Revisiting the effect of molybdenum on pitting resistance of stainless steels Tungsten

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Tungsten (2021) 3:329–337

https://doi.org/10.1007/s42864-021-00099-1 REVIEW PAPER

Revisiting the effect of molybdenum on pitting resistance of stainless steels

Yang‑Ting Sun¹ · Xin Tan¹ · Long‑Lin Lei¹ · Jin Li¹ · Yi‑Ming Jiang¹

Received: 20 November 2020 / Revised: 17 March 2021 / Accepted: 18 March 2021 / Published online: 23 June 2021

Abstract

As an important alloyed element to improve the localized corrosion resistance of stainless steels, Mo has been widely studied by researchers. This article reviews the mechanisms of the effect of Mo on localized corrosion. Two possible effects and corresponding experimental research are described respectively: Mo exists in the passive film as oxide and enhances the film stability (manifesting as lower nucleation probability of localized corrosion and longer delay time of breakdown of the passive film); Mo affects dissolution kinetics and inhibits active dissolution current density (manifesting as smaller metastable pitting current density and size). Then some contradictory results are discussed by considering the pitting model proposedrecently. Strictly speaking, none of the existing experimental results can deny the effect of Mo on the passive film or on the dissolution kinetics. It is reasonable to believe that Mo affects the entire process of localized corrosion rather than a single reaction. Finally, some possible research suggestions are put forward.

Keywords Pitting · Molybdenum · Stainless steels · Localized corrosion

1 Introduction

Stainless steels, which refer to steels containing a certain amount of chromium, are widely used in all walks of life because of their excellent corrosion resistance. Generally, the corrosion resistance of stainless steels mainly comes from the effect of chromium, which helps to form a continuous and dense oxide film. The higher the chromium content in stainless steel, the better the corrosion resistance.

Besides Cr, adding other appropriate alloyed elements is a very important method to improve the localized corrosion resistance of stainless steels [1–3]. Alloyed elements can be beneficial to corrosion resistance only if there is a certain chromium content in stainless steels. Well-known helpful alloyed element includes Ni, Mo, N, and Cu [4–8].

Chromium is the basis of the corrosion resistance of stainless steels; however, some researches have shown that in some extreme environments such as acid environment, chromium may lose its protective effect on stainless steels. In the later stage of localized corrosion development, chromium

may even accelerate the corrosion rate, but at this time, the presence of molybdenum can make stainless steels still have localized corrosion resistance [9]. When the content of chromium in stainless steel is sufficient, molybdenum has an obvious effect on improving the corrosion resistance of stainless steel. And the higher the chromium content in stainless steel, the more obvious the effect of molybdenum [10].

For typical localized corrosion of stainless steels, such as pitting corrosion, crevice corrosion and stress corrosion crack- ing, the presence of molybdenum has a significant inhibition effect [11, 12]. There are many types of elements in stainless steel, and the interaction between the elements is complex;

localized corrosion of stainless steel involves the rupture of the passive film, corrosion growth kinetics and other pro- cesses, so the mechanism itself is complicated; environmen- tal factors such as pH and halogen ion concentration have a various influence on localized corrosion. In such a situation, although many experiments have been conducted to investi- gate the role of molybdenum in the resistance of stainless steel to localized corrosion, there is still no unanimous opinion among researchers so far on the mechanism of molybdenum’s effect on stainless steel. This paper attempts to state some important experimental facts obtained so far and the possible mechanism of the effect of molybdenum based on these experiments, and then carry out some necessary discussion.

Tungsten

www.springer.com/42864

* Jin Li

jinli@fudan.edu.cn

1 Department of Materials Science, Fudan University, Shanghai 200433, China

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2 Two schools of thought

In general, existing investigations tend to focus on two aspects: passive film and dissolution kinetics. Specifically, some investigators believe that the main role of molybdenum is to change the composition of the passive film, thus enhancing the stability of the passive film and improving the localized corrosion resistance; the other is that molybdenum affects the dissolution kinetics of localized corrosion, which reduces the active dissolution rate and makes continuous propagation of localized corrosion difficult to occur.

2.1 Mo enhancing the passive film

The resistance of stainless steel to localized corrosion mainly comes from the uniform and dense passive film on its surface (mainly composed of chromium oxide), and the breakdown of the passive film is a prerequisite for localized corrosion to occur. Therefore, when considering the role of molybdenum in stainless steel, many researchers naturally propose that molybdenum may enhance the property of passive film. Some experiments proved that molybdenum did exist in the passive film and changed the structure of the passive film, and thus the localized corrosion resistance of the passive film was enhanced [13–17]. Furthermore, some

researchers found that the thickness of the passive film increased when molybdenum was present in the passive film [18–20], or molybdenum could reduce the active sites on the surface of the passive film [21]. Some other researchers have found that the presence of molybdenum made it difficult for chloride ions to pass through the passive film, thus preventing the damage caused by aggressive chloride ions attacking the local passive film [22–25].

Olefjord et al. [26] measured the polarization curve of Fe19Cr and Fe24Cr2Mo alloy at 0.5 mol·L⁻¹·H₂SO₄ at 25 ℃ and the results showed that the molybdenum-containing specimens had higher corrosion potential, smaller maximum current and lower passivation potential.

Through analyzing the passive films of the specimens polar- ized to different stages by electron spectroscopy for chemical analysis (ESCA) technique, the following facts were found: the inner layer of passive film of Fe–Cr–Mo alloy was Cr₂O₃, and the outer layer was hydroxides of Fe, Cr and Mo, as shown in Fig. 1. During anodic dissolution, Mo and Cr were enriched on the surface, thus increasing the corrosion resistance of the passivated film (manifesting as a smaller dissolution current density). Mesquita et al. [27] studied the effect of molybdenum on the pitting resistance of stainless steel under alkaline conditions, and detected the presence of molybdenum in the passive films of molybdenum-containing specimens.

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Lynch et al. [17] investigated the passivation mechanism and the effect of ultra-low pressure (ULP) pre-oxidation on Cr and Mo surface enrichment. Polycrystalline American Iron and Steel Institute (AISI) 316L stainless steels were exposure to sulfuric acid, and the surface of the specimens was analyzed by X-ray photoelectron spectroscopy (XPS) before and after electrochemical treatment at open circuit potential (OCP) and anodic passivation. The XPS results of both native oxide and pre-oxidation oxide showed that molybdenum ion existed in passive film, as shown in Fig. 2. Preferential dissolution of oxidized iron occurred at OCP, and the steadystate thickness of the initially formed oxide film decreased; however, the enrichment of Cr³⁺

and Mo^4+/6+ was promoted, mostly in the exchange outer hydroxide layer. Similar results were found in Loable et al.

[20] study, who measured the chemical composition of the passive film of AISI 316 formed at OCP in acidic chloride solutions (pH = 3) by XPS. The result showed that the oxide of Cr and Mo existed in the film, while the iron oxide dissolved under this condition (Fig. 3).

In alkaline solution, the passive film of ferritic stainless steel and duplex stainless steel was enhanced by molybdenum, but molybdenum seems to have no effect on the passive film of austenitic stainless steel [28].

2.2 Mo hindering the dissolution kinetics

The dissolution process after the rupture of the passive film, i.e., after the localized corrosion nucleation, is also the focus of many experimental studies [29–31]. With regard to the effect of molybdenum on dissolution kinetics, it can sim- ply be argued that adding molybdenum to stainless steels is similar to the adding of a molybdate inhibitor to the solution [32, 33]. At the beginning of the localized corrosion initia- tion, molybdenum dissolves into the solution as molybdate ions, which can form insoluble molybdate with cations such as iron ions, thus reducing the concentration of metal cations in the corrosive system and also reducing the concentration of aggressive chloride ions. As a result, the corrosion current density is reduced and the expansion of localized corrosion is hindered, making the nucleation site of the localized corrosion easier to repassivate (more difficult to propagate).

With the propagation of localized corrosion, the concentration of hydrogen ion increases gradually. At a certain time, molybdate ion may react with hydrogen ion to form MoO₂, which consumes hydrogen ion and reduces the system’s aggressiveness, preventing the further expansion of localized corrosion [9].

In the solution containing bromide ions, some researchers found the role of molybdate inhibitor not as effective [34].

Corresponding to this, it was also found that in the similar solution, the role of the alloyed molybdenum in stainless steel was also not as effective [35], which confirmed from

the other side that the alloyed molybdenum and the molybdate inhibitor may have the same mechanism to resist localized corrosion. As shown in Fig. 4, when the molybdenum content is relatively low, stainless steels in chloride solution have smaller pitting potential (E_pit) and larger dissolution current than those in bromide solution; however, when the molybdenum content is relatively high, the results become the opposite.

The experiment conducted by Ilevbare et al. [36] pro- vided a very direct evidence for the effect of molybdenum on dissolution kinetics. As shown in Fig. 5, in the same condition (same temperature, potential, solution pH, chloride ion concentration, etc.), compared with stainless steel with- out molybdenum (SS304GF), the transient current density of metastable pitting of stainless steel with molybdenum (SS316GF) was small, indicating that the metastable pitting grew slowly and the size of metastable pits were small. Thus the stable pitting was more difficult to form.

Some latest research showed that the resistance to localized corrosion of stainless steels containing Mo and N is improved due to the chemical interaction between nitrogen and molybdenum which hindered active dissolution [37].

3 Discussion

In addition to the aforementioned experimental evidence in support of these respective views, investigators also reported some contradictory results, which are also worthy of attention and discussion.

As described in Sect. 2, the effect of molybdenum on the dissolution kinetics can be approximately considered to be the same as that of molybdate inhibitor. That is, molybdenum dissolves to form molybdate in the active dissolution process, and then prevents the expansion of localized corrosion.

However, Lemaitre et al. [38] questioned this mechanism by comparing the effect of alloyed molybdenum and that of molybdate inhibitor. They measured the E_pit of AISI 430 and 304 stainless steels (containing no molybdenum) in different concentrations of NaCl or NaCl + Na₂MoO₄ solutions, and AISI 434 and 316 stainless steels (containing molybdenum) in different concentrations of NaCl solutions, and obtained the logarithm curve of E_pit corresponding to chloride ion concentration under different conditions. In NaCl solution, molybdenum-containing specimens had higher E_pit, but all specimens used in the experiment had the same slope a of E_pit ~ lg [Cl⁻] curve (120 mV·dec⁻¹, the slope of the curve may be related to the dissolution kinetics[38]). In NaCl + Na₂MoO₄ solution, there is a critical concentration of molybdate, below which molybdate will not show inhibition effect on pitting. For AISI 430 specimens, when molybdate ion was above the critical value, the slope of E_pit ~ lg [Cl⁻] curve was 240 mV·dec⁻¹ [38]. However, the slope value of

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AISI 304 was 320 mV·dec⁻¹ [38]. The difference between the E_pit ~ lg [Cl⁻] curves of molybdenum-containing specimens (in NaCl solution) and those of molybdenum-free specimens (in NaCl + Na₂MoO₄ solution) seems to indicate that the speculation that “dissolved molybdenum is deposited on the pit surface in the form of molybdate to prevent its active dissolution” is incorrect. Nevertheless, the above experimental

Fig. 2 a XPS Fe 2p, Cr 2p, Mo 3d core level spectra and their peak fitting for the native oxide-covered 316L surface (Nat) sequentially treated at OCP and under anodic polarization (AP) at Upass = − 0.17 V/Pt in 0.05 mol·L⁻¹·H₂SO₄. The emission angle is 45°. b XPS Fe 2p, Cr 2p, Mo 3d core level spectra and their peak fitting for the ULP pre-oxidized 316L surface sequentially treated at OCP and under AP at Upass = 0.025 V/Pt in 0.05 mol·L⁻¹ H₂SO₄. The emission angle is 45°. Reproduced with permission from Ref.

◂

Fig. 3 XPS spectra of 18Cr-12Ni-3Mo and 18Cr-12Ni-3Mo-0.1N at 90° for the a Fe2p, b Cr2p, c Mo3d3/2-3d5/2, and d N1s ionizations at both acidic. Reproduced with permission from Ref. [20] Copyright 2017, Elsevier

Fig. 4 a Effect of Mo concentration on the E_pit of Fe–18 wt%Cr–Mo ferritic alloys in 1.0 mol·L⁻¹ LiCl and 1.0 mol·L⁻¹ LiBr. b AP of Fe–18 wt%Cr ferritic alloys with 2.0 wt% and 5.1 wt% Mo in bro-

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results can not completely refute the effect of molybdenum on the active dissolution if we think a little more deeply.

Prior to further discussion, it may be of some help to understand the general mechanism of localized corrosion.

Taking pitting corrosion, which is the most typical localized corrosion, as an example, it is generally believed that the propagation process of pitting corrosion consists of three steps. First, the local passive film ruptures, then the metastable pits is generate, and finally, part of the metastable pits develop to stable pits. Regarding the critical step of pitting, Frankel et al. [39] believed that when the specimen is exposed to an aggressive environment, the passive film breakdown is relatively easy, and the passive film ruptures frequently. Therefore the pitting growth kinetics and pitting stability becomes the critical step. On the contrary, when the specimen is in a less aggressive environment, the breakdown of the passive film is more difficult, and once the passive film ruptures, metastable pitting is easy to grow into stable pitting corrosion. Therefore the breakdown of the passive film becomes the critical step, as shown in Fig. 6.

One possible explanation for the difference between the slopes of E_pit ~ lg [Cl⁻] curves of molybdenum-containing specimens (in NaCl solution) and those of molybdenum- free specimens (in NaCl + Na₂MoO₄ solution) is that, in this experiment, the breakdown of the passive film is the critical step for the specimens containing molybdenum. That is to say, the stability of the passive film determines the E_pit of

AISI 434 SS and AISI 316 SS, and the effect of molybdenum on the dissolution kinetics can be ignored, thus leading to different experimental results from adding molybdate inhibi- tors into the solution directly. Another possible explanation is that, the transformation of the alloyed molybdenum in stainless steel to molybdate involves a dissolution process while directly adding molybdate to the solution only involves the diffusion process of molybdate into the pit. Since we have not fully understood the mechanism of this dissolution and diffusion process, we cannot arbitrarily assume that the effect of alloyed molybdenum and molybdate is exactly the same, even if we admit that molybdenum dissolves into molybdate and thus affects the dissolution kinetics.

Carcea et al. [40] studied the active dissolution stage of Ni–Cr–Mo alloy and stainless steel with different Mo com- positions (Mo content 0–13wt%) under diffusion control and found that the Mo content had almost no influence on the limit current density, which seems to be contradictory to that molybdenum have an effect on active dissolution. Under the same experimental condition, the value of the diffusion current density mainly depends on the concentration of metal cations in the pit. At the diffusion control stage of pitting growth, the metal cation concentration in the pit bottom reaches saturation so that diffusion current density reaches its maximum, and the actual current density measured in the experiment is equal to the diffusion current density [41].

At this time, the salt film forms to reduce the dissolution

Fig. 5 a Terminal current density (i) distribution and b diameters of metastable pits on SS304GF and SS316GF as a function of applied potential (E), and the potential is referred to saturated calomel elec-

trode (SCE). Polarization occurred in de-aerated 1 mol·L⁻¹ HCl at a sweep rate of 2 mV·s⁻¹. Reproduced with permission from Ref. [36]

Passive film breakdown Metastable pitting Pit growth

Less aggressive Slow (rate-controlling) Fast Fast

More aggressive Fast Fast Slow (rate-controlling)

Fig. 6 Schematic summary of relative speed of passive film breakdown and pit growth for less and more aggressive condition

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current density, so that the concentration of metal cations in the pit can remain unchanged [42]. In this case, even if molybdenum affects the dissolution current density, this effect may only be reflected in the change of the salt film.

Since the metal cation in the pitting pit is still saturated, the limit current density measured in the experiment does not necessarily change.

The most direct objection to the view that molybdenum enhances passive film performance is that some researchers have not detected the presence of molybdenum element in the passive film of stainless steel [43], and have not found the relationship between the molybdenum content in the passive film and the resistance to pitting corrosion [44]. If we accept the view that both passive film breakdown and pitting growth kinetics can be the critical step of pitting corrosion, these experimental results also become easy to explain. Under the conditions that pitting growth kinetics is the critical step, the breakdown of passive film is very frequent, and the effect of the performance of passive film on pitting growth can be nearly ignored. Therefore, it may be meaningless to measure the content of molybdenum in the passive film under this condition. At this time, molybdenum enhances pitting resistance mainly by affecting dissolution kinetics. Another significant benefit of molybdenum is its ability to remove absorbed sulfur from the surface of the alloy [45]. When sulfur species are present, the alloyed molybdenum in stainless steel may bind to the adsorbed sulfur on the surface of the passive film, and then remove the sulfur by dissolving it, thus improving the stability of the passive film. This mechanism suggests that even if

molybdenum does not enter the passive film directly, existing in the passive film as an oxide such as MoO₂ (in other words, even if the presence of molybdenum element is not detected in the passive film), molybdenum may indirectly enhance the passive film performance by other means.

The complexity of the mechanism of localized corrosion leads to the complexity of the role of molybdenum in it.

We should realize that the role of molybdenum should be more than a simple inference of “enhancing passive film”

or “inhibiting active dissolution”. It is reasonable for us to believe that molybdenum affects the entire process of localized corrosion rather than a single reaction. Some experimental data has proved this [46]. Molybdenum can not only enhance the performance of the passive film (manifesting as lower nucleation probability of localized corrosion and longer delay time of breakdown of the passive film) but also inhibit the active dissolution after the breakdown of the passive film (manifesting as smaller metastable pitting current density and size) [36]. Heon-Young et al. [47] found that Mo increased the protectiveness of passive film of high intersti- tial stainless steels by decreasing the number of point defects in the film. Mo increased Epit and repassivation potentials ( Erp ) due to enhanced passive film and reduced active dissolution rate. And the value of Epit−Erp decreased with Mo content increased, which meant stainless steels with higher Mo content were more likely to repassivate, as shown in Fig. 7.

However, more detailed mechanisms are still inconclu- sive, and further research is needed.

Fig. 7 a Cyclic potentiodynamic polarization curves of 0Mo, 1Mo, and 2Mo alloys measured in 4 mol·L⁻¹ NaCl. b Average pitting and repassivation potentials with standard deviation values (scatter band)

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4 Outlook

Under the conditions that the breakdown of the passive film is the critical step, if there are experiments proving that the presence or content of molybdenum in stainless steel does not change the results such as pitting nucleation fre- quency and passivation current density, these experiments may prove that Mo cannot enhance the performance of passive film or the enhancement is not significant. But there is no such evidence at present. Under the conditions that the pitting growth kinetics is the critical step, if there are experiments proving that the presence or content of molybdenum in stainless steel does not change the results such as E_pit and active dissolution current density, then it may be proved that the effect of Mo on activation dissolution kinetics can be ignored. But there is no such evidence at present, either. It should be noted that the design of these experiments must be based on the premise of figuring out under what circumstances the breakdown of passive film or pitting growth kinetics becomes the critical step, which is obviously not easy.

To further reveal the detailed effect of molybdenum, the following aspects may be worthy of attention in future research. First, since all the commercial stainless steels have rather complex microstructure, they are not proper specimens for us to explore the fundamental and microscopic mechanisms. Adopting a series of carefully designed Fe–Cr–Ni–Mo alloys may be helpful. Second, pitting is a stochastic and abrupt process and it is not easy to fol- low a single pit in a bulk specimen. Thus, usage of one- dimentional artifical pit and controlling the pit geometry may better reveal the role of Mo in the dissolution process.

In addition, the adoption of advanced in-situ electrochemical techniques and characterization methods will be a strong tool to monitor the real-time changes of passive film and the local pitting environment.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (Grant Nos. 51901046, and 51871061).

Author contributions Yang-Ting Sun and Xin Tan wrote the draft;

Long-Lin Lei and Yang-Ting Sun collected the references; Jin Li contributed to conceived the idea of the study. All authors contributed to the writing and revisions.

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Dr. Jin Li is a professor in Depart- ment of materials science, Fudan University, and a guest professor of Institute of Materials Research, China Academy of Engineering Physics. He is cur- rently the vice chairman of Chi- nese Society for Corrosion and Protection. After graduating from Tsinghua University in 1982 with a B.Sc. degree, he received his M.Sc. degree from Institute of Metal Research, Chi- nese Academy of Sciences in 1986, and his Ph.D. degree from University Paris XI in 1990. His expertise is corrosion of stainless steel and corrosion evaluation.