ML082340677

Vermont Yankee July 2008 Evidentiary Hearing-Intervenor Exhibit NEC-JH_71, Bignold, G.J. Et Al, Paper 1, Water Chemistry II, Bnes (1980): 5-18

ML082340677
Person / Time
Site:	Vermont Yankee
Issue date:	07/21/2008
From:	Bignold G, Garbett K, Garnsey R, Woolsey I Entergy Nuclear Vermont Yankee
To:	NRC/SECY/RAS
SECY RAS
References
06-849-03-LR, 50-271-LR, Entergy-Intervenor-NEC-JH_71, RAS M-235
Download: ML082340677 (14)

Text

PAS t41")S5 G. J. Bignold, BSc, PhD, K. Garbett, BSc, PhD, R. Garnsey, BSc, PhD, C.Chem, FRSC, and L S. Woolsey, BSc, PhD, CEGB, Leatherhead NEC-JH_71 Erosion-corrosion of carbon steels has been experienced in the steam generator and secondary water circuits of many reactor systems. Damage has occurred under both single and two-phase water flow conditions, and is associated with severe fluid turbulence at the metal surface. In the most severe cases, this can lead to very high metal wastage rates (>lmm/year), and consequently rapid component failure. The available experience, previous research and current understanding ofithe phenomenon are reviewed, and both experimental and theoretical work in progress at CERL is /

0 described. The pH dependence of the phenomenon under single phase conditions at 148 C is rýported.

I and by using hydrodynamically well characterized specimens, the dependence of erosion-corrosion rate on mass-transfer has been investigated. At 148 0 C, the rate has been found to vary as the ýube of the mass transfer coefficient. This is in agreement with the predictions of a model of the process based on the electrochemical dissolution of magnetite. In order to make quantitative measurements on the process, high precision bore metrology and surface activation of the test specimens has been used extensively, and these measurement techniques are also discussed. I INTRODUCTION has occurred in the steam-water circuits of water

1. -Nuclear steam generators have experienced a and sodium cooled reactors. As a result there is wide variety of corrosion related problems, and growing international interest in erosion-the vulnerability of individual designs to any corrosion phenomena (ref. 4-7). The present particular type of corrosion damage can vary paper therefore attempts to summarize current widely. In all cases, however, the economic experience and understanding of the problem, and penalties resulting from such damage are consider- describe erosion-corrosion work in progress at able, and there is therefore a strong incentive CERL.

to eliminate such problems as far as possible.

To this end, a wide variety of research programmes EROSION-CORROSION are in progress throughout the world.

4. The term erosion-corrosion is slightly

.2. In many nuclear systems, corrosion has misleading and the phenomenon is perhaps better resulted from the generation of aggressive described as flow assisted corrosion. As such solutions via solute concentration processes it is clearly distinguishable from pure erosion (ref.l). This is particularly true in the case or cavitation damage.

of PWR steam generators, for example with the denting, phosphate thinning and tube sheet 5. Erosion-corrosion damage normally occurs at crevice stress-corrosion problems (ref.2). locations where there is severe fluid turbulence

3. In the case of U.K. gas cooled reactor steam adjacent to the metal surface, either as a result of inherently high fluid velocities, or generators, considerable effort has been directed the presence of some feature (bend, orifice etc.)

at understanding and eliminating the possibility generating high levels of turbulence locally.

of corrosion damage resulting from solute Its occurrence is also usually associated with the concentration under two phase flow and dryout use of mild or carbon steel components. The conditions, and the vulnerability of both Magnox attack occurs under both single and two-phase and AGR steam generators to on-load corrosion water conditions, but not in dry steam, which is and stress corrosion has been reviewed very consistent with the general view that the process recently (ref.3). The need for stringent feed- is essentially one of surface dissolution. It water chemical control was recognised and to is frequently, although not invariably, date they have not proved to be a problem. characterized by the occurrence of overlapping However, both Magnox and AGR steam generators horse-shoe shaped pits, giving the surface a have been subject to an entirely different type scalloped appearance, as shown in Plate 1.

of corrosion damage not dependent on any solute However, these pits are normally relatively concentration process, namely erosion-corrosion. shallow in comparison to the general metal Similar erosion-corrosion problems have also wastage in the area concerned. The'oxide present been encountered in other gas cooled reactors on the corroding surface is normally very thin, elsewhere, most notably in France and Japan, but 1 um or less, and often exhibits a polished the problems are not restricted to gas cooled appearance. However, heavy oxide deposition is reactor steam generators, and this type of damage sometimes present on ad acent areas of tube not WMNUMMEAMM.GU1A 0OUWSOI.

Water Chemtt S DOCKETED Z BNES, 1980. Paper)

USNRC DOFFEE b: A (OIO Exhibit No. kLO"LII OFFEREJ)by: Apdfiri*, . CD.,Lý*I*:I August 12, 2008 (11:00am)

OFFICE OF SECRETARY [* * .*?*

RULEMAKINGS AND ADJUDICATIONS STAFF O2k~ 'IDS- C)

PLATE 1. Erosion-corrosion damage produced under two phase conditions in a mild steel riser pipe from a Magnox steam generator. Flow from left to right.

4 PLATE 2. Erosion-corrosion damage downstream of the orifice in a CERL mild steel orifice assembly specimen. Flow from left to right.

OXIDE -

A.'~ 1 10 pm PLATE 3. Metallographic cross section of specimen shown in Plate 2 in region of maximum erosion corrosion loss.

I 6

CORROSION-EROSION LOSS FEEDWATER PH s-0-

to00 II CORROSION-EROSION LOSS. His/yEwr FIGURE 3. Eros ion-corros ion loss rate v pH for a rotating disc at 99°C (ref.26)

FIGURE 1. Temperature dependence of erosion-corrosion losses under two phase conditions (ref. 19)

FLOWPATTIAN AEFEPEIICEI ELCIETy KM P010 1 "+- - -ATIE IfLOCITI OF

+LDI INIIAL. FLOW STOCIAEION ....

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1 I I..... I I. ...

$TAGNATIOP

........ACLE OS voiIs H.D l0 STC ON.O Sl 0.1

'OIT FLO?

POINTSDUE FIP-"

"TOLOTIT[ .002- FLOW VELOCITY 0.

FOMOAFIOW -O.T OFmlOW 0,0 MCOLATS 0,2 P.

NO .. ____ - Olb~lOO PI PiPS FLOW VELOCITY 0.04 cPOIN .TSOED IE CONF.SI, POAWAIOTS TOOIW.---FLOW IIOOWIOI,Wt~bI zo UTAL f~lLOOT, .LCOLAYSO VEOCT POIT.i = 0~IVI~n *PSOU P10 CONFLICOTE0 = lIN TU..4 aASd.Oo OELOOTVCALOUOIIFO.

• • .U5r1p0.

T.W.-~ ool FL.OW TROUGOC *-t.. [ .O~lO[o t~urn..] AUFA1C1O P03010 PO~t LAm OY*

ENOSUE oSCYP00IO 00 FIGURE 2. Influence of flow path configuration FIGURE 5. Arrangement of orifice assembly on erosion-corrosion damage under two-phase specimens in autoclave flow channels.

conditions (ref.13)

. 7

spffering erosion-corrosion attack, particularly Power Station, erosion-corrosion damage at the under two phase conditions. feedwater inlets downstream of the flow control orifices was compounded by flow bypassing

6. Under severe conditions, metal wastage rates through the gap between the threaded ends of the of I mm/year or even higher can be observed in restrictor tubes and the orifice carriers erosion-corrosion situations, so that component (ref.12). In some cases this fluid bypassing failure can be relatively rapid in the worst completely eroded away the restrictor tube end.

cases.

11. In addition to problems within the steam Plant Experience generators themselves, erosion-corrosion damage

7. Under two phase conditions, erosion- has frequently been encountered in wet steam corrosion damage within nuclear steam generators turbines (ref.13) and associated steam pipework has frequently occurred at tube bends, (ref.6), both the feedwater and steam-side of bifurcations or similar features in the steam- feed heaters (ref.14-17) and boiler feed pumps water circuit. Among the earliest reported (ref.7). Clearly therefore the problems are instances of damage of this type were those at very widespread, and not restricted to any one the Tokai Mura plant in late 1968 (ref.8,9). type of nuclear plant.

This station employs dual pressure drum re-circulation type steam generators, and early Current Understanding of Erosion-Corrosion failures occurred at 2140C in swan neck bends and Behaviour tube bifurcations at the outlet end of the mild 12. In spite of the widespread occurrence of steel L.P. evaporator tubes. Some failures also erosion-corrosion problems, as outlined in the occurred at tube bends in the subsequent riser preceeding section, relatively little experimental pipes to the L.P. steam drum external to the or theoretical work on the subject has been steam generator itself, and significant tube reported in the open literature. It is clear, thinning was reported for the last two return however, that erosion-corrosion behaviour depends bends of the serpentine evaporator tube banks on a number of physical and chemical variables.

inside the units. Tube wastage rates as high as These are principally; materials' composition, 1.3 mm/year were found in some cases. Up to the local hydrodynamic conditions including the time of the failures, the boilers had been effects of steam quality, temperature and water operated with hydrazine/ammonia dosing to give a chemistry. Any model of the process should boiler water pH in the range 8.5 to 9.2. Some therefore be capable of explaining the detailed dosing with Na3PO4 was also employed to combat dependence of erosion-corrosion on these chloride ion (200-300 ppb) present in the water parameters. Their general influence on (ref.9). erosion-corrosion behaviour under boiler feed-water conditions is summarised below.

8. Similar failures to these have occurred under steaming conditions in the mild steel 13. Materials' Composition. Erosion-corrosion economiser sections of British Magnox stations, damage is most frequently observed when carbon and in the evaporator sections of once-through or mild steel components are employed. Alloy steam generators such as those at St. Laurent I steels, particularly chrome alloy steels are and II. In the case of St. Laurent II, failures much less susceptible to erosion-corrosion attack, occurred in the 1800 return bends of the mild and austeniFtic stainless steels essentially steel serpentine evaporator towards the end of immune to damage. Relatively small amounts of the evaporation zone, at a temperature of about chromium in the steel improve its erosion 2450C (ref. 4, 10). As in the case of Tokai Mura, resistance quite markedly, although the degree the boiler feedwater was originally dosed with of improvement appears to depend on the severity ammonia and hydrazine to about pH 9.0. However, of the conditions. Thus in tests at 120 0 C, more recently morpholine dosing has been employed involving impingement of a water jet on the because of its lower partition coefficient between sample surface at 58 ms- 1 , 2% Cr steel was found water and steam and higher basicity at high to be at least an order of magnitude more temperature, which should maintain a higher resistant to damage than carbon steel, with solution pH at temperature (ref.ll). higher chrome steels even more resistant (ref.18).

However, practical experience with wet steam

9. Erosion-corrosion problems under two phase turbines and their associated pipework suggests conditions have also been reported to have 24% Cr steel to be about four times more occurred in the steam generator units at Marcoule resistant to attack than mild steel, whilst 12%

and Chinon 2 (ref.4, 10). Cr steel has proved to be virtually unaffected (ref.13).

10. Erosion-corrosion damage in nuclear steam generators under single phase (water) conditions 14. It is likely that other minor alloying or has commonly been associated with boiler feed- trace elements such as copper, nickel, water tube inlets, and in particular those where manganese and silicon would influence resistance orifices have been installed to control the to erosion-corrosion as such elements are known boiler feed flow. Damage of this type has been to affect corrosion resistance of carbon and low experienced at St. Laurent II, with an inlet alloy steels to a wide range of aqueous 0

feedwater temperature of about 125 C (ref. 4,10), environments. However, there appears to be no 0 systematic studies reported in the open and at somewhat higher temperature (up to 246 C) in the case of the Phenix steam generators literature.

(ref. 5, 10). In the case of Hinkley Point 'B' 8

15. Temperature. Erosion-corrosion damage is pH was less than 9.0, but attack was not most prevalent in the temperature range 500 to normally observed with pH >9.2. Similarly, the 0

250 C, Fig. I shows the effect of temperature occurrence of erosion-corrosion damage in wet on relative erosion rates based on data derived steam turbines has been reported to occur only from damage occurring under two phase conditions when the condensate pH is below about pH 9.4 in wet steam turbines (ref.19). This indicates (ref.13,19).

0 maximum damage to occur at around 180 C. How-ever, more recently it has been proposed that 21. The effect of pH on erosion-corrosion under single phase conditions, the maximum is rates has been studied experimentally by 0

close to 140 C (ref.20). Limited studies on the Apblett (ref.26) using a rotating carbon steel effects of temperature under single phase disc over the pH range 8.0 to 9.5 at 99 0 C in conditions have also been reported by Decker, deaerated water. The results are shown in Wagner and Marsh (ref.21) which would appear to Fig. 2, and indicate a tenfold reduction in support this, but there remains some uncertainty wastage rate on increasing the pH from pH 8 to 9.

in the precise variation of erosion-corrosion Similar reductions in rate have also been 0

rates with temperature. For example, rapid two reported for jet impingement studies at 120 C phase erosion damage has frequently been observed (ref.27).

0 at temperatures well in excess of 200 C (e.g.

St. Laurent II), whereas the curve in Fig. 1 22. The effect of oxygen on erosion-corrosion would suggest the problem to be disappearing behaviour as such has not been studied in great rapidly at these temperatures. detail. However, iron release rates from carbon steel in neutral water at 1.85 ms- 1 over the 0

16. Hydrodynamics. Erosion-corrosion damage temperature range 380 to 204 C have been shown has in general been observed at points of to decrease by up to two orders of magnitude hydrodynamic disturbance in the fluid flow. with increasing oxygen content over the range Under single phase conditions damage has <1 to 200 ppb (refs.23, 28-31). It is to be frequently occurred at tube entries in preheaters, expected that erosion-corrosion will at least or downstream of orifices at boiler tube entries, qualitatively follow this type of behaviour.

whereas under two phase conditions the damage has often been associated with bends. Keller 23. Additions of up to 300 ppb oxygen (or more (ref.19) has attempted to rationalize the effects commonly hydrogen peroxide) to neutral feedwater of various flow path configurations on erosion- forms the basis of the neutral oxygen low corrosion damage under two phase conditions by conductivity (NOLC) water chemistry regime used use of an empirical damage factor (Kc) together by a number of power utilities for fossil fired with a reference flow velocity. These are given once thro' boilers (ref.32), and these are .

in Fig. 2. However, it is doubtful that these evidently largely free from erosion-corrosion parameters can be equally well applied to damage damage. More recently, it has been reported under single phase conditions as a result of the that combined NH 3 /H 2 02 dosing of feedwater is differing hydrodynamic flow patterns which would also effective in this respect (ref.33).

occur. More recently at CERL and elsewhere (ref.7) attempts have been made to-relate erosion- Models of Erosion-Corrosion Behaviour corrosion rates in single phase water to local mass transfer rates, and these will be discussed 24. Keller (ref.19) has proposed an empirical subsequently. equation for predicting erosion-corrosion losses from carbon steel, based on observations in wet

17. In view of the critical dependence of steam turbines. This has the form erosion-corrosion damage on fluid flow and turbulence, it is surprising that no detailed s = f(T).f(x).c.K - K studies have been reported of the effect of flow c s (1) velocity and turbulence on erosion-corrosion rates. where s is the maximum local depth of material Some studies have been made at high temperature 0

loss in mm/10 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.

(>28 0C) (ref.22-24), but these are outside the range normally associated with erosion-corrosion f(T) is a dimensionless variable denoting the attack. influence of temperature on erosion-corrosion damage. A plot of f(T) is shown 18.' Water Chemistry. Several aspects of water in Fig. 1.

chemistry are thought to influence erosion-corro-sion behaviour. The effect of pH and oxygen con- f(x) is a dimensionless variable denoting the tent of the water have been examined, but other influence of steam wetness on erosion-components such as hydrazine and dissolved iron corrosion loss. For sub-cooled water it are also expected to exert a significant has been suggested that this has a value influence on the process (ref.25). of unity, but for two phase mixtures it has the form f(x) - (1 - x)K x, where x is the

19. Most instances of erosion-corrosion steam fraction and 0 < Kx < 1. A value of damage have occurred with a deoxygenated volatile Kx = 0.5 is evidently considered the most alkali dosed water chemistry. appropriate one. °

20. In studies of erosion-corrosion damage in Kc is a variable factor accounting for the feed heaters (ref.14,15), it was found that effect of local geometry on the fluid flow.

attack occurred predominantly when the feedwater Values of Kc in mm.s/m 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> are 9

( (

iI

\.

0 Air release valve Deilorized tertank p..ump 9 L..n L.a ion ea.hange exchange Main circulation pump Conduclivity cell T

Cartridge tiller -

i---o0 F-- I Tank P antP9 an PC lank PCPCPressurizer OT I I I Venturi5 jPlatea 2 Plate j Tube Tube a' o e1 i I samplesa S I iP li sa'mole sample 2 10 Kg/h I IT .Or

-OT P e Pu T Venturl 1I Venturi 2' Healer 1 Up to 1031 Kg/h each plate sample T

aN2 Caln Maivs l I Mains su/pl Int amsc Venluri4 Vents ri3 consrpply - seso oln I ae InI 20B Kg/h either sample, not both 1.al.r2P TC T

P Pressure gauge Molar cooling n o'celodctvtycl P T Thermometer TC Thermometer/control F Flow indicator/conlrol PC Pressure control ConducyivityI L Level indicator/control FIGURE 4. Isothermal Rig flow diagram

S00 400 300 100 700 80 a 60 60 40 r,,

40 u 20 is 20[

10 0 S tO Is in 25 30 TEDPERATURE. °C DEPTH. rn FIGURE 6. Activity/depth curve for 56Co produced FIGURE 8. Variation of magnetite solubility in an iron matrix by a 10.8 MeV proton beam with temperature and pH at 1 bar hydrogen inclined at 100 to the surface partial pressure. (Ref. 42) 4.0 r-E I-a I..

LI-2.0 --

0 a

Zr D 1.0 I-a 0 4 5 6 TUBE DIAMETERS BEYONDORIFICE FIGURE 7. Variation of mass transfer coefficient in a tube downstream of an orifice ISO 160 1)0 110 00 O t DISTANCEFROMSPECIMENOUTLET.

FIGURE 9. Erosion-corrosion loss profile in tube downstream of an orifice (Diametral circumferential locations shown. Orifice located 185 mm from specimen outlet) 11

given in Fig. 2. behaviour under two-phase conditions.

-I ms c is the fluid velocity in Experimental Facility

28. Experimental studies of erosion-corrosion K is a constant which the first term must are being carried out using a high velocity s exceed before erosion-corrosion is observed. isothermal water circulation loop, referred to 4 as the isothermal rig for short (ref.37). This A value of 1 mm/l0 hours has been given by Keller (ref.19). facility consists basically of a main circulation loop, a secondary water clean up loop and a Equation (1) does not include any influence from pressurizer loop, together with ancillary changes in water chemistry, although as indicated make-up/dosing and chemical sampling systems. A earlier, these have a very marked effect on the flow diagram for the rig is shown in Fig. 4.

rate and occurrence of erosion-corrosion damage.

It is also very doubtful that it can be applied in its present form to single phase erosion- 29. Four specimen flow channels are incorporated corrosion damage, since many instances of damage in the rig, two specimen autoclaves in the main have occurred under single phase conditions which loop, and two tube specimens within the secondary would not have been predicted by equation (1). polishing loop.

25. Discussions of some mechanistic aspects of 30. The rig is principally constructed of Type erosion-corrosion attack has been given by Homig 316 stainless steel, with the exception of the (ref.34) and Bohnsack (ref.35), who concluded that pressurizer vessel (24 Cr 1 Mo ferritic steel),

the process is due to dissolution of the metal the heater elements (Inconel) and some parts of 2

surface to give Fe + ions in solution, which are the main circulating pump (Incoloy 825, stellite continually removed by the turbulent fluid flow. and ferobestos). The rig is designed to operate However, both these authors restrict themselves over the following range of physical conditions:

largely to discussion of the dissolution at 0 250C, which is well below the temperatures at Temperature, up to 350 C which erosion-corrosion attack is normally 2 2 encountered.

0 At 25 C, Fe(OH) 2 is normally Pressure, up to 21.78MNm- (3160 lbf in- )

considered to be the corrosion product involved in the dissolution process in deoxygenated water, Autoclave up to 1031 kg h per autoclave but at temperatures higher than about 1OOC, this flowrates, is converted to Fe30 4 via the Schikorr reaction:

Tube specimen up to 208 kg h-1 total 3 Fe(OH)2 ÷ Fe304 + 2H 2 (2) flowrate, The rate of this reaction increases with tempera- Bypass flowrate,up to 103 kg h-1 ture, and magnetite is typically the phase observed on surfaces undergoing erosion-corrosion Pressurizer up to 20 kg h-1 attack at temperatures above about 1200C. As a flowrate, result, erosion-corrosion attack at these Once the rig water has been pressurized and water higher temperatures has been attributed to rapid circulation achieved using the main pump, dissolution of the unstable Fe(OH) 2 intermediate control of the physical operating parameters of (ref.36).

the rig is largely automatic, with the variables

26. Very recently attempts to produce a model of interest (flow rate, temperature, pressure, water level etc.) being recorded by a dedicated of erosion-corrosion based on calculated mass of magnetite CAMAC data logger.

transfer rates and the solubility have been made by Gilich et al. (ref.7). Work

31. The rig incorporates four methods for to produce a more satisfactory model is also in progress at CERL, and this is outlined in controlling the water chemistry, namely ion However, at present there exchange, chemical dosing, blowdown and deaeration.

subsequent sections.

Data on the chemical composition of the water is no completely satisfactory model of erosion-within the rig is derived mainly from continuous corrosion behaviour which is capable of rationali-the diverse factors chemical monitoring of sample streams which can sing the effect of all influencing the process.

be drawn from a large number of different sampling points around the rig. The exception to this is CERL EROSION-CORROSION STUDIES the direct measurement of conductivity before and after the. ion-exchange columns. To date, all the

27. The work currently in progress at CERL on experimental work carried out on the rig has been erosion-corrosion is directed at establishing a with an ammonia dosed deoxygenated water chemistry consistent set of experimental data from which regime, and for these conditions it has been found it is possible to make accurate predictions of convenient to work with the cation exchange resins plant-behaviour, and to develop a satisfactory of the mixed bed ion-exchange columns converted theoretical model of the process capable of to their ammonium ion form.

rationalizing the experimental work. At present, both experimental and theoretical studies are

32. In its present form, the rig is capable of concerned entirely with erosion-corrosion in operating within the following limits of physical single phase water, although it is to be hoped and chemical control parameters.

that the results of the work can be applied with certain limitations to erosion-corrosion 12

+/- loc Temperature at test specimens hydrodynamically. They can therefore be used for precise'correlation of erosion-corrosion and Flow to test specimens +/- 1% mass transfer behaviour (see subsequent discussion). The particular specimens used pH of circulation water* +/- 0.1 pH unit permit behaviour to be studied at five different potential erosion-corrosion sites; the

-1 Conductivity of water after < 0.6 pS cm tube inlet, the jet reattachment zone downstream cation exchange+ of the orifice, downstream of a tube expansion, and in two different diameter straight tube 1

Dissolved iron in circulating

< 10 jigkg-sections (i.e. two different flow velocities).

0 water at 148 C The specimens also have the advantage that being essentially straight tube test pieces, it 1

Dissolved active silica in

< 10 jigkg-is possible to use high accuracy bore dianetral 0

circulating water at 148 C measurements to characterize the erosion loss profile throughout the specimen.

Dissolved oxygen in circulating < 6 jig water at 1480C g Erosion-Corrosion Monitoring Methods

37. Simple weight change measurements are

• Dependent on pH of circulating water, values possible on all the test specimens described, given for pH 9.0. At higher pH, the precision except the tube specimen channels themselves.

of pH control improves, and dissolved Fe levels However, most of the effort to date has been fall. concentrated on monitoring damage produced in the orifice assembly specimens, and this has

+ Upper limit of conductivity, due to very slow been done principally by the use of high sampling rate. accuracy bore diametral measurements, and thin layer surface activation methods.

Test Specimens

33. A variety of erosion-corrosion test 38. Bore Metrology. Measurements of bore specimens can be incorporated into the isothermal diameter have been made on test specimens using loop, using both the autoclave and tube specimen a "Diatest" internal bore measuring instrument.

flow channels. This instrument permits diametral measurements to be made with a precision of +/-1 jam, and on a

34. The tube specimen channels are provided uniform tube surface, the reproducibility was with couplings for the attachment of tubing better than +/-2 um. The tubes used in the present between two points 2 m apart. Initially straight work are typically either drawn, or machined 3 mnm bore mild steel test specimens and stainless from bar material and have a honed surface finish.

steel dummy specimens have been incorporated, but In both cases, the quality of the tubes used is it is possible to incorporate bent tubes, bore sufficiently good to permit measurements to be expansions and constrictions and a variety of made with the reproducibility quoted above.

other options in this area of the rig.

39. On non-uniform tubes, or heavily eroded

35. Four plate type specimens, 195 x 12 x 1 mm surfaces where the diameter changes rapidly, the can be incorporated into each of the autoclave reproducibility of measurement is reduced, flow channels using stainless steel specimen principally due to the relatively poor longitudinal holders. These hold the precision (+/-0.5 mm) with which measurements are specimens with a 1 mm gap between them, and made at present. Measures are currently in allow rig water to flow along their length. How- hand to improve this by using an automated ever, it is possible to incorporate other types measuring procedure. Nevertheless, in all cases of test specimens in the autoclave flow channels, to date it has been possible to produce highly and most of the work to date has involved the use accurate bore loss profiles from the test of orifice assembly specimens. Up to three such specimens.

assemblies can be accommodated in each autoclave flow channel, as shown in Fig. 5. To minimise 40. Surface Activated-Specimens. Erosion-interaction between specimens in series with one corrosion losses of a number of specimens have another, a baffle plate can be inserted, as shown been monitored in situ by the use of thin layer in Fig. 5, and this also serves as an impingement activation of the specimen. To date this has specimen. It is possible to incorporate up to only been employed with orifice assemblies, but three orifice assemblies in parallel on the inlet can in principal be used for any type of Grayloc seal of the autoclave, and in this way specimen.

interactions between adjacent tubes could be studied, in addition to increasing the total 41. The technique consists of activating to a number of specimens. This does, however, reduce known depth an area of the specimen surface by the flow through any one specimen to one third of high energy charged particle bombardment (ref.38).

that through the specimen on the autoclave outlet Metal loss from the specimen can then be deter-Grayloc seal. mined by monitoring the loss im activity from the specimen surface as erosion-corrosion proceeds.

36. The advantage of using this type of orifice In the present work, small areas of the internal assembly is that experiments can be performed tube surface (5 to 10 mm x 1.75 mm) have been on specimens which accurately simulate plant activated by bombardment with 10.8 MeV protons components, and which are well characterised at angles of 100 or 200 to the tube surface.

13

I&

14 0 MUS AVERAGI

&4 X ACM MCA" UTE 0 It I- 44 .

(0) a I0 4.2 an 42 IB

• f a

a C 4.4

-4.1 2.5 2s Ia 11 i9 is IIN = - 4" we 60  ?" 0 IM1,*FLWRAU.kaM 15". b FICURE 10. Velocity dependence of maximum FIGURE 12. Time dependence of erosion-corroslon erosion-corrosion rate observed downstream of loss for a surface activated specimen.

an orifice at 157 0 C, and pH 9.05 10.0 10 GEOMETRY FLOWRATES

- ORIFICE.2.72 m DIA

- TUBE .8.34 mmI.D.

2

- :?0 kg0 . x

- 74 ki h

- 28701 0

.i!I I

"k, 1.0 Iio 1

\ %0 9

U. S 4.

4. i S

S N. I 5-A 0.1 A. o s A '4.

S 0

0

.0 .. v, i l n I l l I i 0.1 0.0 4.0 9.1 4. 9 9.4

. 9.5 P4 4.7 MASSTRANSFER COEFFICIENT. m 32 101 PHI FIGURE 11. Correlation of erosion corrosion FIGURE 13. gH dependence of erosion-corrosion rate downstream of orifice with corresponding rates at 157 C. Solid symbols, maximum rates mass transfer coefficient throughout the from surface activated specimens, open symbols erosion-corrosion zone average rates 14

56 The bombardment of Fe with high energy prc)tons where Sh in equation (6) refers to the maximum 56 Sherwood number observed downstream of the produces Co which has a half-life of 77.3 days, and the principal y-ray emitted on decay has an orifice, and ReN the orifice Reynolds number.

energy of 845 keV. The maximum depths of The overall variation of mass transfer activation for bombardment with protons at Oo0 coefficient K in the tube downstream of the orifice is illustrated in Fig. 7.

and 200 are around 35 and 70 pm respectivel3 Deeper activation is possible by bombarding 46.

clearlyThebe value of C in equation (3) may normally to the tube surface, or by increasi determined experimentally for any 56 the incident proton energy. The total Co at given erosion-corrosion situation, and for most activity versus depth curve for bombardment situations of practical interest will be very 100 to the tube surface is shown in Fig. 6. 1 low, probably less than 10 pg kg- . However, the concentration of iron in solution at the

42. Loss of material from the specimen sui-face -e oxide-solution interface cannot be so easily has been determined in-situ by monitoring tl evaluated. In the first instance, it might be y-ray emissions from the sample using a scintillation detector placed in close proxi [mity assumed that this term may be equated with the to the autoclave containing the active speci .men. equilibrium solubility of the surface 0 oxide, These have permitted measurements of erosion loss which at temperatures above about 100 C is usually taken to be magnetite. If however some to be made as a function of time with an acc :uracy rhof metastable intermediate oxide such as Fe(OH) 2 of +/-0.1 pm in the case of an activation dept is invoked, then a different solubility would 35 um.

be appropriate.

43. Full details of the experimental techn]ique 47. The solubility of magnetite is known to be will be reported elsewhere (ref.39).

dependent on temperature, pH and hydrogen Theoretical Work partial pressure (ref. 42). Fig. 8 shows the variation in magnetite solubility with pH and

44. If erosion-corrosion is controlled sol ely temperature at 1 bar partial pressure of by the rate of mass transfer of Fe from the hydrogen (1585 pg kg-') derived from the data of eroding surface, then the erosion-corrosion rate Sweeton and Baes. However, these solubilities may be expected to vary according to are much higher than would be anticipated in operating plant, where the partial pressure of dm d-m = K(C - Cb) (3) hydrogen would be much lower ("'I-5 pgkg- 1 ).

Under these circumstances the equilibrium Where K = mass transfer coefficient solubility of magnetite, when taken with the expected mass transfer coefficients is far too C = concentration of iron in solution at low to explain the observed erosion-corrosion s the oxide-solution interface rates (ref.43). This analysis would indicate Cb concentration of iron in the bulk that the solubility of the surface oxide is solution much higher than that expected for magnetite in equilibrium with the bulk partial pressure of dm rate of metal loss. hydrogen. It is possible, however, that the dt solubility may be sufficiently enhanced locally

45. The value of the mass transfer coeffic:Lent by the high equivalent partial pressure of K varies with the local hydrodynamic conditicMs. hydrogen which results from the high local Its dependence on these is usually expressed in corrosion rate. Once established, the high dimensionless form using the corresponding local solubility in turn assists in maintaining Sherwood number Sh, where Sh = KD/D, D = duel the high erosion rate. Electrochemically this diameter and D = diffusion coefficient for i3.on is equivalent to the dissolution process in solution. This is normally expressed in :erms occurring at relatively negative potentials, of the Reynolds (Re) and Schmidt (Sc) nunberz Sin which is in agreement with the general observation empirical correlations of the form that actively eroding areas are normally covered with magnetite, whereas nearby non-eroding Sh = ctResScy (4) surfaces are frequently covered with haematite.

This possibility may be analysed theoretically where a, 0 andy are constants determined by in the following manner:

experiment; y typically has a value around 1/ '3, whilst the value of 6 is usually in the range 48. At equilibrium the dissolution of 2/3 to 7/8. Correlations of this type are magnetite to form Fel+ ions in solution (the already available for a number of hydrodynami ,c dominant species under the conditions of interest) situations of concern in erosion-corrosion, a nd may be expressed as:

two of particular interest in the present wor sk Fe 3 04 +2H 2 0 +2H+ +2e - 3Fe(OH) ' 3Fe 2 + +60H-are those for turbulent flow in straight pipe (ref.40), and downstream of an orifice (ref.4 1). (7)

These have the form:

for which the appropriate Nernst-equation is 0 86 0 33 Straight pipes: Sh =0.0165 Re" Sc" (5) E = E ;_Rfn 1- 3 (8)

Downstream of 0 067 0.33 an orifice: Shmax 0.27 P eN Sc (6) [H1 2

15

which gives 51. In the case of specimens undergoing very rapid erosion-corrosion wastage, the films are

[Fe2 2+ [H

- K2

- x-2F(E e !1 -- 2 3:T

- E f (9 sufficiently thin to exhibit interference colours.

With lower erosion-corrosion rates, however, the eroding surface is black, as for non-eroding areas of the tube surface.

[Fe (OH)j2 Hl1

52. Micropitting of the tube surface to a depth of about 5 pm is evident in the erosion where K2 = 2 Fe i zone shown in Plate 3, and this is associated with accelerated attack of the pearlite grains The cathodic current ic of the corrosion reaction of the steel. Effects of this type have also resulting from hydrogen discharge at the surface been observed in plant specimens.

of the magnetite film may be expected to vary exponentially with the surface potential E of the 53. Most of the work to date has involved the use of mild steel orifice assembly specimens, film, as follows:

-FB exp ) (10) and Fig. 9 shows a typical erosion-corrosion loss profile downstream of the orifice, obtained using the bore measuring technique outlined previously. The general similarity to the mass If the anodic current ia at this potential is limited by the rate of removal of Fe 2 + ions transfer profile shown in Fig. 7 is i-mmediately apparent. However, it is clear that the from the surface, and since ia + ic = 0, straight tube losses are quite small, whereas Fig. 7 shows the mass transfer coefficient decays asymtotically to that appropriate to the straight 2 FK Lpe2+

2 Cb PB exp )* (1) tube, which is about 1/3 to 1/4 of that at the mass transfer maximum. It is important to note If c,<< [Fe + a B = 1 then substituting however, that the maxima in both curves occurs from equation (9) and eliminating E gives approximately 2 tube diameters beyond the orifice.

54. Experiments exposing several specimens at different flow rates under the same conditions 0

2FE may be used to establish the flow and hence mass Fe I 2 exp transfer dependence of the erosion rate, and K

K23B2 Fig 10 shows the velocity dependence obtained at 148 0 C using pairs of specimens at three different flow rates. The slope the plot indicates a V2 In this case the Fe2+ solubility of magnetite at dependence of erosion rate on flow, which the surface is dependent on the square of the mass according to equation (6) would indicate a transfer coefficient K, giving an overall dependence on mass transfer coefficient cubed.

dependence of the erosion rate on the cube of Further confirmation of this K3 dependence is the mass transfer coefficient, through equation shown in Fig. 11, where the erosion loss (3). profiles of individual specimens have been compared point by point with the corresponding mass

49. This treatment may be extended to include transfer profile of the type shown in Fig. 7.

all soluble iron species under the conditions From this it is seen that not only do the of interest, and the effects of a non negligible maximum losses downstream of the orifice conform bulk concentration of iron. The expressions with the K3 dependence, but the erosion-corrosion become more complex in this case, but still rates over nearly the whole profile of the indicate a dependence of erosion-corrosion rate specimens correlate with K3.

on the cube of the mass transfer coefficient (plus smaller terms in K2 and K). Further 55. Whilst this alone does not substantiate analysis of the mechanistic aspects of erosion- the theoretical treatment outlined in the corrosion is still under consideration, but this previous section, it does provide strong support rather unexpected dependence of the rate on the for the type of mechanism invoked, and indicates cube of the mass transfer coefficient is born that further development of the theory along out by experiment. these lines should prove very fruitful.

Results 56. Fig. 12 shows the erosion-corrosion loss of an orifice assembly specimen downstream of

50. Plate 2 shows the erosion-corrosion zone the orifice as a function of time, determined downstream of the orifice generated in a mild from the activity loss of a surface activated steel orifice assembly test specimen. Although spot in the erosion-corrosion zone. It is the surface loss at the erosion maximum is evident that under the particular conditions relatively large (0-150 um), scalloping of the used, there is a substantial initia.;ion time surface, of the type shown in Plate 1 has not before any erosion-corrosion loss is observed.

yet developed. However, the oxide film present Once initiated, the erosion-corrosion rare in the eroded area is extremely thin, as shown rose rapidly to a high value, and then remained in Plate 3. constant for most of the remainder of the test (the reduction in rate towards the end of the 16

test shown in Fig. 12 is thought to be due to 60. Since mass-transfer coefficients can be changes in experimental conditions). This type calculated for a wide variety of hydrodynamic of behaviour has been observed on a number of situations, at least under single phase occasions, although the initiation time can vary conditions, it should be possible to use widely with the experimental conditions, generally correlations of this type to predict plant being much shorter under more aggressive behaviour over a wide range of conditions.

erosion-corrosion conditions. The cause of such initiation periods is not certain at present. 61. Increasing pH has been shown to markedly In some cases this most likely represents the reduce erosion-corrosion rates over the range time taken to remove a thin oxide film produced 9.05 to 9.65, in agreement with other studies of during start-up of the rig, when specimens are the effect at lower temperatures. In many plant exposed to low flow for a few hours. In other situations, therefore, this option should prove cases it is thought that thin air formed oxides effective in controlling erosion-corrosion produced during welding of the test specimens damage. It is likely to be especially useful were responsible. However, in some cases, an when other options such as materials change or initial loss of a few microns has been observed, oxygen addition are not feasible.

after which no loss has occurred for up to 200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br />, before true erosion-corrosion attack has 62. Details of the mechanism of erosion-been initiated with a continuing linear loss as corrosion damage have still to be established, a function of time. This would suggest that but the use of surface activation in the present initiation is more complex than simply removing work has proved to be extremely valuable for a pre-existing oxide film, and may indicate monitoring losses in-situ. Using this technique changes occur in initially formed films under it has been possible to establish the linearity erosion-corrosion conditions. of erosion-corrosion loss as a function of time,

.after some initiation period, and it will.

57. The pH dependence of erosion-corrosion rates undoubtedly be useful in studying erosion-has also been investigated using orifice assemblieýs corrosion behaviour under transient conditions.

and Fig. 13 shows the results obtained at 1480C. In conjunction with electrochemical techniques, The upper limit of the data is essentially therefore, it should prove very valuable in derived from the maximum linear rates observed elucidating aspects of the corrosion mechanism.

using surface activated specimens. The rates de.rived- from other specimens are average rates, AQGS*OW-LE DG* iNT S which are in general lover as a result of a significant but unknown initiation time. Th1e 63. We wisb to t-hank J.. Ashford, C.H. de Whalley, erosion-corrosion rates decrease by a factor of D. Libaert and R. Sale for their assistzce with about 7 over the pH range 9.05 to 9.65, which is the experi.e-ntal work described, and M.W.E. Coney 4

equivalent to a variation with[H+i1. . This is for helpful discussions on aspects of ---. s-tr-nsfer a somewhat higher dependence than that seen by behaviour in turbulent flow4, Apblett (ref.26) at 99°C, where the erosion-corrosion rate varies as"H+l i.0. 64. This paper is published with the permission of the Central Electricity Generating Board.

SUMMARY

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