On the performance of commercially available corrosion-resistant nickel alloys: a review

1 Introduction

Most of the globally produced nickel (Ni) (>60%) is used in the manufacturing of stainless steel. Only approximately 13% of the world production of Ni is used for the fabrication of Ni-based alloys. The most common Ni-based alloys are known by their commercial names such as Hastelloy, Inconel, Monel, and Incoloy alloys although they all have a unique unified numbering system (UNS) number. Ni-based alloys are usually divided in two main groups: high-temperature alloys, designed to withstand high-temperature and dry or gaseous corrosion (Du et al., 2006; Sequeira & Hocking, 1981; Sui et al., 2009; Zhao et al., 2006), and corrosion-resistant alloys (CRAs), intended to resist to low-temperature aqueous corrosion (Alves, 2012; Alves & Heubner, 2010; Friend, 1980; Heubner, 2012; Revie, 2011; Wahl, 2012). The corrosion performance of several commercially available CRAs with the focus on the applicability of different Ni-alloys in a variety of acidic and caustic environments (either oxidizing or reducing) and temperatures will be addressed in this paper.

On the basis of our own experience and on data reported in the literature, it is clear that although most of the Ni-based alloys have good mechanical properties and corrosion resistance, their corrosive degradation is generally a problem when they are used at typical industrial operation conditions. Therefore, updated information on their corrosion performance, in a general sense, is appropriate, as reported here, by covering old and recent corrosion results along with proper morphological evidence.

Corrosion-resistant Ni-based alloys can be grouped according to their chemical composition: commercially pure Ni, nickel-copper (Ni-Cu) alloys, nickel-molybdenum (Ni-Mo) alloys, nickel-chromium-molybdenum (Ni-Cr-Mo) alloys, and nickel-chromium-iron (Ni-Cr-Fe) alloys. Table 1 gives the composition of the most popular wrought Ni-based alloys. Although the Ni-based alloys in Table 1 contain a significant fraction of other elements, these alloys still retain the face-centered cubic lattice structure (fcc or gamma) of the Ni base element, which is responsible for properties such as excellent ductility, malleability, and formability.

2 Commercial Ni

Commercial pure Ni normally contains at least 99% of Ni (Table 1). Ni is a metal nobler than Fe but less noble than Cu (Pourbaix, 1974). Thermodynamically (i.e. according to Pourbaix diagram), Ni is shown to dissolve as Ni²⁺ at pH <9, to form nickel (II) oxide (NiO) at pH 9–12, and to dissolve as HNiO₂^- at pH >12. NiO is shown to be stable approximately between -0.5 and +0.3 V versus normal hydrogen electrode (NHE). In practice, it is found that Ni has good corrosion resistance in environments such as cold and hot aqueous caustic solutions. Ni is also highly tolerant to cold reducing agents because of the slow discharge of H₂ from its surface. However, as pointed out by the two main theories on the exact nature of the passive film on Ni, either the film is entirely NiO with a small amount of nonstoichiometry giving rise to Ni³⁺ and cation vacancies or it consists of an inner layer of NiO and an outer hydrous layer of Ni(OH)₂ (Schutze et al., 2015; Sikora & Macdonald, 2002). The p-type character of the film is consistent with the diagnostic criteria obtained from the point defect model (Memming, 2001) for a passive film, in which the majority defect is the metal vacancy. On the upper end of the Ni passive state and on the Ni transpassive state, explored semiconductive properties by dc and ac data are consistent with the formation of a thick, porous anodic oxide or oxihydroxide film overlaying a barrier layer of reduced thickness, with charge transfer being due to the oxidation ejection of Ni³⁺ or Ni⁴⁺ species from the barrier layer or oxygen evolution within the pores, or both. In the presence of oxidants (such as dissolved air) in acids and in neutral aqueous solutions, the free corrosion potential moves to the zone of high anodic potentials (in other words, to the passive-transpassive region) where the Ni passive film is less protective, and in these conditions, the corrosion rate of the metal increases (Chawla & Gupta, 1993; Friend, 1980; Pourbaix, 1974). The p-type character of the film at low anodic potentials is also consistent with the fact that the corrosion rate of Ni in reducing acids increases rapidly as the temperature and the acid concentration increase (Schmickler, 1996; Schutze et al., 2015).

In general, electrodeposited nanocrystalline Ni retains the high corrosion resistance of conventional polycrystalline Ni. However, some differences were observed and mainly attributed to the differences in intercrystalline surface fraction and intercrystalline defect site distribution for nanocrystalline and polycrystalline materials. For example, nanocrystalline Ni corrodes more uniformly with higher passive current densities, whereas polycrystalline Ni suffers severe localized corrosion at higher anodic potentials. The beneficial effects of nanocrystalline processing for corrosion resistance are receiving considerable attention as a result of their unique physical, chemical, and mechanical properties (El-Sherik, 1993; Koch et al., 2007; Rofagha et al., 1991).

The widest industrial application of commercial pure Ni is in the handling of cold and hot caustic solutions. It has been shown that the corrosion rate of alloyed Ni in caustic solutions is greater than that of commercially available pure Ni. Ni can also be used in reducing acids such as hydrochloric acid (HCl) at room temperature. The corrosion rate of Ni 200 in boiling reducing acids, e.g. sulfuric acid (H₂SO₄), is high, and therefore other alloys such as B-2 or B-3 alloys are recommended for this application. Moreover, it has been shown that in the presence of oxidizing acids, e.g. boiling nitric acid (HNO₃), the corrosion rate of Ni 200 is high, whereas alloys containing Cr would be passivated and, consequently, have low corrosion rates.

Commercial pure Ni is not susceptible to stress corrosion cracking (SCC) except under heavily cold operating conditions in concentrated caustic solutions at high temperatures (>250°C) (Crook, 2007; Hoffmeister, 2011). Furthermore, commercial Ni is not susceptible to hydrogen embrittlement (i.e. loss of ductility leading to brittleness after exposure to H₂) because the solubility and diffusivity of H in Ni are low and this material has low mechanical strength. The yield stress (YS), i.e. the stress level at which a material stops behaving elastically, is 190 MPa in case of annealed Ni 200 at room temperature. Its ultimate tensile stress (UTS), i.e. the maximum stress that it can withstand while being stretched or pulled before failing or breaking, is 465 MPa, its elongation to rupture, i.e. elongation between zero stress and final rupture, is 50% of original specimen length, and the Rockwell B (Vickers) hardness is 60 (160). Ni can suffer pitting corrosion in solutions containing Cl^- and SO₄^-. Corrosion pits are always crystallographic and not well defined because the first stage of pitting, usually explained through the formation of a protective passive film in competition with the nucleation and growth of a chloride layer, is modified by the poor protectiveness of the Ni passive film, as already mentioned (Schutze et al., 2015; Sikora & Macdonald, 2002; Szklarska-Smialowska, 1986).

3 Nickel-copper (Ni-Cu) alloys

Ni-Cu alloys are generally known as Monel alloys with Monel 400 being the most popular among them. The addition of nobler Cu to Ni increases the nobility of the alloy compared with pure Ni (Friend, 1980) and, consequently, leads to lower corrosion rate of alloys such as Monel 400 than pure Ni in the active region (reducing conditions) and a lower corrosion rate than Cu under oxidizing conditions (Moniz & MacDiarmid, 1997). Conversely, the addition of Cu to Ni increases the corrosion rate under oxidizing conditions because Cu increases the critical current for passivation and the current density in the passive region of potentials of Ni (Hummel & Smith, 1988). In general, Monel 400 can be used to handle reducing acids such as HCl and H₂SO₄ (<80%) from room temperature up to 70°C (Agarwal & Wessel, 2004; Rebak, 2000). Monel 400 is reasonably resistant to corrosion in air-free hydrofluoric acid (HF) (Chawla & Gupta, 1993).

Figure 1 shows the study of corrosion rate in the acid media of Ni 200 and Monel 400, measured by means of immersion tests, as a function of hydrochloric acid concentration at 30°C. As expected, the presence of Cu in acidic media increases the corrosion rate significantly. Furthermore, the presence of air and oxidizing salts considerably increases the corrosion rate of Monel 400 in these acids.

Figure 1:

Corrosion rate of Nickel 200 and Monel 400 in hydrochloric acid solutions at 30°C.

Still, the effect of temperature on the corrosion rate of Monel 400 is not as pronounced as for Ni 200 (Friend, 1980) (Figure 2). At each temperature, as the acid concentration increased between 5% and 60%–80%, the corrosion rate decreased, most likely due to the decreased solubility of oxygen (O₂) in the electrolyte solution with increased acid concentration. Depending on the temperature, at H₂SO₄ concentrations higher than 60%–80%, the corrosion rate increased rapidly, probably because H₂SO₄ becomes oxidizing.

Figure 2:

Temperature effect on corrosion of Nickel 200 and Monel 400 in 5% of hydrochloric acid solution.

Investigation of the effect of the electrolyte composition on the corrosion rate of Monel 400 showed that corrosion rate in HCl is approximately four times higher than that in H₂SO₄ electrolyte of similar concentration. The corrosion rate decreases between 5% and 70% in the H₂SO₄ solution, whereas the corrosion rate increases continuously as the concentration of acid increases in the HCl solution (Rebak, 2000).

One of the most important industrial applications of Monel 400 is in the handling of HF. Braun et al. (1957) studied the effect of acid concentration on the corrosion behavior of Monel 400 at 60°C in liquid, interface, and vapor phase exposures (Figure 3) using a purge gas containing 1% O₂ in N₂ (10000 ppm O₂). It was observed that interface and vapor phase exposures caused corrosion at considerably higher rates and in a nonuniform manner, whereas for liquid exposure, the acid concentration had little effect on corrosion rate. Other laboratory tests of Monel 400 in HF solutions showed that the corrosion rate was high when air or O₂ was present. The effect of aeration is considerably less severe in anhydrous acid than for aqueous solutions. Monel 400 exhibits excellent corrosion resistance in the absence of O₂ up to 149°C. It has been reported that Monel 400 is susceptible to pitting corrosion in seawater, especially under quiescent conditions (Ali & Ambrose, 1991; Bogar & Peterson, 1985; NiDI, 1968). Deepening of the corrosion pits was observed to occasionally stop, while the lateral growing would start.

Figure 3:

Effect of HF concentration on the rate of corrosion of alloy 400 at 60°C adapted from (NiDI, 1968).

Attributed to their low mechanical strength, alloys Ni 200 and 400 are not very susceptible to SCC. The YS of annealed Monel 400 at room temperature is 260 MPa, the UTS is 550 MPa, the elongation to rupture is approximately 50%, and the Rockwell B hardness is 72. However, Monel 400 showed to suffer SCC in acidic mercury (Hg) containing solutions, in liquid Hg, in HF, and in fluorosilicic acid (H₂SiF₆) (Figure 4) (International Nickel Company, 1968). In HF, cracking is transgranular, and the highest susceptibility occurs in the vapor phase, particularly in the presence of air (Pawel, 1994), being reduced with reduction of aeration. Highly stressed alloy 400 is also vulnerable to SCC in ammonia (NH₃) vapor at 300°C (Theus et al., 1982). All types of environmentally induced cracking are reduced in Monel 400 by heat treatment, which eliminates residual stresses, and in cold worked microstructures.

Figure 4:

Transgranular crack of Monel 400 caused by HF medium.

Many other Ni-Cu alloys and Ni-Cu composite coatings are being investigated in acid, neutral, and basic corrosive solutions (Badawy et al., 2005, 2006, 2009, 2010; Brizuela et al., 2006; Druska & Strehblow, 1995; Dutta, 2009; Ghosh et al., 2006; Huot et al., 1991; Ismail et al., 2004; Liu & Zhao, 2004; Liu et al., 2010; Munoz et al., 2004; Nady et al., 2012; Raj, 1992; Rosalbino et al., 2013; San et al., 2011, 2012; Tang et al., 1995; Zhao & Liu, 2005) in which they are finding new applications (Badawy et al., 2005, 2006, 2009, 2010; Druska & Strehblow, 1995; Ismail et al., 2004; Munoz et al., 2004; Nady et al., 2012; San et al., 2011). In addition, recent studies have shown that Ni-Cu composite coatings have exhibited many promising features for further applications, such as improved hardness, increased wear resistance, and protection against corrosion (Brizuela et al., 2006; Ghosh et al., 2006; Liu & Zhao, 2004; Liu et al., 2010; San et al., 2012; Zhao & Liu, 2005). Moreover, studies of Ni-Cu alloys and coatings for their usage in the industrial alkaline water electrolysis, which requires searching for more cheaper and available materials with good catalytic activity, have also been reported (Cardoso et al., 2015; Rosalbino et al., 2013; Solmaz et al., 2009). From the experimental results, it is possible to conclude that the novel Ni-Cu electrodes showed higher intrinsic catalytic activity than the pure.

4 Nickel-molybdenum (Ni-Mo) alloys

Ni-Mo alloys are generally known as Hastelloy B-type alloys. These alloys contain approximately 28% of Mo dissolved in an austenitic Ni matrix (Table 1). In general, the addition of Mo in Ni encourages the formation of a cauliflower-like surface morphology resulting in higher surface area. Jović et al. (2008) studied several Ni-Mo-O alloy powders with different Ni/Mo ratios and reported that with the decrease of the Ni/Mo ratio, cauliflower-type particles with rounded edges are formed while the cracks are more pronounced (Figure 5). The high tensile stresses in the powder and the hydrogen evolution reaction are the main reasons of the large number of cracks (Jović et al., 2008).

Figure 5:

(A, B) A SEM micrographs of the as-deposited powder for the Ni/Mo = 1/3, at different magnification. Reprinted from ref. (Jović et al., 2008) with permission from Elsevier.

Currently, Ni-Mo alloys and Ni-Mo coatings are showing excellent electrocatalytic activity for the alkaline water electrolysis and attracted more and more attention, such as Ni-MoO and Ni-MoO₂, among others (Aaboubi, 2011; Gonzalez-Buch et al., 2016; Highfield et al., 1999; Krstajić et al., 2008, 2011; Navarro-Flores et al., 2005; Tasic et al., 2011; Wang et al., 2015). For example, the Ni₇₁Mo₁₀B₁₉ prepared by melt spinning displayed a Tafel slope of 120 mV decade^-1 and an exchange current density of 0.0162 mA cm^-2 as a cathode electrode for the hydrogen production in 1 m KOH at 25°C, which are excellent results (Rauscher et al., 2016).

It has also been shown that when Mo is added to Ni, the corrosion rate of the resulting alloy in boiling 10% HCl decreased as the amount of Mo increased (Flint, 1960). The series of alloys B (now obsolete), B-2, and B-3 was developed to withstand reducing HCl at all concentrations and temperatures (Chawla & Gupta, 1993). The performance of B-type alloys in HCl is comparable with that of tantalum (Ta) and zirconium (Zr) only (Schweitzer, 1995). Although the B-3 alloy has corrosion resistance similar to that of the B-2 alloy, it has greater thermal stability; thus, the annealing of the B-3 alloy is not necessary after welding (Klarstrom, 1993). Besides HCl solutions, B-2 and B-3 alloys are also used in other corrosive reducing environments such as H₂SO₄, HF, H₃PO₄, acetic acid (CH₃COOH), and formic acid (HCOOH). The major weakness of B-type alloys is their poor resistance to oxidizing environments. Thus, oxidizing impurities in HCl, such as ferric ions (Fe³⁺), are detrimental to the performance of the Ni-Mo and Ni-Cu alloys. Under such conditions, the Ni-Cr-Mo alloys present the best choice because they are tolerant to residuals, although they are temperature limited at the higher acid concentrations. The Ni-Mo alloys have been intensively used for service in pure HCl. This selection is based on evidence that alloys such as B-2 and B-3 exhibit rates of 0.5 mm year^-1 (20 mpy) or less over significant concentrations and temperature ranges (0%–20% HCl, 25°C–120°C). In fact, Mo is the most important alloying element for good performance of Ni-based alloys in pure HCl (reducing conditions). The corrosion rate in boiling HCl decreases as the content of Mo in the alloy increases. For the most part of metals and alloys, the properties and durability of the passive films formed on them are relative to the inherent corrosion resistance of the alloy and the corrosivity of the environment. Accordingly, for Ni-Mo alloys, the polarization resistance that usually reaches a maximum in their passive region in many circumstances decreases with the increase of film-growth potential in their transpassive region because of the oxidant injection of cation defects. This explains why for the oxide-covered Ni-Mo alloys exposed to oxidizing acidic solutions, the corrosion rate increases continuously as the electrochemical potential is increased (Marcus et al., 1988). Figure 6 shows the corrosion rate of the B-3 alloy in deaerated and aerated 1 m HCl solution as a function of temperature. In aerated solution, the corrosion rate is higher than that in the deaerated solution. For the same reason, Ni-Mo alloys cannot be used to handle oxidizing acids such as HNO₃ and chromic acid (H₂CrO₄).

Figure 6:

General rate of corrosion of the B-3 alloy in aerated and deaerated 1 m HCl.

Electrochemical measurements of the corrosion rate of the B-3 alloy as a function of temperature, for deaerated 1 m HCl and 1 m HBr solutions, showed that the effect of temperature is greater in HCl solutions than that in HBr. This means that the corrosion apparent activation energy (E^app) of the B-3 alloy in HBr solutions is lower than that in HCl.

Figure 7 shows the effect of temperature on the corrosion rate of B-3 and C-2000 alloys in deaerated 1 m HCl solution. The corrosion rates of C-2000 and B-3 alloys are similar for temperatures lower than 65°C. However, the increase in the corrosion rate of the C-2000 alloy is more pronounced than that for the B-3 alloy, as the temperature increases higher than 65°C, i.e. the corrosion E^app of the B-3 alloy in HCl is lower than that of the C-2000 alloy.

Figure 7:

Rate of corrosion of B-3 and C-2000 alloys as a function of temperature in 1 m HCl deaerated solutions.

Because passivating oxide films are not formed on the surface of Ni-Mo alloys, they do not suffer localized attack in the classical sense, such as the halide-induced pitting and crevice corrosion of stainless steels. In some service applications, these alloys would not corrode uniformly but might develop some shallow cavities on their surface, as a result of confined enhanced corrosion due to microgalvanic couples with oxidizing agents or other impurities. Ni-Mo alloys show resistance to chloride-induced cracking in boiling magnesium chloride (MgCl₂) solutions (Hodge, 1983; Kolts, 1982). Because of a solid-phase transformation originating ordered intermetallic phases such as Ni₄Mo, the B-2 alloy and, to some extent, the B-3 alloy lose ductility at temperatures between 550°C and 850°C. As a consequence of the precipitation of these ordered phases (which may occur in the heat-affected zone during welding of the B-2 alloy, changing the deformation mechanisms), these alloys may become susceptible to environmentally induced cracking such as hydrogen embrittlement, or intergranular SCC when exposed to organic solvents containing H₂SO₄ at a temperature of 120°C (Takizawa & Sekine, 1985) or to transgranular SCC in the presence of hydroiodic acid (HI) at temperatures higher than 177°C (Sridhar & Cragnolino, 1992).

SCC characteristics of commercially available Ni-Mo alloys in a reducing environment were investigated with the alloys in their mill-annealed state and after heat treatment (HT) to simulate the effects of welding and stress relief in clad vessel fabrication (Nakahara & Shoji, 1996). The cracking mode varied with HT and environment electrochemical potential. Transgranular fissuring in all alloys was found both for mill-annealed and aged materials. At cathodic potentials, namely, 100 and 400 mV below the free corrosion potential, only the aged alloys showed intergranular cracking. This was attributed to hydrogen embrittlement because of the higher amount of intergranular brittle cracking at the lower applied cathodic potential (Nakahara & Shoji, 1996).

5 Nickel-chromium-molybdenum (Ni-Cr-Mo) alloys

Many Ni-Cr-Mo alloys are nowadays commercially available. All these were derived from the original C alloy (N10002) that was introduced in the market in the cast form in 1932. Some of the wrought Ni-Cr-Mo alloys that are currently available are C-276, C-4, C-22, 686, 59, and C-2000 (see Table 1 for compositions). The proportion of elements such as Mo and Cr is different in each of the alloys. Besides Ni, Cr, and Mo, some alloys contain tungsten (W) (C-276, C-22, and 686) or a small amount of Cu (C-2000). These alloys have good corrosion resistance in reducing environments (effect of Mo) and in oxidizing environments (effect of Cr). The role of Cr in Ni-Cr-Mo alloys is the same as in stainless steels; i.e. it promotes the formation of a passive film under oxidizing conditions such as aerated acids. Cu, Mo, and W reduce the corrosion rate of the alloys under active corrosion conditions, with Mo and W also increasing the mechanical strength of these alloys. The C-276 alloy (16Cr-16Mo-4W) is the wrought Ni-Cr-Mo alloy that currently most closely resembles the original C alloy composition, and it is by far the most popular wrought Ni-Cr-Mo alloy. The Hastelloy C-276 alloy was developed in the late 1960s by reducing the carbon (C) and silicon (Si) content of the original C alloy to obtain alloys with increased thermal stability. In many applications, the C-276 alloy was used in the as-welded conditions, whereas under the same conditions, alloy C would have suffered intergranular attack in the heat-affected zone. The reduction of the Si content also led to a decrease in the general corrosion rate of the new alloy. The Hastelloy C-22 alloy, first commercialized in 1984, was an improvement over the C-276 alloy both in corrosion resistance in oxidizing environments and in localized corrosion, e.g. halide-induced pitting and crevice corrosion. The Hastelloy C-2000 alloy was developed in the early 1990s to offer greater resistance to corrosion under reducing conditions than, for example, the C-22 alloy without sacrificing its resistance to oxidizing conditions (Crook, 1996; Crook et al., 1997).

As expected, laboratory tests showed that alloys with the largest quantities of Cr (C-22, C-2000, and 59) perform better under oxidizing conditions, and those with the largest amount of Mo perform better under reducing conditions. Because of their versatility and good corrosion resistance under many different conditions, Ni-Cr-Mo alloys are the most widely used Ni-based alloys (Dillon, 1997). Similarly to Ni-Cu and Ni-Mo alloys, at the same temperature, the corrosion rate of Ni-Cr-Mo alloys is higher in HCl than that in H₂SO₄ of similar composition. In general, the corrosion rates in HCl and H₂SO₄ solutions increase in the order alloy 686 < alloy 59 < alloy C-22, both under reducing conditions and for 80% H₂SO₄ solution. Again, the corrosion rate under reducing conditions decreases as the amount of Mo or Mo + W in the alloys increases. Mo + W amount is as high as 20% in 686 alloy and 16% in alloys 59 and C-22 (Table 1). Ni-Cr-Mo alloys are the most resistant Ni-based alloys to the classic chloride-induced localized corrosion that troubles the stainless steels. Ni-Cr-Mo do not all have the same resistance to localized attack. It is known that the addition of elements such as Cr, Mo, W, and N is beneficial to the resistance of alloys to pitting and crevice corrosion. Still, the correct proportion of the beneficial alloying elements is necessary – too much of one element does not compensate for the lack of another.

Pitting corrosion is an insidious type of localized corrosion in which cavities appear on a smooth passivated surface. Crevice corrosion is another type of localized attack immediately below the metal surface exposed to the bulk solution, under a creviced or occluded region (such as under a gasket). From the environmental point of view, both types of attack depend on the concentration of halides (especially chloride), the electrochemical potential, and the temperature. After identifying the elements that gave stainless steels resistance to pitting corrosion, researchers developed a pitting resistance equivalent (PRE) to rate the resistance of stainless steels to localized corrosion (Agarwal & Köler, 1997; Levey & Vanbennekom, 1995; Sedriks, 1986). As PRE for the alloy increased, the resistance to localized corrosion increased. One of the formulas most commonly used for PRE is as follows:

The dependence of the critical pitting temperature (CPT) in the green death (11.9% H₂SO₄ + 1.3% HCl + 1% FeCl₃ + 1% CuCl₂ boiling at 103°C) and yellow death (4% NaCl + 0.01 m HCl + 0.1% Fe₂(SO₄)₃) solutions on PRE for several alloys is presented in Figure 8.

Figure 8:

CPT as a function of the PRE.

Despite the beneficial effect of Cr and Mo in Ni-based alloys (Asphahani, 1980; Postlethwaite et al., 1988), the relationship between the CPT and the PRE number of these alloys is not linear, especially in the green death solution. For example, the C-276 alloy has a PRE higher than the C-22 alloy and a CPT lower than that of C-22, but this is the alloy that is more resistant to localized corrosion.

As a consequence of its excellent resistance to localized corrosion (Gruss et al., 1998; Rebak & Koon, 1998), the C-22 alloy was selected by the U.S. Department of Energy for the inner wall of the high-level nuclear waste containers to be buried permanently at the Yucca Mountain site (McCright & Clarke, 1998). Like other Ni-based alloys, Ni-Cr-Mo alloys are resistant to chloride-induced SCC (Kolts, 1987).

Ni-based alloys are known to be susceptible to caustic cracking. For example, the performance of the C-276 alloy in a severe test for susceptibility to intergranular attack, under conditions of slow strain rate (Payer et al., 1979), designed to verify mill production only and not to compare alloys for use in applications such as flue gas desulfurization, showed significant rates of corrosion indicating vulnerability to transgranular cracking in 50% NaOH at 147°C. Conversely, for C-shape coupons of the C-22 alloy, submitted to mill annealed and aged for 24 h at 677°C after immersion in 50% NaOH solution at 147°C for 720 h, a high resistance to sensitization was observed.

In Ni-Cr-Mo alloys aged for a long time at temperatures higher than 600°C, long-range ordering reactions and precipitation of an intermetallic μ-phase can occur. The occurrence of these solid-state reactions during aging would reduce the ductility of the Ni-Cr-Mo alloy. It has been reported that aged C-276 alloy is susceptible to hydrogen-induced cracking in environments containing hydrogen sulfide (H₂S) (Kane et al., 1977; Sridhar et al., 1980).

Ni-Cr-Mo alloys find application as electrodes for the H₂ discharge in the production of hydrogen by water electrolysis (Luo et al., 2006). Data regarding phase stability (Turchi et al., 2006), graph theory (McCafferty, 2008), ennoblement studies (Martin et al., 2006), and environmental assisted cracking (Rebak, 2005) of several Ni-Cr-Mo-based alloys are available, showing the significance in terms of the corrosion resistance and mechanical properties of the concerned materials.

6 Nickel-chromium-iron (Ni-Cr-Fe) alloys

Among the main commercial Ni-Cr-Fe alloys, the major ones are alloys 600 and 601, alloy 800 and its variations, alloy 690, chromia/alumina-forming alloy 602CA, and chromia/silica-forming alloy 45TM (Table 1). These alloys find wide application in the chemical and petrochemical industries, among others, mostly at high temperatures. Alloy 45TM, a silicon-containing high Cr-Ni alloy, showed improved sulfidation properties. Ni-Cr-Fe alloys and Ni-Cr-Fe-Mo-Cu alloys find similar applications to the Ni-Cr-Mo alloys (Revie & Uhlig, 2008). In general, Ni-Cr-Fe alloys are more resistant to corrosion than stainless steels but less resistant than Ni-Cr-Mo alloys. For example, alloy 600 can be used to handle hot water containing chlorides because its Ni base makes it resistant to the chloride cracking that affects the stainless steels. Alloy 600, however, has a low Cr content and contains no Mo, and it is therefore not resistant to pitting and crevice corrosion in oxidizing atmospheres and in reducing acids.

Although alloy 600 was widely used for the tubes in the steam generators in nuclear power plants, it has subsequently been found to suffer SCC in pure water at temperatures higher than 300°C and in hot caustic solutions (150°C–200°C). On the other hand, alloy 690, which contains twice as much Cr as alloy 600, has proven to be more resistant to SCC than stainless steels because of its higher Ni content. The Hastelloy G-30 alloy has good resistance to corrosion in reducing acids by virtue of its Mo and Cu content and in oxidizing conditions because of its high Cr content (30%). The presence of Mo in the G-30 alloy gives it a good resistance to localized attack in chloride-containing environments although other alloys, such as C-22, are more resistant than the G-30 alloy to chloride-induced localized attack.

The high Ni content (minimum 72%) of alloy 600 makes it similar to pure Ni for applications such as hot caustic solutions and dry Cl₂. Although it is known that the passivating effect of Cr is less effective in Ni-based alloys than that in Fe-based alloys (stainless steels) (Rebak & Crook, 2000), under some oxidizing conditions, alloy 600 has greater resistance to corrosion than Ni 200 because of the modest Cr content of alloy 600 (15.5%) (Roy et al., 1998).

Table 2 shows the corrosion rates of alloys 600 and 825 and the G-30 alloy in the green death solution (which chemically breaks down at temperatures higher than 120°C) and in the Streicher test (ASTM G 28 A; 236 ml H₂SO₄, 25 g iron(III) sulfate hydrate, Fe₂(SO₄)₃·xH₂O, and 400 ml distilled water). In both solutions, the lowest corrosion rate was observed for the G-30 alloy and the highest for alloy 600. Alloy 600 is not very resistant to localized attack, such as crevice and pitting corrosion, again because of the modest Cr content and absence of other beneficial elements such as Mo or W. The CPT of the G-30 alloy in the yellow death solution is 70°C, whereas the CPT for alloy 825 in the same solution is 25°C. During the screening of candidate materials for nuclear waste containers, by use of potentiodynamic polarization tests in acidic 10% NaCl solution at 90°C, it was shown that the G-30 alloy had much lower susceptibility to pitting corrosion than alloy 825 (Rofagha et al., 1991). The G-30 alloy is more resistant to pitting corrosion than alloy 825 because of its higher Cr and Mo content, plus the additional W (2.5%).

Because of its high Ni content, alloy 600 is resistant to SCC in chloride-containing solutions; however, alloy 600 is susceptible to localized attack in HF-containing environments (Pawel, 1994). Because of its high importance in the nuclear industry, the stress cracking of alloys 600 and 690 in pure water and in caustic solutions has been widely studied in the last 30 years (Staehle, 1996). Probably because of its higher Cr content (29%), alloy 690 is generally more resistant to intergranular or transgranular cracking than alloy 600. The susceptibility to cracking of alloys 600 and 690 depends on environmental and metallurgical factors. In the first group, temperature, level of tensile stresses, presence of H₂ gas, solution pH, and electrochemical potential must be considered, as well as the amount of cold work and heat treatment regarding the second group of factors.

Slow strain-rate tests and U-bend tests have shown that alloy 825 is susceptible to transgranular SCC in 45% MgCl₂ solutions at temperatures higher than 146°C. Alloy 825 can suffer cracking under the aggressive conditions encountered in supercritical water oxidation (SCWO) treatment (Mitton et al., 2000).

Recently, alloys sintered with Ni-Fe-M (M=Cr, Mn, Cu) have been characterized using microstructural and electrochemical techniques in view of their possible applications as electrocatalytic materials for hydrogen evolution reaction (Rosalbino et al., 2013). This is a process in which hydrogen may be produced by true renewable and fully environmentally friendly energy sources, without evolution of the green-house gas, CO₂. The experimental data obtained with the ternary electrode indicated that alloying Ni-Fe with Cr, Mn, and Cu leads to an increase of electrocatalytic activity in hydrogen evolution with respect to the Ni-Fe electrode, and this has been discussed on the basis of the Engel-Brewer valence band theory (Jakšić, 2009) and the electronic structure of alloys. New materials that present better corrosion resistance, are low cost, and are easy to manufacture are being continuously developed. In this context, alloy 33 is an austenitic material (33 Cr-32 Fe-31 Ni) that began to be used in 1995, and it has been studied by dc, ac, Mott-Schottky analysis, and other techniques because of its interest in marine medium and other environments (Cardoso et al., 2008); alloy 690 TT (Ni-30 Cr-10 Fe) was chosen in the mid-1980s as the best heat transfer tubing material, and manufacturing practices to optimize its corrosion resistance for nuclear-pressurized water reactor steam generators have been described (Harrod et al., 2001); amorphous/microcrystalline electrodeposited Fe-25-Ni-25 Cr alloys were studied using electrochemical techniques, X-ray diffractometry, and conversion electron Mössbauer spectroscopy because of their greater importance in recent years as substitutes for stainless steel (Sziraki et al., 2000); and alloy 600, alloy 82/182, and others were subjected to detailed characterization by SEM, TEM, AES, XPS, GZXRD, and ion milling to establish the parameters that are influential on the primary-water SCC initiation and growth (Celin & Tehovnik, 2011; Marioli & Kuwana, 1993; Rosecrans & Duquette, 2001; Ziemniak & Hanson, 2006).

7 Cast Ni-based alloys

With the exception of some high silicon and proprietary grades, cast Ni-based alloys generally have wrought approximate equivalents (Table 3). Besides some chemistry differences to improve castability and soundness, cast and wrought Ni-based alloys are often used in combination because of their similar performance (Gossett & Schweitzer, 1996).

Carbon content is usually a distinguishing factor between the heat- and the corrosion-resistant alloys, but the division may be difficult mainly for alloys used in the 480°C–650°C range. Ni and Ni-Cu, Ni-Cr-Mo, Ni-Mo, and special proprietary Ni-based alloys are used for corrosive applications. Important properties of these alloys are also their strength and resistance to wear and galling. Some Ni-Cr-Fe alloys are also used in high-temperature applications.

Good performance of a cast Ni-based alloy generally depends on the microstructural quality (amount of interdendritic segregation, secondary carbides, and intermetallic phases) of the castings; failures are commonly attributed to inappropriate casting techniques and/or heat treatment. The corrosion rates of cast alloys therefore vary significantly from one cast to another although the overall composition is the same.

Table 4 lists comparative corrosion rates of several cast alloys and the corrosion rates of the corresponding wrought alloys. The corrosion rates of the corresponding wrought alloys are generally lower than those of the cast versions. Some of the Ni-based superalloys originally developed for aerospace applications are also being used for other applications. Among other alloys, despite the small current application volume, alloy 713C (0.12% C, 0.8% Ti, 6.1% Al, 2.2% Nb, 13% Cr, bal Ni), initially developed for turbine rotors, is nowadays used for diesel turbocharger wheels or high-temperature fasteners.

Other studies on the corrosion resistance of cast Ni-based alloys in the presence of Cl^- ions in highly aggressive solutions were described in the open literature (Luce, 1948; Peric et al., 1994; Schussler & Exner, 1993).

8 Summary and conclusions

Ni and Ni alloys are resistant to a wide variety of industrial corrosive environments. Ni by itself is a good corrosion-resistant metal in many useful applications. Moreover, its metallurgical compatibility with several other metals as alloying elements is the basis for many complex nickel-based alloy systems, which show unique resistance to corrosion and high temperatures. These properties are very useful for handling materials degradation in chemical processes present in petrochemical, marine, pulp and paper, agrichemicals, oil and gas, heat treat, energy conversion, and many other industries.

Besides exotic materials such as Ta and Pt, Ni-based alloys show the best performance regarding corrosion by mineral acids, as well as a particular resistance to localized corrosion in environments containing chloride, which are aggressive to stainless steels. Ni-based alloys can be broadly divided into alloys such as Ni-Mo (B-2, B-3) and Ni-Cu (alloy 400), which do not contain Cr and therefore are not passivated under oxidizing conditions, and alloys such as Ni-Cr-Mo (C-22, C-2000, 59, 686, etc.) and Ni-Cr-Fe (G-30, 825, etc.), which form a chromium oxide passive film under oxidizing conditions. Ni-Mo alloys, e.g. B-3, have excellent corrosion resistance in hot reducing acids such as HCl and H₂SO₄ but cannot withstand oxidizing conditions, where HNO₃ and HCl are contaminated with Fe³⁺ ions. Ni-Cr-Mo alloys, such as the C-2000 alloy, are multipurpose alloys that can be used both in reducing and oxidizing conditions. In this paper, the major Ni alloy systems are discussed, including their strengths, weaknesses and important applications.