Cathodic modification of stainless steels with ruthenium: a review of recent advances in making the cheaper option cheaper

1 Introduction

Stainless steels have become so ubiquitous in our daily lives that it has become impossible to enumerate their applications (Lo et al., 2009). In most applications, corrosion resistance merits the choice of stainless steels over other alloy types. Corrosion resistance of stainless steels is due to passivity brought about by the formation of a tenacious and impervious chromium (III) oxide film, typically 1- to 5-nm thick (Olsson et al., 2003; Sudesh et al., 2006). The practical importance of this Cr₂O₃ film lies in its ability to self-heal, therefore making passivity a cost-effective means of ensuring sustained corrosion resistance of stainless steel components. However, the Cr₂O₃ film is prone to dissolution in reducing acidic environments such as those found in flotation, electro-winning, fuel cells, and wastewater treatment.

Numerous investigators (Monnartz, 1911; Tomashov et al., 1970; Agarwala & Biefer, 1972; Chernova et al., 1981; Tomashov et al., 1984) have demonstrated that the passivity of stainless steels in reducing acidic media may be stimulated and sustained by cathodic modification. Cathodic modification involves introducing active cathodes such as platinum group metals (PGMs) into the stainless steel matrix and is comprehensively discussed in the subsequent section. Cathodically modified stainless steels were reported to spontaneously passivate in acids such as sulphuric and hydrochloric acids, even at temperatures >100°C (Tomashov et al., 1970; Streicher, 1977; Chernova et al., 1981). Some researchers have recorded improvements in corrosion resistance by a factor of up to 100.

Although cathodic modification is a promising solution to increase the tolerance of stainless steels in reducing acidic environments, commercialisation of the technique has been constrained by the prohibitive cost of PGMs. Recent attempts to reduce alloying costs of cathodically modified stainless steels have shifted focus to ruthenium. Ru is by far the cheapest of all PGMs, fetching 24 and 13 times less than platinum and palladium, respectively. Pt and Pd were largely preferred as active cathodes by earlier investigators (Hoar, 1958; Bianchi et al., 1968; Agarwala & Biefer, 1972; Chernova et al., 1981; Tomashov et al., 1984).

This paper seeks to review efforts made to optimise alloying costs for stainless steels cathodically modified with Ru. An additional objective is to highlight opportunities for further research. The paper focuses on advances made since 1990. Work done prior to 1990 on cathodic modification using PGMs, in general, is presented comprehensively elsewhere (Hoar, 1958; Streicher, 1977; McGill, 1990; Potgieter, 1991).

The present paper is divided into three main sections; the first presents the theory of cathodic modification, the second section highlights the merits of Ru over other PGMs, and the final section gives a review of studies done on Ru-alloyed stainless steels.

2 Cathodic modification: principles and mechanisms

Tomashov et al. (1948) postulated that in order to achieve passivity and consequently retard corrosion rates, it is necessary, if one rules out the possibility of pit formation or transpassive corrosion, to strive for increased cathodic efficiency, i.e. to reduce cathodic polarisation. The cathodic evolution of hydrogen as per Equation 1 supports the corrosion of stainless steels in reducing acidic media.

The efficiency of this reaction depends on the hydrogen overvoltage of the corroding metal; the higher the hydrogen overvoltage, the lower the exchange current density (i_o), and hence, the rate of the cathodic reaction. Stainless steels have a high overvoltage and i_o for the hydrogen evolution reaction, such that the equilibrium in Equation 1 lies to the left and the corresponding corrosion potential (E_corr) lies mostly in the active region of the polarisation curve in Figure 1. At these potentials (typically <−50 mV vs. SHE), non-protective Cr (II) species are thermodynamically stable (Haupt & Strehblow, 1995), and stainless steels would corrode unabatedly.

Figure 1:

Hypothetical polarisation curve showing the effect of cathodic modification of stainless steels with PGMs on corrosion behaviour in acidic solutions (adapted from Tomashov, 1964; Potgieter et al., 1990; Haupt & Strehblow, 1995).

Cathodic modification via the addition of PGM to the stainless steel matrix lowers the hydrogen overvoltage of the alloy. The consequence of this is an increase in i_o, such that the equilibrium in Equation 1 lies predominantly to the right. In this case, the corresponding E_corr would be anchored in the passive region, where Cr (III) species such as Cr₂O₃ are thermodynamically stable and corrosion rates are marginal.

Cathodic modification presumably inhibits the corrosion of stainless steels in reducing acid media in two ways: first, by catalysing the hydrogen evolution reaction in Equation 1 and lowering the alloy’s hydrogen overvoltage, as described in preceding paragraphs, and second, the PGMs added to the stainless steels decrease the anodic dissolution by blocking the active sites in the crystal lattice of the host metal (Tomashov et al., 1984).

Successful cathodic modification typically presents as an increase in the value of E_corr, a decrease in i_corr, and the disappearance of the anodic “nose” from the polarisation curves indicating spontaneous passivity (Tomashov, 1964; Potgieter et al., 1990) and is markedly influenced by the corrosion behaviour of the stainless steels prior to alloying. The following are prerequisites for a successful cathodic modification:

1. The stainless steel must initially exhibit active-passive behaviour in the acidic environment; otherwise, adding PGMs would serve only to accelerate corrosion. Cathodic modification of titanium- and chromium-based alloys such as Fe25Cr, 304, and 430 stainless steels was reportedly beneficial in H₂SO₄ and HCl (Agarwala & Biefer, 1972; Armstrong et al., 1973; Chernova et al., 1981; Draper & Meyer, 1981).

2. The passive region (Figure 1) of the stainless steel before alloying must be wide, i.e. the passivation potential (E_pass) must be sufficiently negative and the transpassive potential (E_trans) must be sufficiently positive. The latter is essential to eliminate the likelihood of transpassive or pitting corrosion after cathodic modification.

3. The critical current density (i_crit) of the stainless steel before alloying must be sufficiently low. In Figure 1, the dotted curve illustrates a hypothetical case where i_crit is greater than the current of the cathodic reaction. The anodic and cathodic curves will intersect at multiple points, i.e. at A, B, and C. These conditions present two possibilities: passive or active. At higher potentials, the cathodically modified alloy would passivate but actively corrode at lower potentials. The corrosion of such an alloy is unpredictable and undesirable.

3 Cathodic modification with ruthenium

The PGMs that have been used to study the cathodic modification of stainless steels are Pt, Pd, and Ru. The high cost of the first two has necessitated a shift in research focus to Ru. Besides being cheaper, Ru bears other attributes that make it a choice candidate for cathodic modification. For instance, ruthenium has lower hydrogen overvoltage than most PGMs. In fact, the effectiveness of PGMs in improving corrosion resistance in H₂SO₄ and HCl decreases in the following order (Potgieter, 1991):

Ir > Rh > Ru > Pt > Pd > Os

Figure 2 (Potgieter, 1991) shows that alloying stainless steels with Pt, Pd, and Ru increases E_corr, i_o (seen by the shift of the cathodic curve to higher current densities) while decreasing i_crit, implying the increased thermodynamic stability of Cr₂O₃, increased cathodic efficiency, and increased ease of passivation, respectively. This is all consistent with cathodic modification. However, Ru is the only PGM that causes a decrease in passivation current density (i_pass). In Figure 1, i_corr coincides with i_pass, suggesting that alloying with ruthenium could result in lower corrosion rates compared with either Pt or Pd.

Figure 2:

Summary of the effects of alloy additions on the polarisation characteristics of iron-chromium stainless steels in sulphuric acid (Potgieter, 1991). Reprinted with permission from Springer Customer Service Centre GmbH: Springer, Journal of Applied Electrochemistry, Potgieter, Alloys cathodically modified with noble metals, 1991.

The lower i_pass predicted in Figure 2 could be due to the formation of a much more stable passive film in the presence of Ru. Surface analysis of duplex stainless steels passivated in 1 m H₂SO₄ using X-ray photoelectron spectroscopy (XPS) showed that adding 0.3 wt% Ru increased Cr₂O₃:Cr(OH)₃ ratio in the passive film (Myburg et al., 1998). This points to an increased stability of the passive film, as Cr(OH)₃ is relatively soluble and its presence in the Cr₂O₃ film tends to undermine its integrity.

Unlike Pd, which remains a separate metallic phase, there is evidence to suggest that Ru incorporates into the passive film as a hydroxide, an oxide, or an oxyhydroxide. Several researchers (Tjong, 1990; Tjong et al., 1992; Potgieter et al., 1993; Myburg et al., 1998) reported detecting Ru (IV) species in the passive films of ruthenium-alloyed stainless steels exposed to H₂SO₄ and HCl. It is likely that the presence of these Ru (IV) species increases the stability of the Cr₂O₃ passive films, yet little is understood about their nature or their role in stabilising the passive films.

4 Ruthenium-alloyed stainless steels: recent advances

Numerous investigators have demonstrated that additions of ≤0.3 wt% Ru were sufficient to effect substantial corrosion resistance of stainless steels in H₂SO₄ and HCl with concentrations ranging from 0.5 to 2 m (Potgieter & Kincer, 1991; Potgieter & Brooke, 1995; Olubambi et al., 2009; Sherif et al., 2009; Olaseinde et al., 2012). This constitutes huge cost savings when compared to alloying with 0.1–2 wt% Pt or Pd, as reported by earlier researchers (Tomashov et al., 1970; Agarwal & Biefer, 1972; Chernova et al., 1981). Table 1 presents some of the findings reported on Ru-alloyed stainless steels. Equation 2 (Sherif et al., 2009) defines protection efficiency (PE), where CR_ss and CR_alloyed are the corrosion rates of the stainless steels before and after adding Ru, respectively. The results in Table 1 show that the PE of Ru in H₂SO₄ increased with (1) Ru content, (2) temperature, and (3) acid concentration.

During the exposure of cathodically modified alloys, a selective dissolution of the base alloy leads to surface enrichment of the PGMs (Pickering, 1983), thus increasing their availability for the corrosion process. The rate at which the PGMs accumulate on the surface and therefore facilitate improved cathodic efficiency naturally depends on their concentration in the alloy, as demonstrated by Tomashov et al. (1970) on stainless steels having between 0.1 and 0.5 wt% Pd and exposed to 10% H₂SO₄ at ≈100°C. It follows therefore that increasing Ru composition in the stainless steel matrix would increase resistance to corrosion attack.

Increasing temperature has the effect of reducing the oxygen content of a solution. Passivity of stainless steels depends on the availability of oxygen or an oxidising agent to allow the formation of Cr₂O₃ films. However, the presence of Ru enables stainless steels to stimulate passivity even in limited oxygen, and the alloys are unlikely to be affected as significantly by increasing temperatures as pristine stainless steels. It should be noted that the improvement in PE with temperature as reported in Table 1 does not necessarily suggest non-Arrhenius behaviour; the corrosion rates of Ru-alloyed stainless steels in both H₂SO₄ and HCl also increased with temperature (Potgieter & Brooke, 1995; Olubambi et al., 2009; Olaseinde et al., 2012).

Olubambi et al. (2009) reported that the corrosion rates of Ru-alloyed superferritic stainless steels increased with acid concentration. This behaviour is typical and was demonstrated by other researchers studying different alloy systems in H₂SO₄ (Phelps & Vreeland, 1957; Rebak & Crook, 2000; Richardson, 2010). Table 1 shows that the PE of Ru-alloyed stainless steels increased with an increase in acid concentration. These results demonstrate that additions of Ru widen the temperature and concentration ranges that stainless steels can tolerate and therefore present opportunities for wider application of the alloys.

Efforts by Mintek South Africa resulted in the commercialisation of a variant of cathodically modified stainless steels in circa 1995 under the trade name Ruthalloy. Ruthalloy is modelled on the superferritic stainless steel Fe29Cr4Mo and has additions of 0.2 wt% Ru. The alloy showed remarkable corrosion resistance in aggressive media (Table 2) and is an excellent candidate for use in pump components such as impellers and front wear plates exposed to acidic media with high levels of suspended solids (Wolff, 1999).

Despite an impressive corrosion performance in hot and abrasive acidic media, Ruthalloy is largely unknown decades after its commercialisation. A likely reason for this apathy may be the cost of the alloy. Although Ru is the cheapest of the PGMs, alloying with this metal is still rather expensive. For instance, adding only 0.2 wt% Ru to Fe29Cr4Mo would cost at least US $2700 more per tonne of the stainless steel, based on current prices. Inevitably, much research focus over the past 20 or so years has been on reducing the amount of Ru used in alloying stainless steel. Notable efforts include partial substitution of Ru and coating and surface alloying with Ru. These are discussed in the subsequent sections.

4.2 Surface alloying and coating

Potgieter et al. (1992) demonstrated that Ru coatings produced by electron beam evaporation were just as effective in protecting Fe22Cr9Ni 3Mo in 10% H₂SO₄ as bulk alloying with 0.2 wt% Ru. After a 24-h exposure, the PE of the stainless steel with 1-μm Ru film was 80%, that of a 2-μm-thick annealed Ru film was 92%, and that of the bulk-alloyed stainless steel was 95%. These high PE values were reported on the coated stainless steels despite the fact that the ruthenium films spalled severely during exposure.

Similar observations (Figure 3) were made on 304L stainless steel with Ru films produced by radio frequency (RF) sputtering and exposed to 1 m H₂SO₄ (Moyo, 2017). In this case, the Ru films were between 10 and 95 nm in thickness. Spalling of the ruthenium films during corrosion exposure could be attributed to inherent stresses. The primary source of stress in the Ru films was probably residual stress due to the film deposition process. For instance, in RF sputtering, the ballistic effect of atoms and ions during film deposition can induce significant compressive stresses in the resulting film (Cuthrell et al., 1988; Detor et al., 2009). Such stresses can be relieved by annealing.

Figure 3:

Image of Ru-coated 304L stainless steel after 12-h exposure in 1 m H2SO4 at 25°C, as viewed using field emission scanning electron with secondary electron detector (FESEM-SE). The coating was deposited via RF sputtering and annealed prior to corrosion exposure (Moyo, 2017).

However, in both studies of Potgieter et al. (1992) and Moyo (2017), annealing did not seem to prevent spalling of the ruthenium films in the corrosion media. Another possible source of stresses could be the hydrogen gas released during the corrosion process (Equation 1). The release of the gas is forceful and has been associated with stresses ranging from 20 to 200 MPa (Wampler et al., 1976; Nakahara & Okinaka, 1983). In the electrodeposition of Pd from acidic solution, the release of hydrogen was also reported to be extremely forceful and caused the plated coupons to twist (Abys, 2010).

The high corrosion resistance associated with the fractured Ru films (Potgieter et al., 1992; Moyo, 2017) seems to suggest that ruthenium films offered the stainless steels a dual protection (1) by physically isolating the metal from the H₂SO₄ and (2) by cathodic modification. The latter is consistent with the supposition by Chernova et al. (1981) that cathodic modification using PGM surface layers would effectively increase corrosion resistance even if the proposed films were non-continuous. Although this approach holds promise as a cost-effective alternative to bulk alloying, whether these surface layers could provide long-term corrosion protection seems unlikely.

To eliminate the risk of spalling and delamination, a technique such as laser surface alloying (LSA) may be contemplated. This technique involves melting a preplaced powder layer on a substrate using a high-energy beam. As part of the underlying substrate is also melted, good metallurgical bonding is frequently obtained and the resulting surface alloy is not susceptible to spalling, delamination, or interfacial corrosion (van der Merwe et al., 2017).

In their work, Tjong et al. (1997) laser-alloyed a 0.1-mm-thick layer of Ru powder onto the surface of Fe40Cr stainless steel. Although EDS analysis of the resulting surface alloy indicated a surface composition of up to 52 wt% Ru, 30 wt% Fe, and 18 wt% Cr, the surface alloyed stainless steel performed poorer than did Fe40Cr bulk alloyed with 0.2 wt% ruthenium in both 0.5 m HCl and H₂SO₄. This could be because of the microstructural changes associated with LSA. Cooling rates in LSA are very rapid and consequently result in a fine microstructure, as reported by previous researchers (Tjong et al., 1997; Lekala et al., 2012; Pityana et al., 2015; Lekala & van der Merwe, 2017). While reports on the effect of grain size on corrosion resistance are rather inconsistent, it has been demonstrated that grain refinement of stainless steels may increase susceptibility to corrosion (Schino & Kenny, 2002; Ralston & Birbills, 2010). Grain boundaries are high-energy regions prone to corrosion attack, and a fine grain structure increases the surface area for such attack.

In the same study, Tjong et al. (1997) noted that the surface alloy had a predominantly hexagonal close packed (HCP) structure owing to the presence of Ru. Olaseinde (2015) reported similar observations where additions of 10 wt% Ru changed the microstructure of Fe22Cr9Ni3Mo duplex stainless steel from austenite+ferrite to a combination of ferrite and an HCP phase. The corrosion behaviour of the duplex stainless steel with 10 wt% Ru was, however, not analysed.

It is expected that a change in crystallographic orientation would alter surface energy as well as surface atom density and therefore influence corrosion behaviour. A study by Kumar et al. (2005) showed that changes in crystallographic texture on cold-worked AISI 304L stainless steel altered the electrochemical behaviour in H₂SO₄ and NaCl. However, the effect of altering crystal structure, for example, from face-centred cubic (FCC) to HCP, on corrosion is not as obvious and is obscured by the fact that such transformation was a result of alloying known to influence the corrosion rates of metals.

More definite though is the influence of the crystal structure on the mechanical properties of metals. It is widely accepted that alloying with Ru, which has an HCP structure, increases the hardness of the alloyed metals. In addition, the grain refinement associated with LSA presents a unique combination of strength and toughness. However, the erosion-corrosion resistance of stainless steels alloyed with Ru via LSA remains understudied.

Another means of introducing Ru onto stainless steel surfaces while eliminating the risk of delamination is ion implantation. This low-temperature technique offers the ability to alloy any element into the near surface region of any metal regardless of its miscibility and has been successfully used to create numerous corrosion-resistant alloy systems (Lei & Zhu, 2005; Cano et al., 2006; Abreu et al., 2008; Padhy et al., 2010; Escalada et al., 2013).

Independent studies by Tjong and Chu (2007) and Moyo (2017) looked at implanting Ru into Fe24Cr and 304L stainless steels, respectively. The dose of Ru implanted into the stainless steel was between 10¹⁴ and 10¹⁸ ions/cm². Tjong and Chu (2007) used an accelerating voltage of 25 kV, while Moyo (2017) used 50 keV. Although, in both cases, the exact dose of Ru implanted was not quantified to account for sputtering effects, it was sufficient to improve corrosion resistance in H₂SO₄ of concentrations ranging from 0.5 to 4 m.

The study by Moyo (2017) showed that implanted Ru did not successfully induce cathodic modification in HCl but increased the corrosion rates of 304L stainless steel in 0.5 to 2 m of the acid instead. This was contrary to reports by other researchers (Olubambi et al., 2009; Sherif et al., 2009), who noted an improved corrosion resistance of Ru-alloyed stainless steels in HCl. The poor performance reported by Moyo (2017) was attributed to the corrosion behaviour of pristine 304L stainless steel in HCl; its passivation region was too narrow and the value of i_crit was too high for effective cathodic modification (Section 2).

Ion implantation is typically associated with radiation damage as well as high defect density, which may increase susceptibility to anodic dissolution. Radiation damage is particularly marked with ion implantation of heavy metals such as Ru and was reported by Paine and Speriosu (1987) to increase with implantation dose. Annealing has been recommended post ion implantation to minimise the effects of radiation damage (Chang & Ameen, 2011). The benefits of annealing on the corrosion resistance of ruthenium-implanted stainless steels need to be determined.

4.3 Surface alloying and partial substitution

Some researchers have contemplated the combined effect of surface alloying and either co-alloying or partially substituting Ru with relatively cheaper metals. Tjong and Chu (2007) co-alloyed Ru with titanium via ion implantation. Although the resulting alloy performed better than did pristine Fe40Cr ferritic stainless steel in 0.5 m H₂SO₄, adding Ti had no beneficial effect. For instance, the alloy with only Ru spontaneously passivated in the acid, but adding 5×10¹⁷ Ti ions/cm² reduced corrosion potential to more active region and increased i_corr from ≈0.2 to 1 μA/cm². Ti has been shown to exhibit limited corrosion resistance in reducing acidic media such as H₂SO₄ and HCl (Straumanis & Chen, 1951; Hu et al., 2017). This is because Ti, like Cr, depends on the availability of oxygen or an oxidising agent to induce passivation.

Van der Merwe and colleagues (2015, 2017) laser-alloyed 304L stainless steel with a powder mixture of 304L stainless steel and between 0.1 and 5 wt% Ru. In one study (van der Merwe et al., 2017), Ru compositions of up to ≈0.5 wt% improved the corrosion resistance of the stainless steel in 1 m H₂SO₄ at temperatures between 25°C and 45°C. In addition, the study demonstrated that at Ru composition >0.2 wt%, the corrosion rates of the laser-alloyed stainless steel deviated significantly from Arrhenius behaviour: corrosion rate decreased with an increase in exposure temperature. This is inconsistent with the findings of previous researchers (Potgieter & Brooke, 1995; Olubambi et al., 2009; Olaseinde et al., 2012) and seems to suggest a change in temperature dependence of the corrosion mechanism with high Ru compositions. Reactions that conform to Arrhenius’ law are typically limited by kinetics. The inconsistency presented here requires further investigation.

Some of the results obtained in the other study (van der Merwe & Tharandt, 2015) are presented in Table 4. They demonstrated that Ru compositions >≈2.5 wt% were not beneficial in 1 m H₂SO₄. This suggests that high Ru compositions do not necessarily translate to improved corrosion resistance and are in line with the findings by Tjong et al. (1997), which showed that compositions of ≈52 wt% Ru did not induce better corrosion performance than bulk alloying with 0.2 wt% of the PGM. It is intuitive to expect corrosion rates to decrease with the increase in Ru composition, as demonstrated in Table 1 and by other researchers (Potgieter & Brooke, 1995; Olubambi et al., 2009; Sherif et al., 2009; Sherif, 2011). In both cases (Tjong et al., 1997; van der Merwe & Tharandt, 2015), the stainless steels laser-alloyed with high Ru contents exhibited spontaneous passivation, and E_corr values remained much lower than those typically associated with pitting and transpassive corrosion. This means that the unexpectedly higher corrosion rates reported by these researchers were neither a consequence of pitting nor a transpassive attack. It is likely that the corrosion mechanism of the stainless steels with high Ru content is different to that described in Figure 1; experimental work is necessary to confirm this.

Table 4:

Corrosion parameters of ruthenium laser-alloyed 304L stainless steel in 1 m H₂SO₄ at 25°C (van der Merwe & Tharandt, 2015).

Ru (wt%)	Ecorr vs. Ag/AgCl (mV)	icorr (μA/cm2)	Corrosion rate (mm/year)
As received	−257	47.39	0.551
0	−243	287.34	3.841
0.44	−222	14.25	0.106
0.82	77	0.78	0.009
2.44	134	0.59	0.007
2.92	121	1.41	0.016
4.67	15	57.11	0.081

1. © The South African Institute of Mining and Metallurgy (SAIMM).

Van der Merwe and Tharandt (2015) noted a Ru-rich stringer in the Ru laser-alloyed surfaces (Figure 4), the distribution density of which increased with the increase in Ru composition. Ru melts at ≈2330°C, while 304L stainless steel typically melts at between 1400°C and 1450°C. It is likely that the heat produced during LSA was insufficient to smelt the Ru, leaving it unmolten and randomly dispersed in the alloy. The presence of these stringers caused notable variations in the chemical composition of the exposed surfaces, resulting in variable and most likely unpredictable corrosion performance. Similar variability in microstructure and elemental distribution was also noted by Lekala and van der Merwe (2017) on 316L stainless steel LSA with Ru+Ni powders and exposed to 60% H₂SO₄. In service, such unreliable behaviour would be unacceptable. This therefore necessitates research work to minimise or eliminate the formation of these Ru stringers. Optimising parameters such as laser power, beam diameter, and laser scanning seed to increase heat input could be explored.

Figure 4:

(A) Surface and (B) cross-section FESEM-SE images of Ru laser-alloyed 304L stainless steel. Ruthenium composition of the surface alloys was (A) 3 wt% and (B) 5 wt% (van der Merwe & Tharandt, 2015). © The South African Institute of Mining and Metallurgy (SAIMM).

5 Concluding remarks

This review revealed a number of strategies that could potentially increase the economic viability of alloying stainless steels with ruthenium for improved corrosion resistance in non-oxidising acidic media. The most promising of these is partially substituting ruthenium with cheaper metals and surface alloying via ion implantation and LSA. However, much work still needs to be done to optimise these strategies and to understand the mechanisms for corrosion protection, as they are likely to deviate from that proposed by Tomashov et al. (1948). Long-term tolerance of the stainless steels alloyed with ruthenium via these strategies also needs to be ascertained.

At first glance, surface alloying may seem more cost-effective than partially substituting Ru with cheaper metals. However, a cost-benefit analysis needs to be carried out to conclusively decide on this. In addition, combinatorial techniques may be employed to screen low-cost metals that could partially substitute ruthenium without compromising passivation efficiency. It should be noted that techniques such as ion implantation may be difficult and costly to upscale and hence may be limited to small, favourably shaped components. This therefore opens opportunities for research into other strategies such as paint and thermal spray, which can be applied on large structures.