Having discussed the general electrochemical mechanisms of corrosion of steel, we now briefly discuss the effect of the reprocessing media, principally nitric acid, on steel corrosion.
Nitric acid undergoes partial thermal decomposition at room temperature [77]:
4𝐻𝑁𝑂 ⇄ 4𝑁𝑂 + 2𝐻 𝑂 + 𝑂 (1.12)
The nitrogen dioxide then disproportionates into nitric acid and nitrous acid [78]:
2𝑁𝑂 + 𝐻 𝑂 ⇄ 𝐻𝑁𝑂 + 𝐻𝑁𝑂 (1.13)
Nitric acid complicates the passive behaviour of stainless steels. Stainless steels may autocatalytically interact with HNO3 (see below) resulting in a variety of nitrogen-oxygen based reduction products, in
particular strongly oxidising conditions, nitrous acid (HNO2). With increasing nitric acid concentration
the reduction rate of the nitrate derived HNO2 (shown in Equations (1.12) and (1.13)), and thus the
oxidising power of the electrolyte also increases. This behaviour accelerates the corrosion rate due to the nitrous acid-driven oxidation of alloying elements, such as Fe and Cr. Consequently Cr, which, as discussed, is important to passive film stability, depletes from the surface [6], [79].
This HNO3 reduction process has been previously studied on Platinum and on 304L stainless steels in
nitric acid condensates [80]. For concentrations of 1 to 10 mol dm-3 HNO
3 two different mechanisms of
nitric acid reduction have been proposed by Vetter and Schmid [81]–[87].
Vetter [81]–[83] describes the autocatalytic reduction of HNO3 as a heterogeneous process, where the
chemical regeneration of NO2 (electroactive species) occurs at the electrode surface. In this case, stirring
has no influence on the current density due to the adsorbed nature of the reactions.
𝐻𝑁𝑂( )+ 𝐻 ⇄ 𝑁𝑂 + 𝐻 𝑂 (1.14)
𝑁𝑂( )+ 𝑁𝑂( )⇄ 𝑁 𝑂( ) (1.15)
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𝑁𝑂( )+ 𝑒 ⇄ 𝑁𝑂( ) (1.17)
𝑁𝑂( )+ 𝐻 ⇄ 𝐻𝑁𝑂( ) (1.18)
Schmid [84]–[87] describes the reduction of HNO3 as a homogenous process, where the chemical
regeneration of NO+ (electroactive species), occurs in the bulk in a layer near the electrode.
HNO + H ⇄ 𝑁𝑂 + H O (1.19)
𝑁𝑂 + 𝑒 ⇄ 𝑁𝑂 (1.20)
2𝑁𝑂 + 𝐻𝑁𝑂 + 𝐻 𝑂 → 3𝐻𝑁𝑂 (1.21)
If Schmid obtains, then stirring of the solution should provoke a decrease in the current density as the accelerating NO+ species is swept away from the electrode surface.
More recently, Balbaud et al. suggested a mechanism that is dependent on the concentration of acid [5]. Balbaud et al. employed parallel thermodynamic studies (to determine reaction potential) and electrochemical experiments (to determine the key reduction step), resulting in a mechanism that has much in common with Schmid and can be described overall as follows:
𝐻𝑁𝑂( )+ 𝐻 + 𝑒 ⇄ 𝑁𝑂 + 𝐻 𝑂 (1.22)
𝐻𝑁𝑂 + 𝑁𝑂( )⇄ 𝐻𝑁𝑂( )+ 𝑁𝑂 ( ) (1.23)
2𝑁𝑂 ( )+ 𝐻 𝑂 ⇄ 𝐻𝑁𝑂 + 𝐻𝑁𝑂 ( ) (1.24)
Where el indicates solution based species in the near electrode solution volume and ads indicates a species adsorbed at the electrode surface.
Side reactions were also identified:
𝑁𝑂( )⇄ 𝑁𝑂 (1.25)
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𝐻𝑁𝑂( )⇄ 𝐻𝑁𝑂 (1.27)
Balbaud et al. indicate that reactions (1.22) to (1.24) are the elementary steps in the reduction process that occur across the HNO3 concentration range. However, they identify two limiting cases depending on
HNO3 concentration.
At low HNO3 concentrations < 6 mol dm3, reaction (1.23) will be slow, leading to an accumulation of
NO at the electrode surface which may eventually evolve via reaction (1.25). However, the accumulated NO may also react with HNO3 in accordance with reaction (1.29) below, leading to the following overall
mechanism for HNO3 concentrations < 6 mol dm3,
𝐻𝑁𝑂( )+ 𝐻 + 𝑒 ⇄ 𝑁𝑂 + 𝐻 𝑂 (1.28)
𝐻𝑁𝑂 + 2𝑁𝑂 + 𝐻 𝑂 ⇄ 3𝐻𝑁𝑂( ) (1.29)
which is essentially a heterogeneous version of Schmid above with the NO being adsorbed at the electrode surface instead of present in solution near the electrode surface.
At HNO3 concentrations > 6 mol dm-3, as a result of increased HNO3 concentration and increased
thermodynamic stability of the intermediates such as NO2 [6] (see below), reaction (1.23) proceeds fast
enough to produce, in concert with reaction (1.24), an autocatalytic cycle for HNO2 reduction and
regeneration. This ultimately leads to enhanced rates of HNO3 reductionon the electrode surface, at HNO3
concentrations > 6 mol dm-3 via:
𝐻𝑁𝑂( )+ 𝐻 + 𝑒 ↔ 𝑁𝑂 + 𝐻 𝑂 (1.22)
𝐻𝑁𝑂 + 𝑁𝑂( )⇄ 𝐻𝑁𝑂( )+ 𝑁𝑂 ( ) (1.23)
2𝑁𝑂 ( )+ 𝐻 𝑂 ⇄ 𝐻𝑁𝑂 + 𝐻𝑁𝑂 ( ) (1.24)
Whilst this mechanism can be considered heterogeneous with regard to the main product of the electrochemical reduction, NO, the following reactions are heavily dependent upon the supply of HNO3
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Thus, the mechanisms proposed by Balbaud et al. can be considered a hybrid, or surface and solution based reactions, as the regeneration step takes place at the electrode surface. The gaseous species NO and NO2 are adsorbed at the surface and HNO2 is a soluble compound that is formed at the electrode that
diffuses into solution [5], [6], [88].
A third mechanism, most recently described by Lange [7] and essentially revisiting some early studies by Carta and Pigford [8] and Abel and Schmid [89]–[91], suggests the following hybrid mechanism (in 8 mol dm3 HNO
3) based on the Schmid mechanism:
𝐻𝑁𝑂( )+ 𝐻 ⇄ 𝑁𝑂 + 𝐻 𝑂 (1.30) 𝑁𝑂( )+ 𝑒 ⇄ 𝑣𝑁𝑂( ) (1.31) 𝑁𝑂( )⇄ 𝑁𝑂 (1.32) 𝐻𝑁𝑂 + 𝐻 + 𝑁𝑂 ⇄ 2𝑁 𝑂 + 𝐻 𝑂 (1.33) 𝑁 𝑂 → 2𝑁𝑂 (1.34) 𝑁𝑂 + 𝑁𝑂 + 𝐻 𝑂 ⇄ 2𝐻𝑁𝑂 (1.35)
Again, the Lange et al. mechanism reactions (1.30) to (1.35) are a hybrid of surface and solution based reactions.
However, a common view amongst Balbaud et al, Schmid and Lange et al is that the common electrochemical reduction steps, be they homogeneous or heterogeneous, can be summarised as follows:
𝐻𝑁𝑂 + 𝐻 ⇄ 𝐻 𝑂 + 𝑁𝑂 (1.36)
𝑁𝑂 + 𝑒 ⇄ 𝑁𝑂 (1.37)
With the reactions for NO, and thus the degree of autocatalysis in operation, being determined by HNO3
concentration. This will be discussed further in Chapters 3 and 4.
Whilst the electrochemical reduction of HNO3 on inert metals, such as Pt, is otherwise well understood,
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reprocessing streams, on nuclear process steel, such as 316L SS, has not been extensively explored, and that will be the subject of work presented later in this thesis.
Corrosion of Austenitic Stainless Steels
Having discussed corrosion from an electrochemical standpoint, it is now necessary to describe the physical processes of corrosion. Different steel types, geometries and weld points will produce different physical corrosion characteristics that may lead to materials failure in a reprocessing stream. Common areas for materials failure occurs at grain boundaries. A grain boundary is the interface between two regions of the same crystal structure but of a different orientation. Grain boundaries are defects within the crystal structure and tend to be the preferred sites of the onset corrosion [92]. Another common area for material failure is where inclusions are present within the steel itself. Inclusions are local heterogeneities present in alloys. They are produced in the chemical reactions and physical processes that occur during the melting, pouring and rolling etc. of alloy metals [93].
The common types of corrosion processes that may be encountered with stainless steel are described below.