Electrochemical investigation of the influence of nitrogen alloying on pitting corrosion of austenitic stainless steels

Abstract

A systematic investigation has been performed of the influence of nitrogen alloying on the pitting corrosion of austenitic stainless steels. The alloys investigated had a base composition of 20% Cr–25% Ni with 0% or 4.5% Mo. The nitrogen level was varied up to the solid solubility limit at approximately 0.2%. The critical temperature for pitting corrosion increases with nitrogen additions and the effect is more marked in the molybdenum-alloyed steels, indicating a synergistic interaction between the two alloying elements. A number of investigations have been performed to elucidate the mechanisms for the influence of nitrogen on pitting corrosion and for Mo–N synergism.

Introduction

The influence of nitrogen alloying on the corrosion properties of stainless steels has received much attention in recent years, and has also been the subject of a number of reviews 1, 2, 3, 4, 5. There is fairly universal agreement than nitrogen additions improve the pitting corrosion resistance of austenitic stainless steels; increasing the pitting potential in aqueous chloride solutions 6, 7, 8, 9, 10, 11, 12, 13or decreasing the weight loss in immersion testing in FeCl₃ 7, 9, 14, 15. Similar beneficial effects have also been reported for duplex steels 16, 17, 18, austenitic welds [19]Fe–N alloys 4, 20, tool steels [21]and low-alloy steels [22]. The concept of nitrogen–molybdenum synergy, whereby greater beneficial effects are attained by nitrogen additions in the presence of molybdenum, is also widely-acknowledged 1, 9, 14, 23, 24, 25, 26, 27, 28, 29, 30, 31. Only in exceptional cases have negative effects of nitrogen alloying on pitting resistance been reported 32, 33.

There is less consensus regarding the effect of nitrogen alloying on general corrosion resistance or active dissolution. In an early work, Osozawa and Okato 7, 8demonstrated that nitrogen had a smaller beneficial effect on the critical current density for passivation in 0.5 M H₂SO₄+0.05 M NaCl than on the pitting potential when compared to other alloying elements. Nitrogen alloying has been shown in several studies 2, 10, 27, 34, 35to decrease the critical current density in HCl. For 17 Cr–12 Ni–2.5 Mo steels, the largest effect has been seen around 4 M HCl in stagnant solution [2]or 5 M for rotating disc electrodes [34]. Similar beneficial effects have been noted for various steels in 0.1 M HCl+0.4 M NaCl [12]and 7.5 M H₂SO₄ [13]. However, nitrogen has also been reported to have a negligible effect on the active peak for 6% Mo austenitic stainless steels in 0.5 M HCl+2 M NaCl [27]and for 17 Cr–13 Ni steels in 0.5 M H₂SO₄ [36].

In terms of passive film properties, nitrogen has been observed to decrease the passive current density 10, 12or to have a negligible effect 6, 27. A decrease in passive film capacity has been noted for N-implanted 18–8 steels in 0.5 M NaCl [13]but the converse for nitrided iron in 0.01 M H₂SO₄+0.99 M Na₂SO₄ [37]. There is, however, a general consensus that nitrogen accelerates repassivation, both for Fe–N alloys 4, 20, 37and for stainless steels 8, 27, particularly in the presence of chloride [38].

There has been extensive discussion in the literature about nitrogens effect on corrosion properties and the proposed mechanisms may be divided into the following categories:

Chernova [39]attributed the beneficial effect of nitrogen to structural homogenisation caused by the elimination of delta ferrite, and Janik Czachor [6]observed that nitrogen modified the ratio or composition of carbide phases which acted as pit initiation sites. In sensitised stainless steels 40, 41and in welds, particularly of duplex stainless steels [42], microstructural modification is often the dominant effect of nitrogen on corrosion properties. However, these arguments are not readily applicable to austenitic stainless steels which have a homogenous, single-phase structure.

Osozawa and Okato [8]were the first to detect the presence of NH⁺₄ following dissolution of nitrogen-containing stainless steels in pitting tests in 20% FeCl₃. They proposed the idea that the formation of ammonium ions consumes protons and thereby increases the pH in incipient pits and promotes repassivation. This electrochemical nitrogen dissolution reaction may be represented as:

$$\text{[N]+4H}{}^{\mathrm{+}}\text{+3e}{}^{\mathrm{-}}\text{→NH}{}^{\mathrm{+}}{}_4\text{.}$$

Subsequent works 22, 30, 35, 43, 44, 45have confirmed NH⁺₄ formation in a variety of media and have demonstrated that the NH⁺₄ generation rate increases with the amount of nitrogen in the steel [44]and decreases with increasing applied potential [35]. In XPS studies, NH⁺₄ (or NH₃) has been detected at passive 1, 11, 27, 29, 46, 47, active 1, 48, 35and transpassive [1]potentials for stainless steels. Similar surface analysis results have also led to the suggestion that the same mechanism may even be operative for aluminium implanted with nitrogen [49].

Willenbruch et al. [50]noted that nitriding caused a remarkable reduction in the active dissolution of chromium and suggested that surface CrN may act as a precursor to passive film formation and also form NH⁺₄ /NH₃ on dissolution via the reaction:

$$\text{2CrN+3H}{}_2\text{O→Cr}{}_2\text{O}{}_3\text{+2NH}{}_3\text{(ligands)}\text{.}$$

(2)It was also pointed out [51]that this would counteract the drop in surface pH which would otherwise be a consequence of metal hydration or oxide formation. Experimental evidence for the possibility of such a mechanism is provided by the work of Kolotyrkin et al. [43]who observed Cr₂N precipitates to form NH⁺₄ on dissolution. However, they also noted that such nitrides were very reactive and resulted in a more rapid release of chromium than did dissolution of the pure metal.

A mechanism based on ammonium formation from chemical dissolution of nitrides circumvents objections 10, 11, 52to the mechanism of Section 1.2on the grounds that Eq. (1)is a cathodic reaction which should cease at sufficiently anodic potentials, behaviour which is difficult to reconcile with the observation that the effect of nitrogen is larger in molybdenum-alloyed steels for which the pitting potential is higher 10, 52.

The inhibiting effects of nitrates are widely recognised 14, 53, 54, 55and Truman [53]suggested speculatively that nitrate formation during dissolution could inhibit pit growth. Kamachi Mudali [19]proposed that NH⁺₄ from Eq. (1)could form inhibiting species, such as NO⁻ ₂ or NO⁻ ₃, by processes such as:

$$\text{NH}{}^{\mathrm{+}}{}_4\text{+2H}{}_2\text{O→NO}{}^{\mathrm{-}}{}_2\text{+8H}{}^{\mathrm{+}}\text{+6e}{}^{\mathrm{-}}\text{.}$$

(3)In this context, it may however be noted that the widely-accepted mechanism 52, 10, 54, 55for the inhibiting role of nitrate is itself via acid consumption and ammonium formation, i.e. 54, 55:

$$\text{NO}{}^{\mathrm{-}}{}_3\text{+10H}{}^{\mathrm{+}}\text{+8e}{}^{\mathrm{-}}\text{→3H}{}_2\text{O+NH}{}^{\mathrm{+}}{}_4\text{.}$$

The concept that nitrogen is incorporated in and improves the properties of the passive film was suggested in early works by Truman [9]and Bandy [14]and also invoked by Song [13]to explain impedance data. Indications of a nitride in the passive film have been presented by Sadough Vanini et al. [36], who argued that dispersed particles of chromium nitride in the passive film were responsible for the detrimental effect of nitrogen on passivation observed in their investigation. However, other surface analytical studies 1, 10, 11, 12, 27, 46, 47, 48, 56, 57, 58have provided little evidence of significant amounts of nitrogen within the passive film.

Nitrogen enrichment has been observed at the metal–film interface in a number of works. Lu et al. 27, 56first reported an apparent nitrogen concentration at the metal side of the interface which was seven times the level in the bulk alloy and concluded, from XPS binding energy data, that this was in an uncharged form. Later works by Clayton et al. 1, 10, 11, 12, 48, 57, 58confirmed interfacial nitrogen enrichment but identified this as a negatively-charged state, such as a nitride phase. They found evidence of Ni–Mo–N synergism 12, 51, 57, 58and attributed this to the formation of a stable mixed nitride surface layer, such as Ni₂Mo₃N.

In contrast, Olefjord et al. [59]suggested, on the basis of XPS studies, that a thin intermetallic layer formed on the active surface or beneath the passive film may be responsible for improving corrosion properties. Modelling was used [48]to predict the occurrence of Ni–Mo and weaker Ni–Cr bonding and it was suggested that nitrogen may increase the strength of this bonding. Olefjord and Wegrelius [47]found interfacial nitrogen to correspond to 12–20% of an atomic layer but argued against the formation of chromium nitrides on thermodynamic grounds (energetically unfavourable compared to chromium oxide) and against Ni₂Mo₃N, since the measured molybdenum content was too low.

A different interpretation of the role of interfacial nitrogen accumulation was made by Grabke [4]. A comparison with AES, XPS and LEED analysis of surface segregation on Fe–N single crystals at elevated temperatures [60]was used to demonstrate that the interfacial nitrogen species is negatively charged. On this basis, he proposed that nitrogen decreases the potential gradient across the film and repels chloride ions, also that desorption of aggressive ions is induced by segregated N ^δ− just after the local failure of the passive film. It was further suggested that this negatively charged nitrogen probably converts directly to ammonium via the reaction:

$$\text{N}{}^{\mathrm{-}}{}_3\text{+4H}{}^{\mathrm{+}}\text{→NH}{}^{\mathrm{+}}{}_4\text{.}$$

The idea that significant transient anodic segregation of nitrogen may occur during dissolution was first proposed by Newman et al. [27]and subsequently demonstrated in a number of surface analytical studies 1, 35, 48. Bandy et al. [10]put forward two reasons why both nitrogen and molybdenum should accumulate on anodically dissolving surfaces: that they are thermodynamically more noble than iron and that their dissolution reactions are slow, multi-electron processes. It was also pointed out by these authors and by Newman and Shahrabi [2]that, because the nitrogen dissolution reaction Eq. (1)is cathodic, nitrogen will accumulate at a dissolving surface, particularly at the higher potentials to which molybdenum displaces dissolution. They therefore suggested that nitrogen will tend to accumulate at active surface sites, such as kink and step sites, and stifle active dissolution. Newman and Shahrabi [2]took this argument a step further in claiming this could lead to a fundamentally different passivation process, associated with nitrogen blockage of dissolution sites, which was both reversible and insensitive to solution flow and which could occur below the potential at which Cr₂O₃ forms.

Nitrogen has frequently been reported to accelerate the anodic dissolution of iron in acidic solutions 4, 20, 38, 50, 58, 61, although the converse has been reported in neutral solutions 38, 61. It has also been demonstrated to stimulate the selective dissolution of iron from stainless steels [12]. Conversely, nitriding pure chromium decreases the dissolution rate 16, 38, 50and may even completely eliminate the active peak [50]while, in steels, nitrogen can enhance surface accumulation of chromium [22]and suppress its release into solution [16]. The dissolution of molybdenum is largely unaffected in the active range [50]by nitriding but suppressed at transpassive potentials [28].

These observations have been used to support the idea that nitrogen modifies the surface composition of stainless steels and thereby facilitates repassivation. Lu et al. [29]pointed out that the higher stability of Cr and Mo nitrides in acidic solutions than in neutral solutions may accelerate the anodic segregation of beneficial elements during localized corrosion, therefore building up a more resistive surface layer at pit sites. They also suggested 28, 29that nitrogen can inhibit the transpassive dissolution of molybdenum, possibly by nitride formation, and effectively retain Mo in the passive film. Surface analytical studies have, however, provided somewhat conflicting evidence. Clayton et al. [1]found nitrogen to increase the concentration of Cr and Mo in the passive film and noted that Ni, Mo and N all promoted the selective dissolution of iron; but Olefjord and Wegrelius [47]observed no effect of nitrogen on either the film or the underlying metallic composition.

In sulphate solutions, Clayton et al. 1, 11suggested that NH⁺₄ may form an ammonium sulphate layer which may act as a temporary prepassive barrier between the metal and the solution. In a related vein, Newman and Ajjawi [55]have demonstrated that nitrate inhibition of stainless steels in chloride solutions involves salt film formation.

Ives et al. [62]suggested that in chlorinated water systems, ammonium may react with free chlorine to form combined chlorine species which are less effective oxidants than free chlorine.

In view of the recognised importance of nitrogen–molybdenum synergism in improving localised corrosion resistance, surprisingly few papers have addressed possible mechanisms for this interaction. The first suggestion, put forward by Newman et al. in 1984 [27], was that Mo and N may form a surface array which inhibits further dissolution. This idea of cosegregation is also implicit in the arguments in Section 1.8above, namely that nitrogen modifies the dissolution and therefore surface concentration of alloying elements in stainless steels also, in the reasoning from Section 1.6, that nitride or nitrogen-promoted intermetallic surface bonding occurs.

Arguments based on solution chemistry have also been put forward. Lu et al. [29]suggested that the formation of NH₃ ligands raises the local pH and favours the formation of molybdate, which enhances the passivity of stainless steels. Olsson [46]showed evidence of interaction between highly mobile Mo and N ions near the passive layer region and formation of a passive layer which is independent of the underlying phase. His tentative explanation of the synergistic interaction was that not only does nitrogen buffer the pH and stabilise molybdates, but also that molybdates would assist in the formation of ammonium, since the presence of molybdate in the passive film is an important parameter for the deprotonation of the film [63].

Bandy et al. [10]and Newman and Shahrabi [2]suggested that synergistic interaction is observed because molybdenum displaces metal dissolution (and pitting) to higher potentials. At sufficiently high potentials, nitrogen will accumulate at kink sites because the dissolution reaction Eq. (1)is cathodic, and it may be that the cross-over point of the anodic dissolution and nitrogen reduction reactions is thereby shifted to current densities below the critical value for pit initiation. Likewise, Olefjord and Wegrelius [47]suggested that enrichment of Mo at the metal–electrolyte interface of initiated pits decreases the dissolution rate to such an extent that formation of NH₃ /NH⁺₄ can compensate for the pH drop.

Finally, Newman and Shahrabi [2]speculatively suggested that there may be a specific interaction between Mo and N, analogous to molybdenums role in biological nitrogen fixation.

The aim of the present work 64, 65is to contribute to the elucidation of nitrogen mechanisms via electrochemical investigation of a series of experimental alloys with systematic variation in Mo and N contents. Pitting, passive film development and dissolution behaviour in acidic chloride solutions (which have a bearing on the dissolution conditions inside a pit or crevice) are examined.