase in thepH of the crevice solution, as demonstrated by thepresent experimental data. According to a potential-vs-pH diagram of Fe,24the pipe surface can becomepassivated with the formation of a ferrosoferric oxide(Fe3O4) film in the pH range 9 to 13. When thecrevice solution becomes sufficiently alkaline, the

steel passivates and the anodic polarization curveshifts from the initial behavior of curve A to curve A(Figure 12). The corrosion rate is decreased to a lowvalue of icorr, and the corrosion potential in thecrevice shifts to a positive value of Ecorr. Thispassivation mechanism is consistent with the presentcathodic protection data of a pH increase in thecrevice to 10 to 12 and a shift in the local crevicepotential at the 3:00 and 6:00 positions to a morepositive value of 0.5 VSCEto 0.6 VSCE. To furtherelaborate the passivation mechanism in a high-resistivity medium, the anodic and cathodic

polarization curves of the present pipe steel in the

test soil water were measured using a laboratory celland a computer-interfaced potentiostat. Themeasurement was carried out at 50C and at twocontrolled pH values (7.9 and 12.3) corresponding tothe initial and final pH values of the crevice solution

in the cathodic protection experiments. Results areshown in Figure 13, in which A1 and C1 representthe anodic and cathodic polarization curves in the pH7.9 water, and A2 and C2 represent the anodic andcathodic polarization curves in the pH 12.3 water. AtpH 7.9, curve A1 indicates the steel was corroding inan active state. The cathodic curve (C1) exhibited alimiting current regime for the O2 reduction reaction,followed by the onset of H2evolution at 1.0 VSCE.This result implied that the corrosion of pipe steel inneutral soil water was controlled by the diffusion ofO2to the cathodic sites. At pH 12.3, the anodic

polarization curve (A2) exhibited a passive potentialregime similar to the vertical portion of curve A,shown in Figure 12, and the cathodic polarizationcurve (C2) exhibited a Tafel potential-currentrelationship for O2 reduction in the neighborhood ofthe corrosion potential. This result indicated that, inthe pH 12.3 water, the corrosion of steel was takingplace in the passive state, and the corrosion rate ascalculated using the Tafel extrapolation method wasnearly 10 times smaller than that in the pH 7.9 water.The passivation mechanism also was discussed byCherry and Gould for the cathodic protection of steel

in a crevice filled with a dilute NaCl solution.25

FIGURE 13. Anodic (A) and cathodic (C) polarization curves ofa pipe steel in aerated soil water at controlled pH values of 7.9

and 12.3.

FIGURE 12. Schematic of a potential-vs-current diagram

showing the changes in corrosion rate and corrosion potentialduring cathodic protection of steel inside a crevice. Curve A anodic polarization of steel in neutral crevice solution; Curve

A anodic polarization of steel showing passivity aftersufficient alkali has been introduced into the crevice by cathodicprotection, Curve C cathodic polarization at the initial stageof corrosion; Curve C cathodic polarization curve after O2concentration in the crevice has been reduced by cathodicprotection.


13/13


C

CONCLUSIONS

Experiments were conducted to determine theeffectiveness of cathodic protection to mitigateexternal corrosion of a steel pipe beneath adisbonded coating with a holiday. The relative merits

of using PC vs DC were examined for a steel pipehaving a crevice that had a thickness-to-length ratioof 1:500. The pipe was buried in a wet soil in whichthe resistivity of the soil water was 3,050 -cm to4,400 -cm. A model crevice cell was used toexamine the effect of solution resistivity on thebehavior of the crevice potential for solutionresistivities from 158 -cm to 4,200 -cm. Cathodic protection by means of an impressedcurrent was shown to be potentially effective inmitigating the external corrosion of steel pipebeneath a disbonded coating with a holiday in a high-

resistivity soil. In the present study, the protectionmechanism was identified to be: (a) cathodicpolarization of pipe surface in the immediate vicinityof the holiday opening to a corrosion immunitypotential, (b) depletion of dissolved O2 in the solutioninside the crevice, and (c) a pH increase in thecrevice and passivation of the steel surface in thealkaline environment. The applied cell voltage is an important parameterfor cathodic protection in a high-resistivityenvironment. A low cell voltage would not permitsufficient polarization of the steel surface within thecrevice. A high voltage induced excessive H2evolution at the holiday opening, which blocked thepenetration of cathodic protection current into thecrevice. In the present laboratory cell, the optimal cellvoltage settings were: (a) PC of a frequency of 1 Hzand a duty cycle of 0.5 at Von= 15 V to 20 V and(b) DC at a controlled cell voltage of 15 V to 20 V. Cathodic protection increases the pH anddecreases the resistivity of solution inside thecrevice. The maximum crevice pH measured in thepresent experiment was 12.3. The resistivity of thecrevice solution was reduced from an initial valueof 3,050 -cm to 4,400 -cm to a final value of

200 -cm to 600 -cm. The chemical changes werefavorable to the cathodic protection of the steel pipebeneath the disbonded coating. With a properly controlled voltage, both DC andpulsed cathodic protection can protect a steel pipeburied in high-resistivity soil. There was no significantdifference in overall protection effectiveness betweenthe DC- and PC-controlled modes. A low-frequencyPC of 1 Hz at a duty cycle of 0.5 provided betterprotection than a high-frequency PC of 10 Hz. Whenusing PC, the protective current and energy

consumption were smaller and the pH distribution inthe crevice was more uniform than when using DCcathodic protection.

ACKNOWLEDGMENTS

The authors acknowledge the support of AlyeskaPipeline Service Co. (TAPS); E.D. Burger (ARCOExploration and Production Technology); and themembers of a TAPS task group including L. Bone(ARCO Exploration and Production Technology),J. Hotchkiss and E.W. Klechka (Alyeska PipelineService Co.), J. Nunn (Mobil Pipe Line Co.),E. Pierson (Exxon Pipeline Co.), S.N. Smith (ExxonProduction Research), and T. Widin (BP Research).

REFERENCES

1. G. Mills, MP 28, 12 (1988): p. 13.

2. M. Roche, J.P. Samaran, MP 27, 11 (1987): p. 28.3. H.M. Smith, M.F. Bird, R.H. Penna, MP 28, 11 (1988): p. 19.4. A.C. Toncre, N. Ahmad, MP 19, 6 (1980): p. 39.5. R.R. Fessler, Oil Gas J. 74, 7 (1976): p. 81.6. A.W. Peabody, Control of Pipeline Corrosion (Houston, TX: NACE,

1967), p. 26.7. R.L. Wenk, Proc. 5th Symp. Line Pipe Research (Arlington, VA:

American Gas Association, 1974), p. T-15.8. M.H. Peterson, T.J. Lennox Jr., Corrosion 29 (1973): p. 406.9. W.E. Berry, Stress Corrosion Cracking Laboratory Experiences, Proc.

5th Symp. on Line Pipe Research (Arlington, VA: American GasAssociation, 1977), p. 43.

10. A.A. Fletcher, L. Fletcher, R.J. Morrison, The Effect of Pipe SurfaceOxide upon Crevice Polarization and Stress Corrosion Cracking UnderFusion-Bonded, Low-Density Polyethylene Coatings, 5th Int. Conf. onthe Internal and External Protection of Pipes, paper F4 (Bedford, UK:BHRA Fluid Eng. 1983).

11. R.R. Fessler, A.J. Markworth, R.N. Parkins, Corrosion 39 (1983):p. 20.12. A.C. Toncre, MP 23, 8 (1984): p. 22.13. M.D. Orton, MP 24, 6 (1985): p. 17.14. J. McCoy, MP 28, 2 (1989): p. 16.15. G. Sabde, F. Gan, D.-T. Chin, J. Chin. I. Ch. E. 24 (1993): p. 417.16. E.W. Haycock, D.M. Seid, Cathodic Protection by Intermittent and

Pulsed Currents, CORROSION/73, paper no. 69 (Houston, TX:NACE, 1973).

17. T. M. Doniguian, Oil Gas J. 80, 26 (1982): p. 221.18. E.E. Fletcher, T.J. Barlo, A.J. Markworth, E.W. Brooman, W.E. Berry,

R.N. Parkins, W.C. McGary, R.R. Fessler, Relative Merits of PulsedRectifiers for Controlling Stress Corrosion Cracking, NG-18 ReportNo. 127 to the American Gas Association (Columbus, OH: BattelleColumbus Laboratories, 1982).

19. R. Juchniewicz, J. Szukalski, E. Stankiewicz, B. Electrochem. 3(1987), p. 531.

20. B.N. Bauman,Protection of Well Casings by Pulsed CathodicCurrents, NACE-CRCW Meeting, Saskatoon, Saskatchewan, Canada,Feb. 19-21, 1991.

21. J-C. Puippe, F. Leaman, eds., Theory and Practice of Pulse Plating,(Orlando, FL: American Electroplaters and Surface Finishers Society,1986).

22. D.-T. Chin, Periodic Electrode Processes: AC Corrosion and PulsePlating, in Trends in Electrochemistry, Vol. 1 (Sreekanteswaram,Trivandrum, India: Council of Scientific Research Integration, 1992),p. 215.

23. NACE Standard RP01-69, Recommended Practice, Control ofExternal Corrosion on Underground or Submersed Metallic pipingSystems (Houston, TX: NACE, 1983).

24. H.H. Uhlig, R.W. Revie, Corrosion and Corrosion Control (New York,NY: John Wiley & Sons, 1985), p. 414.

25. R.W. Cherry, A.N. Gould, MP 29, 8 (1990): p. 22.

disbonded coatings.pdf

Documents