b) Clasificación de los productos de apoyo

In terms of electrochemical behaviour, described in section 1.6.1, altering the pH of the electrolyte can affect the current density, the passivation potential and the pitting potential. The most probable natural oxide phases formed on the surface of a stainless steel change with pH and this behaviour can be predicted and understood using Pourbaix diagrams, see section 1.6.2.

Acids tend to form soluble salts when interacting with metals causing increased dissolution of the surface. Fe dissolves rapidly in acidic solutions and if a passive film develops on a steel under such conditions it is enriched with Cr at the outer surface [51]. Indeed, mass loss has been observed in acidic solutions even at potentials where the Cr-rich passive layer is growing due to the selective dissolution of iron at low pH and potential [52].

If the pH is increased to more alkaline conditions there is a significant lowering of the disolution rates of stainless steels. Passive layers grown under alkaline conditions tend to be thicker and more stable [31]. Specifically, the stability of Fe and its oxides at alkaline pHs is greatly increased, resulting in the outer sections of the passive layer becoming enriched with Fe, Fe cations travelling through the passive film at greater rates than Cr cations [52]. As stated in the previous section the addition of OH− ions to the solution is also an effective way of inhibiting

corrosion due to their affinity for the metal surface.

Figure 1.17: Tafel plot illustrating the effect of pH increasing from P1 to P2.

With regards to the voltammetric behaviour of stainless steel as a function of pH, Figure 1.17, both the current density in the passive range and the passivation potential are decreased with increased pH (i.e. towards alkaline conditions). As well as this the onset of transpassivity shifts to higher potentials increasing the potential range over which stainless steel are expected to be passive [52] [25] [53]. From the Nernst equation, the potential range over which the passive layer exists increases by ≈ 60 mV with each unit increase in pH [54]. Thus, the selection of an alkaline pH for the optimum storage of stainless steel clad appears justified and as noted previously AGR fuel pins have been held in storage ponds of pH'11.4 without pin failure for periods as long as 20 years.

1.8.3

Effect of Electrolyte Temperature on the Corrosion

Behaviour of Stainless Steels

For the majority of chemical reactions an increase in the rate of formation of reaction products is seen with increasing temperature. The general influence of

1.8. CORROSION BEHAVIOUR OF STAINLESS STEELS 33 temperature on corrosion processes can be described by three effects: (i) changes in the dissociation of water, (ii) changes in the breakdown potential of the passive film and (iii) alterations in the physical and chemical properties of the passive film. Considering first the effect of temperature on the dissociation of water, Table 1.3 shows the effect of varying temperature on the pH of a solution of pH=11.4 at 24

◦_{C. The dissociation of water increases with temperature and results in consequent}

decrease in solution pH. As discussed in section 1.8.2 as the pH becomes more acidic the passive potential range of stainless steels is reduced.

Temperature /◦C Ionisation constant for water pH

-log10Kw Kw 0 14.9435 1.1389×10−15 _12.34 10 14.5346 2.9201×10−15 _11.93 20 14.1669 6.8093×10−15 _11.57 24 14 1×10−14 _11.4 30 13.833 1.4689×10−14 _11.23 40 13.5348 2.9188×10−14 _10.93 50 13.2617 5.4739×10−14 _10.66 60 13.0171 9.6139×10−14 _10.42 90 12.4 3.9811×10−13 _9.8 100 12.2899 5.13×10−13 9.69

Table 1.3: Effect of temperature on the pH of a solution of pH=11.4 at 24◦C [2].

Secondly, cathodic shifts in the breakdown potential are commonly observed with an increase in temperature [55] [56]. Increasing the temperature to 70◦C can shift the breakdown potential by a approximately 200 mV [2]. It must be noted that this is a greater difference than would be expected solely as a result of the reduction in pH derived from increased water dissociation with increasing temperature (described above) [2]. Thus, the shift in breakdown potential most likely is also due to increased solubility of metal oxides at elevated temperatures. Figure 1.18 demonstrates the expected voltammetric shifts in behaviour of a typical steel under heating in an aqueous solution. As the temperature increases from T1 to T2 there is an increase in current density at all applied potentials, an anodic movement in the corrosion potential and a cathodic shift in the

breakdown potential, i.e. higher steel corrosion rates and a smaller passive window as temperature increases.

Finally, it has also been reported that the physical and chemical properties of the passive layer may be affected by temperature [55]. Thicker oxide layers may form but said layers have increased porosity and thus offer less corrosion resistance [57]. Thus, even under passive conditions general corrosion rates of stainless steel have been seen to increase progressively with temperature. For the temperatures of interest in this thesis, previous work by Langevoort et al. on 304 SS suggests that the passive layer may be slightly more enriched with Cr as temperature increases [58].

Figure 1.18: Tafel plot illustrating the effect of temperature increasing from T1 to T2 [2]. E0 is the standard electrode potential, i0 is the current associated with the

standard electrode potential and Ecorr is the corrosion potential

1.8.4

Effect of Radiolysis Products on the Corrosion