99020115

CORROSION–Vol. 55, No. 2 115

CORROSION SCIENCE SECTION

Submitted for publication December 1997; in revised form,September 1998. Presented as paper no. 726 at CORROSION/98,March 1998, San Diego, CA.

* Center for Marine Materials, Department of Ocean Engineering,Florida Atlantic University, Boca Raton, FL 33431.

Effect of Velocity on Current Densityfor Cathodically Polarized Steel in Seawater

D. Hugus and W.H. Hartt*

ABSTRACT

A series of cathodic polarization experiments was performedin which sections of UNS G10230 steel pipe were galvani-cally coupled through an external resistor to an aluminumanode ring. These cells were incorporated into a seawaterflow loop such that the hydrodynamic conditions were de-fined quantitatively. Seawater velocities for the experimentswere 0.03, 0.09, and 0.30 m/s. From the results, the trend ofsteady-state potential vs current density at a given velocitywas characterized in terms of the recently proposed slopeparameter approach to cathodic protection (CP) design. Also,an expression for the velocity dependence of the mainte-nance current density was developed. Anomalous behaviorwas encountered for some specimens tested at velocities of0.09 m/s and 0.30 m/s in that these exhibited a period ofdepolarization followed, in some cases, by repolarization.This behavior was reviewed in terms of transitory calcareousdeposit protectiveness, possibly reflecting the relativeamounts of calcium and magnesium in the deposits at aparticular time.

KEY WORDS: calcareous deposits, cathodic protection,design, offshore environments, seawater, structural steel,UNS G10230, velocity

INTRODUCTION

Cathodic protection (CP), with or without coatings,has served for decades as the corrosion control

method of choice for steel members and structures inseawater. In the past, incentives for optimizing thisprotection have been relatively modest, but the ad-vent of deep-water petroleum production activitiesand environmental concerns have prompted researchthat has led to designs for rapid polarization,1-4 im-proved understanding of calcareous deposits andstructure current demand,5-10 and development of theslope parameter approach to CP design.11-13 Through-out these studies and the evolution of various designpractices,14-16 the maintenance or mean currentdensity (im), the time-averaged current density overthe design life (T) of the CP system, has remained akey parameter. This term is represented in currentdesign practices by:15-16

im =NC

AcT (1)

where N is the requisite number of anodes, C is an-ode current capacity, and Ac is cathode surface area.

In the slope parameter format, this expressionbecomes:11-13

im =RawC

TS (2)

where Ra and w are the resistance and weight of theindividual anodes, respectively, and S is the slopeparameter. With development of this latter equation,

0010-9312/99/000027/$5.00+$0.50/0© 1999, NACE International

116 CORROSION–FEBRUARY 1999


the design process was elevated from an empiricalprocedure in which overdesign by some undeter-mined amount is implicit to one which isfirst-principles based.12 Still to be developed, how-ever, is an improved method for projecting im, sincethis presently is determined only by prior serviceexperience in comparable ocean locations or byexperiments and test exposures that simulate suchexperience. Even information developed from thesemay be inadequate or misleading, as evidenced bythe fact that the existing recommended practice listsim for the Gulf of Mexico as 55 mA/m2 (5 mA/ft2),15

whereas long-term steady-state values an order ofmagnitude or more lower have been reported.17

It has been established from corrosion principlesthat CP current demand for a structure and, hence,im are governed by oxygen availability or by the rateof oxygen transport across the diffusional and hydro-dynamic boundary layers as affected by temperature,relative water movement (velocity), and surface films(coatings, calcareous deposits, and fouling). Particu-larly absent are studies that incorporate seawatervelocity as a control variable and studies where thisparameter has been characterized hydrodynamically.Exceptions include the research of Wolfson andHartt,18 Smith, et al.,19 and Gartland, et al.,20 each ofwhom potentiostatically polarized steel specimensunder controlled seawater flow conditions. However,the potentiostatic nature of these tests was suchthat, while information was gained regarding calcare-ous deposits and the capability for these to reducecurrent demand as exposure time progressed, theresults are not quantitatively relatable to galvanic CPwhere both potential and current density changewith time.

A procedure for addressing the combined influ-ences of electrolyte flow and presence of calcareousdeposits upon current density has been proposedbased upon the Sherwood (Sh), Schmidt (Sc), andReynolds (Re) numbers.19-20 Thus, for flow alonga straight, circular cross-section pipe, Re isexpressed as:

Re =vdν (3)

where v is the average fluid velocity, d is the insidepipe diameter, and ν is the kinematic viscosity.

Under these conditions, flow is considered tur-bulent if Re exceeds 2,100 and laminar at lowervalues. Sc, in contrast, characterizes the diffusionalnature of the electrolyte according to:

Sc =νD

(4)

where D in the present case is the oxygen diffusivity.

TABLE 1Chemical Composition of the Aluminum Anode Material

Element Composition (wt%)

Zn 1.444In < 0.0010Hg 0.0433Si 0.036Cu 0.00345Fe 0.038Cd 0.0016B < 0.03Sn 0.0033Mg 0.0075Cr 0.002Ni 0.0025Mn 0.004Ti 0.003V 0.003Bi 0.0023Al Bal.

TABLE 2Chemical Composition of the Steel Cathode Pipe Material

Element Composition (wt%)

C 0.23Mn 0.39Si 0.04Ni 0.01Cr 0.03Mo < 0.01S 0.011P 0.008Cu 0.02

(a)

(b)

FIGURE 1. Schematic of: (a) a steel cathode specimen and (b) analuminum anode specimen after machining.



Sh is determined from:

Sh = 0.03 Re 0.8 × Sc

0.33 (5)

and this can be incorporated into Fick’s first lawsuch that:21

iL =

DnFc

x/Sh + t/p (6)

where iL is the limiting current density for oxygenconcentration polarization (equivalent to im), n and Fhave their normal meanings, c is the bulk dissolvedoxygen concentration, x is the average length of thepipe, t is the deposit thickness, and p is the porosityconstant of the calcareous deposit (unitless).

In conjunction with development of the slopeparameter approach to design of galvanic CP systems,an experimental methodology was developed thatfacilitates quantitative characterization of polarizationin terms of field-relevant parameters.11 The objectiveof the present research was to extend this approachto experiments where flow was controlled and quanti-fied for the purpose of characterizing im in terms ofelectrochemical and hydrodynamic parameters.

EXPERIMENTAL

Materials and Specimens — Anodes for the ex-periments were machined into a ring shape from anAl-Zn-Hg (Galvalum I†) casting. The cathodes werecylinders that were sectioned from a 3.7-m length ofUNS G10230(1) steel pipe 10 cm (4 in.) in nominaldiameter with the internal surface polished to a 600-grit finish. Interior surface area of the steel cathodeswas 506 cm2 and was 43 cm2 for the anodes, suchthat the surface area ratio (anode-to-cathode) was0.085. Both specimen types were degreased afterpreparation. Final dimensions are shown in Figure 1.Chemical compositions of the aluminum and steelare presented in Tables 1 and 2, respectively.

Test Cells and System — A Schedule 80 polyvinylchloride (PVC) slip flange was compression-fittedabout each end of the steel cathode specimens, andthe joint between the two was sealed at the outersurface. A machine screw was soldered to the outersteel surface at mid-length. Anode rings were drilledand tapped on the outside surface and fitted with amachine screw. In both cases, these screws served assites for electrical connection.

Figure 2 shows the test cell configuration, whichwas comprised of a steel cathode with flanges, an

aluminum anode ring, a PVC pipe section withflanges, an external resistor between the anode andcathode, and a silver-silver chloride (Ag-AgCl) refer-ence electrode. The external resistor was used toaffect a particular value for S. The Ag-AgCl referenceelectrodes were fabricated from silver wire 0.5 mm indiameter and molten AgCl. These were potted in athreaded PVC cap using silicone and then positionedinto a length of PVC pipe with end flanges of similardimensions as for the steel cathode such that the tipof the reference electrode extended ≈ 5 mm below theinterior PVC pipe surface. Different sections werebolted together across an anode ring such that awatertight seal was created. All components had thesame internal diameter, and caution was taken toensure that these aligned with one another.

Figure 3 illustrates the test system used for thefirst set of experiments (Set 1). This consisted of anupper reservoir tank that gravity fed seawater froman elevation of ≈ 2 m through three flow lines, each

(1) UNS numbers are listed in Metals and Alloys in the UnifiedNumbering System, published by the Society of AutomotiveEngineers (SAE) and cosponsored by ASTM.

† Trade name.

FIGURE 2. Side-view schematic of the test cell layout.

FIGURE 3. Side-view schematic of the first experimental setup.



of which was inclined at a 22° angle. Each flow lineconsisted of four test cells mounted in parallel(Figure 2). From the flow lines, the water emptiedinto a lower reservoir and was pumped back to theupper reservoir.

Upon completing the first set of experiments andopening the system, it was determined that corrosionproducts from the anode had settled along the bot-tom of the line and that these extended onto theinitial portion of the adjacent cathode. Also, corro-sion products on the marginally protected andunderprotected cathodes indicated the flow had aspiral component, apparently because of the rela-tively short distance from the upstream cell to thetest sections. Consequently, the test system wasmodified for the second set of tests (Set 2). Thismodification included positioning the flow lines verti-cally, adding 13 mm-diameter PVC pipe sectionswithin the PVC portion of the lines as flow straight-eners, and repositioning the anodes so that thesewere downstream rather than upstream from thecorresponding cathode. Figure 4 is a schematic ofthis arrangement.

Flow rate was controlled by a gate valve in eachline such that average velocities of 0.03, 0.09, and0.30 m/s were achieved, as determined by time mea-surements of flow volume for each leg. The seawaterwas pumped continuously to the laboratory from a

buried offshore wellpoint, which had been shown bymonitoring during an annual cycle to be typical ofsemitropical ocean water.22 In the flow system, thiswater was replenished at an exchange rate of five tosix turnovers per day. This was considered sufficientto maintain the water quality in view of the aerationthat resulted from the pumping and because therates of calcium (Ca2+) and magnesium (Mg2+) deple-tion via calcareous deposition were calculated to besmall compared to those of replenishment.

Calculation of Re (Equation [3]) indicated thatflow in the 10-cm test line was turbulent at each ofthe three velocities. At the same time, flow in the flowstraighteners that were added for Set 2 was laminar.Consequently, this may have resulted in a laminar-to-turbulent flow transition along the steel cathodespecimens in the Set 2 experiments.

(a)

(b)

(c)

FIGURE 5. Plot of: (a) f vs time, (b) i vs time, and (c) f vs i forSpecimen I-32/03 (S = 0.32 Ω-m2, and v = 0.03 m/s).

FIGURE 4. Side-view schematic of the second experimental setup.



External resistors were sized such that onespecimen in each flow line had S values of 0.32, 0.63,1.94, and 3.85 Ω-m2 during Set 1 and 0.081, 0.17,0.32, and 0.63 Ω-m2 for Set 2. Cathode potential andvoltage drop across the external resistors were moni-tored during the experiments and were recordedusing a computer-based data acquisition system.

RESULTS AND DISCUSSION

Polarization DataPotential (f) and current density (i) for all

specimens decreased during the initial 100 h or

so of exposure and approached steady-statevalues as illustrated by the example in Figure 5,which plots f vs time, i vs time, and f vs i. Thisbehavior was similar to that reported from similargalvanic CP experiments and was attributed toprogressive oxygen concentration polarizationin conjunction with formation of calcareousdeposits.11-12 Previous studies have shown thisf-i data trend (Figure 5[c]) to conform to therelationship:

φc = Rt× Ac i c + φa (7)

TABLE 4Steady-State Potentials for the Steel Cathode Specimens in Set 2

S v Steady-State PotentialSpecimen ( Ω-m2) (m/s) (VAg-AgCl )

II-081/03 0.081 0.03 –1.05II-081/09 0.081 0.09 –0.98II-081/30 0.081 0.30 (A)

II-17/03 0.17 0.03 –1.04II-17/09 0.17 0.09 –0.99II-17/30 0.17 0.30 (B,C)

II-32/03 0.32 0.03 –1.05II-32/09 0.32 0.09 –0.96(C)

II-32/30 0.32 0.30 (B,C)

II-63/03 0.63 0.03 –1.04II-63/09 0.63 0.09 –0.97(B,C)

II-63/30 0.63 0.30 –0.78(B)

(A) Exception 3 (see text)(B) Exception 1, Type 1 (see text).(C) Exception 1, Type 2 (see text).

TABLE 3Steady-State Potentials for the Steel Cathode Specimens in Set 1

S v Steady-State PotentialSpecimen ( Ω-m2) (m/s) (VAg-AgCl )

I-32/03 0.32 0.03 –1.04I-32/09 0.32 0.09 –0.98I-32/30 0.32 0.30 –0.88(A)

I-63/03 0.63 0.03 –1.00I-63/09 0.63 0.09 –0.90I-63/30 0.63 0.30 –0.84(A)

I-194/03 1.94 0.03 –0.89I-194/09 1.94 0.09 –0.78(A,B)

I-194/30 1.94 0.30 –0.67(B)

I-385/03 3.85 0.03 –0.71I-385/09 3.85 0.09 –0.68(B)

I-385/30 3.85 0.30 –0.66

(A) Exception 2 (see text).(B) Exception 1, Type 1 (see text).



where fc is the cathode potential, Rt is the totalcircuit resistance (dominated in the present case bymagnitude of the external resistor), and fa is theanode potential.

Thus, the f-i trend is projected to be linear withS = Rt x Ac and with intersection of the vertical axis atfa, provided S and fa are constant with time. Thedeparture from linearity in Figure 5(c) probablyreflects a period of anode activation during the initialstages of the test (upper right portion of the curve)and partial passivation of the anode once i becamelow (lower left).

The above example where f and i decreasedmonotonically to steady-state values was termednormal behavior, as contrasted with exceptions thatalso were noted. Tables 3 and 4 list steady-statepotentials for the test specimens in experiments in

Sets 1 and 2, respectively, and indicate that suchexceptions to normal polarization occurred mostly atthe highest velocity (0.30 m/s) and, to a lesserdegree, at the intermediate velocity (0.09 m/s). Alltests at the lowest velocity exhibited normal behav-ior. These occurrences of exceptional behavior werecategorized according to:

— Category 1: cathode depolarization followed byrepolarization (f vs i linear);

— Category 2: cathode depolarization withoutsubsequent repolarization (f vs i linear); and

— Category 3: cathode depolarization followed byrepolarization (f vs i nonlinear).

In addition, two types of Category 1 behaviorwere encountered. The first, which was designated asType 1, occurred relatively early in the tests and wasof short duration. The second (Type 2) occurred sub-

(a)

(b)

(c)

(a)

(b)

(c)

FIGURE 6. Plot of: (a) f vs time, (b) i vs time, and (c) f vs i forSpecimen II-63/09 (S = 0.63 Ω-m2, and v = 0.09 m/s).

FIGURE 7. Plot of: (a) f vs time, (b) i vs time, and (c) f vs i forSpecimen I-63/30 (S = 0.63 Ω-m2, and v = 0.30 m/s).



sequently and lasted longer. Figures 6 through 8illustrate examples of each category. Thus, in thecase of Figure 6, two depolarization/repolarizationevents were noted, where the first (Category 1/Type1) occurred in the 100-h to 200-h timeframe, and thesecond (Category 1/Type 2) occurred at 500 h to2,500 h. Figure 7 shows an example where both Cat-egory 1/Type 1 and Category 2 behaviors occurred.Figure 8 shows an example of Category 3. The f-iplots for the Categories 1 and 2 behavior (Figures 6and 7) were essentially linear and, as such, indicatedthat circuit resistance and fa remained constantthroughout the experiments. Instances of Category 2behavior were noted only in Set 1 experiments. Sincethe duration of these experiments was shorter thanfor Set 2 tests (1,850 h compared to 4,500 h), it waspossible that Category 2 behavior was the same asfor Category 3 but that there was insufficient time forrepolarization in the former case. The nonlineartrend in the case of Category 3, in contrast, indicatedRt, fa, or both changed (increased) as these experi-ments progressed. To investigate this, fa and fc

(current-on values) were recorded periodically over a2-week period. The potential difference between thesetwo (fc – fa) was determined and compared with thevoltage drop across the external resistors. Results areshown in Table 5, which indicates that the magni-tude of these two parameters was essentially thesame. It was concluded from this that a circuit resis-tance increase, as could result from corrosionproduct accumulation upon the anode calcareousdeposits upon the cathode, or a combination of thesetwo, was not a factor and that polarization of theanode caused the f-i nonlinearity.

In such experiments, the cathode would be ex-pected to control with regard to current. That thiswas the case in instances of normal and Categories 1and 2 behavior was indicated by the fact that itracked f (i.e., a positive f excursion was accompa-nied by an i increase and visa versa [cf. Figures 6and 7]). If this was the case, then some developmen-tal aspect in the formation of the calcareous deposit,whereby its protectiveness was compromised at firstand later restored, must have been responsible.

A similar trend as for Category 1/Type 1 andCategory 2 behavior occurred also in Category 3(Figure 8) to ~ 1,500 h. Beyond this, i decreased withtime, while fc remained about the same (slightlynegative to –0.80 VAg-AgCl) to ~ 2,850 h and then in-creased to ≈ –0.70 VAg-AgCl. During this same period,fa increased to near this same value (–0.70 VAg-AgCl).This trend, coupled with the fact that the peak cur-rent density upon the anode (1,500 h into the test)was 10 A/m2, suggested the critical i may have beenreached and that the anode partially passivated.

In evaluating data for the various specimens,consideration was given to the fact that the experi-mental setup was changed between the Set 1 and Set

2 tests. However, no distinctions in the f-i-velocitytrends between the two sets were apparent. Thus, thedata were analyzed assuming they conformed to acommon population.

Velocity Dependence of PolarizationFigure 9 shows f-i data for each of the three

velocities in cases where steady-state was achieved.Also included are results from previous tests thatwere performed in a 2-L test cell with a seawaterexchange rate of 150 mL/min, for which velocity wasconsidered “quiescent.”11 The three Set 2 specimensfor which f-i behavior was nonlinear (Table 4) werenot included in Figure 9 because steady-state wasnot achieved for these. In each case, a trend curvewas added. Thus, the quiescent velocity data exhib-ited a relatively well-defined sigmoidal trend. The0.03-m/s and 0.09-m/s data were displaced progres-sively toward higher velocity but showed a trend

(a)

(b)

(c)

FIGURE 8. Plot of: (a) f vs time, (b) i vs time, and (c) f vs i forSpecimen II-081/30 (S = 0.081 Ω-m2, and v = 0.30 m/s).



similar to that of the quiescent curve, although scat-ter was greater and there were gaps in the data forthe two higher velocities. The 0.30-m/s results indi-cated a progressive current density increase assteady-state potential became more negative.

The maximum steady-state current density atthe lower two velocities (quiescent and 0.03 m/s)occurred near –0.80 VAg-AgCl (that this was so was lessapparent in the 0.03-m/s case), whereas this maxi-mum was at ~ –0.90 VAg-AgCl for 0.09 m/s and at aneven lower f, assuming such a peak occurred at all,for 0.30 m/s. Such a trend (f at which the peak ioccurred decreasing with increasing velocity above0.03 m/s) indicated rapid polarization. Achievementof a low im1-4 may have become more difficult, if notimpossible, at higher velocities.

The data in Figure 9 indicated that some speci-mens tested at different velocities developed nearlythe same steady-state potential, –0.78, –0.88, and–0.98 VAg-AgCl. This facilitated projection of a steady-state i-vs-velocity relationship (Figure 10). In doingthis, the quiescent water movement data wereassigned zero velocity.11 A best fit line, which con-formed to the relationship:

i m = 852v + 48 (8)

was fitted to the data. On this basis, an order of mag-nitude increase in velocity caused about a fourfoldincrease in i.

Calcareous Deposit PropertiesPorosity — An attempt was made to evaluate the

calcareous deposit porosity constant (p) in Equation(6). To accomplish this, Re was determined for thepresent experimental setup assuming v = 8.81 x

10–7 m2/s and Sc (Equation [4]) assuming an oxygendiffusivity (D) of 2.72 x 10–5 cm2/s. From these, theSh (Equation [5]) was calculated based upon an as-sumed bulk oxygen concentration (c) of 5 mg/L.Deposit thickness values were taken from previousresearch for velocities and f that were comparable tothose of the present test conditions,18 and the cur-rent densities (iL) were those measured in the presentstudy. Table 6 lists these tvalues, and accordingly,Table 7 indicates the calculated p values. Althoughresults were limited, they showed that p increasedwith increasing velocity and with decreasing f. Also,the ratio of thickness-to-porosity was 3.3 to 5.7 timesgreater than the average x-to-Sh ratio. This indicatedthe resistance to diffusion afforded by the calcareousdeposit was of greater significance than that of theboundary layer.

Composition and Morphology — Following theexperiments, one specimen from each flow line of Set1 and all Set 2 specimens were analyzed by scanningelectron microscopy (SEM) and energy-dispersivex-ray analysis (EDX). Specimens in the former case(Set 1) were selected such that the steady-state f wasapproximately the same for each. Figures 11 through13 illustrate the morphology for these, where for thefirst (Figure 11, Specimen I-194/03) the deposit wasrelatively coarse, with EDX indicating this was pre-dominantly calcium. Figure 12, on the other hand,shows the morphology for Specimen I-63/09 andthat this exhibited an inner deposit similar to that inFigure 11 and a finer-grain outer one. EDX revealedthe former to be calcium-rich and the latter predomi-nantly magnesium. Figure 13 shows the deposit forSpecimen I-63/30, which was mostly fine grain andmagnesium-rich. These micrographs suggested therewas a tendency for deposits that formed under theseconditions (steady-state f of ≈ –0.98 VAg-AgCl) to transi-

TABLE 5Comparison of φc – φa and Voltage Drop

Across the External Resistors for Set 2 Specimens

∆V AcrossS Time φc φa φc – φa Resistor

(Ω-m2) (h) (V) (V) (V) (V)

3,740 –0.689 –0.700 0.011 0.0110.081 3,915 –0.709 –0.720 0.011 0.011

4,035 –0.695 –0.707 0.012 0.0124,165 –0.695 –0.706 0.010 0.011

3,740 –0.669 –0.691 0.022 0.0210.17 3,915 –0.673 –0.693 0.020 0.020

4,035 –0.671 –0.689 0.018 0.0194,165 –0.67 –0.687 0.017 0.016

3,740 –0.832 –0.952 0.120 0.1190.32 3,915 –0.796 –0.926 0.130 0.130

4,035 –0.775 –0.900 0.125 0.1254,165 –0.777 –0.897 0.120 0.119



tion from calcium- to magnesium-rich with increas-ing velocity. The calcium-rich deposits werepresumably aragonite (CaCO3), and the magnesiumones were brucite (Mg[OH]2).

Set 2 specimens tested at 0.03 m/s velocity allpolarized to –1.04 VAg-AgCl or –1.05 VAg-AgCl, irrespectiveof S (Table 4). Figure 14 illustrates the deposit mor-phology for Specimen II-17/03 and reveals this to besimilar to what has been reported historically forcomparable test conditions.5,9-10 EDX showed thecomposition to be calcium-rich with only a trace ofmagnesium. The deposits for other Set 2 specimenstested at this same velocity were similar to this withregard to morphology and composition.

Steady-state f for Set 2, 0.09-m/s velocity speci-mens were in the range –0.96 VAg-AgCl to –0.99 VAg-AgCl.An example of these for Specimen II-63/09 is shownin Figure 15. Here, the morphology was a mixture ofcoarse and fine grain particles that appeared similarto those for Specimen I-63/09 (Figure 12). However,EDX showed both to be predominantly calcium.

Set 2 specimens tested at 0.30-m/s velocityexhibited a relatively broad range of steady-state f(Table 4) because some of these depolarized andothers did not. In this regard, Specimens II-081/30and II-17/30 had final f that were positive to–0.70 VAg-AgCl. Consistent with this, relatively littlecalcareous deposit was present. Figure 16 showsexamples of the deposit morphology on the othertwo specimens (Specimens II-32/30 and II-63/30)and that these also were a mixture of coarse- andfine-grain particles. EDX revealed these to becalcium-rich.

Previous studies of calcareous deposits haveascribed significance to the relative amounts of cal-cium and magnesium, which often has beencharacterized in terms of the calcium-to-magnesiumratio.23 Apparently, the calcium-rich phase (aragoniteor calcite) serves as a more resistant barrier to oxy-gen diffusion than does the magnesium one (brucite),although this concept has been challenged in thecase of the thin, inner Mg(OH)2 film that invariably ispresent beneath the CaCO3 layer but not necessarilydetected.10 Because of gaps in the polarization dataand because deposit compositional information wasavailable for only two exposure times, it was not pos-sible to construct a comprehensive evolutionarymodel of the calcareous deposits and to interrelatethis to the f-i history. However, the depolarizationand depolarization/repolarization trends in Figures 6through 8, coupled with the finding that the calcium-magnesium ratio was often low after 1,850 h buthigh at 4,500 h, suggests that, subsequent to initialrapid polarization, Mg(OH)2 precipitated in somecases upon specimens exposed to the higher veloci-ties and caused current demand for these toincrease, which resulted in depolarization. Subject tothe anode potential remaining sufficiently negative,

TABLE 6Calcareous Deposit Thickness

at Different Potentials and Velocities18

Seawater DepositVelocity Potential Thickness

(m/s) (V SCE) (mm)

–1.03 0.180.30 –0.93 0.07

–0.78 0.04

–1.03 0.060.08 –0.93 0.05

–0.78 0.03

TABLE 7Calculated Values for p

ApproximatePotential(VAg-AgCl ) 0.09 m/s 0.30 m/s

–0.78 0.035 0.061–0.89 0.086 0.104

p(unitless)

FIGURE 9. Plot of steady-state f vs steady-state i for specimens ofthe present tests.

FIGURE 10. Plot of steady-state i as a function of VAg-AgCl.



these specimens subsequently repolarized in re-sponse to CaCO3 being reestablished as the principledeposit phase. The cause of such a velocity depen-dent deposit compositional cycle was not determined.Also unclear was why such a cycle has apparentlynot been evident from data acquired in conjunctionwith monitoring programs on offshore structures forlocations where water movements were comparableto those in the present study. Possible explanationsfor this are that:

— Data of this type from actual structures arelimited, and the exposure conditions for where theydo exist may not have been the same as for thepresent experiments;

— Velocity was continuous in the present experi-ments; whereas in actual service, it may be near zero

during slack periods. Deposit initiation and growth isexpected to be enhanced during times of low or nilvelocity; and

— Macrofouling, which was not present in theselaboratory experiments since the seawater was sandfiltered, may provide an additional barrier to watermovement on actual offshore structures such thatthe effective velocity at the steel surface is lower thanwas the case for the present specimens.

These same factors may explain also why arelatively sensitive dependence of im upon velocity(Equation [8]) was realized from the present experi-ments, such that calculated values for thisparameter exceeded what has typically been reportedfor actual offshore structures. For example, the aver-age of the three 0.09-m/s data points in Figure 10

(a) (b)

(a) (b)

FIGURE 11. SEM micrographs of the calcareous deposit that formed on Specimen I-194/03 (S = 1.94 Ω-m2, and v = 0.03 m/s).




was 145 mA/m2, which exceeds typical steady-statevalues for Gulf of Mexico structures by an order ofmagnitude or more. Accordingly, deposits that formduring periods of nil water movement may be rela-tively protective, such that, once these areestablished, the velocity dependence of current den-sity is more modest than projected by Equation (8),as has been reported.20

CONCLUSIONS

In most cases, polarization conformed to what wastermed a normal trend where f and i decreased withtime to a steady-state value. Exceptions were notedat the two higher velocities, however, where the ini-

(a) (b)

(a) (b)

tial polarization was followed by depolarization and,in some cases, repolarization. Apparently, some as-pect of the higher flow rates caused the calcareousdeposits to become less protective for some periodand for current demand to increase. The trend of f vs i at the two lower velocities re-vealed a sigmoidal curve with a peak current densitybeing achieved for 0.03-m/s tests at ~ –0.80 VAg-AgCl

and for 0.09 m/s at ~ –0.90 VAg-AgCl. At the highestvelocity (0.30 m/s), i increased with decreasingsteady-state f for the entire range of S values forwhich results were available. For specimens that reached steady-state f in therange –0.78 VAg-AgCl to –0.98 VAg-AgCl, im increased withv according to Equation (8). This velocity dependence


FIGURE 14. SEM micrographs of the calcareous deposit that formed on Specimen II-17/03 (S = 0.17 Ω-m2, and v = 0.03m/s).



of im exceeds what typically is realized in service,possibly because velocity was continuous during thepresent tests such that there were no nil water move-ment periods during which a more protectivecalcareous deposit could have formed. Based upon thickness values for calcareous de-posits taken from the literature, the deposit porosityconstant was calculated to be in the range 0.035 to0.104 (unitless). Consequently, the restriction tooxygen availability afforded by the deposits was 3.3to 5.7 times greater than otherwise would have beenthe case. For Set 1 specimens (exposure time = 1,850 h)tested at the two higher velocities (0.09 m/s and0.30 m/s), the calcareous deposit was comprised of a

magnesium-rich as well as a calcium-rich phase. Inthe case of Set 2 specimens (exposure time = 4,500 h),the deposit was calcium-rich with only traceamounts of magnesium. The depolarization/repolar-ization behavior noted may have been related to anevolutionary compositional aspect of calcareous de-posit development where the relative amounts ofcalcium and magnesium varied with time.

ACKNOWLEDGMENTS

The authors acknowledge financial support ofthe Florida Sea Grant Program of the NationalOceanographic and Atmospheric Administration(project R/C-D-16).

(a) (b)

(a) (b)

FIGURE 15. SEM micrographs of the calcareous deposit that formed on Specimen II-63/09 (S = 0.63 Ω-m2, and v = 0.09 m/s).

FIGURE 16. SEM micrographs of the calcareous deposit that formed on: (a) Specimen II-63/03 (S = 0.63 Ω-m2, and v = 0.30m/s) and (b) Specimen II-32/30 (S = 0.32 Ω-m2, and v = 0.30 m/s).



REFERENCES

1. T. Foster, V.G. Moores, “Cathodic Protection Current Demandof Various Alloys in Seawater,” CORROSION/86, paper no. 295(Houston, TX: NACE, 1986).

2. S. Evans, MP 27, 2 (1988): p. 9.3. C.F. Schrieber, J. Reding, “Application Methods for Rapid

Polarization of Offshore Structures,” CORROSION/90, paperno. 381 (Houston, TX, NACE, 1990).

4. K.P. Fischer, T. Sydberger, R. Lye, “Field Testing of Deep-WaterCathodic Protection on the Norwegian Continental Shelf,”CORROSION/87, paper no. 67 (Houston, TX: NACE, 1987).

5. W.H. Hartt, C.H. Culberson, S.W. Smith, Corrosion 40 (1984):p. 609.

6. H.-S. Lin, S.C. Dexter, Corrosion 44 (1988): p. 615.7. J.E. Finnegan, K.P. Fischer, “Calcareous Deposits: Calcium and

Magnesium Ion Concentrations,” CORROSION/89, paper no.581 (Houston, TX: NACE, 1989).

8. K.P. Fischer, J.E. Finnegan, “Cathodic Protection Behavior ofSteel in Seawater and the Protective Properties of the Calcare-ous Deposits,” CORROSION/89, paper no. 582 (Houston, TX:NACE, 1989).

9. J.S. Luo, R.U. Lee, T.Y. Chen, W.H. Hartt, S.W. Smith, Corro-sion 47 (1991): p. 189.

10. K.E. Mantel, W.H. Hartt, T.Y. Chen, Corrosion 48 (1992): p. 489.

11. W. Wang, W.H. Hartt, S. Chen, Corrosion 52 (1996): p. 419.12. W.H. Hartt, S. Chen, D.W. Townley, Corrosion 54 (1998), p. 317.13. D.W. Townley, “Unified Design Equation for Offshore Cathodic

Protection,” CORROSION/97, paper no. 473 (Houston, TX:NACE, 1997).

14. NACE Standard RP0176-76, “Corrosion Control of Steel-FixedOffshore Platforms Associated with Petroleum Production”(Houston, TX: NACE, 1976).

15. NACE Standard RP0176-94, “Corrosion Control of Steel-FixedOffshore Platforms Associated with Petroleum Production”(Houston, TX: NACE, 1994).

16. Recommended Practice RP B401, “Cathodic Protection Design”(Hovilc, Norway: Det Norske Veritas Industri NorRe AS, 1993).

17. M.W. Mateer, K.J. Kennelley, “Design of Platform Anode Retro-fits Using Measured Structure Current Density,” CORROSION/93, paper no 526 (Houston, TX: NACE, 1993).

18. S.L. Wolfson, W.H. Hartt, Corrosion 37 (1981): p. 70.19. S.W. Smith, K.M. McCabe, D.W. Black, Corrosion 45 (1989):

p. 790.20. P.O. Gartland, E. Bardal, R.E. Andersen, R. Johnsen, Corrosion

40 (1984): p. 127.21. T.K. Ross, D.H. Jones, J. Appl. Chem. 12 (1962): p. 314.22. W.H. Hartt, “Fatigue of Welded Structural Steel in Seawater,”

paper no. 3982, Proc. Offshore Technol. Conf., Houston, May,1981 (Dallas, TX: Society of Petroleum Engineers, 1981).

23. R.A. Humble, Corrosion 4 (1948): p. 358.

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