CORROSION MONITORING AND CONTROL IN REFINERY … · 2020. 4. 13. · Paper No. 512 CORROSION MONITORING AND CONTROL IN REFINERY PROCESS UNITS Alec Groysman Oil Refineries Ltd. P.O

Paper No.

512

CORROSION MONITORING AND CONTROLIN REFINERY PROCESS UNITS

Alec GroysmanOil Refineries Ltd.

P.O. Box 431000 Haifa, Israel

Avihu HiramHIRAM - Process Control Eng. Ltd

22 Martin Buber St34861 Haifa, Israel

eMail:hiram@netvision. net.il

ABSTRACT

This paper describes the experience gained in “corrosion monitoring” in the overheadsystems of three crude dktillation units of a refinery in Haifa, Israel.

The data of electrical resistance (ER) probes, connected to an on-line data acquisitionsystem were compared with the mass loss method and chemical analysis of accumulated sourwater tier condensation in the overhead system,

SEM & EDS anrdysis of films and deposits formed on the coupons showed that ironsulfide with impurities of chlorides are responsible for the comosion extent. The corrosion wasless than 5 MPY when a uniform tenacious iron sulfide tilms of 10-50 microns thickness wereformed. The severe corrosion occurred when deposits and non-uniform films of more than 80-100 microns thickness were formed.

Special attention was given to ER probes which were connected on-line to the processunits and enabled the operators to react immediately to any change in corrosion rates.

KEYWORDS: corrosion monitoring, crude distillation unit, mass loss method(coupons), electrical resistance @R) probes, on-line corrosion monitoringsystem, Distributed Control System (DCS), accumulated condensed sour water.

Copyright01997 by NACE 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-S340. 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.

Avihu Hiram - Invoice INV-601527-XGK0G3, downloaded on 11/9/2012 1:35:54 PM - Single-user license only, copying and networking prohibited.

INTRODUCTION

Crude oil contains different species which while beii processed reacted or convertedinto substances corrosive to alloys.

Water is condensed during the production processes and gases like hydrogen chloride(I-ICI) and hydrogen sulfide (H,S) are dissolved in this water, various salts (chlorides,sulfides, hydrosulfides) and oxide-hydroxide deposits formed on the metal surhces result insevere corrosion in the overhead system of the crude distiition unit. Air coolers, heatexchangers, condemers and pipes made of carbon steel suffkr km general corrosiorqpitting corrosio~ and under deposit corrosion.

Prevention of corrosion damage and possible detrimental eflkcts on the environmentdepend on knowledge of the corrosion situation at the units, that is “corrosion behavior” ofmetallic equipment, especially their corrosion rates and corrosion forms.

Various types of corrosion monitoring methods were recommended fix follow-up inthe overhead system of dkdlation units [1,2]. We did not tind literature comparing thevarious types of monitoring results and how ef%ctive it is in evaluating real time corrosion.

SEM & EDS method is widely used for the identitlcation of corrosion products [3],but we did not find in the literature, how this method is particularly used fir the chemicalidedfkation of corrosion products and morphology of coupons’ surfhces f.iom theoverhead system.

Given this apparent lack of well published basis for using various corrosionmonitoring techniques, we attempted to use our practical experience to define ways how toutilize these tools to monitor Real Time Corrosion in the Equipment.

Corrosion follow-up in the overhead systems of three crude distillation units consistedof on-line monitoring with ER-probe measurements, long-term mass loss couponmewurements, visual examination of coupons, identification of deposits fbrmed on CQuponsand investigation of coupons’ surt%cesby means of SEM & EDS, and chemical analysis ofaccumulated condensed sour water.

The collected intlmnation is analyzd by experts and conclusions are drawn as of therequired changes in the anti-corrosion treatment program The datz their treatment, theanalysis and the conclusions are presented in a periodic report by the corrosion engineer tothe management, to the operation people, to the maintenance and to the technical services.Some of the information is also presented in an on line tiormation system and are madeavailable to all concerning stti.

These also serve the goal of improving anti-corrosion meSSUre$ prediction ofequipment service Ii@ decision of needed maintenance and shutdown period.

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EXPERIMENTAL PROCEDURE

A. MASS LOSS (ML) METHOD

Retractable corrosion probes with coupons were used, The mass loss of couponsmade of a carbon steel (the same material the corrosion of which we want to monitor) stripspecimen and their corrosion rates were measured every 40-60 days. The exposure perioddepended on the corrosion, In some cases, where the corrosion rate was too low to bemeasur@ we had to extend this period up to 190 days.

After the retraction of the coupons, hydrochloric acid (5%) with organic corrosioninhibitor was used for cleaning coupons’ surthces from corrosion products and W.Analytical weighing was used for definition of the mass loss of the specimen.

B. ELECTRICAL RESISTANCE (ER) METHODRetractable ER-probes were used fix measuring changes of electricrd resistance of

sensor made of carbon steel wire or cylinder and thus corrosion rate. The ER-probes werelocated m close proximity to the corrosion coupons, close to the wall of heat exchangers orpipes as possible. The measuring of changes in the electrical resistance of the sensor wasdone by portable device or continually monitored by an on-line monitoring (acquisition)system.

The corrosion rates were calculated from the slope of line “Dial reading of ER-probeversus Measuring time”.

The on-line monitoring included also data collecting systeq calculations, performingand utilization of received data.

C. CHEMICAL ANALYSIS OF ACCUMULATED CONDENSED SOUR WATER

The chemical analysis of corrodents (pH, chlorides, sulfldes, sulfhtes) and corrosionproduct (iron) in accumulated water was carried-out five times a week by means ofconventional laboratory methods. The data of pH measurements under laboratoryconditions were compared with on-line pH-meters which are mounted in the processstreams as part of the operating monitoring and control system. The data obtained in thelaboratory were also made available to the techuical team.

D. VISUAL EXAMINATION OF COUPONS

Visual inspection of coupons’ surthces was done every 40-60 or 190 days, tierremoving from the places of their mounting in the system

The presence of tilm and deposits fbrmed on the coupons’ surfhces, their unifiorrnity,pits or other corrosion phenomena or damages were descrii.

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‘The film and deposit thickness on the specimen was measured by means of specialdevice for measmhg non-conductive or non-magnetic ilhns on the iron. Some expertise andexperkme needed in order to make out the important tits which should be considered inthe anti-corrosion efforts.

E. THE IDENTIFICATION OF CORROSION PRODUCTS ON THE COUPONS

The chemical composition of corrosion products, films and other deposits formed oncoupons’ surhces as well as morphology of these surfhces after removing of deposits wasM by IIBXUISof SEM & EDS. The device JSM-5400 of JEOL was used fix thesepurposes.

RESULTS AND THEIR DISCUSSION

The arrangement of corrosion probes in the overhead systems of a typical crudedistillation units is presented m Figure 1. The general scheme of corrosion monitoring logic,the relationship betsveen its various methods and its posslMlities are descrii in F~e 2.

I. CHEMICAL ANALYSIS OF ACCUMULATED SOUR WATER

‘h chemical analysis of accumulated sour water formed after condensation ofoverhead gases from the atmospheric dihlhtion colurnm+has been used for many years tomake out the details about corrosion “behavior” in the overhead systems. The chemicalanalysis of the corrodents (p~ chlorides, sulfides, suliktes) and the corrosion product(dissolved iron mainly) give relatively fhst and reliable “picture” of both sides of thecorrosion system. These data fimilitate improving neutralization and inlrdition treatment inthe overhead system and improving the desalting process of the crude.

The chemical data of accumulated sour water do not reflect exactly the real corrosionsituation in the overhead system. The tit reason is that the most vulnerable place from thecorrosion point of view is situated in the air coolers and condensers - heat exchangerswhere the shock condensation is taking place (see Figure 1). It is clear that there aredi.tlierences between pH, corrodent content in the overhead (m the place of shockcondensation) and below in the accumulated water drum (the water sampling place for thechemical analysis). The second reason is that the iron content in sour water does not alwaysreflect real corrosion situation. For example, iron sulfide formed on the surhce of carbonsteel may dissolve as a result of pH changes or concentrations of other species and not as aresult of a corrosion process.

The chemical data enable the indication of corrosion situation and general Mormationin the overhead system. It is acceptable to run the system with pH values of 5.5-6.5 subjectto cldondes and iron content lower than 20 ppm and 0.5 ppm respectively. Informationabout corrodents and corrosion product content enable improving anti-corrosion treatment.

51214


High changes of pH values as well as content of chlorides and iron in sour waterindicate an unstable conditions in processing and as a result bad corrosion “behavior”. Inspite of this, there is no iniiormation about the real corrosion rates and the actual corrosionforms. There is no way to forecast, km the chemical analysis of the sour water only, thetube service life and shut-down periods.

The chemical method gives qualitative rather than quantitative information of thecorrosion situAon. Therefore the metal loss coupons, visual examinatio~ identification ofcorrosion products and iilrns and ER - probes were utilized to complete the “picture” of thecorrosion in the overhead system of the crude distillation units.

II. MASS LOSS (ML) METHOD

This method is descriid in literature [4,5]. There is no universal method forcorrosion follow-up. Every method has advantages and d~vantages, limits and benefits.Mass loss method enables memwing the average corrosion rate over a period. Visual-on enables detining the presence of deposits and pits. The salt deposits on thecoupon aurt%cesshows the necessity of increase washing m the overhead system.

Chemical identiktion of films and deposits firmed on the surf%cespecimen enablesascertaining corrosion mechanisms, that is the causes of corrosion process and thusCbinkh@ or preventing its spreading.

The results of mass loss method corrosion follow-up that have been collected for thelast two years are presented in F- 3.

The strip coupons show the corrosion situation only in the places of its mounting. Thecorrosion situation may dif.lkrin the other places of the system. Therefore the more couponswe insert in the overhead system the better “picture” we are going to get of the realcorrosion situation.

The exposure period of coupons dependa on the expected corrosion rate values,There is no sense in taking measurements and hence having no mea.ni@d value when thecorrosion rates are too low to determine. The exposure period for coupons was first set to30 days and then extended up to 60 days and 190 days when corrosion rates were low(thousandths and hundredths mdy), m order to receive reliable and correct data. Theobtained data show that corrosion rate of carbon steel before the air cooler is higher thantier its heat exchangers (see Fw. 3).

This tit indicates that condensation occurs kefore air cooler.

One should note that corrosion rate in some cases is twice the permissible value(accepted, dangerous or limiting). This permissible corrosion rate was determined as 0.11nnrdy (5 MPY) for the carbon steel tube bundles of air coolers, heat exchangers andcondensers. It is very important to achieve a correct interpretation of mass loss coupon

51215


data. The thickness of tube bundles m many cases is 2.336 mm. The planned Me of aircoolers, condensers and heat exchangers m refineries is 15 years [6]. Thus the accepted orpermissible corrosion rate for this equipment is:

0.7 * 2.336 Lnrn/15y = 0.11 MM/y(5 my),

where 0.7 - wear and tear coefficient, that is the maximum percent (70??) thatthickness of tubes are allowed to be diminished up to its replacement. It means thatcorrosion rate up to 0.11 nudy is acceptable.

This value cotildes with the one NACE recommendation as an accepted lowcorrosion rate for the equipment in the oil industry [4].

SEM & EDS analysis of corrosion products and films formed on coupons

In addition to corrosion rate values, coupons enable defining corrosion form andmechanism based on surfhce state (morphology) and chemical identification of films andcorrosion products formed.

General corrosion with uniform tilms, and pitting corrosion under non-unifbrm filmsand deposits have been found.

The ardysis of corrosion products made by means of SEM & EDS, showed thepresence of iro~ suli%r, oxyg~ s.msll quantity (traces) of chloride and sometimes evenbromide (Figures 4,5).

The obtained data point out that iron suliides (probably FeS and some other formsFeJ+.) covering the carbon steel surfiwe are responsible for the formation of protective iihn,

The presence of nitrogen in the films is problematic because of its likely smallquantities and close energy with oxygen atom The presence of nitrogen suggests a possibleinhibitive film formation by mans of filming amine. The chloride akaline salts are generallywater soluble. In spite of this, traces of chlorides were found in the deposits and films (seeFigure 4). The presenm of bromides in the deposits in some cases (see F~e 5) maybeexplained by the usage of W tankers with sea water during the transportation of cmdeoil.

TIE pits formed under films and deposits on the coupons’ surfhces can be explainedby the presence of non-unifbrm iron sulfide ilm or ammonium and amine chloride deposits.If this film were perfkct, tiorm and tenacious not more than 50 microns of thickness, thecorrosion was less than 5 MPY (0.1 1 mndy).

When deposits and non-uniftmn films of 100 microns and more were formed, thecoupons, arranged in the air cooler and condensers were severely corroded. Generally pits

51216


were Rmned under non-unifbrm films and deposits. The iron sulfide iilm is cathodic to iron.If this film were perfect, the galvanic pair does not work, therefore corrosion is negligible. Ifthis tl.hnwere non-unifiorm and impmfkct, the corrosion agents ingress in impmtkctions andsevere pittii corrosion occurred under ilms.

The presence of annnonia or amines and chlorides m the overhead system results inthe formation of ammonium and amine chloride deposits which may hydrolyze withformation of hydrochloric acid under deposits.

The pH values decrease to 2-3 and results in corrosion under deposits and m its turnpitting corrosion. The corrosion rate under deposits can be as nmch as 100 times as thegeneral corrosion rate. In order to prevent the formation of these deposits, ammonia usageis mininimd and generous water wash is applied. The deposits consisted of chloride salts onthe coupon surfhces is an indicator of wash quality.

In cases where the deposits and sediments were consisted of iron sulfides water washcould not be effbctive, because the sulfide salts are not water soluble.

The mass loss method has some limitations.

Corrosion coupons examination are unable to ditlkrentiate between long term steadycorrosion rates and corrosion which has occurred rapidly over a short period of time, due toupsets or other events.

This method gives the overall average corrosion rate during some, usually longperiod. If corrosion rate was low (thousandths mm/y), the exposure time should beprolonged up to as long as halfa year and it is impossible to retrieve any data during all thisVerylong period.

The other deficiency is that a lot of time has to be spent for preparation of couponspecimen surfhces befbre and after exposure, and fir calculations. Not every one can usethis method and highly skilled and well traiued people are needed to get meaningiid data.

The use of ER-probes overcomes most of these drawbacks.

IIL ELECTRICAL RESISTANCE (ER) METHOD

A. Manual registration method.

Retrievable ER-probe was mounted parallel to mass loss coupons in the overheadsystem of the crude distillation unit. A portable device was used for registration of thechanges of wire sensor electrical resistance [7]. Corrosion rates were calculated accordingto slope of an average line “dial reading of ER-probe - exposure time” (Figure 6).

51217


Curve fitting by regression analysis was carried out for the data received from thisdevice.

ER-method gives results considerably quicker, aud less effimts are needed mcomparison to the mass loss method.

Properly selected ER-Probes enable data to be read close enough to give instantinformation about events m the “ER-probe litk”. The various results may be received for theM%rent periods (Figure 7). Phenomena like sudden reversing the corrosion process mightbe indicated R values go to the opposite direction - decreasing @ii 8). Findingreasonable explanation for this phenomena is always problematic. One of the explanationsfor such fluctuations is that protecting tlrns on the sensor surilice are formed and destroyedperiodically. Another possibility is the periodic formation of films with various electricalconductivity. If the memring period taken is long (the same period as fbr mass lossmethod) then identical results are received.

As a rule the corrosion rates are lower when measuring periods are longer (see Fig.8). The similar results may be received by means of the mass loss method. When theexposure period of coupons is increased the average corrosion rate is diminkhed. Generallythis relationship occurs when dependence of specimen mass loss versus time is descrii bya curve with “saturation” (Fii 9). This phenomenon often occurs when carbon steel isunder neutral or near neutral solutions. Comparison of the results received by means of twomethods, mass loss and E~ during long-term periods shows their fidl codhnity (Figurelo).

B. On-line corrosion monitoring.

A number of ER-probes with 4-20 ma output as a fimction of R changes wereinstalled m the overhead of two crude distillation units (see F- 1). The installation ofthese devices gave us the ability to measure and monitor the corrosion rates m and aroundthe places they were mounted at, to get accurate - real time fked back data about theprocess and from the anti-corrosion measures taken - all these to reduce the time needed toreact to changes in the plant (processing) and to minimim the corrosion damages.

ER-method is one of the corrosion monitoring ways which better enables operators toknow the exact and real situation of the equipment with regards to the corrosion and decidequicker how to improve the treatments such as desalting, maintaining the right processconditions and the right parameters of the anti-corrosion program injection of soda andother types of neutdimrs, inhibiting agents etc.

The philosophy of on-line ER corrosion monitoring incMes the data collectingsyst~ their treatment @ormiag the calculation), utilization the data and theiiinterpretation.

51218


The analog signals (4-20 ma) are sent from the probe are proportional to the un-corroded matter left and are calibrated to give an indication of total corrosion of the steelsensors in microns.

The outputs of these probes are monitored via the existing DCS(Honeywell/TDC3000) and are also sent to the Plant Mormation System (PI) where theyare analyzed. Figure 11 shows these data as collected and stored within the PI System. ThePI System enables easy access to the history data base with every simple PC which isconnected to the local net or by dialing-in tiom remote PCs. Bringing this intbrmation ontothe PC makes possible the manipulating with all kind of analysis tools not only from withinthe PI System but from outside it on other systems which might be available on any PC inthe network (e.g. commercial available spreadsheets and various Stattilcal software).

Performing the cakulations. Based on the field imtrume nt readings corrosion ratesare calculated by means of WRmmtiation of received data regarding given period. In orderto make mean@@d calculations it is essential to fist tilter the noise from the data. Anumber of approaches were practiced.

The simplest way is to take all the readings that were collected within the MormationSystem and find the regression line which best fits the data (Figure 12).

Another technique which is more suitable fbr on-line calculations is Mtering byaveraging over periods of time and making the corrosion calculations between separatedaverages. There are two parameters to adjust in the application of this algorithm the lengthof the period to be averaged to give representing values and the time to separate betweenthese tsvo values (Figure 13).

The calculated corrosion rates are depended on measured period and these data arepresented for various periods (Figure 14). The title of each trend rndicates how many hourswere averaged and what is the d-e in time between these averages. The calculation wasdone continuously and the results were stored in the system history.

Long periods (longer than 30 days) give integral corrosion rate values coinciding withthe data of mass loss method but the latter does not allow to follow changes in corrosion asit happens during this period. on-line ER data allow adyzing during various small periods(Figure 14). These data reflect process changes as it reflects the corrosion and enables theexperts to find the reasons for these upsets and to try to improve or elimiuate the causes andminimiz the corrosion damages.

4. Actual Equipment Inspection Data

After four years of service actual equipment inspection of heat exchanger tubes wastaken at planned turnaround.

51219


The tube bundles in the overhead atmospheric unit are under reduction conditions

~zs atmoS@e@ during nod operation. men the equipment is opened for inspectionthey are exposed to normal atmosphere (with oxygen), and oxidation processes immediatelytake place. The black deposits of iron sultide inside the air cooler becomes reddish ironhydroxides within several hours after opening of the cooler. Therefore it is important tohave the inspection of the tube bundle done as soon as it is opened to the atmosphere.Visual exarnination had been done immediately afler opening the heat exchangers. The tubeswere found to be covered with black deposits of various thickness. The chemical analysisshowed the deposits consisted of iron sulfides and iron oxides. These deposits wereremoved by water jet, and tube surt%cewas then visually inspected.

The general condition of the tubes’ sties was good, but shallow pits of various sizewere discovered under the thick deposits. The pits on the tubes were very mild and of nodangerous for the on going service of the heat exchangers. These pits required no specialattention and no speci6c action to be taken

The actual equipment inspection findiis matched the available on line monitoringdata.

The ability to react i%stto the signals given by the on line monitoring system enabledthe anti-corrosion program to be adjusted in real time and helped to maintain the equipmentunder good service conditions.

CONCLUSIONS

1. The experience of corrosion monitoring in the overhead system of the crude distillationunits in the reiinery has been described.

2. Mass loss coupons and ER-probes showed good coincidence.

3. On-1ine, real time ER monitoring with data acquisition system data analysis andcalculations allowed following the corrosion and to react immediately and accurately tominimim the damages.

4. The combination of these methods enabled anti-corrosion program included crudedesalting, neutralization and inhibitio~ to be optimize~ to be more precise and moreefficient. Wfi these data it is possible to better predict tube service life and to plan themaintenance shut-down p$XiOdS.

5. SEM & EDS analysis showed that iron sultide films and deposits of variable contentwere formed on the coupons installed in air coolers aud condensers in the overhead ofthe atmospheric cohmms.

6. The corrosion was less than 0.11 mudyr ( 5 MPY) when a uniform tenacious ironsulfide films of 10-50 microns thickness were fbrmed. The severe corrosion occurred

512110


when deposits and non-tiorm Wns of more than 80-100 microns thickness wereformed.

7. The actual equipment inspection iindings showed general good condition of tubesbundles and thus matched the available on line monitoring data,

8. The availab@ of on-line corrosion monitoring system enabled better look after anti-corrosion treatment and made the overall program much more successfid.

REFERENCES

1. Russel C. Strong, Raymond A. Stephenso~ Ara J. Bagdaaar@ James E. Feather,Robert B. Hti “Crude Unit Corrosion and Corrosion Control”, CORROSION/96,paper no. 615, @otiOQ TXNACE Internatio~ 1996).

2. Milton P. Ramo% Luiz A. Corre% “On-Line Corrosion Control in RefineryOverhead System”, CORROSION/95, paper no. 335, (Houstonj TXNACEInternatiod 1995).

3. Thomas Mebrahtuj K.J. Del Rossij “SEM and XPS Characterization of the CarbonSteel SurfSee Paasivation Film in Anhydrous Hydrogen Fluoride Media”,CORROSION/95, paper no. 341, (Housto~ lXNACE Internatio@ 1995).

4. RP 0775-87 NACE. “Preparation and Installation of Corrosion Coupons andInterpretation of Test Data in Oiltleld Operations”.

5. ASTM G4-84. “Standard Method for Conducting Corrosion Coupon Tests in PlantEquipment”.

6. R.A. White and E.F. Ehrnke, “Materials Selection for Refineries and AssociatedFacilities” (NACE, 1991), p.131.

7. RP 0189-89 NACE. “On-Line Monitoring of Cooling Waters”.

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512112


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512113


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512/15


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512116


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512/20


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— Linear [dai1507) I

10.00~ 1 4 I I 1

17-Nov 22-Nov 27-Nov 02-Dec

Figure 42. Dial reading and linear

07-Dec 12-Dee 17-Dee

regression.


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Dial reading

I * I

2 HtmrS / 2 Ifays

i /’%”i%’ , ,-- . --- - - - ---

‘---: --- :5 Htmrs ./ 5 Hays. !3.3DIICI. ----: ---- ----;---- 1+--- ----: ---- ----; ---- .___; ----

1 I I I I $I I >—-- . - - - - -- - - --, I

6 HmrS / & Ilays. 0.3000-. ----:---___~-_---_. .__-; --- --------- __-. -;--.” ---; .-_.

t , ,n , 1

11--- ---:---- ----? --- -;. =-- -- - . --i $

7 Hcmrs } 7 Ilays0.3000- ____;---- -__-;----- .__-;---- __-j--_- -_-_;___ .---;----

-----i?

0.- ---:”-1 --’-+.-------- -- - .-*..- - - ---- L.-__, ---, : i ,

Ilimsm•1 32.357MIGRIIN

llli11506 R2❑ -2. E-03MM/YmR

l17i11506 . R5❑ 0.07225MM/YElm

DA11506 R6❑ 0,0’7244MMmm

llFJ1506 R7❑ 0.06903MM?fwm

17-Nov-94 00:00:00 12hridiv 24-Ncw-!14 00:00:00

Figure 14. Corrctsinn rate vs TimE base


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