22
Paper No. 6 CORROSIOIUC)L The NACE International Annual Conference and Exposition THE FORMATION OF PROTECTIVE FeCO~ CORROSION PRODUCT LAYERS IN CO, CORROSION E.W.J. van Hunnik, B.F.M. Pots and E. L.J.A. Hendriksen Koninklijke/Shell-Laboratorium. Amsterdam (Shell Research B. V.) PO. Box 38000 1030 BN Amsterdam The Netherlands ABSTRACT Current models often over-predict COZ corrosion rates for wet gas and oil transport systems. One of the main reasons for this is the fact that the formation of corrosion product scales is not properly taken into account. An evaluation of in-house and literature experiments has been carried out to develop an equation for iron carbonate precipitation kinetics. Modelling the kinetics of this precipitation process offers a number of applications in the area of corrosion prediction and control. The ultimate goal of the work is to establish the conditions under which stable, protective corrosion product layers form. A good indicator for the formation was found to be the so-called scaling tendency, which is defined as the ratio between the precipitation and corrosion flux. To form reliable scales, the bulk scaling tendency should be high enough for protective film formation and the local scaling tendency, in case of film damage, should be high enough for film repair. Both processes have been modelled and it was found that above approximately 80 “C reliable scales are often easily formed whereas below this temperature, pH values of at least 6 are required. Copyright C1996@ ltAcE International. Requests for permission to publish this manuscript in any form, in part or in whole must be made in writing to NACE International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A. ohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

006 the Formation of Protective Feco3 Corrosion Product Layers in Co2 Corrosion (51300-96006-Sg)[1]

Embed Size (px)

DESCRIPTION

006 the Formation of Protective Feco3 Corrosion Product Layers in

Citation preview

Paper No.

6CORROSIOIUC)LThe NACE International Annual Conference and Exposition

THE FORMATION OF PROTECTIVE FeCO~ CORROSION PRODUCT LAYERS

IN CO, CORROSION

E.W.J. van Hunnik, B.F.M. Pots and E.L.J.A. Hendriksen

Koninklijke/Shell-Laboratorium. Amsterdam(Shell Research B.V.)

PO. Box 380001030 BN Amsterdam

The Netherlands

ABSTRACT

Current models often over-predict COZ corrosion rates for wet gas and oil transport systems. One of themain reasons for this is the fact that the formation of corrosion product scales is not properly taken intoaccount. An evaluation of in-house and literature experiments has been carried out to develop anequation for iron carbonate precipitation kinetics.

Modelling the kinetics of this precipitation process offers a number of applications in the area ofcorrosion prediction and control. The ultimate goal of the work is to establish the conditions underwhich stable, protective corrosion product layers form. A good indicator for the formation was found tobe the so-called scaling tendency, which is defined as the ratio between the precipitation and corrosionflux. To form reliable scales, the bulk scaling tendency should be high enough for protective filmformation and the local scaling tendency, in case of film damage, should be high enough for film repair.Both processes have been modelled and it was found that above approximately 80 “C reliable scales areoften easily formed whereas below this temperature, pH values of at least 6 are required.

CopyrightC1996@ ltAcE International. Requests for permission to publish this manuscript in any form, in part or in whole must be made in writing to NACEInternational, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in thispaper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

The precipitation model also results in improved prediction of the solution pH which allows bettercorrosion rate descriptions even if the scales are not protective. In some cases the changes in pH maylead to reductions of 40% in the predicted corrosion rates. Other applications of iron carbonateprecipitation modelling are also discussed.

Kevwords:. carbon dioxide corrosion, corrosion prediction, iron carbonate precipitation, scale repair,supersaturation, scaling tendency.

INTRODUCTION

Reliable prediction and control of corrosion is key to cost-effective and safe design of facilities for thegas and oil industry. As an example, current tools for the prediction of COZ corrosion in pipelines 1‘2arestill based on “worst case” assumptions, which may lead to unnecessary extra capital or operationalexpenditures to combat potential corrosion. These extra costs can stem from the use of expensivecorrosion resistant steels, too much corrosion allowance (extra steel wall thickness), or application of acorrosion inhibitor. One area where there might be scope for design improvements concerns thedescription of the formation of corrosion product layers (scaling), which may limit the progress ofinternal pipeline corrosion. Currently the potential protective properties of scales cannot be taken intc~account in predictive models as adequate understanding of their stability and reliability, e.g. under fluidflow conditions, is lacking.

This paper deals with the precipitation of iron carbonate, FeC03. In facilities for oil and gas production,‘handling and transport, iron carbonate is often the main corrosion product. It forms at the wall of apipeline if the product of ferrous ion concentration, [Fe2+], and carbonate ion concentration, [CQZ-l,exceeds the volubility product. The precipitation kinetics is known to be relatively slow and allowsmuch higher ferrous ion concentrations than would be dictated by thermodynamic equilibrium. ‘Thiscondition is known as supersaturation. Literature on the precipitation kinetics of FeC03 is ratherlimited< and additional corrosion experiments were carried out. This information enabled thedevelopment of an improved description of the kinetics. A number of potential application examples,for which knowledge of the FeC03 precipitation process is relevant, will be dealt with.

EXPERIMENTAL

Test facilities

Experiments were carried out employing an autoclave, a low-flow loop with a fluid velocity of 0.1 m/s,and a high-flow loop with a velocity of 4 m/s.

Temperatures were between 40 ‘C and 106 “C, C02 pressures between 0.55 and 5.6 bar. Allexperiments were performed oxygen free in a sodium chloride solution ( 1 wt90). The pH wascontinuously monitored in all experiments. In the 40°C experiments the iron content was also frequentlymeasured by photo-spectroscopy.

Twelve experiments were performed in the two flow loops at 40 “C. In each flow loop, tests wereperformed for six different microstructure to examine the influence of flow and microstructure on

6/2

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

carbonate precipitation: four normalised steels and two quenched and tempered. Chemicalcompositions and heat treatments are given in Table 1.

Precipitation rate

The precipitation rate of FeCOs was determined from the deviation from the linear increase of the ironcounts with time during the corrosion experiments. This deviation starts after the saturation pH forFeC03 precipitation is exceeded. The limxu part of the curve is used to calculate the corrosion ratewhich was checked with weight loss measurements. The rate refers to the non-scaling part of the testsand is assumed to be constant over this period and during the early part of the subsequent precipitationperiod. Obviously, only the first part of the deviation from linearity can be used. Once the scale starts tobecome protective, the corrosion rate drops and it is no longer possible to determine the precipitationrate from iron counts.

In theory the precipitation and corrosion rate can be derived from the increase of pH during the non-sualing period. In practice pH measurements are often too insensitive for this purpose, however.Experimental errors in the pH have been encountered and are most likely caused by scaling on the pHelectrodes.

FeC03 PRECIPITATION DATA BANK

Table 2 lists the above-mentioned in-house experiments and relevant literature precipitation data cases.A distinction is made between data based on corrosion experiments with C02 and data based onchemistry experiments with salts (absence of metallic iron). All corrosion experiments are basedconstant inventory tests with no net in- or out-flux of iron. Although additional experimentsavailable from literature, their results were not employed because of incomplete data (no pH and nocounts or no corrosion rate), inaccurate data or inconsistencies in the data published.

The three experiments by Johnson and Tomson3 are referred to as scanning experiments, in which

onareron

thetemperature was varied between approximately 35 “C and 80 ‘C. Since this rndces comparison withother data difficult, it was decided to, artificially, split the experiments into “separate” experiments at40, 50, 60, 70 and 800C and to derive precipitation rates for these temperatures.

In total 3’7experimental cases were examined: 15 originating from the Johnson and Tomson data. 19 in-house experiments, one experiment by Dunlop etal.q and two experiments by Dugstads.

NIODELLING IRON CARBONATE PRECIPITATION

All concenh-ations in this section refer to bulk concentrations. Although the precipitation in practice willbe determined by local surface concentrations, description of the precipitation kinetics using bulkconcentrations is preferred since these can be measured easily. In the section ‘model for scale repair’local concentrations are discussed as repair of damaged scales is determined by the local concentrationsrather than the bulk. concentrations,,

6i3

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

Precipitation rate equation

Johnson and ‘Tomson3 (J&T) developed a model for the precipitation kinetics of FeCO~ in which theprecipitation rate (in kmol/m3s) follows from:

~{ F+ (w, )“’-~:’}’[F;’+]Pr,c= k, ~ (a~a (1)

with k, the temperature-dependent rate constant, A/V the surface/volume ratio, K.,P the volubilityproduct of FeC03 and

,,

[16[Fe’+]

Fe’+ =&

(2)

The activity coefficients of iron and carbonate ions are assumed to equal unity, allowing the use ofconcentrations rather than activities

[Fe”’+],,,, = k, ${([Fe2+][COj”] )””5- K$’}2 = k, $K,P(S”’ -1)’

where

(3)

(4)

is the supersaturation level.

As will be demonstrated in this paper, the J&T equation does not provide a good description over awide mnge of supersaturation levels; it was fitted only to data at low levels of supersaturation, i.e. S<2and becomes inaccurate at large values for S. In corrosion processes the supersaturation varies fromvalues just above 1 for high temperatures to values as high as 103 for low temperatures. This requiresthe functional relationship with the supersaturation level to be properly described over a wide range.For instance, fitting the data to the simple linear relation between precipitation rate and supersaturationlevel:

[F;’+] = k,$K,P (S-l)prec

(5)

can easily lead to discrepancies up to a fmtor of 10 over the mentioned range.

Various other alternative equations were examined. An improved precipitation rate equation wasobtained by making the rate proportional to the supersaturation levels of both ferrous ions andcarbonate ions:

F;’+]pr,ccc ([Fe’+] -[Fe2+]Ja,)([C@] - ~Oj-l..,) (6)

leading to:

6/4

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

[Fi’+ ],,,, =+kp(s-l)(l- s-’) (7)

This equation will be further referred to as KSLA equation.

Note that implicitly the assumption has been made that the precipitation rate of FeCO~ for corrosionsystems is controlled by kinetics and not by nucleation. The fact that the experiments, irrespective ofsteel microstructure, could in fact be described by modelling only precipitate growth indicates thatnucleation is less important in modelling iron carbonate precipitation.

Precipitation rate constant

The rate constants for all experiments were derived from the measured precipitation rates via theprecipitation equation and appear to follow Arrhenius’ law with temperature (Figures 1 and 2).Deviations from the Arrhenius line are somewhat larger for the J&T equation. Excluding the Johnsonand Tomson precipitation data and thus limiting the analysis to corrosion experiments only, the J&Tequation systematically over-predicts the precipitation rate. Precipitation data based on iron countsprove to scatter less around the Arrhenius line than data based on pH.

The results were fitted according to the Arrhenius equation:

(8)

with:

Johnson and Tomson KSLA equationequation

average 26* average 2&

A 56.3 4 52.4 3

B 127.3 12 119.8 9

* 20 = 95% confidence level

The constant from the J&T equation is within the quoted accuracy range for this constant given byJohnson and Tomsonq.

Supersaturation dependence

On the basis of just the corrosion experiments in the databank it is impossible to distinguish whichequation should be used since the supersaturation levels in most corrosion experiments are relativelyhigh (> 10), and both precipitation equations converge to:

[Fe”’+],,,, cc S (9)

6/5

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

To distinguish between them, one needs to compare, for example, the large (12j set of in-house 400Cexperiments at high supersaturation levels, of the order of 103, with the Johnson and Tomson 40°C dataat low supersaturation levels (less than 2).

The Figures 3 and 4 plot the ratio of the measured and calculated precipitation rates versus thesupersaturation level. The J&T equation overestimates the precipitation rate at higher supersaturationlevels whereas the KSLA equation gives a better overall description, irrespective of supersaturationlevel.

Reduction of corrosion due to FeCO~ precipitation

The formation of FeC03 layers will eventually lead to at least a global reduction of the corrosion rate.The exact corrosion reduction is difficu~t to predict in view of the many factors involved, like the typeof steel, the flow velocity (both shear stress and mass transport effects), temperature, COZ pressure,formation water composition, etc. It is clear that a full description of the influence of precipitation oncorrosion rate is far too complicated. However, prediction of the corrosion rate reduction may bepossible under specific conditions.

A further observation is that corrosion can only be reduced if the precipitation rate is of the order of thecorrosion rate, If iron precipitation would be much more slowly than iron dissolution, the steel surfwewould be corroded away before a protective, dense layer could form.

The concept of scaling tendency is introduced here and is defined as the ratio between precipitation rateand corrosion rate (expressed in the same units). When the scaling tendency exceeds a critical value, aFeCO~ film starts to grow and corrosion will reduce. This critical value is referred to as the criticalscaling tendency. In Figure 5 the values for the critical scaling tendencies at which the effectiveprotection by precipitation starts are given for the various microstructure and carbon contents studied.The small differences between the low and high flow experiments are probably not significant, alsoconsidering the fact that these experiments were performed in different flow loops with differentsurface/volume ratios, probably leading to slightiy different critical scaling tendencies. The results showthat the. critical scaling tendency decreases when the carbon content in the steel increases. It is knownbthat carbon steels having a carbon content higher than approximate y ().15% can form acementite/carbide network which remains on the surface after corrosion of the ferrite phase. Thisnetwork can lead to a higher local supersaturation level, resulting in protective fti formation at lowerbulk supersaturation. Apart from the higher local supersaturation level, the type of corrosion reductionwill also be influenced by the fact whether or not there remains a network on the steel surface. Thiseffect will be discussed in the next sections.

For all experiments the critical scaling tendency ratio exceeds the value of 0.4. In the experiments withArmco iron and quenched and tempered steels at higher flow velocities no corrosion reduction wasobserved during the experiment (the maximum scaling tendency attained in these experiments wasapproximately ().7).

Low carbon steels

The reduction of the corrosion process by scaling can be visualized in different ways. Localised FeCO~sites develop first. Only at these sites corrosion is reduced. Assuming further coverage continues afterearlier sites have reached a minimum scale thickness, the time for full coverage follows from the ironmass balance. During the second stage, the thickness of the layer increases but growth will continuously

616

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

slow down due to the corrosion reduction by the scale layer. As a minimum the model requires twoparameters, which could be the layer thickness for full surface coverage and a layer thickness at whichthe corrosion rate is reduced to a certain fraction of the original rate.

Network-forming steels

For ferritic-pearlitic steels with more than O.15% C, the corrosion reduction mechanism is different.Once the scaling tendency exceeds a critical value, the precipitation of iron carbonate in the pearlitenetwork starts. The most simple description would be to assume that the corrosion rate is a linearfunction of the porosity. The porosity follows from the ferrite dissolution and iron carbonateprecipitation in the network.

The corrosion rate in this study is assumed constant to limit the number of adjustable parameters. Forthe experiments discussed in this report a constant corrosion rate is an adequate approximation sincemost experiments were carried out at conditions which resulted in a reasonably pH-independentcorrosion rate: high pH (scaling conditions) and low flow rates.

PRACTICAL APPLICATION

Ferrous ion mass balance

For actual systems, the precipitation equation needs to be incorporated into the ferrous ion massbalance:

{ }

[F’’’+lnrw=[Fe2+10ki+ [F;2+lin-[F;2+lou+[F;2+lcr-[F;2+lpre “ (10)

The mass balance can be used for both closed systems (constant inventory and most laboratoryexperiments) and open systems (once-through and actual field systems). Examples of open and closedsystems are discussed below.

Open systems

In an open system (for example a pipeline) the change of the iron concentration with time is lessrelevant since the time required to develop a steady state situation is normally negligible compared tothe lifetime of a pipeline. In Figure 6 an example of the calculated pH and iron concentration along apipeline is given. It is clear that the iron concentration and therefore the pH increase with distance asthe iron concentration builds up. Due to the increased iron concentration and pH (e.g. carbonateconcentration) the precipitation rate increases up to a point where the precipitation rate and corrosionrate balance (10 km), beyond this point a steady state situation is reached and the iron concentrationand pH can only decrease when the corrosion rate drops due to protective film formation.

In an open system (pipeline, tubing etc.) there is also an influence of flow, v, on the mass balance. Thefollowing formulas are used for the in-flux of dissolved iron:

617

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

[F;’+].=[Fe2+lupsre.,.~and out-flux:

(11)

(12?

closed systems

In a closecl system there is no net in- or out-flux of iron, leaving only the corrosion ancl precipitation

terms in the iron mass balance. Figures 7 to 10 show a few examples of experimental data fitted withthe precipitation equations discussed in this report. The figures show that accurate simulation ofexperiments IS in some cases possible. It is clear from the mass balance that after long exposure a kind

‘Feold) should be reached in which the amount of iron released byof steady state situation (Fe..W–corrosion is equal to the amount of iron precipitated. [n the case of the formation of a protective FeCOllayer, the corrosion rate will tend to decrease to zero and the iron content will reach the saturationlevel. However, when no protective layer is formed the. corrosion and precipitation rate will balance at ahigher iron level (see Figure 8).

Prediction of protective scale formation

The ultimate aim of our studies on precipitation kinetics is to enable the prediction of the formation ofprotective corrosion product layers and the resulting reduction of the corrosion rate. By combiningmodels for corrosion rate and precipitation kinetics the appearance of FeCO! layers can be predicted.To judge under what conditions full credit of protective iron carbonate layers can be taken, thefollowing aspects should be clarified:

~formation of protective corrosion product layers;“ stability of these layers;“adherence to the steel surface of these layers;orepair of damaged scales.

The formation of protective scales can be describedscales is relatively easy to predict; requirement for

with the equations in this paper. The stability of thea protective stable scale is that the bulk conditions

are at least saturated. No credit should be taken for scales which grow under local surfacesupersaturation conditions if the bulk remains undersaturated since this would mean that once fullyprotective fdms are formed and the local iron and carbonate gradients disappear (no corrosion), thescales would dissolve. The adherence of the scales is a function of various parameters like steelmicrostructure and flow rate. Some information has already been gathered but further work would beneeded to fully cover this issue.

The last requirement is that a scale once damaged should repair reasonably quickly. This repairbehaviour is no longer controlled by the bulk supersaturation since the bulk phase in a scaled systemshould be close to saturation. This means that repair should be possible at a lowbut probably a high local supersaturation at the damaged spot (inside a pit or

6J8

bulk supersaturationmesa attack type of

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

location). Modelling the local concentrations at the scratched surface to map the appeamnce of scalerepair as a function of process parameters is discussed in the next section.

MODEL FOR SCALE REPAIR

The reliability of protection by a scale layer depends on its ability to self-repair after damage. For aproper description of the repair of a scale, it is required to consider wall concentrations rather than bulk

concentrations, however, since the precipitation process is driven from the wall, where salt volubilitywill be exceeded first.

The Fez+ concentration at the wall is controlled by the corrosion rate, precipitation kinetics and flow-controlled mass transfer via the mass transport equation for the ferrous ions (Figure 11) which for asteady state situation and no bulk source terms simplifies to:

(13)

where j stands for the ferrous ion flux from or to the walk the subscript D, cor, and prec stand for thecontributions to the flux from mass transfer, corrosion, and precipitation, respectively.

The mass transfer is influenced by the presence of other charged species in the solution. A model forthis multi-component mass transport was presented last year7. The same model also provides theconcentrations of the other species, including C03Z-, and in addition the (non-scaling) corrosion rate.For C02 corrosion, the rate is controlled by one or a combination of the following mechanisms: masstransfer of protons, mass transport of carbonic acid, hydration of carbon dioxide, and charge transferkinetics. Hydration is controlling at low velocities or higher pH’s, mass ~ansfer at intermediatevelocities or low pH ‘s. At high flow velocities, electrochemical charge transfer is limiting. Linking ofthe multi-component mass mansfer model with the precipitation model for FeCO? allows solvingsimultaneously the precipitation and corrosion equations

It is postulated that repair is possible if the scaling tendency locally exceeds a critical value. For a ratioof one, each dissolved iron atom would immediately precipitate as iron carbonate and repair is expectedto occur. Even at lower values of the scaling tendency, a protective scale may form in view of thevolume of an iron carbonate molecule being larger than the volume of an iron atom. For illustrationpurposes it is assumed that a scaling tendency of 0.5 would suffice for the scale to become protective.

Figure 12 shows the temperatures at which, according to the model outlined above, the scalingtendency equals ().5. The area above this temperature line corresponds to repair conditions, whereasbelow the line non-repair conditions would prevail. The most critical parameters are the flow velocity,which controls the mass transfer, and the pH. Obviously, it makes a difference whether an increase ofthe pH above the saturation pH is due to the presence of ferrous ions or another salt, like sodiumbicarbonate. Figure 12 applies to pH’s controlled by increased Fez+ levels (i.e. corrosion), while Figure

13 would apply to sodium bicarbonate addition. The saturation pH is typically pH=5.3.

In a pipeline the most critical situation occurs when the complete wall is protected by a scale layerexcept for a few spots where the scale is damaged. The bulk pH is then close to or slightly above theFeCOS saturation pH, say pH=5.5. It is clear from the Figures 12 and 13 that for temperatures belowsay 80 ‘C and for normal liquid flow velocities in the range from say 0.3 to 3 m/s, repair of damagedspots is unlikely. Repair by addition of sodium bicarbonate is only effective at these temperatures at

6/9

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

relatively high concentrations (pH values of at least 6). Addition of ferrous ions would be moreeffective but it is doubtful whether the dissolved Fez+ would reach the damaged spots furtherdownstream, as premature precipitation could occur.

The repair model has a number of shortcomings. First, it only applies to a smooth surface. Secondly,adherence needs to be considered, which, among other things, depends on the formation kinetics andsteel microstructure. Steel microstructure plays more than one role. Ferntic/pearlitic steels, for instance,show a relatively high flow-independent corrosion rate, once part of the ferritic top layer has dissolvedand active cathodic iron carbides have developed. In combination with the stagnant conditions withinthe ktrnellar structures this gives a high local scaling tendency. Furthermore, these cementite structuresprovide good anchor sites for crystal growth. Another shortcoming of the above model 1s that the effectof the transport processes through the (porous) scale layer is not considered.

OTHER APPLICATIONS OF PRECIPITATION MODELLING

Improved pH prediction

The pH is an important parameter for corrosion prediction. Normally, the higher the pH, the lower thecorrosion rate. The pH of a wet gas or oil/water transport system is often assumed to equal the

saturation pH for FeCOq precipitation. Since the kinetics of precipitation is very slow below say 60 “C,the pH can reach much higher levels in practice (supersaturation) as discussed in this paper. At thebeginning of a pipeline the iron build-up due to corrosion is relatively low and the pH will normally belower than the saturation pH. By incorporating the FeCOs precipitation kinetics, the pH along thepipeline can be calculated more accurately leading to a better corrosion prediction (see Figure 14). Thescale in these calculations is assumed non-protective.

Guidelines for inspection

IVlore work will be required to establish the correct relationship between precipitation rate andcorrosion reduction, but obviously, the ingher the precipitation rate, the more likely the reduction ofcorrosion by corrosion product layer formation. A parameter which would be of direct use foroperations already is the scaling tendency. By calculating the scaling tendency along a pipeline thecritical parts (from a scaling point of view) of the system. having a low scaling tendency, can bepredicted. These parts require special attention during inspection since the chance of insufficientprotection by corrosion product scales is greater than for parts having a higher scaling tendency. InFigure 15 the scaling tendency along a gas transport pipeline is shown. The scaling tendency is low atthe beginning of the line and around approximately 20 km. Along the line the temperature decreasesleading to slower precipitation kinetics and the iron content builds up, leading to a higher driving forcefor scaling. These counter acting factors lead to the minimum in scaling tendency around 20 km. Itwould, therefore, be recommended to pay special attention to this part of the line during inspection andlikewise to the very early parts of the line. By using the methods described above, installations whichrely on the formation of protective scales can be monitored properly.

Top of the line corrosion

When FeCO~ scaling is chosen as a corrosion control option, top-of-the-line corrosion is potentially aserious problem. In principle perfect protection of the top of the line by FeCOq is impossible when

6/1O

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

condensation takes place since the fresh condensed water will dissolve the FeCOl layer. This means thata minimum corrosion is required to increase the iron content in the condensed water phase to at leastthe saturation level or:

[1C3 ~Vcod ~Fe2+ =Vsat cor

(14)

where VCon~is the condensation rate (1/s), VCO,the corrosion rate and C3 the correction factor to convertmol Fe2+ per litre into mm/y corrosion. With this equation the minimum corrosion rate under scalingconditions for the top of the line can be calculated. At very high condensation rates no protective filmcan form and the corrosion rate can of course no longer be described by the equation given above, butwill simply equal the non-scaling corrosion.

CONCLUSIONS

To improve on the prediction of COZ corrosion rates, the kinetics of iron carbonate precipitation havebeen examined in detail, using both literature data and in-house experimental results. It was found thatcorrosion and precipitation experiments can be adequately described by using the proposed ironcarbonate precipitation kinetics equation.

A model for evaluation of’ the repair characteristics of damaged scales as a function of operational andenvironmental conditions has been proposed,, Calculations with this model show that damaged scales incondensed water systems will only repair at temperatures of at least approximately 800C. At lowertemperatures scale repair in only possible if the pH is increased to values of at least 6.

Several applications of iron carbonate precipitation modelling like improved pH prediction andguidelines for inspection are also discussed.

REFERENCES

1 Waard, C. de and Lotz, U.: “Prediction of COZ corrosion of carbon steel”, NACECORROSION/93, paper 93.069

2 Waard, C. de, Lotz, U. and Dugstad, A.: “Influence of liquid flow velocity on COZcorrosion: a semi-empirical model”, NACE CORROSION/95, paper 95.128

3 Johnson, N1.L. and Tomson, M. B.: “Ferrous carbonate precipitation kinetics and its impactCOZ corrosion”, NACE CORROSION/91, paper 91.268.

4 Dunlop, A. K., Hassell, H.L. and, Rhodes, P. R.: “Fundamental considerations in sweet gaswell corrosion”, NACE CORROSION/83, paper 83.046.

5 Dugstad, A.: “The importance of FeC03 supersaturation on the COZ corrosion of carbonsteels”, NACE CORROSION/92, paper 92.014.

6 Dugstad, A. and Lunde, L.: “Parametric study of COZ corrosion of carbon steel”, NACECORROSION/94, paper 94.014.

7 Pots, B. F. M.: “Mechanistic models for the prediction of COZ corrosion rates under multi-phase flow conditions”, NACE CORROSION/95, paper 95.137.

6/11

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

Table 1. Chemical analysis and heat treatment of the steels

X65 0.09

C22 0.18normalised

C22 ().18Q&T

C45 ().46normalised

C45 0.46Q&T

Mn

<().04

1.50

0.58

0.58

0“77

().77

Si

N.A.

0.30

0.22

0.27

0.27

P

<0.004

0.014

().010

0.010

0.013

0.013

s

<0.003

<0.002

0.011

0.011

0.023

0.023

Heat treatment

1 h, 670 “C, aircooled

1 h, 920 ‘C, aircooled

1 h, 895 “C, aircooled

1 h, 850 “C, waterquenched,1 h, 600 ‘C, aircooled

1 h, 855 “C, aircooled

1 h, 830 “C, waterquenched,1 h, 600 “C, aircooled

6112

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

Table 2. Experimental data set used for evaluation of precipitation kinetics

Experiment Temp. pa), Type of experiment k, m J&T equat]on k, m KSI.A equa[lmr

(W:) (bar) (kg2/(mol mz s)) (kg’/(mo[ mz s))

1, in-house 93 4 (“orroslon, autnclave (pH measurements) 5.5105 ~g 10$

2, m-house 106 3.2 COrrOs]On, autoclave (pH measurements) 1.1108 3.5 I o’

-3, m-house 88 3.7 Corros]on. autoclave (pkl measurements) 1.510’ 1.0 ).OO

4, )1-hmlse 92 5 Corrosmn. autoclave (pHmeasurements) 4.0104 3.() 10”

5, u]-house 95 5 Corrosion, autoclave (pH measurements) 6.5105 fr.s 105

6, ir-house 45 5.6 Corrosion, autoclave {pH measurements) 9.4102 9.5102

7, m-house 80 0.55 Cormslon, autoclave (pH measurements) 4.8104 4.2104

8-19, m-house 40 1 (’orrosion, flow [oop (mm measurements) 6.3102 6.7 Ioz

(12 experuneuts)

Dugstmll’ 20 1 (-0 rros]on, autrrciav e (iron measurements ) 45 41

[)ugstad25 80 1 Comosmn, autcudav e (mm measurements) 3.810’ ‘3.(I 10’

Dunlop’ 82 96 (fugaeity) Corrosion, autoclave (iron measurements) 5.0 lo~ 4.5105

farl’ 40 No gas phase PrWipitatlmt 2.3103 6.010:

J&T2’ 40 No gas phase I%erp@tiOn 2.6 10] 4.0 I 0’

J&T?I‘ 40 N<)gas phase PrecipKatimr 1,5103 4.3 I(Y

.l&T1 ‘ 50 No gas phase PreapitatiOn 1.1 104 3.1 I(+

.LQT2’ so No gas phase Preapltatlrm 1.0104 3.310’

J&’~3’ 50 NOgas phase Preeipitatmn 6.61 (Y 2.1 10’

J&T13 60 No gas phase Preerpltat]on 4.6104 1.3104

J&T2’ 60 Nogas phase Prwipltation 3,9104 1.I 104

J&T3’ 60 Nogasphase Precipitation 2.610’ 6.7103

J&T13 70 NrI gas phase Precipitation 1.8105 4.7104

J&r2’ 70 NOgas phase Precipitation 1.3105 3.5104

J&T?~ 70 Nogas phase Preapitatlon 9.3104 2.4104

J&T13 80 No gas phase Precipitation 6.4105 1.6105

J& ’I”,Z.’ 80 No gas phase Precipitation 4.2105 1.1 10’

J&T33 80 No gas phase Precipitation 3.1105 7.8104

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

1,0E+09

1,0E+08

- 1.0E+072~ 1,0E+06

: 1.0E+05

~ 1,0E+04

~ 1,0E+03

X 1,0E+02

l, OE+O1

1,OE+OO

A

■ Corrosion, Femeasurements

A Corrosion, pHmeasurements

o Precipitationexperiments

— Best fit

, ! [ !i

0.3 032 0,34 0,36 0,38 0.4 0.42

1/RT (kmol/J)

FIGURE 1

1,0E+08

1,0E+07

~ 1,0E+0601E 1,0E+055~ 1,0E+04&~ 1.0E+03.~ 1,0E+02

1,OE+O1

1,OE+OO

A

0.3 0.32 0,34 0,36 0.38 0,4 0.42

1/RT (kmol/J)

FIGURE 2

■ Corrosion, Femeasurements

A Corrosion, pHmeasurements

o Precipitation

experiments

— Best fit

Fittd t-ate constunt versus the inwrsc’ of tempct-atut-e, using the KSLA equation

6/14

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

la) T

1 AA

0,01

Supersaturation

■ Corrosion, Fe ~ Corrosion, pH o Precipitation experiments

measurements measurements

FIGURE 3Rulio ojrnemured und culculuted precipitation constantly versu.y sl{/]ef-Lsatl~r(ltiotlf)onl .1012/!s0/1ujldTotnsotl equation

o

6■

. Ie

10 1(KI o 1Occl 10000❑

o0

1

Supersaturation

■ Corrosion, Fe e Corrosion, pH o Precipitation experiments

measurements measurements

FIG URE 4Rutio of rwuxurcd titld cul(wlutd prmipitatioi~ comtmts vet-sus supersutwatioil j“om KSLA equution

6/1 5

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

~ Q&T steels, 0,1 m/s

~ F-P steels, 0,1 m/s

F-P steels, 4 m/s1

0,1t

I01 I 1!

( I

o 0,1 0.2 0.3 0,4 o,~

Carbon conteni (wt%)

6/1 6

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

A) 120

lal

EQ 80g

zg!6Qco

hlg

20

0

B)

02468101214161820 222426283032

Distance (km)

120

20

0

—--- ———— —.

47-

r024681012141618 20222426283032

Distance (km)

6

5

— PH I

‘—Fe

— pH

2

1

0

FIGURE 6E.Yuwlpl(Iof culculatcd irotl utld pH curve alo~lg a pipeline in cme of protective scule forrnutiot)(u) UIILi

HO protection by irotl carbotlutc scules (b) (80 ‘C, jloMJ2 rids, I bar CO1)

6fl 7

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

8~

702

603

302

2CCI

lm

o

1 I

o 50 1(XI 150 2m 250 3CII

Time (h)

—.

1(XI--

0 I I

FIG URE 8

0 2m 4(XI 603 800 1OMI

Time (h)

‘Fecalc.

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

%33

8C0

7m

= 603-iig 503

$ 4CII

3CCI

203

1(XI

0+

o

— Fe talc

I~ Fe mess,1

I , I 1,

{

5 10 15 20 25 30

~me (h)

FI ❑ Fe mess,

1

6fl 9

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

} Mass transfer

1x

h CorroQon

r Precipitation

)el DiffusionWa!i layer

boundary

6/20

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

120

1

pH=5.5

pH=6

PH=6.5

120

100

uoG 80L3~CJ& 60Gms.--5 40:

20

0

0.01 0.1 1 10

Flow velocity, mls

FIGURE 13Scaling tentperaturefrom scale repair mode[ling, ,rjj~ ~:,~~i!-(~li’i{l(I(i[lili<]jlof.y(j(liunl I>ic(lt-l>(]tl(lt(’.

(])CO: = 1 bar, }Iydrauiic diameter D = [).1 mj

6121

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.

0.25 ,.\

0,3 6.5k .AAAb A

*AA AA AA AA

AA AA

,..A AAA

,* AA

.’

-----k

+.

-.---

-.+-

.+-

: 0.15 i-.--- ---

0.05 :i

o

0 5000 10000 15000 20000 25000 30000 35000

Distance, m

_ Corros!on rate with pH — — Corrosion rate with pH

from precipitation from saturation iron

model carbotlate

P-

6

t.

5.5

pH

‘5

4.5

4

40000

pH from saturation iron A pH from prectpltat!oncarbonate model

FIG URE 14

‘, -,

Disfance along the pipeline (km)

6/22

Mohamed Nechachbi - Invoice INV-653417-GSCFM3, downloaded on 4/4/2013 12:03:17 AM - Single-user license only, copying and networking prohibited.