ML082530370
| ML082530370 | |
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| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 08/12/2008 |
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| SECY RAS | |
| References | |
| 06-849-03-LR, 50-271-LR, Entergy-Applicant-E4-19-VY, RAS M-334 | |
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Text
2.
Effects of chemistry on corrosion-erosion of steels in water and wet steam Ph. Berge, J. Ducreux, and P. Saint-Paul, EDF, Moret-sur-Loing It has often been observed, particularly in steam production plants that numerous cases of degrada-tion of steels occur when in contact with water or wet st'eam circulating at high velocity in feed or discharge pumps, water reheaters, etc. (ref. 1). When the phenomenon occurs without any mechani-cal wear of the metal or the oxide from the impact of solid particles (abrasion) or droplets (erosion),
it is called corrosion-erosion. The phenomenon usually occurs between 100 and 250 0C, as has been confirmed by an empirical study of the thermal and hydraulic factors which govern it (ref.2).
Corrosion rates can reach 1 to 2 mm/year, for a carbon steel pipe where water treated with ammonia circulates at about pE 9, at 200*C, and at a velocity of 5 to 10 m/s. Usually the remedy is either a change in the steel quality, for example, the choice of a chromium steel, or the lowering of the flow velocity of the liquid or the wet steam. But in certain cases the only possible action is by chemical treatment of the water. In fact the part played by the water chemistry on the development and even perhaps on the occurrenceof the phenomenon has been noted, but without it being possible to make a quantitative estimation and without the mechanism of this action being clearly established.
In this study we propose to evaluate the part played by the factors solely connected to the chemis-try of water, with respect to tie kinetics of the corrosion-erosion phenomenon.
MECHANISM OF THE PHENOMENON Whereas oxidation of steels in dry steam follows a parabolic development, at least when an homoge-neous oxide formed adheres to the metal, in the case of oxidation occuring in circulating water, it is quite different. This difference is due to the fact that ferrous oxides which are formed either directly by the action of water on steel, or according to a more recent hypothesis by re-duction by hydrogen of the magnetite formed (ref. 3-7),
have a high solubility in water. One can then observe a linear rate of oxidation after a relatively short time (Fig. 1) (ref. 4).
The slope of the linear curve depends on the nature of the steel, the temperature, the hydraulic con-Sditions and the water chemistry. This linear function is due to the fact that the oxide layer formed attains a constant protection, i.e. a constant thickness in this case. The metal is thus oxidized in the form of soluble ferrous species. Dissolution by reduction of the magne-tite layer, by the equation Fe304 + (2-b)H+ +
H2 layer.
The dissolving rate, following the reaction (1),
for a~given pH and hydrogen concentration, is proportional to Cn
- Cn (ref. 8) where Ce is eq the concentration of ferrous iron in equillbrium (the equation being reversible) and C the concen-tration of ferrous iron at the surface of the oxide layer, n depends on. the order of the reac-tion (1').
The following calculations have assu-med, as a first hypothesis, n -
1, which leads to a relatively simple expression of the corro-sion rate, by stating that the mass transfer (ferrous iron) towards solution is equal to X(C - CO) where X is the mass. transfer coeffi-cient and C0 the concentration of ferrous iron in the solution.
dm A (C - Co) dt dm E 2K (Ceq -
C)
(2)
(3) from which one gets b
3 b) 2 O (1) dm 2Kk (C
-C dt". +2K+-X eq a
(4)
,is accompanied by the formation of a new magneti-te-layerlbf the same thickness by direct oxida-tijon.of the metal. This oxidation is itself accompanied by the formation of soluble iron which diffuses towards the exterior of the layer in a quantity more or less equal to half the mass of oxidized iron. One can therefore assume that corrosion is equal to twice the rate of dissolu-tion of-the external surface of the magnetite where K depends on the dissolving rate of the*
magnetite under given chemical and hydraulic
.conditions.
The tests-in process should make it possible either to' verify. the proportionality between the corrosion and the difference (Cea - Co), 'or to determine a factor.n giving a mofe complex axpres-sion of corrosion.
Water Cheinistir 1, BNES, 1980. Paper 2
".19
DOCKETED USNRC August 12, 2008 (11:00am)
OFFICE OF SECRETARY RULEMAKINGS AND ADJUDICATIONS STAFF U.S. NUCLEAR REGULATORY COMMISSI N In the Matterof *.A'*Jv V-,
4'e LLL-.-
Docket No.
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' f ---- fcial Exhibit No. -
Vy OFFERED by
@aricemee Intervenor-NRC Staff Other IDENTIED onikr*Z(O1 witness/Panel M LC Aon Taken:
REJECTED WITHDRAWN
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Chemistry and corrosion in steam generating circuits DISCUSSION
- These hypotheses only take into account the mass transfer of soluble iron in the aqueous phase in the vicinity of the surface. It can either be a diffusion in pressurized water, or in the case of wet steam, a renewal of the liquid layer, necessarily present if this mechanism is involved.
It is not applicable to any mechanism involving a mc-hanical~degradation of the oxide.
In the case when the solution is saturated in ferrous iron, for example under isothermic and static conditions (autoclave), the long term corrosion rate is practically nil (Fig. 1).
When the solution contains ccmplexing agents, for example chlorides, the value of C cannot be considered as a resultant of tha equilibrium of the reaction (1),
and the corrosion is much higher.
In the presence of oxygen, ferrous iron oxidi-zes into ferric iron which has a very-low solu-bility and a different protective oxide forms which can markedly slow down the corrosion rate (ref. 9-11).
If the metal is protectec by an oxide formed under not very turbulent conditions, starting up of rapid linear kinetics can take a certain time, called the incubation period of the phenomenon.
EVALUATION OF THE PART PLAYED BY pH ON CORROSION The value of Ceq depends on the pH of the solu-tion, the hydrogen concentration, and the tempe-rature C
= KI (H) 2(H2)1/3 + K2 H+ (H2) 1/3 +
K3 ~
K4 1H)
/3 K3 (H2)
/3 +
(H2 The value of the constants has been determined by F.H. Sweeton and C.F. Baes (ref. 3).
As an example, we have calculated the values of Ceq in.boiler water treated with ammonia or mor-pholine, at 250*C. This casecorresponds to conditions of a once-through steam generator of an electric power plant, which developed severe corrosion-erosion at the end of the evaporation zone, the steam quality being about 90 % with an ammoniated water at pH 8.9 -
9.
Table 1 shows the corrosion rate in different pH according to (4),
for C0 = 0. The pH is raised either with NH3,. at different concentration, or with morpholine, the dissociation coefficient of which at 250*C being higher, and the partition coefficient between water and steam more favou-rable than for NH3.
The measurement of hydrogen produced in the gene-rator has made it possible to ascertain that cor-relation with the calculation was reasonably good over the whole range of pH tested around the working pH level having shown corrosion (Fig. 2).
EXPERIMENTAL STUDY The Ciroco test loop (Fig. 3) makes it possible to evaluate various physical and chemical factors governing, corrosion-erosion in pressurized water; Most of the tests made to date have investigated the behaviour of mild steel (either preoxidized or not) exposed to a flow of demineralized, deaerated and alkalized water.
Before entering the test section, water is alka-lized with ammonia or morpholine at a pH within the range of 8.5 to 9.7. It is also conditioned
.with 20 ppb of hydrazine, pressurized at about 15 MPa and heated to 2250C. Flowing through a converging nozzle the jet of water speeds up to 60 m/s and impinges the flat surface of the test specimen at an angle of incidence of 45*.
Four similar nozzles are set up one after another-so that four samples can be tested at the same time under reasonably comparable conditions. Aftez cooling, the full water flow is polished in a mixed bed ion exchanger, so that any complexing agents (chloride, sulfate) and iron compounds (oxides, hydroxides, etc.) are removed as far as possible. It is then'necessary to adjust the pH with ammonia or morpholine as required.
The rig tubing and the main components are made of stainless steel or Inconel 600.
TABLE I Ratio between corrosion at various pH under steam generator conditions of a graphite-gaz reactor (2500C - 90% steam) and in water at 2500C and that obtained at neutral pH (rf. 12.13).
pH at 250C 8.5 8.8 9.0 9.1 9.2 9.3 9.4, 9.5 9.6 With ammonia (steam quality 90%)
0.88 0.77 0.64 0.58 0.50 0.43 0.37 0.31 0.27 (steam quality 0%)
0.55 0.50 0.39 0.34 0.29 0.25 0.22 0.20 0.18 With morpholine (in water or wet steam )
0.50 0.31 0.23 0.20 0.17 0.16 0.14 0.13 0.13 The partition coefficient of morpholine in water and steam being taken as 1.
9n
4 Total corrosion (mg/dm2) loin/a 10000 8lam SMI/s Soo
,,X 2
s nozzle xyt.--.
- test
- I in autoclave Time (h) 0 W
500 1000 1500 200O Figure 1 Effect of flow rate on corrosion in circulating 300 0C water of low alloyed Mn Mo steel [4]
Heat exchanger
- Mixed bed:
- Htxca Additives i~njection Circulatlng heater pum Measures of pH, conductivity, 07 H 2. N2 H4, Fe, Cl0 S04 Figure 3 : Scheme of CIROCO LOOP and lost section.
H 22 production img/h) 1,5 X Measured 0
Calculated
\\./
Loss of metal S
pH at 250C 9.2 alcalizing agent: ammonia A, morpholine M preoxidized specimen
- 400.
not preoxidized specimen +
0.5 pH at 250 0C 6.0 6,5 7,0 Figure 2 ; Effect of pH on the H2 production in a steam generator of a graphite--gaz reactor.
100 20O Figure 4 : CIROCO LOOP corrosion curves.
21
Chemistry and corrosion in steam generating circuits Under the above conditions, the loss.of metal obtained during a few hundred hours by corrosion-erosion is easily measurable by weighing each steel specimen. By continuous or recurring water
- sampling, it is possible to check the variations of the following chemical parameters : pH, catio-nic conductivity, chloride, sulfate, iron, hydra-zine, dissolved hydrogen and dissolved oxygen concentrations.
The steel surface preparation determines two dif-ferent types of corrosion versus time curves (Fig. 4)
- when the steel has not been preoxidi-zed the loss of metal due to corrosion-erosion is proportional to the time ; in contrast when
-the steel has been preoxidized by exposure in high temperature static water, corrosion-erosion starts after some delay.
Taking into account the above theory, one can assume the rate of metal loss due to corrosion-erosion strongly depends on the pH.
When the steel specimens have not been preoxidized, for a given temperature, hydraulics conditions and hydrogen concentration, when the water has not been contaminated by ccmplexing agents, the corrosion-erosion rate depends linearly on the ferrous iron concentration at equilibrium, gien by thepH (Fig. 5).
(a) "
(b)
Figure 6 :CIROCO LOOP specimen after testing a - Macrograph (after 80 h at PHol 8.5 with ammonia) b
- SEM examination (selection atack of the peilite).
Working conditions at the test section will be as follows :
Wet steam quality
- Temperature Flow rate 80 to 100 %
200 to 250 5 C 50 to 100 kg/h Ferrous ions CO (ppb) oncentrationat equillbrium (1)
Scale,:A B
C pH at2250Cat 25°Cat 25PC (1) This concentration is calculated as a.
6.0 3
function of pH at 2250C (A scale) 6 1 and Cscales show the PH t2°C 2
0 necessary to obtain a given pH at 2 25°C (A scale) either with morphollne oe with ammonia 90 2.0.
1,0-clk
+
6,5190 iQO; 7,0495 12 3
I1mghI Corrosion rate Figure 5
- Ci ROCO LOOP correlation between the corrosion rate and the soluble iron concentration at equilibrium.
Impingement velocity on target 50 to 70 m/s The water will be demineralized, deaerated and conditioned before being turned into slightly superheated steam. Then the steam will be par-tially condensed and the emulsion sent into the test section, then drained off after cooling.
An ultrasonic device will raa3: it possible to follow continuously the loss of metal.
CONcLUSION The study undertaken has confirmed the very important part played by water chemistry on corrosion-erosion of steel.
The formula d 2K (C
Co) which makes d t
=K) ef-
,AIC.q o
it-possible to quantify the pH role on corrosion and which has been established taking into ac-count certain hypotheses on the kinetic order of the Schikorr reaction, seems acceptable from the data obtained in the plant or in the labora-tory.
REFERENCES
- 1. "Chimie de l'eau et corrosion dans les cir-cuits eau-vapeur des centrales nucldaires".
Colloque ADERP-EdF, Seillac, Mars 1980.
- 2. KELLER H. VGB, Kraftwerkstechnik 54,
- 1974, 5,'
292-295.
- 3.
SWEETON F.H. and BAES C.F.
"The solubility of magnetite and hydrolysis of ferrous ions in aqueous solutions elevated temperatures".
J. Chem'. Thermodynamics,
- 1970, 2, 479-500.
- 4.
BERGE Ph. M4canisme de l'oxydation des aciers dans l'eau a haute temperature et de la forma-tion de ddp6ts d'oxyde. Congr6s d'Ermenonville, 13-17 Mars 1972.
- 5.
STYRIKOVICH M.A.,
MARTYNOVA OJ. et al Teploenergetika 19, 1972, H.9.S.,
85-87.
Some steel specimens which have undergone corro-sion-erosion either in the Ciroco test loop or in power plants during normal operations look very similar when observed with a Scanning Electron Microscope ; in some cases the perlite seems to corrode more rapidly than the ferrite (Fig. 6).
Furthermore it has been observed that if there is chloride or oxygen present in the water, the corrosion rate is modified as indicated above.
REMARKS
-In order to verify that the phenomena observed in the Ciroco test loop under monophasic condi-tions are the same as those observed under dipha-sic conditions, a new program has been initiated.
Another test loop in stainless steel giving wet steam at the required temperature and steam:qua-lity, in under construction.
i
- 6. STURLA P. S. Nationale Speisewasserkonferenz in Prag vom 2 bis 4, Oktober 1973.
- 7. BIGNOLD G.J., GARNSEY R. and MANN G.M.W. High temperature aqueous corrosion of iron develop-ment of theories of equilibrium solution phase transport through a porous oxide. Corrosion.
Science, 1972, Vol. 12, 325-332.
- 8.
BERGE Ph.,
RIBON C. and SAINT-PAUL P.
"Effect of hydrogen on the corrosion of steels in high temperature water.Corrosion, 1976, 32, June, 223.
- 9. BOHNSACK G. Sonderheft VGB Speisewassertagung 1975, 15-19.
- 10.
FREIER R.K. Wasser, Vol. 8, 1971, 443-458.
- 11.
BRUSH E.G.,
PEARL W.L. Corrosion and corro-sion product release in neutral feedwater.
Corrosion,,Vol. 28, 1972, 129-136.
- 12. POCOCK F.S. "Control of iron pickup on cycles utilizing carbon steel feedwater heaters". Proc.
Amer. Power Conference, 28, 1966, 758-771.
- 13.
MESMER R.E. No publied results.
Addendum to Paper 2 Mathematic study of diffusion and kinetic equations for different values of n, the order of reaction.
+
1/3 Fe 3 04 + (2-b) H* +
.H2 Fe(OH)b 4
.+
b) H2 0 b = 0,1,2,3 Considering the predomifiant ferrous species at equilibrium (for instance Fe (OH)3 in a basic medium according to the model of Sweaton and Baes) one can write down (1)
I,=
X (C-Co) 2K (Ceqn
. Cn) or otherwise (II) th X (C Co)
Ceq =
(Cn +
(C-Co))
lln 2K Although the explicit function i = f (Ceq) can be calculated for n = 1,2,3, its study is not very easy forn - 2 and 3.
One had better look for the shape of the curve i Ai f (Ceq) by study-ing the system (II) of parametric equations," f and Ceq being considered as functions'of the same parameter C.
One has then to calculate the first and second derivatives du,"dCeq ; d 2 Ceq dc dc dc 1Z and deduce from them the concavity of the curve,-
the points of inflexion, and the behaviour at infinity.
The main results are summarized in the following diagrams (Fig.i).
The comparison between these theoretical results and-the trend of experimental measurements may help us to determine the order of n of reaction.
n-2 mh Asmtt Gradient at the origin Gradient -)
(Ceq -Cc)
MKo)k 4KCo4AL co co+j, cq n.13 m
Asympltet Gradient-).
Gradient at thle ttontehf origin (cea - Co)
- aK~o~ XI 2n o'
)
co Oaq n-i 2KX m - T (cc-co)
./
Fig.l 23