Hydrogen evolution from corrosion of iron and steel in low/intermediate level waste repositories, P Kreis

Tags: NAGRA NTB, hydrogen evolution, pore water, corrosion rates, pH, mercury column, iron and steel, current density, neutral media, evolution rate, WASTE REPOSITORIES, carbon content, corrosion rate, anoxic conditions, hydrogen production, passive layer, test series, test cell, pore waters, NTB, KOH, corrosion of iron, Wasserstoffentwicklungsraten von korrodierendem Eisen, Corrosion behaviour, gas generation
Content: ... 1'111111111 111
n 9 ro A
III
National Cooperative for the Disposal of Radioactive Waste
Technical Report 91 -21
HYDROGEN EVOLUTION FROM CORROSION OF IRON AND STEEL IN LOW/intermediate level WASTE REPOSITORIES
P. KREIS
AUGUST 1991
SULZER INNOTEC, Winterthur, Switzerland
Hardstrasse 73, CH-5430 Wettingen/Switzerland, Telephone + 41-56-371111
This report was prepared as an account of work sponsored by Nagra. The viewpoints presented and conclusions reached are those of the author and do not necessarily represent those of Nagra. "Copyright (c) 1991 by Nagra, Wettingen (Switzerland). / All rights reserved. All parts of this work are protected by copyright. Any utilisation outwith the remit of the copyright Law is unlawful and liable to prosecution. This applies in particular to translations, storage and processing in electronic systems and programs, microfilms, reproductions, etc."
NAGRA NTB 91-21
-I -
CON TEN T S
Summary
II
Zusammenfassung
IV
Resume
VI
1.
Introduction
1
2.
Objectives
1
3.
Corrosion mechanism of iron and steel
and survey of available experimental results 2
4.
Experimental
4
4.1 Measuring technique and apparatus
4
4.2 Experimental program
6
4.2.1 Media
6
4.2.2 Specimen material
8
4.2.3 Tests performed
8
4.3
Calculations
11
5.
Results
14
5.1 Blank tests
16
5.2 Tests with alkaline media
18
5.3 Tests with neutral media
21
6.
Discussion and Conclusions
26
7.
References
28
NAGRA NTB 91-21
- II -
HYDROGEN EVOLUTION FROM CORROSION OF IRON AND STEEL IN LOW/INTERMEDIATE LEVEL WASTE REPOSITORIES SUMMARY The production of hydrogen from the corrosion of iron or steel is an important issue in low/intermediate level nuclear waste repositories where large quantities of iron and steel (e.g. as drums and reinforcing steel) accompany the waste. Most of the iron in low/intermediate level repositories is in a cementitious environment. A reviw of the literature on the corrosion of iron and steel at high pH values, in particular in cementitious environments, points to hydrogen evolution rates between 22 and 220 mmol(H2 )/(m2 *a). There is some indication that the rates might be lower but for normal engineering applications there has been no practical need to demonstrate this, and hence a lower rate cannot be assumed on current evidence. Near field analysis shows that hydrogen evolved at these rates will not be dissipated by diffusion of dissolved hydrogen. This may have various consequences, for example displacement of water from the near field and, if the gas cannot escape, mechanical disruption of the host formation cannot be ruled out because of equilibrium pressures in excess of 500 bar. In the present work the hydrogen evolution rates of corroding iron in strongly alkaline and neutral media were measured during a period of at least 6000 hours, using a volumetric method. The sensitivity of this method is sufficiently high (0.1 mmol/(m2 *a) to determine lower hydrogen evolution rates than those previously assumed for iron and steel in cement. Selected as alkaline media were on the one hand solutions of the free bases NaOH, KOH, Ca(OH)2' and on the other hand three synthetic cement pore waters. The pH of the solutions ranged from 12.5 to 13.
NAGRA NTB 91-21
- III -
In general the hydrogen evolution rates found were below
the 22
220 mmol (H2 ) / (m2 *a) postulated above. Never-
theless despite similar pH there were distinct differences
between the corrosion behaviour of the monovalent (NaOH,
KOH) and bivalent Ca(OH)2 solutions.
While the Ca (OH) 2 solution shows a practically constant hydrogen evolution rate of 1 - 2 mmol/(m2 *a) throughout the measuring period, the monovalent solutions reach a maximum around 13 mmol/ (m2 *a) after about 2000 hours, dropping below 4 rnmol/(m2 *a) in the course of 6000 h.
Accordingly an aged cement pore water containing only Ca(OH)2 as free base produces hydrogen at a constant value of 0.5 - 1 rnmol/ (m2 *a). It must be assumed that new- cement pore waters containing much free NaOH and KOH behave according to the free bases, although the measuring time of 6000 hours was too short for any exact statements.
Employed as neutral media for the tests were Boettstein
water (a synthetic granitic deep water), bidistilled water
and a solution of pH 8.5 containing 8000 ppm chloride. All
three media revealed a similar picture in principle,
qualititatively and quantitatively. After a starting phase
of about 1000 hours with hydrogen evolution rates of 80 -
100 mmol/(m2*a), the values begin to fall in the course of
6000 hours to 10
20 rnmol/ (m2 *a). The results measured
during the starting phase agree in order of magnitude with
earlier results (Schenk 1988), but since corrosion measure-
ments covering several thousand hours are rare or nonexis-
tent, a comparison with results from earlier published
works is difficult.
Keywords:
Corrosion, Hydrogen evolution, Cement pore water, anerobic corrosion
NAGRA NTB 91-21
- IV -
ZUSAMMENFASSUNG
Die Produktion von Wasserstoff durch Korrosion von Eisen und Stahl ist ein wichtiger Aspekt bei Endlagern fьr schwach- und mittelradioaktive Abfдlle, da grosse Mengen an Eisen und Stahl in den Lagern enthalten sind.
Der grцsste Teil des Eisens in Endlagern fьr schwach- und
mittelaktive Abfдlle befindet sich in einer zementhaItigen
Umgebung. Literaturangaben betreffend die Korrosion von
Eisen und Stahl bei hohen pH-Werten, d.h. speziell in
zementhaItiger Umgebung, deuten auf Wasserstoffentwick-
lungsraten von 22
220 mmol (H2 ) / (m2 *a) hin. Es gibt
Hinweise, dass die Korrosionsraten sogar eher noch tiefer
liegen. Bisher war allerdings kein Bedarf vorhanden, dieser
Vermutung nachzugehen, da fьr normale technische
Anwendungen die oben erwдhnten Korrosionsraten nicht von
Bedeutung sind.
Sicherheitsanalysen zeigen hingegen, dass in einem Endlager fьr schwach- und mittelaktive Abfдlle der Wasserstoff, in diesen Raten produziert, nicht genьgend schnell durch Diffusion abgebaut wird. Dies kann verschiedene Konsequenzen haben. Einerseits kцnnen sich Gastaschen bilden, die das Wasser verdrдngen, andererseits, falls die Gasblasen nicht entweichen kцnnen, kann bei einem Gleichgewichtsdruck von mehr als 500bar bereits eine mechanische Zerstцrung des Felsens resp. eine Beschдdigung des Endlagers erfolgen. Aus diesen Grьnden ist eine genaue Bestimmung kleiner Korrosionsraten sehr wichtig.
In der vorliegenden Arbeit wurden mit einer volumetrischen Methode die Wasserstoffentwicklungsraten von korrodierendem Eisen in stark alkalischen und neutrAlen Medien ьber einen Zeitraum von mindestens 6000h gemessen. Die Empfindlichkeit der Methode ist genьgend hoch (0.1 mmol/(m2 *a)), um kleinere Wasserstoffentwicklungsraten als jene fьr Eisen und Stahl in Zement bisher angenommen, zu bestimmen.
Als alkalische Medien wurden einerseits Lцsungen der freien Basen NaOH, KOH, Ca (OH) 2' andererseits drei synthetische Zementporenwasser gewдhlt. Der pR der Lцsungen lag zwischen 12.5 - 13.
NAGRA NTB 91-21
- V-
Generell liegen die gefundenen Wasserstoffentwicklungsraten unter den oben postulierten 22 - 220 mmol(H2 )/(m2 *a). Hingegen ergaben sich trotz дhnlichem pH deutliche Unter- schiede im Korrosionsverhalten zwischen den einwertigen Basen (NaOH, KOH) und der zweiwertigen Ca(OH)2.
So weist die Lцsung mit Ca(OH)2 in der gesamten Messperiode eine praktisch konstante Wasserstoffentwicklungsrate von 1 - 2 mmol/ (m2 *a) auf. Demgegenьber weisen die einwertigen Basen nach ca. 2000h ein Wasserstoffentwicklungsmaximum von ca. 13 mmol/ (m2 *a), das aber im Verlauf von 6000h unter 4mmol/(m2 *a) sinkt.
Ein gealtertes Zementporenwasser, das nur Ca(OH)2 als freie
Base enthдlt, produziert dementsprechend eine konstante
Wasserstoffmenge von 0.5
1 mmol/(m2 *a). Bei jungen
Zementporenwдssern, die viel freie NaOH und KOH enthalten,
ist anzunehmen, dass sie sich entsprechend den Lцsungen mit
freien Basen verhalten. Fьr eine genaue Aussage erwies sich
allerdings die Messzeit von 6000h als zu kurz.
Als neutrale Medien fьr die Versuche dienten Boettstein-
wasser (ein synthetisches granitisches Tiefenwasser) ,
bidestilliertes Wasser und eine Lцsung mit pH 8.5, die
8000ppm Chlorid enthдlt. Grundsдtzlich zeigten alle 3
Medien qualitativ und quantitativ ein дhnliches Bild. Nach
einer Startphase von ca. 1000h mit Wasserstoffentwick-
lungsraten von 80 - 100 mmol/ (m2 *a) sinken die Werte im
Verlauf von 6000 h auf 10
20 mmol/ (m2 *a). Die in der
Startphase gemessenen Werte stimmen grцssenordnungsmдssig
mit frьher gefundenen Resultaten (Schenk 1988) ьberein. Da
jedoch Korrosionsmessungen im Bereich von mehreren tausend
Stunden selten bzw. inexistent sind, ist ein Vergleich mit
Resultaten aus frьheren, publizierten Arbeiten schwierig.
Stichwцrter:
Korrosion, Wasserstoffentwicklung, Zementporenwasser, anerobe Korrosion
NAGRA NTB 91-21
- VI -
RESUME La production d'hydrogиne rйsultant de la corrosion du fer et de l'acier prйsents en quantitйs importantes dans un dйpфt final pour dйchets de faible et moyenne activitй doit кtre prise en compte dans toute analyse de sыretй. La plus grande partie du fer prйsent dans un dйpфt pour dйchets de moyenne activitй se trouve dans un environnement contenant du ciment. Selon la litйrature, les vitesses de formation d' hydrogиne lors de la corrosion du fer et de l'acier а des pH йlevйs, en particulier en prйsence de ciment, sont de l'ordre de 22 а 220 mmol H2 /(m2 *a). Certains auteurs mentionnent des vitesses de corrosion encore plus basses. Jusqu'а prйsent toutefois, l'intйrкt а йtudier des vitesses de corrosion aussi basses йtait pratiquement nul, vu que dans les applications techniques normales elles ne jouent aucun rфle. Les analyses de sыretй montrent cependant que la diffusion dans les dйpфts pour dйchets moyennement et faiblement actifs est insuffisante pour йliminer l'hydrogиne produit selon une cinйtique comme celle citйe ci-dessus. Ceci peut avoir des consйquences diverses. D'un cфtй, des poches de gaz dйplaзant l'eau de la surface mйtallique peuvent se former, d'un autre cфtй, des pression d'йquilibre de gaz de l'ordre de 500 bar peuvent conduire а une destruction mйcanique du rocher, voir mкme du dйpфt final. Voilа pourquoi la dйtermination exacte de vitesses de corrosion faibles est, dans le prйsent cas, trиs importante. Dans la prйsente йtude on a utilisй une mйthode volumйtrique pour dйterminer la cinйtique d'йvolution de l'hydrogиne lors de la corrosion du fer en mileu neutre resp. alcalin sur une pйriode de plus de 6000 heures. La sensibilitй de la mйthode est suffisante pour dйterminer des vitesses de corrosion du fer dans le ciment plus faibles que celles gйnйralement admises jusqu'а prйsent. Les йtudes ont йtй rйalisйes dans des milieux contenant les bases libres NaOH, KOH et Ca (OH) 2 ainsi que dans trois solutions synthйtiques simulant l'eau dans les pores du bйton. Le pH des solutions йtait de 12,5 а 13.
NAGRA NTB 91-21
- VII -
Les vitesses de production d' hydrogиne dйterminйes sont infйrieures de 22 а 200 mmol H2 / (m2 *a) citйes ci-dessus. Toutefois, des diffйrences manifestes ont йtй observйes entre les bases monovalentes (NaOH, KOH) et bivalentes (Ca (OH) 2) . Ainsi, la solution Ca(OH)2 produit pendant toute la durйe d'essai de l' hydrogиne а une vitesse constante de 1 а 2 mmol/(m2 *a), alors que les bases monovalentes montrent un maximum de la production d'hydrogиne de l'ordre de 13 mmol/(m2 *a) aprиs 2000 heures, tombant а 4 mmol/(m2 *a) pour des temps d'exposition de 6000 heures. Une eau de pore conditionnйe ne contenant que du Ca (OH) 2 comme base libre produit en consйquence 0,5 а 1 mmol d 'hydrogиne par m2 et annйe. Une eau de pore " jeune" contenant encore des bases monovalentes NaOH et KOH en quantitйs importantes devrait ein principe se comporter comme les bases libres correspondantes. Cependant, la durйe des essais йtait insuffisante pour vйrifier cette hypothиse. Pour simuler une solution neutre, on a utilisй de l'eau de Bottstein (eau synthйtique en provenance d'un gisement granitique en grande profondeur), de l'eau bidistillйe ainsi qu'une solution а pH 8,5 contenant 8000 ppm de chlorures. En principe ces trois solutions conduisent а des rйsul tats similaires: Aprиs une phase de dйmarrage d'environ 1000 heures, charactйrisйe par une vitesse d'йvolution d'hydrogиne de 80 а 100 mmol/(m2xa); cette derniиre s'abaisse а 10 а 20 mmol/(m2xa) au cours de 6000 h. L'ordre des valeurs dйtйrminйes dans la phase initiale correspond а celui trouvй dans une йtude antйrieure (Schenk 1988); toutefois comme des essais de corrosion sur des durйes de plusieurs milliers d'heures sont rares resp. inexistants, une comparaison avec des rйsultats publiйs dans la litйrature est difficile. Mots clef: Corrosion, formation d'hydrogиne, eau de pore de ciment, corrosion anaerobique
NAGRA NTB 91-21
-1 -
1.
INTRODUCTION
Gas generation from the waste itself and other components of the near-field of a repository is an important issue in the design and safety analysis of deep repositories (Zuidema & Hoglund 1988; Neretnieks 1985), in particular for the case of low/intermediate level wastes: it has been shown (Zuidema et ale 1989 and references therein) that in this type of waste the corrosion of metals, particularly of iron and steel (e.g. waste drums and rebars), will be the dominant gas generation mechanism even for general corrosion rates between 1 and 0.1 ~m/a; the gas generation rate in this case will be such that the other near field components (backfill, cavern lining) have to be specially designed to ensure that this gas can leave the near-field before an appreciable gas pressure builds up (Iriya et ale 1990) .
In current projects in Switzerland for instance one requirement is that the intrinsic permeability of the cementi tious backfill material will be at least 10 -15 m2 (considerably higher than that of ordinary concrete). In order to be able to optimize the backfill material composition, and for safety analysis purposes, it is essential to be able to quantify as accurately as possible the hydrogen evolution from corrosion processes in the reposit.ory environment as well as understand the mechanisms involved in order to be able to extrapolate short-term experimental results to repository time scales.
2.
OBJECTIVES
The purpose of this work was on the one hand to provide a brief overview of the already existing results concerning corrosion in alkaline media, and on the other hand by using suitable methodology to determine as accurately as possible the corrosion rates of iron in a medium corresponding to that in an ultimate repository for wastes of low and medium-level radioactivity.
NAGRA NTB 91-21
-2 -
3.
CORROSION MECHANISM OF IRON AND STEEL IN CEMENT PORE
WATER AND SURVEY OF AVAILABLE EXPERIMENTAL RESULTS
Iron and steel are in the passive state in alkaline solutions such as cement pore water, even under fully anoxic conditions (Grauer 1988). In the passive condition, iron is covered by a non-porous oxide film only a few nanometers thick. The corrosion rate is thus practically independent of the electrochemical potential and is governed by the dissolution rate of the passive layer in the surrounding medium. The passive layer is an electron conductor which means that cathodic partial reactions (e.g. hydrogen evolution, or oxygen reduction if oxygen is present) can occur with the surface in dynamic equilibrium.
Danish authors assume that steel in cement under anoxic conditions is corroding in the active state, i.e. there is no passive layer (Hansson 1984; Preece 1982). They did however conclude that the corrosion current density was of the same order of magnitude as that for the passive state. This is not in agreement with the German school who maintain that steel in alkaline oxygen-free conditions is in a stable passive state (Grubitsch et al. 1970; Heusler et al. 1958) .
On first impressions, this contradiction may appear to be purely academic, as the relevant parameter for the repository, the corrosion rate, is predicted to be the same for both mechanisms. Despite this, the argument does have a practical relevance. If the steel was corroding in the active state, then the corrosion rate would be expected to be dependent upon the material, since the exchange current density for the H2/H20 reaction is strongly affected by impurities. If hydrogen evolves on a passive surface then the effect of steel composition would only be slight.
Stable passivity in anoxic conditions in the pH range 12.5 to 13 is supported by numerous observations. The first point to note is that there is no significant difference between pure iron and technical steels. Further evidence for stable passivity is the form of the polarization diagrams (Grubitsch et al. 1970; Heusler et al. 1958; Kaesche 1965) which indicate no change in mechanism; an activepassive transition is only observed at significantly higher pH levels in sodium hydroxide.
NAGRA NTB 91-21
-3-
There is a comprehensive body of literature on the corrosion of steel in concrete, but interest tends to be focussed upon carbonated concrete and the effect of chloride (Crane 1983). For engineering purposes, the corrosion rate in alkaline, chloridefree concrete of the order of a few ~m·a-1 is low enough to be neglected, and there is, therefore, little interest in determining it accurately. Low corrosion rates are most readily measured by electrochemical methods, but these are not straightforward on passive systems. It should be noticed that at a typical current density of 0.1 ~A·cm-2 (equivalent to a corrosion rate of ca. 1.2 ~m·a-1) only a very small amount of material reacts in normal experimental time scales. Per hour only 0.14 nm of iron corrode, equivalent to only about half an unit cell! It is clear that a constant passive layer thickness and corrosion rate will only be reached in days rather than hours. Since corrosion studies in anoxic alkaline solutions are few, values obtained in the presence of oxygen have also been evaluated. This is justifiable for steel in the passive state as the current density on passive steel has been shown to be independent of potential (e.g. Heusler et al. 1958) . Heusler et al. (1958) report a steady state current density for passive iron in a borate buffer at pH 9.3 of 7 ~A·cm-2, and the same value for O.ln NaOH at 50°C. Kaesche (1965) and Grubisch et al. (1970) produced stationary polarization curves for pure iron and steel in anoxic saturated Ca(OH)2 1 using holding times of 5 and 16.7 h respectively. The corrosion current densities were found to be < 0.2 ~A·cm-2 and 0.05-0.1 ~A·cm-2. Active-passive transitions were not observed. Iron would appear to be in the stable passive state at the corrosion potential. No difference was found between the behaviour of pure iron and technical steels. A Spanish group used the polarization resistance technique to determine the corrosion rate for steel in cement (Gonzales et.al. 1980). Oxygen was present. The measured values for non-carbonated cement without additives were in the range of 0.05-0.1 ~A·cm-2. Hansson (1981) also used the polarization resistance technique and obtained a current density after 10 months of 0.1 - 0.3 ~A·cm-2 for specimen under anoxic conditions. Extrapolation of stationary polarization
NAGRA NTB 91-21
-4 -
curves yielded a current density of 0.1 ~A·cm-2. Preece et ale (1981) reported a corrosion current density of about 0.01 ~A·cm-2 using the stationary polarization technique on specimen embedded in cement under anoxic conditions.
The measured corrosion current densities for steel in cement are in the range 0.01-0.1 ~A·cm2, whereby no
significant differences were found between the values in
aerated and oxygen free environments. This is equivalent
to a hydrogen evolution rate of 22
220 nunolj (m2 *a) .
However, from the evidence available it is not safe to
assume that the lower rate is correct for iron and steel in
repositories even in the longterm. Nevertheless, as corrosion current densities around 10 ~A·cm-2 were measured
independently on several occasions, further effort to
reduce this uncertainty appears worthwhile. Electrochemical
methods are not well suited for the longterm measurements
required, whereas direct measurement of the hydrogen
evolved becomes more accurate with increasing sensitivity
as the observation time increases.
4.
EXPERIMENTAL
4.1 Measuring technique and apparatus The corrosion rates of iron were determined by direct measurement of the evolving hydrogen. Whereas in earlier experiments a gas chromatography method had been used (Schenk 1988), in this work a volumetric method was employed. This method was described for the first time by Schikorr in 1929. Here the evolving hydrogen is determined in a closed glass cell containing an iron specimen and a corresponding corrosion medium, by measuring the build-up of gas pressure. This method allows very long measuring times, because even the tiniest escape of hydrogen is virtually ruled out. The glass cell used is shown in Fig. 1. It consists of the actual corrosion chamber on the right and a U-tube serving to measure the pressure rise.
NAGRA NTB 91-21
-5-
The volume of the left hand chamber (5) is about 27 ml and that of the right hand corrosion chamber (1) is 85 - 90 mI. The U-tube has an internal diametre of 3mm. The volumes are determined for each individual cell. 50 60 ml of test solution and several metres in total in cut lengths of wire to give a surface area of about 0.08 m2 are placed in the right hand chamber (1). Mercury is then introduced into the U-bend section (3) and some water above the mercury on the left hand side. The cells are evacuated for 2 hours to degas the solution before the two arms are sealed under vacuum. The maximum usable mercury column is about 190 mm. In the initial evacuated state the pressure on both sides of the mercury is equal to the vapour pressure of water. Any hydrogen evolved will be detected by a ,rise in the pressure on the right hand side. The cells are held at room temperature which over the long measurement period was 21 ± 3 °e. The presence of water on both sides of the mercury column provides automatic correction for variation in vapour pressure with temperature. Corrections were made for the thermal expansion of the gas in the corrosion chamber and the increase in volume in this chamber due to depression of the mercury column. No corrections were made for thermal expansion of the solution, mercury or glass, changes in the gas SOlubility due to temperature and vapour pressure differences between water and the test solutions. These errors are below 1% and were thus neglected. The mercury column could be read to ±0.5 mm equivalent to ±0.007 mmol(H2 ) .m- 2 for 0.08 m2 iron surface in the cell.
NAGRA NTB 91-21
-6-
5 4< 3 1 Test solution, iron wire 2 Evacuated gas room accumulates hydrogen 3 Mercury column 4 Water 5 Evacuated space Fig. 1: Corrosion test cell 4.2 Experimental program 4.2.1 Media The test media employed are indicated in Tables 1, 2 and 3.
NAGRA NTB 91-21
- 7-
Table 1: Test media
Series 1 Series 2
A) Boettstein water (Table 2)
X
X*
B) 8000 ppm Cl pH 8.5 (Table 2)
X
X*
C) Bidistilled water
X
D) 0.1 N NaOH
pH 13
X
E) Ca(OH)2 saturated pH 12.8
X
F) NaOH
pH 12.8
X
G) KOH
pH 12.8
X
H) Cement pore water Ia pH 13.2
X
I) Cement pore water Ib pH 12.9
X
X
J) Cement pore water II pH 12.5
X
X
* with these media, two different types of iron were tested' (cf. 4.2.2)
Table 2: chemical composition of Boettstein water (a) and 8000 ppm chloride solution, pH 8.5 (b) (in ~g/g).
Na K
Mg Ca
Cl
F S04
(a) Boett.
4800 54
3
1100 8100 3.8 1820
(b) Chloride 5188 - -
--
--
8000
-- --
- - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - ----- - --- - - - -- - - - - - - - - - - - -
The pH of this solution was adjusted to 8.5 by a buffer
system in contact with air (log pC02 = - 3 .5) 0.1% Na2C03 : 0.1% NaHC03 1 : 100
Table 3: Chemical composition of the synthetic cement pore waters.
mmol/l
KOH NaOH Na2S04 CaS04 Ca(OH)2 CaC03Mg(OH)2
Pore water Ia 180 114
3
sat. sat. sat.
Pore water Ib 60 34
3
sat. sat. sat.
Pore water II 3
3
sat. sat. sat.
The three synthetic cement pore waters correspond to different aging stages with the free alkalinity reduced by the diluting effect of Groundwater Flow. The chemical composition of the pore waters are based on model calculations carried out by Berner (1987).
NAGRA NTB 91-21
- 8-
4.2.2 Specimen material The majority of the tests were performed on lengths of pure iron wire of 0.57 rom in diameter, with a carbon content of 0.91% and at least 99% iron. To enable the influence of the purity of the specimen material on the hydrogen evolution to be assessed, in test series 2 (Boettstein water and solution with 8000 ppm CI-) a wire grade with 0.36% carbon was tested additionally (marked * in Table 1). This wire had a diameter of 1 rom. The wires were cut into lengths of 80 rom and degreased with Chlorothene and alcohol, pickled in 10% Hel and rinsed thoroughly in water and alcohol, before being dried under vacuum and put into the cell.
4.2.3 Tests perfor.med
The tests for measuring the hydrogen production by corroding iron under anaerobic Gonditions and alkaline pH were divided into two series (cf. Table 1). Each test consisted of three individual measurments.
In a first series measurments were performed in 0.1 N sodium hydroxide solution at pH 13 and two synthetic cement pore waters with pH between 12.5 and 13 (Table 1, solutions D, I and J). It was to be investigated whether the corrosion depends solely on the pH, or whether the cations and anions present in the cement pore waters (K+,' Ca+, S042+) exercise a decisive influence on the corrosion behaviour of the iron.
Besides these alkaline media, neutral media were tested as well, i. e. :
- Boettstein water (synthetic depth water)
(A)
- a solution with 8000 mg Cl-jl buffered with carbonate (B)
- bidistilled water
(C)
These media provided a comparison between the results obtained by this method and those arrived at by the gas chromatography measurments performed earlier (Schenk 1988) .
NAGRA NTB 91-21
-9-
Cells with high corrosion rates (neutral media and sodium hydroxide solution) reached their maximum hydrogen capacity before the planned minimum measuring time of 6000 hours. In this first measuring series the measuring time was extended by carefully shaking the glasses before the hydrogen came through the mercury column, in order to transfer the gas into the other vessel under control. The measurements from these cells were then corrected for the gas compression on the left-hand side. Because the different corrosion behaviour of the cement pore water II (J) and the 0.1 N NaOH (D) could not be explained in the first test series, the influence of the cations Na, K and Ca on the corrosion behaviour was investigated in a second series with alkaline media at the same pH. The measurements with the synthetic cement pore waters were repeated and supplemented by another strongly alkaline medium. In this phase the tests with neutral media were confined to the Boettstein water (A) and the 8000 ppm CI/I solution at pH 8.5 (B). The influence of the test material on the hydrogen evolution was to be examined in this series, i.e. iron specimens with two different carbon contents were tested. In this second test series the measuring time of the corrosion cell with high hydrogen evolution rates was extended by an other procedure: after reaching its maximum hydrogen pressure the capillary of the reaction chamber was broken under vacuum and the cell evacuated anew. Through this procedure the hydrogen pressure alternated several times between 0 and 0.25 bar during the measuring phase. In this way an attempt was made to estimate the influence of the hydrogen partial pressure on the reaction. Paralleling these two long-time series, a few blank cells were employed in a third series aimed at establishing the sensitivity of the methodology. In particular it was to be ascertained that no parasitic reactions in the test cells were falsifying the measured hydrogen formation.
NAGRA NTB 91-21
- 10 -
No iron specimens were put into the test cell for these blank tests, but the hydrogen was filled straight into chamber (2) in Fig. 1 in gaseous form to simulate a cell at its maximum hydrogen capacity. The hydrogen partial pressure was adjusted to about 180 rom Hg. In a first phase only three kinds of blank cells were tested: 1. filled with bidistilled water 2. filled with hydrogen on the corrosion side (chamber 2) but without water (neither in chamber 2 nor rising tube 4) 3. analogous to cell 2 but with non-degassed water on both sides of the mercury column. The first kind of blank cell showed no change in the mercury column between 100 and 13 000 hours. The second one also gave a constant pressure reading right from the start. The third kind on the other hand revealed a constant loss of H2 in the corrosion part of the cell, necessitating a further investigation with blank cells. Possible reasons of the loss established were: a) Hydrogen dissolves partially in water or mercury b) Hydrogen diffuses through (moist) mercury c) Hydrogen is adsorbed on the cell walls d) Hydrogen reacts with contaminants present in the mercury (such as oxides) e) Hydrogen reacts with the residual dissolved oxygen present in the water. To provide clarity on these points the following tests were made: 4. As with cell 3, but using mercury of high purity. This was to reveal any reaction of the hydrogen with impurities in the mercury. 5. As cell 3 , but wi th large amount of mercury in the corrosion chamber (1). This was to reveal any influence of the surface mercury/water like reaction on the surface or diffusion hydrogen into mercury.
NAGRA NTB 91-21
- 11 -
6. As cell 3, but with some air left in the hydrogen atmosphere. This was to reveal any reaction of oxygen with hydrogen or mercury. 7. As cell 3, but without water in the left-hand chamber (rising tube 4). This was to reveal any diffusion of hydrogen through moist mercury. In all blank tests where water was present, attention was given to complete degassing of the bidistilled water (except test 6).
4.3 Calculations The calculation of the hydrogen generated is performed with the help of the ideal gas law. As mentioned above, the values were corrected for the thermal expansion and volume increase of the gas. Regardless of the pressure the solubility of the hydrogen (Perry 1954) in the test solution amounted to 5% of the quantity produced, and was not taken into account in the calculation.
4.3.1 Calculating the hydrogen production
In a first step the height of the mercury column (+ water column) must be converted into the pressure units (pascals) :
(PmmHg * 101320)/760 = PPascal
(1 )
In a second step the volume increase of the gas chamber due to the falling mercury column is calculated:
v Vo + [(mmHg/2)*(dK/2)2*n]
(2)
V Vo mmHg dK
volume initial volume mercury column capillary diameter
[m3 ] [m3 ] [m] [m]
NAGRA NTB 91-21
- 12 -
After this the mole number of the hydrogen produced is obtained directly:
(p*V) /R*T n
(3)
R= ideal gas constant p~ pressure [pascal] V= gas volume [m3 ] T= temperature [Kelvin] n= mole number [mol]
To allow a comparison of the resul ts, the mole number is divided by the surface of the iron specimen, giving the hydrogen production in mOl/m2 .
n/A mol/m2
(4 )
A = surface area of iron specimen [m2 ]
4.3.2 Calculating the corrosion rates in nanometres
From formula (4) the mole number of the hydrogen generation per square metre is already known. Assuming the iron is oxidized only to Fe 2 + (in accordance with formula A)
--->
(A)
--->
(B)
this corresponds at the same time to the mole number of the corroded iron per m2 . A simple conversion then gives the corrosion rate in nm.
nm
(5 )
n mole number
MM mole mass
A
surface
d density
(Fe = H2 ) Fe (= O.05585kg) Fe [m2 ] Fe [7855kg/m3 ]
General Conversion factors:
formula A: Immol H2/m2 formula B: Immol H2/m2 Immol H2/m2
7.1 nm Fe corrosion *) 5.3 nm Fe corrosion 22.4 ml H2/m2 (lbari 273K)
*) used in this paper as conversion factor
NAGRA NTB 91-21
- 13 -
4.3.3 Calculating the hydrogen production after equalization of the mercury column As mentioned above, some of the test cells reached the maximum measurable pressure before the end of the planned test time. On these cells, in the first test series part of the gas was transferred under control into the top lefthand vessel (Fig. 2). To enable the pressure behaviour in these cells to be determined, the readings must be corrected with the following calculations. The pressure in the corrosion chamber (PA) is given then by the height of the mercury column and the gas pressure on the (PB) in these cases.
(--.J Vacuum
LIP Fig. 2
~m Medium Water
·
Mercury
We have:
PB + pi
i.e.
+ pi
(6 )
The sum nA + nB = nTot is known. Likewise pi can be deter- mined (and converted into pascals)
+ pi
(7)
NAGRA NTB 91-21
- 14 -
(8 ) (9 ) From nA and nB the gas pressure (PB) may be calculated: (10 ) If these values are added in formula (1), the hydrogen evolution can be calculated as before with the formulas 2 and 3.
5.
EXPERIMENTAL RESULTS
The results of the hydrogen evolution experiments are presented as both the total hydrogen evolved and the hydrogen evolution rate. The evolution rate was derived from the total hydrogen curve. The corrosion values are provided as a guide and were calculated from the hydrogen evolution values. All curves plotted are mean values from three individual determinations.
The reproduceability between the separate parallel tests is shown in Fig. 3, 4, 5. In these figures, curves of each individuel parallel tests are plotted.
As one can see the main differences are varying corrosion at the beginning of the tests (Fig. 4) and different incubation time (Fig. 5). It is assumed that this behaviour is caused by slight variation of the remaining oxygen content in the test solution and by the preparation of the test material. But as the measuring time increases these starting effects cease and the values for the hydrogen evolution rates reach an average reproduceability of ± 10%.
NAGRA NTB 91-21
- 15 -
mmol/m2
mmol/(m2*a)
6~----------------~--~16
4
3
2
.,
........·· ........-13
+Tot. hydrogen evolution · Hydrogen evolution rate o~~--------------------~o
o
23468 7
Time [1000h) .
mmol/m2
mmoV(m2*a)
26.-------------------~--~76
Media: Boettatein water
+ Tot. hydrogen evolution
20 ..~ .. ~~.~..,.. ~I.lJ.~~~...~~~......
80
16
46
16
o ., ~~~--~--~~--~--~--~o
2
3
4
6
Time (1000h]
Fig. 3: Examples of total hydrogen evolution and hydrogen evolution rates with good reproducibility between individual determination.
Fig. 4: Total hydrogen evolution and evolution rate of Ca(OH)2· Different behaviour in starting phase of test.
mmol/m2
mmol/(m2*a)
... M.~.I~:. q~(9.H)~ . pH.J~~~... + Tot. hydrogen evolutio Hydrogen evolution r ·
0.8 . i ......·........ ·· ...... ·.... ·
0.8 .
3,8
0,4
2.4
0,2
1.2
O~--~~--------------~O
2
3
466
7
Time [1000h)
NAGRA NTB 91-21
- 16 -
mmol/m2 1,5.----------------,
Media: Cement pore water la
Tot hydrogen e\'OIutlon 1,2
~ ........··.···.·... ····
O,tf-..............·........·..................·" ........·
/ o,e 1- ............................. ,',." .. ,................... ................. ,/t. ,..· f · . /
0.8 .
mmol/(m2*a) 5~----------------------~
o
2
3
4
(f
e
7
o
234 5 e
Time (1000)
Time (1000)
Fig. 5: Total hydrogen evolution and evolution rate of cement pore water la. Different incubation time between individual cells.
5.1 Tests with blank cells
ommol/m2
nmO
-0,05
-0,4
-0,1
-0,'6
\
-0,8
~~:C~I~J/ -1,2 -0,2 -1,6
-0,25
, -0,3 0
-2 23 4 6 6 7
Time (100Oh)
Fig. 6 depicts the hydrogen evolution of a blank cell (type 3) containing bidistilled water and hydrogen in its corrosion chamber. Within 1000 hours the offtake is 4 ~molf equivalent to a hydrogen consumption of 0.05 mmol/m2 or a negative hydrogen evolution rate of 0.43 mmol/(m2 *a) referred to the standard surface of 0.08 m2 . After 5000 hours the loss becomes stabilized.
Fig. 6: "Hydrogen evolution" in a blank cell with nondegassed water (type 3) .
NAGRA NTB 91-21
- 17 -
Fig. 7 depicts the results of blank cell types 4 to 7, also containing water and hydrogen in their corrosion chambers. Special interest attaches to the combinations
4) hydrogen/high-purity mercury reaction 5) hydrogen/mercury reaction 6) hydrogen/oxygen reaction 7) diffusion of hydrogen through moist Hg
As will be shown, only the cell still containing oxygen residues reveals a constant and significant loss of oxygen. All other cells still show a constant mercury column after some days.
On the strength of these results it may be concluded that any disturbances, such as diffusion of hydrogen through mercury, adsorption of hydrogen on cell walls, dissolution of hydrogen in water or mercury, affect the measuring accuracy of the method only to a subordinate degreee after a starting phase lasting a few days. On the other hand, great attention must be given to complete degassing. Apparently blank cell 3 still contained 5 ppm 02' which was completely transformed only after 5000 hours.
This effect may have minor importance at cells which contain iron specimen. The oxygen present there is consumed rapidly by the corrosion of iron. But there is the possibility that the incubation time is influenced by oxygen.
o,osmmollm2
nm O,3S
2 mmol/(m2*a)
nm/a 14
o~.- - - - - - - - - 1 0 1
10,5 7
-0,1
~~
-0,36 -0,7
3,6 0 O~----+-A-------I ~/-3.5
.Ceillyp3\ ..o~15
-1,05
x Celltyp 6
\ __
2 1 , 4 ~ O~ L--_ _- - ' - - _ - - - ' -_ _-'----' .... o123
Time (1000h]
-1
-7
· Celltyp 3
X Celltyp 6 -10,5
-2~--~---~----~-14
o
1
2
3
Time [1000h]
Fig. 7a: "Hydrogen evolution" of different blank cells containing small amounts of oxygen.
NAGRA NTB 91-21
- 18 -
mmol/m2
nm
2 mmol/(m2*a)
nm/a 14
0,04
0,28
1,5
10,6
0,02
1 0,14
7
3,5
0
0
-0,02 -0,04
*Celltyp 4 oCelityp 6 <> Celltyp 7
-0,14 -0,28
0
1
2
3
Time (1000h]
-0,5 -1 -1,5 -2 0
0
*' Celltyp 4 o CeUtyp 6 <> Celltyp 7
-3,5 -7 -10,5
1
2
Time [1000h]
-14 3
Fig. 7b: "Hydrogen evolution " of blank cells without traces of oxygen
From the evaluation of all blank tests it can thus be assu- med that under the Boundary Conditions adopted here the sensitivity of the method is about 0.4 mmol/(m2 *a) when the values are derived from short time hydrogen evolution measurments (cf. Fig 7b). At longer test periods (cf. Fig. 8) and larger derivation interval, the sensitivity increases to about 0.1 mmol/(m2 *a}.
5.2
Tests with alkaline media
As expected the corrosion rates of iron in alkaline media are very low and well below the 22 - 220 mmol/(m2 *a) postu- lated in the literature. All the same, with similar pH of the solutions but different chemical composition, very dif- ferent corrosion behaviours of the media result.
In Fig. 8 the results of the first series of long-time tests in alkaline media are depicted. With O.lN NaOH pH 13, in the first 3000 hours a hydrogen evolution rate of 10 mmol/ (m2 *a) resul ts, which subsequently declines steeply, however. On the other hand the iron in cement pore water II at pH 12.5 corrodes during 12000 hours at a virtually constant hydrogen evolution rate of 0.9 mmol/(m2 *a). Throughout the entire duration of the test (of series 1 and 2), cement pore water Ib with a pH of 12.9
NAGRA NTB 91-21
- 19 -
generated no measurable quantity of hydrogen. With all media, the single iron wires did not stick together, so the specimen surface was constant during the whole period.
smmol H2/m2
nmas
4
28
+ NaOH pH 13
3
* Pore water II pH 12.6 21 o Pore water Ib pH 12.9
2
14
1
7
0
0
0 2 4 6 8 10 12 14
Time [1000h]
mrnol/(m2*a) 16
nm/a 112
12
+ NaOH pH 13
84
* Pore water II pH 12.6
o Pore water Ib pH 12.
8
56
4
28
0 0 2 4 6 8 10 12 14 Time [1000h]
Fig. 8: Results of the first test series with alkaline media. Fig. 9 shows the effects of the three main components of the cement pore water NaOH, KOH and Ca (OH) 2 at pH 12.8 established in the second measuring series. The results of the monovalent bases NaOH and KOH agree very closely with the sodium hydroxide solution of the first test. On the other hand the bivalent base Ca(OH)2 gives a distinctly lower, constant evolution rate similar to cement water II. The results of the three synthetic cement pore waters from series 2 are depicted in Fig. 10. The behaviour of pore waters Ib and II is analogous to that in test series 1. However the pore water Ia used as test medium in test series 2, equivalent to a-new cement pore water, gives a qualitatively similar picture to the media based on NaOH (D) and KOH (G) in Fig. 9. After a starting phase during which no gas generation was measured, the corrosion rises suddenly above that of cement pore water II.
NAGRA NTB 91-21
- 20 -
5mmol H2/m2 + NaOH pH 12.8 * KOH pH 12.8 4 a Ca(OH)2 pH 12.8 3 2
nm35 / 28 21 14
14 mmol/(m2*a) 12 10
nm/a + NaOH pH 12.8 * KOH pH 12.8 84 a Ca(OH)2 12.8
8
56
6
4
28
1
7
oo
1
2
3
4
0 567
Time [1000h]
~~--~------------~o -2~~--~~--~~--~~ o 1 234 567 Time [1000h)
Fig. 9: Comparison of the three main components of cement pore water: NaOH, KOH, Ca(OH)2'
1,2m,-m-o-l -H-2/-m-2----------n-m,
. Pore water II pH 12.6
8
1 0 Pore water la pH 13.2
x Pore water Ib pH 12.9
0,8
6
5 mmol/(m2*a) Por. water II 4 o Pore water la x Pore water Ib 3
nm/a 35 30 25 20
0,6
4
2
15
10
0,4
1
2
5
0,2
0
0
O~~~~~~~~~O o123 4 5 6 7 Time (1000h)
-5 -1~~--~~--~~~~~ o 1 23456 7 Time [1000h}
Fig. 10: Comparison of different types of cement pore water.
On the strength of the results obtained, it can be assumed that the corrosion rate of iron in alkaline media depends mainly on the components NaOH, KOH and Ca(OH)2'
NAGRA NTB 91-21
- 21 -
Even at higher pH values the monovalent bases NaOH and KOH cause a relatively large evolution of hydrogen after a certain incubation period, which however falls off again after a certain time, as may be seen in Figs. 8 and 9. On the other hand the Ca(OH)2 solution has no incubation period, with a practically constant but low corrosion rate over 6000 hours. Apparently the Ca(OH)2 is involved in the formation of a passive layer which reduces the evolution of hydrogen, whereas this does not happen with the monovalent bases. The behaviour of the synthetic cement pore waters follows a basic pattern: With cement pore water II, which as free base contains above all Ca(OH)2' a low and constant production of hydrogen is measured throughout the measuring period. The analogy between the monovalent bases and cement pore waters Ia and Ib is less clear. Nevertheless one might imagine that both media behave accordingly to the pattern of NaOH, though the inactive starting phase is several times longer. Under this assumption therefore, an onset of hydrogen evolution corresponding to that of NaOH should be expected with medium Ib as well as Ia.
5.3
Tests with neutral media
The corrosion rates arrived at experimentally in the first long-time series with the media Boettstein water, 8000 ppm Cl- at pH 8.5 and bidistelled water, are depicted in Fig. 8.
The qualitative behaviour of all three media is roughly similar. In a starting phase lasting about 1000 hours, corrosion rates of 70 - 100 mmol/(m2 *a) occur. After this the hydrogen evolution drops continually, reaching a constant minimum of about 5 9 mmol/ (m2 *a) after about 4000 hours.
In previous gas chromatography measurements carried out on iron specimen containing a.1%C, a corrosion rate of 180 mmol/(m2 *a) was found for Boettstein water at 25 °C (Schenk 1988). Allowing for the fact that the measuring time was less than 500 hours and the specimens were prepared differently (polished), the values established here for the starting phase agree closely with the gas chromatography measurements.
NAGRA NTB 91-21
- 22 -
At the end of the test, the specimen was covered by a greyblue layer, but the single iron wires not sticking together. So the specimen surface was constant throughout the test period. However the pH values of the different neutral media increased during the measuring period. The finally obtained values were:
Solution 8000ppm Cl-, pH 8.5 Boettstein water Bidistilled water
pH 10.1 pH 8.7 pH 8.6
So the decrease of the hydrogen evolution rate can not only be explained by a build up of a covering layer, but also by a slightly increasing pH value.
mmolH2/m2
nm
16~----------------~112
14
12
84
10
8
56
6
4
. 8000ppm CI- pH8.6 28
(> 8oett. water
2
°Bidiat. water
° ° 0 123 4 56 78 Time [1000h]
mmol/(m2*a)
",m/a
100
8000ppm CI- pH8.
(> eoeU. water
°Bidiat. water
0,7
10
0,07
°1 '-----__-'--__-'--_--'--_---l 0,007 246 8 Time [1000h)
Fig. 11: Comparison of corrosion rates in three neutral media. The infltience of the purity of the iron specimens is depicted in Fig. 12. It will be seen that both in Boettstein water and in the NaCl solution the ~pecimen with higher carbon content (0.91 %) corrodes faster in the starting phase. On the other hand after 5000 hours these specimens tend to generate less
NAGRA NTB 91-21
- 23 -
hydrogen than the purer iron specimen. Apparently, owing to the higher corrosion rate on the specimens with higher carbon content a more compact covering layer forms, inhibiting the hydrogen evolution more after a few thousand hours. Nevertheless the generally higher values for the hydrogen evolution (about factor 2,cf. Figs.11 and 13) compared with series 1 are conspicious. As mentioned before, in series 2 the measuring time of a corrosion cell was extended by exhausting the hydrogen generated upon reaching the maximum capacity. In this way the hydrogen pressure alternated several times between 0 and 0.25 bar. The disturbances possibly triggered by this are recognizable in Fig. 12 as irregularities in the curves for the hydrogen evolution rates. Accordingly there are two possible explanations for the generally higher corrosion rates in series 2: - the system is disturbed briefly whilst exhausting the hydrogen, possibly damaging slightly the covering layer of the iron surface, - the hydrogen generated inhibits the reaction already at relatively modest pressures of 0.25 bar.
25 mmol H2/m2 20 16 10
nm 176 140 106 70
mmol/(m2*a)
Jjm/a
Media: Boett.water Specime 0.36'" carbon o Specime 0.914Xt carbon
100
0,7
6
Media: 9oett.water
36
. SpecIme: 0.36% carbon
o Specime: 0.91'" Carbon
oo
1
234
0 667
Time [1000h]
10 0 , 0 7 "----L_.....l-_.L-.---L_--L-----l o12 3 4 66 Time [1000h]
Fig. 12a: Corrosion behaviour of iron specimens with different carbon contents in Boettstein water.
NAGRA NTB 91-21
- 24 -
25mmol H2/m2 Media: 8000ppm CI- pH 8.6 o Specime: 0.38" carbon 20 x Specime: 0.9'Wo carbon
nm 175 140
15
105
70
5
35
'-~--~~--~~~~~O 123 4 567 Time [1000h1
mmol/(m2*a)
101m! a
Media: 8000ppm CI- pH 8.6 OSpecime: 0.38% carbon XSpecime: 0.9ft. carbon
10o
1
234
5
0,07 6
Time [1000h1
Fig.12b: Corrosion behaviour of iron specimen with different carbon content in NaCl-solution pH 8.5
20 mmol H2!m2 16 10
nm 150 100
5
50 . eo.tt 1at aerie
o eoett 2nd aerie
O~--~----~----~--~O
o
2
4
6
8
Time [1000h)
mmol/(m2*a)
IJm/a
100
. 90ett 1at aerie o 90ett 2nd aerie 0,7
0,07 1 0,007 '----~-.....I.__~_--J o 2 468 Time [1000h)
Fig. 13a: Comparison between 1st and 2nd measuring series with Boettstein water
NAGRA NTB 91-21
- 25 -
20 mmol H2/m2 15 10
nm 150 100
5
50 ~ 8000ppm 01 1st 88r.
x 8000ppm 01 2nd 88r.
o~--~----~--~----~o 02468 Time (1000h)
mmol/(m2*a)
IJm/a
° ~ 8000ppm 01 1st s·. x 8000ppm 01 2nd aer ,7
0,07 1 0,007 L-_--1-_-..J.._ _-'--_---l o 246 8 Time [1000h)
Fig. 13b: Comparison between 1st and 2nd measuring series with NaCl-solution pH 8.5
NAGRA NTB 91-21
- 26 -
6. DISCUSSION AND CONCLUSIONS The results show that it takes several thousand hours for many of the hydrogen evolution rates to even approach a constant value. This point is important when evaluating values from other work where time scales in the thousand hour range are rarely considered. This is also a reason for the considerably lower hydrogen evolution rates found for iron in Bottstein water in this paper than the 185 mmol/(m2 .a) reported from short term experiments (Schenk 1988). Initial rates reported here are lower at ca. 70 mmol/(m2 .a) but within expected experimental scatter. The corrosion rate should not be expected to decrease with time since the corroding agent is water itself in a reducing environment, and the corrosion is not regulated by solubility effects due to secondary transformation of the corrosion products as the passive film is metastable with respect to an Fe3-x04 phase. However it must be assumed that the hydrogen generated begins to inhibit the corrosion of the iron as the pressure rises. But further measurements would be needed to establish the order of magnitude of a possible pressure influence on the iron oxdation, because no more exact statements can be made on the strength of the results to date. Of prime interest within the scope of this work are the results with the alkaline media, since they represent the conditions in ultimate repositories for low- and mediumlevel active wastes. Surprisingly the corrosion behaviour with similar pH depends very much on the chemical composition of the corrosion medium. With Ca(OH)2 as base, at pH 12.8 there is a constant hydrogen evolution rate of 1 - 2 mmol/ (m2 *a) during the whole measuring period. As the free bases NaOH and KOH in an ultimate repository may be expected to get washed out by the flowing groundwater after a certain time, and the pH is controlled mainly by the Ca(OH)2' this value would appear to be of importance in final repositories.
NAGRA NTB 91-21
- 27 -
Accordingly the cement pore water II, equivalent to an aged
water virtually free of NaOH and KOH, generates hydrogen at
a constant rate of 0.5
1 mmol/ (m2 *a) throughout the
measuring time of 12 000 hours.
However it must not be overlooked that with the monovalent bases NaOH and KOH, likewise at pH 12.8, hydrogen evolution rates of 10 mmol/m2xa also were measured during a relatively short time. Nor can the behaviour of new cement pore waters, containing larger amounts of KOH and NaOH, be estimated accurately as yet, because in this case even the measuring time of 6000 hours appears to be too short.
Since the mechanism for the different behaviour of mono- and bivalent bases is unclear, it would be wise to assume conservatively a hydrogen evolution rate of 10 mmol/(m2 *a) in alkaline media.
Generally speaking the analysis method employed allows the measurement of very low hydrogen evolution rates over a period of some thousands of hours. Although the results of blank tests show that in the absence of oxygen the measuring accuracy is retained over several thousand hours, it must be assumed that the method tends to underestimate rather than overestimate the evolution of hydrogen due to diffusion of hydrogen into iron. In view of this, the measured values should be regarded as the minimum corrosion rates to be expected.
NAGRA NTB 91-21
- 28 -
7. REFERENCES BERNER U. (PSI), pers. communication CRANE A.P. (Ed.), Corrosion of Reinforcement in Concrete Construction. Ellis Horwood, Chichester, England, 1983. GONZALES J.A., ALGABA S., ANDRADE C., Brit. Corros. J. 15, 135, (1980). GRAUER R., "The Corrosion Behaviour of Carbon Steel in Portland Cement". Technical Report NTB 88-02E, Nagra, Baden, Switzerland, 1988. GRUBITSCH H., MIKLAUTZ H., HILBERT F., Werkstoffe und Korrosion 21, 485, (1970). HANSSON C.M., Hydrogen Evolution in Anaerobic Concrete Resulting from Corrosion of Steel Reinforcement. SKB Teknisk PM Nr. 30. SKB Stockholm, 1981. HANSSON C.M., Cement and Concrete Res. 14, 574, (1984). HEUSLER K.E., WElL K.G., BONHOEFFER F., Z. phys. Chem. N.F. 15,149, (1958). IRIYA K., JACOBS F., KNECHT B., WITTMANN F. H. , "Cement it i ous Materials for L/ILW Repositories: Investigation of Gas Transport Properties", to be published in 'Extended and Updated Selected Papers from the SMIRT-10 Conference Seminar on Structural Mechanics and material properties in Radioactive Waste Repositories' in Nucl. Eng. & Design, 1990. KAESCHE H., Arch. Eisenhuttenwesen lQ, 911, (1965). NERETNIEKS I., "Some Aspects of the Use of Iron Canisters in Deep Lying Repositories", Technical Report NTB 85 - 35, Nagra, Baden, Switzerland, 1985 PERRY J.H. Chemical engineers handbook, Third edition, MacGraw-Hill Book Company, Inc. ,1950
NAGRA NTB 91-21
- 29 -
PREECE C.M. , "Corrosion of Steel in Concrete". Technical Report KBS TR 82-19. Karnbranslesakerhet, Stockholm, Sweden 1982. PREECE C.M., Arup H., Gronvold F.O., The Influence of Hydrogen and carbon dioxide on the Corrosion of Steel in Anaerobic Waste Storage Silos. KBS Report SFR 81-09. KBS, Stockholm, Sweden, 1981. PREECE C. M., GRONVOLD F. 0 ., FROLUND T., The Influence of Concrete Type on the Electrochemical Behaviour of Steel in Concrete. In ref 1: A.P.Crane, Corrosion of Reinforcement in Concrete Construction, p.393. RODWELL W.R., "Near-Field Gas Migration: A Preliminary Review", Safety Studies Report NSS/R200, Nirex Radioactive waste disposal, Harwell (UK), Nirex Ltd. 1989. SCHENK R., "Untersuchung uber die Wasserstoffbildung durch Eisenkorrosion unter Endlagerbedingungen", Technical report NTB 86-24, Nagra, Baden, Switzerland 1988 SCHIKORR G., Zeitschrift fur Elektrochemie 35, 62, (1929). SIMPSON J.P., SCHENK R., Corrosion Induced Hydrogen Evolution on High Level Waste Overpack Materials in Synthetic Groundwaters and Chloride Solutions. Mat. Res. Soc. Symp. Proc. Vol 127, Eds. W. Lutze, R. C. Ewing, 1989 Materials Research Society. ZUIDEMA P., HOGLUND L.O., "Impact of Production and Release of Gas in a L/ILW Repository: A Summary of the Work Performed in the Nagra Programme", in Proceedings of an NEA Workshop on the Near-Field Assessment of Repositories for Low and Medium Level Radioactive Waste. Baden, Switzerland, 23-25 Nov. 1987, Paris, OECD/NEA, 1988, 97-114. ZUIDEMA P., VAN DORP F., KNECHT B., "Gas Formation and Release in Repositories for Low- and Intermediate Level Waste Repositories: An Issue of Potentially Decisive Importance", published in Proc. Int. Symp. on the Safety Assessment of Radioactive Waste Repositories, Paris, 9-13 Oct. 1989, Paris OECD 1990.

P Kreis

File: hydrogen-evolution-from-corrosion-of-iron-and-steel-in-lowintermediate.pdf
Author: P Kreis
Published: Thu Oct 21 14:18:04 2010
Pages: 38
File size: 1.73 Mb


Without a doubt, 16 pages, 0.09 Mb

Sport and the 1916 Rising, 9 pages, 0.43 Mb

Kurzweil 3000 Version 10, 54 pages, 0.62 Mb
Copyright © 2018 doc.uments.com