Enchapes para muros

The groundwaters of the sedimentary clay DGR are expected to be highly saline and to contain anions such as HCO3-/CO32- and SO42-. Some anion species, such as HCO3-/CO32-, are known to increase the stability of FeII species while others, such as Cl-, are thought to stabilize FeIII species [36]. Consequently, the products formed on the steel container surface are expected to have a complex dependence on the dissolved [O2], pH, and anion content of the solution.

1.4.3.1.

The Effects of Chloride

Several authors have previously shown that Cl- _{has the ability to accelerate the conversion of} Fe2+ to Fe3+ in the presence of trace O2 [39-49]. Kurimura et al. [43] proposed that this oxidation was catalyzed by chelation and could be facilitated by a ligand bridging mechanism involving Cl- as observed for the oxidation of Cr2+ to Cr3+ [50]. Figure 1-16 shows that in the presence of trace O2, GRs can form as intermediate corrosion products. However, in addition to further oxidation to produce FeOOH species, GR is also metastable with respect to Fe3O4 at pH values greater than 5 (as expected for DGR groundwaters) making Fe3O4 a likely conversion product of GRs via dehydration and oxidation [51]. In the presence of high [Cl-], an excess of Cl- may be incorporated into the GR structure leading to further oxidation of FeII to FeIIIin order for the

structure to remain neutral. This increase in the Cl- to Fe ratio is seen to cause the formation of γ-FeOOH [46, 51-55].

Refait and Genin [46] studied the effect of Cl- to OH- ratios on the distribution of iron corrosion products. They observed that an increase in the [Cl-] of the exposure environment favoured the formation of γ-FeOOH at the expense of Fe3O4. Taylor [56] suggested that this effect was due to the preferential adsorption of Cl- over OH- at the Fe surface which would hinder the formation of Fe3O4. Therefore, it would appear that a competition exists between the formation of Fe3O4 at low [Cl-] and the formation of γ-FeOOH at high [Cl-], equation 1-50.

(1-50)

Furthermore, at extremely high [Cl-_{] the increased Cl}- _{intercalation into a GR structure has been} shown to cause preferential oxidation to β-FeOOH rather than γ-FeOOH [52]. This is not surprising as β-FeOOH is known to only form in Cl- containing environments. The Cl- anion is incorporated into the FeOOH structure which aids in stabilization of the octahedral tunnels, Figure 1-12.

1.4.3.2.

The Effects of Bicarbonate/Carbonate

The presence of HCO3-/CO32- has been shown to accelerate both the anodic and cathodic reactions involved in corrosion. Acceleration of the cathodic reaction is due to the increased availability of H+ from the dissociation of HCO3- while the acceleration of the anodic reaction is due to stabilization of Fe2+ species through complexation to form species such as FeHCO3+, Fe(HCO3)2, and Fe(CO3)22- [57-64]. Additionally, an increase in the HCO3-/CO32- content of the exposure environment is shown to favour CO32- containing corrosion products [59]. Depending on the conditions, siderite (FeCO3) is known to form competitively with chukanovite (Fe2(OH)2CO3) [32, 65-67]. FeCO3 is known to be the major corrosion product formed on steel pipelines exposed to groundwater saturated soils [68]. Fe2(OH)2CO3 has been found as a corrosion product of steel in oxygen-poor clay environments such as those expected within the

DGR [31, 32, 59, 65, 69-75]. Previous authors have shown that high [Fe2+], moderate [HCO3- /CO32-], and slightly alkaline pH will promote the formation of Fe2(OH)2CO3 over FeCO3 [31, 33, 59, 67, 71, 73, 76]. Observation of the Pourbaix diagram for Fe in HCO3-/CO32- containing environments shows that Fe2(OH)2CO3 is favoured in slightly alkaline conditions but that it is metastable with respect to FeCO3, Figure 1-17. Consequently, it is possible that over extended periods of time Fe2(OH)2CO3 may thermodynamically convert to FeCO3. This is consistent with the observation of inner Fe2(OH)2CO3 layers and outer FeCO3 layers found on archaeological artefacts exposed to anoxic carbonated groundwaters [32, 33, 59, 65, 67, 77].

Figure 1-17: Pourbaix diagram of iron in carbonate containing aqueous media at 25°C for equilibria involving Fe2(OH)2CO3 (dotted lines) and FeCO3 (solid lines) [67].

1.4.3.3.

The Effects of Sulphate

Previous studies of the effects of SO42- on the corrosion of carbon steel suggest that SO42- is an aggressive anion towards steel corrosion in anoxic and alkaline environments [59, 78-85]. Zhu et al. [85] suggest that this is due to the loss of protective oxides due to an accelerated dissolution caused by replacement of adsorbed OH- ions by SO42- leading to a situation in which a small anode is coupled to a large cathode. Others have attributed its aggressiveness to an increase in solution conductivity [86] or to the formation of complexes with iron such as those observed in the case of HCO3-/CO32- [79]. Furthermore, several studies have shown that carbon steel is susceptible to localized pitting events in the presence of SO42- [81, 84, 85]. However, these studies were performed in solutions containing HCO3- with only little Cl- which may have changed the behaviour of the steel in comparison to if it had been exposed to SO42- only.

1.5.

THESIS OBJECTIVES

The overall goal of this thesis was to investigate the corrosion behaviour of A516 Gr70 carbon steel in a variety of solutions containing the species anticipated in the groundwater of a sedimentary clay DGR. In particular, the effects of groundwater anions such as Cl-, HCO3-/CO32-, and SO42- on the corrosion behaviour and corrosion product compositions and morphologies were studied. In addition, the effects of trace levels of O2 were studied in order to investigate how corrosion might progress if saturated conditions are achieved before all the available O2 has been consumed. The overall thesis objectives are:

 To determine whether corrosion on the inside of a failed container is influenced by water radiolysis products produced by the radioactive decay processes occurring in the fuel waste form.

 To study the effects of HCO3-/CO32- and SO42- in highly concentrated Cl- solutions on the corrosion behaviour of A516 Gr70 carbon steel under anoxic and near anoxic conditions.  To identify the corrosion products formed on the steel surface after exposure to

solutions with different compositions.

 To develop a long term understanding of the evolution of the corrosion behaviour of A516 Gr70 carbon steel in highly concentrated Cl- solutions under conditions expected in sedimentary clay groundwaters.