Cathodic Properties ofDifferent Stainless Steels

in Natural Seawater*

ROYJOHNSEN and EINAR BARDAL*

AbstractThe cathodic properties of a number of staínless steels, which were exposed to natural sea-water flowing at 0 to 2.5 m/s and polarized to potentials from —300 to —950 mV SCE, havebeen studied. The current density development at constant potential and the free corrosionpotential during the exposure time were recorded continuously. At the end of the exposureperiod, after approximately 28 to 36 days of exposure, polarization curves were determined.After one to three weeks of exposure, depending on the water velocity, microbiological activi-ty on the surface caused an increase in the current density requirement of the specimen. Anexplanation for the mechanism behind the current density increase caused by slime produc-tion from marine bacteria may be increased exchange current density, i o. There was nomeasurable calcareous deposit on the stainless steel surfaces at the end of the exposureperiods.

IntroductionAustenitic stainless steels have often been used with goodresults in chloride-containing environments. However, somecorrosion failures have been reported for AISI 304 and 316 innatural seawater, where they are particularly likely to undergocrevice corrosion. Crevices can be the result of design, localcovering, welding slag, or surface defects in weids. In manycases, when stainless steel has not been seriously attacked inseawater despite the susceptibility to crevice corrosion, thereason is the use of multi-alloy systems. In such systems, lessnoble materials such as mild steel operate as sacrificialanodes and suppress crevice corrosion. In the literature,cathodic protection has been described as an effective meansof protecting stainless steel against crevice corrosion. 1-6 Thedrawback in most of the published work is that they do notdeal with the current requirement dictated by potential, sea-water velocity, and temperature.

The objective of this work was to estimate cathodic cur-rent density requirement on stainless steel at different poten-tials, seawater velocities, and temperatures. Such informationabout the cathodic properties is important with respect to:

• Estimation of crevice corrosion rates. The cathodicreaction is concentrated on the passive free sur-face outside the crevice and usually controls thecorrosion rate to a great extent.

• Estimation of corrosion rates on less noble mate-rials in galvanic contact with stainless steel.

• Estimation of the current density requirement forcathodic protection systems to prevent crevicecorrosion of stainless steels.

*Submitted for publication December 1983; revised August1984.

*The Corrosion Centre, SINTEF, N-7034 Trondheim-NTH,Norway.

In a recently published paper, 7 the authors described ex-perimental results for an AISI 316 steel in 3% NaCI solutionand natural seawater. The present article contains resultsfrom experiments with six different stainless steels in natura)seawater. In addition, the results from Sandvik 5R60 have beencompared with resuits for the same steel in 3% NaCI solu-tions.

Experimental ProcedureThe experiments were conducted in a closed loop with a

total water volume of 1050 L. The electrolyte was saturatedwith air and was exchanged after each experiment. Thus, theseawater was kept for about one month in the laboratory dur-ing each experiment. The velocity of the solution varied fromstagnant to 2.5 m/s. The water flow was maintained by an im-peller connected to an electrical motor with variable speed,and the velocity was measured by a Pitot tube. Chemical com-position of the alloys investigated is shown in Table 1.

Figure 1 shows a schematic view of the flow channel, withthe specimens mounted. The test section of the flow channelwas 57 cm long x 41 cm wide x 21 cm high, and it is posi-tioned in such a way that the flow pattern through the sectionis uniform, with no expansion, contraction, or directionchanges of the flowing water. Three specimens of each alloy,approximately 10 x 100 mm (metal A) and 10 x 35 mm (metalB-F), were mounted and electrically insulated from each otherin PVC holders (40 cm long x 16 cm high) and connected toelectrical conductors.

These specimens, which are shown in Figures 1(b) and1(c), were assumed to have approximately the same boundarylayer thickness because of their relatively equal distance fromthe inlet edge of the PVC holder. The specimens were polar-ized potentiostatically to potentials from —300 to —950 mVSCE during the exposure time. In addition, specimens approx-imately 20 x 100 or 20 x 55 mm [shown to the right in Figures1(b) and (c)] were mounted in the PVC holder. These specimenswere used to measure free corrosion potentials during the ex-periments.

0010-9312/85/000080/$3.00/0296 © 1985, National Association of Corrosion Engineers CORROSION—NACE

TABLE 1 - Chemical Composition of the Alloys (wt%)

Motel C Si Mn P S Cr Ni Mo N Cu Ti Name

A 0.05 0.54 1.6 0.03 0.01 18.2 12.9 2.5 - 0.04 - Sandvik 5R60B 0.017 0.33 0.48 0.017 0.001 20.0 17.8 6.1 0.12 0.7 - Avesta 254 SMOC 0.012 0.03 1.74 0.016 0.003 26.9 30.9 3.49 0.06 1.04 - Sandvik Sanicro 28D 0.016 0.49 0.73 0.025 0.001 20.1 18.6 6.1 0.22 0.18 - NU 984LNE 0.017 0.43 1.53 0.22 0.003 22.0 5.55 2.96 0.14 - - Sandvik SAF 2205F 0.016 0.25 0.24 0.023 0.002 25.6 4.0 4.1 0.10 0.22 0.37 NU Monit

Counterelectrodes

Vertical oE FloM'section Specimens àirectior

Pitot-fl

Borisontal E PVC holder

section ° ce e 00

Probes for

Agar-bridgea)

4 Flow direction

9N non-

ó^^gó gó F0000 B

4C

320 mu 320 mc.

b) c)

FIGURE 1 - (a) Schematic view of the flow channel withspecimens mounted, (b) PVC holder for metal A, and (c)PVC holder for metal (A)B.F.

The specimens were exposed to a solution of 3% NaCI intap water (metal A) or natural seawater from 70 m depth in theTrondheimsfjord (metal A-F). Data for the experiments areshown in Table 2.

Immediately before the specimens were exposed to theelectrolyte, the surfaces were polished with abrasive paper

(grade 320-800), degreased, and cleaned with acetone and cot-ton wool.

Each specimen was polarized to actual potential with anAMEL 551 potentiostat. One potentiostat was used for severalspecimens at the same constant potential. A saturatedcalomel reference electrode and a platinum counter electrodewere connected to each potentiostat. The counter electrodewas placed in a separate box and connected to the loop via anagar bridge (Figure 1) to prevent chlorine gas produced at thecounter electrode from polluting the electrolyte.

During the experiments, the electric current supply to thepotentiostatically polarized specimens was continuously re-corded as a potential drop over a constant resistance ofknown value. In addition, the corrosion potential on freely cor-roding exposed specimens was recorded at regular intervals.

At the end of each experiment, a cathodic polarizationcurve for each specimen was recorded with stepwise polariza-tion changes, 25 mV per 5 min, from the previously fixed poten-tial down to -900 mV SCE.

ResuitsThe cathodic current density as a function of exposure

time for metal A in 3% NaCI solution polarized potentio-statically to -300 mV SCE and -750 mV SCE and withvelocities of 0.5 m/s, 1.2 m/s, and 2.5 m/s is presented in Figure2. The current density was stabilized within an exposure periodof 12 days. Figure 2(a) contains in addition, a plot of data froman earlier experiment with metal B under the same environ-mental conditions except for velocity, which was zero .8 Thisexperiment was run for approximately 30 days.

The cathodic current density as a function of exposuretime for specimens of different alloys polarized to fixed poten-tials (-300, -400, and -500 mV SCE) in natural seawaterwith a velocity of 0.5 m/s is shown in Figure 3. Figure 4 showsa comparison between the current density development formetal A/meta) F at four velocities from 0 to 2.5 m/s. As il-lustrated in these figures, the cathodic current density in-creased from a low level (the original level was usually approx-imately 1 mA/m2) to a maximum between 300 and 1200 mA/m 2

after an exposure period of one to three weeks, depending on

TABLE 2 - Experimental Variables

Velocity Potential (mV SCE) Exposure Time Temperature PMetal (mts) 1 2 3 Solution (days) pH (C) Slcm

0.5 -300 -500 -750 3% NaCIA 1.2 -300 -500 -750 in tap 12 7.8 20±2 23.0

2.5 -300 -500 -750 water

0 -300 -400 -500 natural 30A-F 0.5 -300 -400 -500 seawater 28 8.0 20±2 23.5

1.2 -300 -400 -500 30

A 1.2 -850 -900 -950 natural 28 8.0 20±2 23.52.5 -300 -400 -500 seawater 36

Vol. 41, No. 5, May 1985 297

1400

rv\ 1200

1000

✓ 800

600

400

200

0 TTTT(''n 4 R n la >n >A 1, i

Exposure time (days)0 4 8 12 16 20 24 28 32 36

Exposure time (days)

b)

t

1400

1200

1000

y 800

600

v 400

200

0

3

1400

1200rvE

qe 1000

N 800

600

t 400

u 200

0

Exposure time (days)

Exposure time (days)

d)

4 8 12 16 20 24 28 32 36Exposure time (days)

1400

1200

1000

N 800

600

y 400

u 200

1400

1200

E

1000

u 800

i 600

400

200

0

E

t4000

2000

0

3

4 8 12 16 20 24 28 32 36

Exposure time (days)

984 LNTrr

SANICRO 28

0 4 8 12 16 20 24 28 32 3E

8

0 1

5

0 2 4 6 8 10 12 20 30

Exposuee time (days)

a)

6000

`V e5000

T4000

3000

2000

1000

E-

0 2 4 6 B 10 12

Exposure time (days)

b)

FIGURE 2 — Cathodic current density on Sandvik 5R60as a function of exposure time in 3% NaCI solutionpolarized to (a) – 300 mV SCE and (b) – 750 mV SCE dur-ing the exposure period. Velocity: (❑) 0.5 mis, (•) 1.2mis, (0) 2.5 mis, and (X) 254 SMO in stagnant solution.

1400

1400

1200

ry 1200E

1000

1000

800

800

t 600

600

400

y 400

200 U 200

0

0 4 8 12 16 20 24 28 32 36

Exposure time (days)

FIGURE 4 — Cathodic current density as a function ofexposure time for (a) Monit and (b) through (d) Sandvik5R60 in naturel seawater at velocities of: (a) 0 mis, (b) 0.5mis, (c) 1.2 mis, and (d) 2.5 ris. Polarized to: ( ❑) –300,(•) –400,(0) –500 mV SCE.

0 4 8 12 16 20 24 28 32 36

4 8 12 16 20 24 28 32 36

Exposure time (deys)

Exposure time (davs 1

FIGURE 3 — Cathodic current density as a function ofexposure time in naturel seawater at a velocity of 0.5mis for metal A-F polarized to: (❑) – 300, (•) – 400, and(0) –500 mV SCE.

the water velocity. After the maximum value was reached, thecurrent density decreased. It had not stabilized at the end ofthe experimental period.

It stainless steel is protected cathodically by sacrificialanodes such as zinc or aluminum, the potential may be sup-pressed to between –800 and –1050 mV SCE, depending onthe IR drop in the electrolyte. To simulate this situation, metalA at a seawater vetocity of 1.2 mis was polarized in one experi-ment to –850, –900, and –950 mV SCE. The current density

expemore time (days) expesure time )days(

a) b)

FIGURE 5 — Cathodic current density as a function ofexposure time for Sandvik 5R60 in naturel seawater witha velocity of 1.2 mis, polarized to: (a) (❑) –300, (•)–400, (0) –500 mV SCE and (b) (L1) –850, (•) –900,(0) –950 mV SCE.

development during the exposure time is shown in Figure 5along with results from another experiment with the same en-vironmental conditions, except for potentials, which in thiscase were – 300, – 400, and – 500 mV SCE.

Cathodic polarization curves at the end of the exposureperiod for metal A in natural seawater at velocities of 0.5 and1.2 m/s and for metal C at a seawater velocity of 0.5 m/s areshown in Figure 6. These curves show a limiting current den-sity of 800 to 1200 mA/m2 at potentials from –300 to – 900 mVSCE. The development of the corrosion potential for metals A-Fduring exposure in natural seawater at a velocity of 0.5 mis isshown in Figure 7.

After 8 to 15 days, a thin, bright surface film was observedon the specimens in natural seawater in all of the experiments.The film from metal A exposed in a velocity of 0.5 m/s was ex-amined at the end of the experiment after the polarizationcurves were recorded. A chemical analysis showed that thedried deposit contained almost 90% organic material. Thethickness of the dried deposit was estimated to be 0.01 to 0.03mm.

HE

t

1400

1200

1000

N 800

600

400

u 200

0 MONIT, r-r

298 CORROSION-NACE

Cucrent density (mA/m a l

-300

-400

> -500

-600

1 -700

-800

-900

Current density (m8/)

b)

Csrrent density (4/n)

FIGURE 6 — Cathodic polarization curves at the end ofthe exposure perlod for (a) Sandvik 5R60, (b) Sanicro 28at a velocity of 0.5 mis, and (c) Sandvik 5R60 at a velocityof 1.2 mis in natural seawater. Rest potential during theexposure period: (❑) -300, (0) -400, (•) -500 mVSCE.

300

200

m 100

0

-100

Só -200a

-300

-400

8 12 16 20 24 28 32 36

Exposure time (days)

FIGURE 7 — Potential measured on freely exposedspecimens in natural seawater at a velocity of 0.5 mis.(0) Sandvik 5R60, (0) 254 SMO, (v) Sanicro 28, (+)984LN, (X) SAF 2205, ( ❑ ) Monit.

DiscussionThe results from the experiments in 3% NaCI solution

show a differente in the current density development betweenthe specimens polarized to - 300 mV SCE and those polarizedto -750 mV SCE (Figure 2). This difference is caused by theelectrochemical reaction on the specimens. The cathodicreaction in the actual potential range is dominated by the ox-ygen reduction process:

02 + 2H 20 + 4e 4(OH) - (1)

In 3% NaCI solutions, this reaction is under activation controlat - 300 mV SCE; at potentials below - 500 to - 600 mV SCE,it is limited by the diffusion of oxygen to the surface. When

this occurs on a bare surface, the observed cathodic currentdensity becomes independent of the potential and is equal tothe limiting current density, iL:

cbiL= ZF

SD: D (2)

where z = number of electrons per mole reactant in Equation(1), F = Faraday constant, cb = concentration of oxygen inthe bulk of the electrolyte, 8D = thickness of the diffusionboundary layer, and D = diffusion constant of oxygen in theelectrolyte.

The thickness of the diffusion boundary layer is related tothe relative velocity, U, between the solution and the surfaceaccording to:9

Laminar flow: 8DL « U -0 .5 (3)

Turbulent flow: 5DT a U -0 .9 (4)

In the presence of a surface layer of some substance,Equation (2) for the limiting current density must be replacedby10

cb

^ L ZF 5D:D + SS:DS (5)

where 8S = the thickness of the surface layer and DS = ox-ygen permeability of the surface layer.

In Equation (5), the ratios 3D:D and 8S:DS act as diffusionresistances for oxygen transport through the electrolyte diffu-sion Iayer and the surface layer, respectively.

We assume that in a NaCI solution there is no deposit onthe metal surface. According to Equations (2) through (4) then,the limiting current density is proportional to U°• 5 or U0 •9 ,depending on the flow conditions.

The transition from laminar to turbulent hydrodynamicboundary layer on a plane surface is reported to occur at aReynolds number (Re) of 3.10 5 to 5.10. The Reynolds numberis described by

UxRex = v (6)

where v = kinematic viscosity of the solution, _ 10 -6 m2/sfor NaCI solution at 25 C, and x = the distance from the in letedge of the plan plate, =0.32 m for the specimens in Figure1(b) and (c).

From this, we calculate the transition velocity to be be-tween 1.0 and 1.6 m/s. Thus, for the experimental velocities wehave applied:

v = 0.5 m/s — laminar flowv = 1.2 m/s — transition regimev = 2.5 m/s — turbulent flow

This can explain the unstable current density of the speci-men polarized to - 750 mV SCE at a velocity of 1.2 mis, whichis shown in Figure 2(b). The other results in this figure are alsoin good agreement with the theoretical relationship described.

An activation-controlled electrochemical reaction doesnot depend on the relative velocity to the same extent. In thiscase, the reaction rate is determined primarily by the ex-change current density, which is sensitive to the surface con-ditions. The differences between the curves in Figure 2(a) aretherefore probably caused by more or less arbitrary differ-ences in surface conditions.

The applied exposure period of 12 days in the experimentswith 3% NaCI solution is a relatively short period. However,earlier experiments with 254 SMO (meta) B) in stagnant solu-tion under the same environmental conditions8 show that the

Vol. 41, No. 5, May 1985 299

current density requirement is approximately constant for theexposure period of 8 to 30 days.

As shown in Figure 3, the different alloys in natural sea-water show the same pattern of current density developmentduring the exposure time. This was also reflected in thepolarization curves, which with only a few exceptions showeda limiting current density with small current variations in thepotential range from actual f ixed potential down to —900 mVSCE at the end of each experiment. From this, we may con-clude that the cathodic properties of stainless steel are inde-pendent of the composition of the alloy.

According to the theory, there is a logarithmic relation-ship between current density and potential for an activation-controlled reaction. For a cathodic reaction, this means in-creased current density when lowering the potential. The cur-rent density for a diffusion-controlled reaction, however, isequal to the limiting current density and negligibly influencedby the potential level. The current density-time measurementsreferred to in Figures 3 through 5 show no systematic connec-tion between current density requirement and potential levelat the end of each exposure period. These curves areregistered on different surfaces. Even if the electrochemicalreaction on these surfaces is the oxygen reduction process,which in the actual potential range is assumed to be under dif-fusion control at the end of each exposure period, the surfacelayer probably differs from one surface to another. Thiscauses a variation in the effective resistance for oxygentransport to the metal. According to Equation (5), this resultsin different values for the limiting current density.

The polarization curves in Figure 6 are registered as adirect continuation of the current density-time curves. Thismeans that these curves are also determined by the differentsurface conditions.

The current density requirement in 3% NaCI solutionbecame approximately constant for each potential after aweek of exposure, with much higher values occurring at —750mV SCE than at — 300 mV SCE. The cathodic current density-time curves in natural seawater, shown in Figure 3, behaved ina different way; after approximately constant values for thefirst eight days, the current density increased the next fourdays, in the worst cases more than 1000 times. At highervelocities, the increase of the current density was delayedseveral days. This is shown in Figure 4 for four velocities in therange between 0 and 2.5 m/s. After reaching a maximum value,the current density decreased.

Behavior of stainless steel in natural seawater relative tothat described above has been reported in the literature. 1 ' 11 ' 12

Mollica, et al., 11-12 have measured free corrosion potentials ofdifferent stainless steel tubes containing seawater with inter-nal flow rates of 0.3 to 5.2 m/s. Measured potentials increasedafter 10 to 15 days in all of their experiments (compare Figure 7from the present work). The development is explained in differ-ent ways, but the present authors agree with Mollica that thedevelopment is caused by a change of the cathodic propertiesof stainless steel as a result of microbiological activity on thesurface. 11-14

Film Formation on Specimens in Seawater 14

When a specimen is submerged in seawater, a thin filmimmediately forms on the surface. This film consists of asingle layer of organic macromolecules. The film, whose thick-ness varies from 200 to 800 Á, changes the properties of thesurface, especially electrostatic charge and wetting proper-ties. This film is the site of the next step in the development,the formation by marine bacteria of the so-called primary film.The primary film builds up on the surface after several days'exposure in seawater. The marine bacteria must be adsorbedon the surface before the film starts growing. A series of fac-tors such as the properties and structure of the surface, thesupply of food from seawater, pH on the surface, and tempera-ture are assumed to influence the adsorption. How the dif-

ferent factors affect primary film formation has not yet beenexamined systematically.

Experiments indicate that bacteria cells, after weak bind-ing to the surface, begin to produce an extracellular slime.This slime helps the bacteria stick to the surface. According tothe literature, the slime consists of sour polysaccharines. Avery important property of the slime is its ability to bind heavymetal ions from Co, Ni, Zn, Ca, Mg, and Cu. 13

Development of Current Density DuringCathodic Polarization of Stainless Steel

The oxygen reduction is the dominating cathodic reactionin the actual potential range. The exchange current density, i o ,for this reaction is very sensitive to impurities in the elec-trolyte. Some organic 4-coordinated metal complexes of Fe,Co, and Ni have shown unusual catalytic properties in the ox-ygen reduction. 15 A combination of this with the propertiesand the development of the slime can explain the currentdensity-exposure time curves shown in Figures 3 and 4. Figure8 shows a schematic view of the relationship between thedevelopment of current density and the movement of the ac-tivation controlled part of the overvoltage curve to the right,and the limiting current density, iL, to the left, as a result ofmicrobiological slime activity on the surface.

With Figure 8 as a basis, we may postulate:

t < t1 Initial organic film formation followed byprimary film formation; bacteria in theprimary film start production of slime.

tl < t < t2 Production of slime on the surface resultsin accumulation of heavy metal ions inthe slime. This increases the io of thecathodic reaction. Simultaneously, iLdecreases because of an increase in theeffective diffusion layer thickness.

t = t3 The cathodic reaction on surfaces in thepotential range below —300 mV SCE (butabove the potential where the hydrogenreduction reaction must be taken into ac-count) has come under diffusion control.This is the result of the movement of theTafel region of the overvoltage curve com-bined with the decrease of iL.

t > t3 Diffusion control (confirmed by thepolarization curves in Figure 6). The cur-rent density is equal to iL. This is reducedby growth of the slime film.

For an activation-controlled electrochemical reaction,one would expect an increasing current density with decreas-ing fixed potential at a certain time, as shown for times t < t 3

in Figure 8. The left portions of the experimental i-t curves aremostly in agreement with this (Figure 4). This would be moreclear if a different scale had been used on the ordinate axis.

For times t >— t 3, however, the current is diffusion-limited(experimentally confirmed by the polarization curves recordedat the end of the exposure, with examples shown in Figure 6)and potential dependence should not exist. In this case, thecurrent will depend on the thickness and diffusion propertiesof the film, and the variation from potential to potential of theright part of the experimental curves in Figures 3 and 4 mustbe explained by more or less arbitrary variations of the organicsurface films.

It is seen from the overvoltage curves in Figure 8 that thecorrosion potential Eo, ... E3 [potential at the intersection be-tween the anodic curve (i = ip) and the cathodic curve] in-creases with time. Thus, the mechanisms described and cur-rent changes also explain the potential increase with time,which was measured on the specimens exposed in seawater

300 CORROSION—NACE

.,emeat aVethex„creaseasurface

E3 £i 1m thicknessE2

E, t t3 l i

ó Eoa0 t2

t2

I i ^ i

^ Ii^,

E-300

E-900 I 'E-soo

ip 'L3 WC-re t density

t0t1 t 2 t3

Exposu re time

FIGURE 8 - The relationship between the variation incurrent density on cathodically protected stainless steelin seawater and the movement of the activation•controlled portion of the overvoltage curve to the right,and the limiting current density iL to the left.

(Figure 7). The potential increase facilitates initiation ofdeposit corrosion, and the liability to deposit and crevice cor-rosion of stainless steel in seawater may be explained partlyby the change of cathodic properties.

As mentioned above, the cathodic properties seem to beindependent of alloy composition. The best indication of thisis Figure 3. Therefore, the differences in corrosion potentialsin Figure 7 are probably caused by differences in the anodicreactions, particularly the passive current density, ip. Decreas-ing ip raises the corrosion potential (see Figure 8).

Diffusion Properties and Composition of FilmMild steel, submerged in seawater and cathodically pro-

tected, develops calcareous deposits (mainly CaCO 3 and

Mg(OH)2) on the surface when pH exceeds certain values. Ex-periments in seawater at a velocity of 0.7 mis and current den-sity of 1000 mA/m2 have shown that surfaces without rust werecovered by calcareous deposits within a few hours. In this

work, the calcareous deposit reduced the limiting current den-sity of the oxygen reduction to about one order of magnitudeless than the final iL on stainless steel.

Table 3 contains a calculation of theoretical maximumprimary film thickness. This calculation is based on results formetal A in 3% NaCI solution and natural seawater. The follow-ing observations and assumptions have been used:

1. The results in 3% NaCI solutions at -750 mV SCEshowed a systematic increase in current density with in-creased water velocity (Figure 2). The cathodic reaction isunder diffusion control at this potential. The results in Figure2(b) can therefore be used to calculate iL at different watervelocities for a surface without any deposit.

2. The polarization curves for metal A in natural seawater[Figures 6(a) and (c)] indicate that the cathodic reaction wasunder diffusion control in the potential range between -300and -900 mV SCE after an exposure period of 28 to 30 days.The curves show that the current density at - 800 mV SCE wasan average of 900 mA/m2 at a water velocity of 0.5 mis and 1150mA/m2 at 1.2 mis. These current density values are consideredthe limiting current densities at the end of the exposureperiod.

3. The diffusion coefficient, DS, for oxygen transportthrough the film is assumed to be equal to the diffusion coeffi-cient, D, in seawater.

In a dried condition, the thickness of the deposit on metalA was 0.01 to 0.03 mm. The primary film contains 80 to 90%water.14 If we assume that there is a direct relationship be-tween percent water content and film thickness, we are able toestimate the film thickness before drying to be approximately0.1 mm, or more exactly, 0.1 to 0.2 mm on the average.

The calculation referred to in Table 3 showed a thicknessof 0.12 to 0.16 mm, depending on the water velocity. The diffu-sion resistance through the primary film must be greater thanor equal to the diffusion resistance in seawater. An additionalcalcareous deposit on the surface would have further increasedthe effective diffusion resistance for oxygen transport. On thebasis of the calculation, therefore, we may conclude that nocalcareous deposit has built up on the stainless steel sur-

TABLE 3 - Calculation of Maximum Primary Film Thickness( 1

U (m/s)

1.2 2200 mA/m2 iL 1. Limiting current density in 3% NaCI solution from theresuits at - 750 mV SCE in Figure 2(b).

0.5 2700 mA/m2

1.2 1.1 x 10 -4 m cb II. Diffusion Iayer thickness for oxygen transport to a sur-8D = ZFD - face without film. Calculation based on results from 1.

0.5 8.9 x 10 5 m IL1

1.2 900 mA/m2iL2 III. Limiting current density in natural seawater at -800 mV

0.5 1150 mA/m2 SCE for metal A in Figure 6.

1.2 6.3 x 104 sim gSZFcb 5D IV. Diffusion resistance for oxygen transport through the

0.5 4.8 x 10° s/m DS - iL2 D primary film. Calculation based on the resuits Erom 1-111.

1.2 1.6 x 10 -4 mSg V. Maximum primary film thickness with diffusion coeffi-

0.5 1.2 x 10 -4 mcient in water equal to diffusion coefficient in the film.

I1 ISandvik 5R60 exposed to natural seawater at 20 C for 28 days.

NOTE: Z = 4, Cb = 0.25 mol/m 3, D = 2.5 • 10 - s m2/s, F = 96500 As.

Vol. 41, No. 5, May 1985 301

faces. This was confirmed by a chemical analysis of the drieddeposit, which showed that the concentration of Mg and Ca inproportion to Na was approximately the same as in seawater.

The Influence of the VelocityThe relative motion between the seawater and the meta)

surface causes a shear stress, which may affect the film for-mation on the surface. This may explain why the tops of thecurves in Figure 4 move to the right when the seawater velocityincreases. This means a longer exposure time is requiredbefore the slime production resuits in an effective depolariza-tion of the cathodic reaction.

The maximum current value for the experiment in stag-nant solution is lower than the results in flowing solutions(Figure 4). This can be explained by a combination of in-creased diffusion boundary thickness and increased organicfilm thickness in stagnant solution. According to Equation (5),the result will be a decrease in limiting current density.

The decrease in maximum value when the seawatervetocity is changed from 1.2 to 2.5 m/s is not as easy to ex-plain. It seems that the movement of the activation-controlledpart of the polarization curves, which dominate the left portionof the curves in Figure 4, is slower at the highest vetocity,while the decrease of the diffusion-limiting current density isless affected by the velocity. The reason for this is not clear,but the lower tendency to affect the activation-controlled partof the polarization area leads to a smaller increase in free cor-rosion potential, which may explain why the tendency towarddeposit corrosion is lower at a high vetocity.

There is probably a critica) shear stress above which theslime production on the surface is prevented. Mollica, et al., 12

have found such a critical value for internal pipe f low.In a previous section, it was pointed out that calcareous

deposit as a result of cathodic protection of stainless steel inseawater was insignficant compared with such deposits whenmild steel is cathodically protected in the same environmentalconditions. One possible reason is that the organic film mayprevent calcareous deposits from sticking to the surface.

Cathodic Protection ofHigh Alloy Stainless Steel

Lately, oil companies have shown interest in using highalloy stainless steel in offshore applications. Even thoughtests and practical experience indicate excellent corrosionresistance, there has also been registered crevice corrosiondamage on 254 SMO, Sanicro 28, and Monit in seawater."

Our test results indicate that the current density require-ment for cathodic protection at potentials lower than - 300mV SCE is essentially higher than the current density require-ment in connection with cathodic protection of mild steel inseawater. This is primarily due to the absence of calcareousdeposits in our experiments. The current density had notstabilized at the end of the exposure period. It the tests wererun for a sufficient period of time, a much lower steady-statecurrent density might have been found. To determine thesteady-state current density, we would probably have to runthe experiments for 6 to 12 months or more.

One possible way of reducing the current density require-ment is to raise the potential level. Experience18 fromlaboratory tests have shown that the potential on the highalloy stainless steel, 254 SMO and Sanicro 28, can be as high as+ 100 mV SCE without any danger of crevice corrosion initia-tion in seawater. The current density requirement on surfacesat this potential will probably be much lower.

Conclusions1. Microbiological activity on stainless steel in natural

seawater moves the activation-controlled part of the over-voltage curve for the oxygen reaction to the right after one tothree weeks' exposure. This increases the free corrosionpotential, which increases the tendency toward initiation ofdeposit corrosion. This finding explains at least partly why

stainless steel is more susceptible to crevice corrosion innatural seawater than in 3% NaCI solution.

2. The cathodic properties of stainless steels in naturalseawater are independent of the alloy composition.

3. Examination of the cathodic properties of stainlesssteel to be used in natural seawater cannot be done in 3%NaCI solution.

4. Calcareous deposits on stainless steel as a result ofcathodic protection in natural seawater were not observedeven with current densities greater than 1000 mA/m2 at flowrates of 0.5 and 1.2 m/s.

5. In the absence of calcareous deposits, the limiting cur-rent density of the oxygen reduction is determined by thethickness of a slime layer (approximately 0.1 mm) and the dif-fusion boundary layer outside the slime.

AcknowledgmentThese investigations were conducted as part of the proj-

ect "Quantification of Corrosion Properties" and financed byThe Royal Norwegian Council for Scientific and IndustrialResearch (NTNF), Kvaerner Brug, Norwegian Water Resourcesand Electricity Board (NVE), Lyse Kraftverk, and TheNorwegian Institute of Technology (NTH).

We would like to thank steering committee members andcolleagues at SINTEF/NTH for encouragement and fruitfuldiscussions.

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