13.2 Corrosion Process

Molten salts are a class of high temperature liquids that range from the low‐melting systems such as the LiNO₃–KNO₃ eutectic (m.p. 120 °C) and molten organic salts to the high‐melting systems of molten metal oxides, some of which have melting points in excess of 1400 °C. Three broad classes of molten salts may be distinguished. These are the simple ionic liquids such as molten halides and halide mixtures; the simple oxyanionic liquids such as molten nitrates, sulfates, and carbonates; and the complex polymeric oxyanionic liquids such as molten phosphates, borates, and silicates. Molten halides and oxysalts are the most interesting melts with regard to their occurrence in molten salt corrosion processes.

Molten salts are liquids with some characteristics that are different from those of liquids at room temperature. Molten salt studies are very important for understanding the liquid state because molten salts consist of ions and the principal forces between particles are coulombic interactions. The existence of coulombic interactions in molten salts is demonstrated by very high melting and boiling points, surface tensions, and electrical conductivities, in comparison with these properties of other liquids. Other properties of the pure molten salts, or ionic liquids, or molten electrolytes, are of the same order of magnitude as for nonpolar liquids, although many ionic liquids exist only at high temperatures. These properties are density, viscosity, refractive index, compressibility, vapor pressure, heat of vaporization, heat of fusion, heat capacity, etc. (Blander 1964; Galasiu et al. 1999; Sundheim 1964).

In general, the ionic character of the solid crystalline form persists in the molten state, although local association reactions may take place. The range of coexistence of metal–molten salt systems depends on two simple factors, namely, the relative electronation–de‐electronation potentials of the various constituents and their relative basicities. A measure of the basic or acidic strength of the system is given by

13.1

where pO²⁻ is equivalent to the pH for protolytic solvents. A high value of pO²⁻ indicates an acidic (and corrosive to metals) melt and a low value a basic melt. The self‐dissociation constants of pure oxyanionic melts indicate the acidic strengths of the liquids. pO²⁻ for . This shows that molten nitrates are more acidic than molten carbonates (at comparable temperatures). pO²⁻ values for oxyanionic species in dilute solution provide a useful means for predicting acid–base reactions between different species. The numerical value of the term pO²⁻ will depend upon the units of concentration employed: convenient ones are molarities, molalities, mole fractions, or mole ratios.

Many studies have been published on the corrosion in fused salts in terms of acid–base properties of the melts (Lewis 1971). There have been many different methods used in the determination of pO²⁻ values. Most of the measurements have been restricted to the low temperature systems of molten nitrates and chlorides. Acid–base reactions in molten sulfates are not so well documented.

The high electrical conductivity of many molten salts makes them particularly suitable as media for electrochemical investigation (Galasiu et al. 1999).

Electrochemical studies in oxyanion melts have been initially confined to nitrate and carbonate systems. Nitrates, being low melting, are convenient media to work with, and carbonates have considerable technological importance as electrolytes for high temperature fuel cells. Molten sulfates have low vapor pressure and high melting points, and their thermal stability depends on the nature of cations, alkali sulfates being the more stable. They appear to be convenient media for electrochemical studies, and they have received much attention due to their practical importance. For example, the sulfate–chloride provides a molten salt system over a wide range of temperatures relevant to gas turbine conditions, thus promoting many studies (Hocking et al. 1989). It follows from what has been said that effects analogous to “electrochemical” or “oxygen concentration” corrosion in aqueous systems can occur in salt melts. Accordingly, in metal/melt systems one possible way of ensuring adequate corrosion resistance is to choose conditions such that the metal is passive, which requires that it should become covered with an adherent, compact, insoluble film or deposit, preventing direct contact of the metal with its environment. Any melt that reacts with a metal to give a corrosion product insoluble in the melt is in principle capable of passivating the metal, e.g. passivity can be expected to occur in oxidizing salts in which metal oxides are sparingly soluble. Thus, iron is highly resistant to alkali nitrate melts because it becomes passive, and passivity has also been observed by electrode potential measurements of an iron electrode in chloride melts containing nitrates (Littlewood and Argent 1961), although in this case the oxide corrosion product is not particularly protective. In general, fused salts are “good” solvents for inorganic compounds so that passivity is not likely to be a widely encountered phenomenon.

“Wash‐line” attack is also a common feature of corrosion by molten salts in contact with air, because the anodic and cathodic reactions will not necessarily occur at the same metal site, and “anodic” and “cathodic” areas can develop as in aqueous solutions.

When a temperature gradient exists in a system containing metal in contact with molten salt, thermal potentials are set up, causing removal of metal at high temperature points and deposition of metal at cooler places. This mass transfer is essentially different in nature from that met in liquid metal corrosion, which is simply a temperature‐solubility effect. In fused salts, both the corrosion and deposition reactions are electrolytic, and it has been shown that an electrical path is necessary between the hot and cold regions of the metal. Edeleanu and Gibson suggest that this type of mass transfer be called “Faradaic mass transfer” to indicate that it requires an electrolytic current (Edeleanu and Gibson 1960).

Mass transfer deposits can lead to blockages in non‐isothermal circulating systems, as in the case of liquid metal corrosion. In fused salts, the effect can be reduced by keeping contamination of the melt by metal ions to a minimum, e.g. by eliminating oxidizing impurities or by maintaining reducing conditions over the melt.

Corrosion of alloys at high temperatures is complicated by effects due to diffusion, particularly where the alloy components have different affinities for the environment, and corrosion of an alloy in a fused salt at high temperature often exhibits features similar to those of internal oxidation. Selective removal of the less noble component occurs, and as it diffuses outwards, vacancies move inward and segregate to form visible voids (Kirkendall effect) (Sequeira and Amaral 2014). Since diffusion rates are faster at grain boundaries than in the grains, voids tend to form at the grain boundaries, and specimens often have the appearance of having undergone ordinary intercrystalline corrosion. More careful examination has shown, however, that in the case of Fe–18Cr–8Ni corroding in a fused 50–50 NaCl–KCl melt at 800 °C in the presence of air, the attack is not continuous at the boundaries, and the voids formed are not in communication with each other. In high nickel alloys, a greater proportion of voids is formed within the grains, and the appearance of intercrystalline attack is less marked. When Inconel is exposed to fused sodium hydroxide, a two‐phase corrosion product layer is formed, resulting from growth of the reaction product – a mixture of oxides and oxysalts – into the network of channels.

Selective removal of the less noble constituent has been demonstrated by chemical analysis in the case of nickel‐rich alloys in fused caustic soda or fused fluorides and by etching effects and X‐ray microanalysis for Fe–18Cr–8Ni steels in fused alkali chlorides. This type of excessive damage can occur with quite small total amounts of corrosion, and in this sense its effect on the mechanical properties of the alloy is comparable with the notorious effect of intercrystalline disintegration in the stainless steels.

13.3 Thermodynamic Diagrams

The thermodynamic diagrams of Pourbaix (1949) have been particularly useful in understanding the behavior of metals in contact with aqueous solutions. Pourbaix plots equilibrium potential against pH, and the diagrams divide themselves into regions of stability at different solid phases (compounds of the metal in question).

In molten salts also, free energies can be expressed as equilibrium potentials, and there are a number of functions of composition that might be used as the other variable. In this context, available thermodynamic data (Glushko 1984; JANAF 1971; Kelley 1949; Kubaschewski and Evans 1985) support the calculations. The oxygen ions are generally quite important in matters of molten salt corrosion, so the function pO²⁻ (defined by the expression 13.1) is often used as the equivalent to the pH in aqueous environments. A typical E versus pO²⁻ diagram for cobalt in molten sodium sulfate at 900 °C is shown in Figure 13.1 (Sequeira 2003). Areas of corrosion, immunity, and passivation are evident. More recent investigation on the thermodynamics of the molten salts is not only academic but also of practical interest (Barin 1993; Berkani and Gaune‐Escard 2011; Gaune‐Escard 2002). These studies are being used to further understand the behavior of metallic materials in molten salts (see also Section 3.3).

In the construction of E/pO²⁻ diagrams, there are two basic requirements, a reference scale of potential and a suitable standard state for oxidation activity. The first requirement has been satisfied by setting E° = 0 and dE°/dT = 0 for an appropriate reference electrode in the melt under consideration. For example, for nitrate and sulfate melts, the following electrode processes were assumed as basis for corresponding reference electrodes in these melts:

13.2

13.3

The second requirement is rather more difficult to fulfill since a satisfactory and unambiguous oxygen electrode of the type

13.4

is not yet experimentally established in oxyanionic melts. In fact, apart from other difficulties, peroxide and superoxide ions have been identified in the melt (Mamantov 1969; Sequeira 1989), which enhances the problem.

Actually, the E versus pO²⁻ diagram is probably more useful than the Pourbaix diagram because of the absence of kinetic limitations at elevated temperatures. The following problems, however, do exist:

• Molten salt electrode reactions and the concomitant thermodynamic data are not readily available.
  
• Products from the reactions are often lost by vaporization.
  
• Diagrams based on pure component thermodynamic data are unrealistic because of departure from ideality.
  
• Lack of passivity even where predictions would show passive behavior.
  
• The stable existence of oxides other than the O²⁻ species.

13.4 Corrosion Rate Measurements

From what has been said already, it is clear that determinations of “corrosion rates” from small‐scale experiments must be treated with great caution. If the metal cannot passivate, it will corrode until it becomes immune, at which point the corrosion rate will fall to zero; between initial exposure and the attainment of immunity, the corrosion rate will be continually changing. If, on the other hand, it is impossible for the metal to come to equilibrium with the melt, then the rate of corrosion, although probably constant, will be primarily controlled by diffusion and interphase mass transfer rates, and the geometry of the system will be an overriding factor. For this reason, it is not always possible to correlate the results of different workers under apparently similar conditions, nor can such results be expected to correspond particularly closely to the amount of corrosion encountered in larger‐scale apparatus (Ozeryanaya 1985; Skelton and Horton 1999; Zeng et al. 2001).

It is not worthwhile, therefore, to give a digest of experimentally determined corrosion rates, but the reader is referred to typical data (Delong et al. 2002; Evans 1960; Janz and Tompkins 1979) for further information on this topic.

One interesting feature of comparative experiments with a series of salts having a common anion is that the aggressiveness of the salts toward metals is dependent on the nature of the cation. The aggressiveness of chloride melts in contact with air is in the order

In the case of CaCl₂ and NaCl, the order corresponds with the corrosion behavior expected from cathodic polarization curves. The order of aggressiveness of chlorides can also be explained on the basis of redox potentials of the melts, calculated on thermodynamic grounds from the free energies of formation of the appropriate oxides and chlorides. The order of aggressiveness of nitrates is complicated by passivity effects, while that of alkalis in contact with air is

This is the reverse order of the aggressiveness of chlorides and indicates that the mechanism of corrosion in the two systems is different, i.e. in the latter case it involves the discharge of hydrogen as in acid aqueous solutions.

13.5 Test Methods

A number of kinetic and thermodynamic studies have been carried out in capsule‐type containers. These studies can determine the nature of the corroding species and the corrosion products under static isothermal conditions and do provide some much needed information. However, to provide the information needed for an actual flowing system, corrosion studies must be conducted in thermal convection loops or forced convection loops, which will include the effects of thermal gradients, flow, chemistry changes, and surface area effects. These loops can also include electrochemical probes and gas monitors (Koger 1987).

The corrosion process is mainly electrochemical in nature because of the excellent ionic conductivity of most molten salts. Therefore, the techniques and processes used in the electrochemical area to study processes in molten electrolytes (e.g. galvanostatic computational, chronoamperometry, chronopotentiometry, linear, cyclic, and square wave voltammetry, scanning electrochemical microscopy) also apply to studies of molten salt corrosion. Furnaces, cells, electrodes, and purification are particularly important aspects and deserve the following information.

• Furnace and controls: The general experimental procedures in molten salt electrochemistry are common to most high temperature measurements and have been extensively reviewed by Bockris et al. (1959).
  

  The most common type of high temperature apparatus is based on the conventional vertical wire wound furnace, which is cheap to build and simple and safe to operate up to 1600 °C. The heating element of Nichrome, Kanthal, molybdenum strip, etc. is wound on a refractory tube and embedded in thermal insulant.

  
  

  Metallic shields should be placed inside the refractory tube primarily to reduce electrical noise and also to smooth out temperature gradients within the hot zone of the furnace. Temperatures are measured by chromel–alumel or platinum–platinum 13% rhodium thermocouples sheated in pyrex, supremax, or alumina, depending on temperature. Proportional or high–low controllers usually control furnace temperatures.

  
  
• Electrochemical cell: Molten salt systems are normally contained in sealed envelopes of glass, silica, or alumina, depending on the temperature. Electrodes can be introduced through ground glass joints placed on the cold side of the cell, without losing the controlled atmosphere in the envelope. The choice of materials directly in contact with the melt is particularly important since “acidic” materials like silica can modify the pO²⁻ of the melt through their buffering action; hence metallic (platinum, gold) containers are preferable. However, nonmetallic containers (especially of recrystallized alumina) have been widely used (Brown et al. 1970; Galasiu et al. 1999; Hocking et al. 1989). A typical high temperature cell assembly is shown in Figure 13.2 (Sequeira and Hocking 1978a).
  
• Electrodes: Gold, silver, and, most commonly, platinum, as foil or wire, are employed for redox (electronation–de‐electronation) electrodes as well as for counterelectrodes. When the melt container is metallic, it may also act as the counterelectrode. The working metal electrodes may be in form of rod, foil, or wire; their adequate insulation, at lower temperatures, can be obtained with a refractory insulator (e.g. boron nitride insulating shield), and above 800 °C, it is thought that the use of cordierite will avoid the triple contact metal–melt–atmosphere, will suppress possible crevices on the edges of the metal electrode, and would keep constant the exposed area of the specimen. Cordierite bodies of general formula 2MgO·2Al₂O₃·5SiO₂ in addition to imperviousness to moisture, high surface resistance, high mechanical strength, and high puncture strength are characterized by a low glassy constant and a low amount of alkali so as to ensure negligible ionic conductivity. In our laboratory, cordierite specimens were tested for sulfate attack at T > 800 °C, and crucible experiments still being used. These tests showed cordierite to be completely insoluble and highly thermal shock resistant in Na₂SO₄–NaCl melts, so the production of a cordierite gasket was developed, and a specimen holder was proposed as illustrated in Figure 13.3.
  
• Reference electrodes: The behavior of a working electrode, either anode or cathode, is usually studied by measuring its potential with reference to a third electrode at constant potential, and the main problem in carrying out such electrochemical measurements in molten salts has been the development of a suitable reference electrode.
  

  Minh and Redey (1987) have published an extensive chapter on molten salt reference electrodes. The commonest type of reference electrode in fused salts is a silver wire in contact with a solution of silver ions of known concentration in the solvent and separated from the bulk melt by a conductive barrier.

  
  

  A paper by Danner and Rey (1961) describes a silver–silver sulfate reference electrode system useful to 1300 °C. Above the melting point of silver, a liquid silver pool was employed. This electrode was found to be the most satisfactory reference electrode for use in sulfate melts at temperatures up to 1000 °C.

It consists of a silver wire dipped into a solution of silver sulfate in Li₂SO₄–K₂SO₄ eutectic (m.p. 535 °C) in concentration ranging from 1 to 10 mol% and isolated from the melt by a pythagoras sheath:

The pythagoras porcelain acts as a solid K⁺‐ion conducting membrane. Pythagoras may be replaced by mullite (2Al₂O₃·SiO₂), pyrex, or supremax glass at lower temperatures. The mullite sheath is conductive to sodium ions, and it was verified that it stood up well to the melt (Brown et al. 1970). Pyrex or supremax glass rapidly develops a brown coloration, and corresponding reference electrode potentials drift with time. Dissolved silver sulfate may be obtained either through anodic dissolution of a silver wire (which sometimes is difficult because no satisfactory container for the cathode compartment can easily be found) or by simple dissolution of silver sulfate. A white precipitate of silver sulfate may be prepared by the addition of Analar sulfuric acid to an aqueous solution of Analar silver nitrate. Addition of silver ions to the solution by dissolution of silver oxide must be avoided because it decomposes thermally at 340 °C. The concentration of Ag₂SO₄ must be large enough to buffer the system but not so large as to cause a significant liquid junction potential.

For measurements over the longer periods of time, it is recommended to use electrodes with more than 1 mol% Ag₂SO₄ because of their higher potential stability. The atmosphere inside the reference half‐cell may or may not be controlled and maintained in static or dynamic (slow bubbling) conditions. A criterion of the thermodynamically reversible e.m.f. properties of such reference electrodes is the micro‐polarization test (Ives and Janz 1961). The relatively poor performance of these electrodes may be discerned in the local recrystallizations of the silver wire as well as in the decrease of the resistivity of the diaphragm over a period of days.

A reference electrode (Figure 13.4) has been developed by Sequeira and Hocking (1978a) that is similar to that described by Danner and Rey (1961) but differs in that the pythagoras capsule used by them is replaced by a mullite capsule conductive to sodium cations. Mullite has relatively poor thermal shock resistance, and the capsules fractured if brought from 900 °C to room temperature in much less than one hour, with Na₂SO₄. Crucible tests showed that mullite is slightly dissolved in molten Na₂SO₄ (see Table 13.2), but the weight loss decreases strongly with time (as it is shown in Table 13.3) so that the mullite sheaths are useful, after aging, as membrane junctions for the reference half‐cells. Therefore, the main advantages of the mullite electrode are that it is not so reactive with the molten Na₂SO₄ as the pythagoras electrode and it is reversible to sodium ions in the melt under study. In common with the Danner electrodes, it also has the advantage that salts in the sheath cannot intermix with those outside the sheath.

The potential of this reference electrode is the same whether evacuated or merely closed at the top by a PVC bulb through which the wire passes; closing is essential to prevent SO₃ escape from Ag₂SO₄ ( = 0.012 atm at 900 °C). Reference electrodes are best stored at a red heat; long cooling times to ambient are necessary to prevent cracking. Against an Au wire electrode at 900 °C, an “ideal” reference electrode has a potential of −160 mV. Its reproducibility, stability, reversibility, and unpolarizability was tested by Sequeira (1979) and found satisfactory for corrosion studies.

• Purification: Molten salts, whether used for experimental purposes or in actual systems, must be kept free of contaminants. This task, which includes initial makeup, transfer, and operation, is specific for each type of molten salt. For example, for nitrates with a melting point of approximately 220 °C purging with argon flowing above and through the salt at 250–300 °C removes significant amounts of water vapor. Another purification method used for this same type of salt consisted of bubbling pure dry oxygen gas through the 350 °C melt for two hours and then bubbling pure dry nitrogen for 30 minutes to remove the oxygen. All metals that contact the molten salt during purification must be carefully selected to avoid contamination from transfer tubes, thermocouple wells, the makeup vessel, and the container itself. This selection process may be an experiment in itself (Bratland 1987; Reavis 1987; White 1983).

More recently other electrochemical techniques have been largely used to study corrosion and other electrode processes in molten salts (Hamel et al. 2004; Keppert et al. 2008; Kerridge and Polyakov 1998; Polovov et al. 2008; Sarou‐Kanian et al. 2009; Sørlie et al. 1995).

13.6 Fluorides

Interest in molten fluorides stems from their importance in nuclear technology and their use in the production of fluorine, electrodeposition of refractory metals, formation of corrosion‐resistant diffusion coatings, and fluorination by electrochemical techniques. Most studies in alkali metal fluorides and other fluorides are rather recent and in connection with the development of molten salt reactors (Naumov and Bychkov 1996) and electrodeposition of silicon and the refractory metals. Corrosion in many fluoride molten salt melts is accelerated because protective surface films are not formed. In fact, the fluoride salts act as excellent fluxes and dissolve the various corrosion products (Lantelme and Groult 2013; Wang et al. 2014; Yaxin and Chaolin 2014). The design of a practicable system using molten fluoride salts, therefore, demands the selection of salt constituents, such as lithium fluoride (LiF), beryllium fluoride (BeF₂), uranium tetrafluoride (UF₄), and thorium fluoride (ThF₄), which are not appreciably reduced by available structural metals and alloys (Koger 1987).

Corrosion data reveal clearly that in reactions with structural metals, M:

13.5

Chromium is much more readily attacked than iron, nickel, or molybdenum.

Nickel‐based alloys, more specifically Hastelloy N (Ni–6.5Mo–6.9Cr–4.5Fe) and its modifications, are considered the most promising for use in molten salts and have received the most attention. Stainless steels, having more chromium than Hastelloy N, are more susceptible to corrosion by fluoride melts but can be considered for some applications (Keiser et al. 1979).

Misra and Whittenberger (1987) reported corrosion data for a variety of commercial alloys in molten LiF–19.5CaF₂, which was being considered for a heat storage medium in an advanced solar space power system, at 797 °C for 500 hours. The tests were conducted in alumina crucibles with argon as a cover gas. Results are tabulated in Table 13.4. For nickel‐based alloys, chromium was detrimental. No influence of chromium, however, was noted in iron‐based alloys.

Moisture, a common impurity in fluoride salts, can produce gaseous HF and increase corrosion attack (Tasaka et al. 1998). Therefore, it is important to reduce its level in the salt, resulting in decreased corrosion rates.

Recently, corrosion of Cu and Mg was also investigated in HF–KF mixtures (Germanaz et al. 1989) because of their use as conducting busbars in fluorine electrowinning. Copper busbars are preferred in low acidic mixtures, while magnesium is a more corrosion‐resistant material in high acidic and low temperature mixtures.

13.7 Chlorides

Molten chlorides are widely used for electrowinning of metals, alloys, and gases, for annealing and normalizing of steels in high temperature batteries, etc.

Colom and Bodalo have investigated the corrosion of mild steel (1971) and Armco iron (1972) in molten LiCl–KCl eutectic as a function of the water content of the melt and of the temperature. Corrosion rates fell rapidly to a constant value with time (i.e. a passivating film is formed) and increased with rising temperature between 400 and 800 °C. The oxidation kinetics followed first a parabolic and then a linear rate law. The corrosion rate seemed to be scarcely affected by traces of water in the melt in the case of mild steel (it was enhanced by traces of water in the case of Armco iron), but, whereas the corrosion product in the dry melt was found to be Fe₂O₄, in the humid bath, both Fe₂O₃ and Fe₂O₄ were formed. Cathodic polarization waves indicate that the corrosion reaction is diffusion controlled and the diffusing species is Fe³⁺. This interpretation requires further support in view of the known electrochemistry of iron in LiCl–KCl eutectic mixture. The rate of corrosion is lowered by cathodic protection.

Hoff (1971) has developed the theory for the corrosion of metals in molten salts under a temperature gradient. Dissolution of a metal on hot parts and recrystallization on the colder parts are caused by the thermoelectric effect. The equations of electrode kinetics can be used to obtain the theoretical relations. The temperature dependence of diffusion and of complex formation leads to a current distribution along the surface of the metal, showing a distinct maximum at the point where recrystallization occurs. The theory is tested using an aluminum wire in AlCl₃–NaCl–KCl in the temperature range of 215–420 °C.

Feng and Melendres (1982) have shown that Fe, Co, Ni, Cu, and Mo are considerably less corroded in molten LiCl–KCl eutectic when this melt contains lithium oxide that is due to oxide film formation.

Lai et al. (1985) evaluated various wrought iron‐, nickel‐, and cobalt‐based alloys in a NaCl–KCl–BACl₂ salt bath at 840 °C for one month. Surprisingly, two high nickel alloys (alloys 600 and 601) suffered more corrosion attack than stainless steels such as Types 304 and 310. Co–Ni–Cr–W, Fe–Ni–Co–Cr, and Ni–Cr–Fe–Mo alloys performed best. Laboratory testing in a simple salt bath failed to reveal the correlation between alloying elements and performance. Tests were conducted at 840 °C for 100 hours in a NaCl salt bath with fresh salt bath for each test run.

Similar to the field test results, Co–Ni–Cr–W and Fe–Ni–Co–Cr alloys performed best.

Smyrl and Blackburn (1975) have been concerned with the stress corrosion cracking phenomena of the Ti–8Al–1Mo–1V alloy in molten LiCl–KCl at 350 °C.

More recently, Atmani and Rameau (1987) have described a tensile apparatus suitable for corrosion tests in molten salts. The behavior of 304L stainless steel was studied in molten NaCl–CaCl₂ at 570 °C using either a constant strain rate or a constant load technique. Intergranular corrosion fracture was shown, and the role of M₂₃C₆ precipitation in the crack propagation was evidenced.

Coyle et al. (1985) conducted corrosion tests on various commercial alloys at 900 °C in the molten 33NaCl–21.5KCl–45.5MgCl₂ eutectic. After 144 hours of exposure, 8 of the 15 Fe‐, Ni‐, Co‐based alloys evaluated were consumed. The remaining seven alloys disintegrated after a total of 456 hours of exposure. The authors concluded that the chloride salt was too aggressive to be used at 900 °C for a solar thermal energy system (Table 13.5).

Corrosion mechanisms in chloride‐based salts were also reviewed to better understand the practical implications of using these salts in thermal solar power systems (Abramov 2010; Indacochea 2001; Masset 2010; Oryshich and Kostyrko 1985). They are susceptible to high corrosion rates in the presence of moisture and oxygen, but they can be used for high temperature heat transfer fluids (HTFs) primarily due to economic considerations and for the thermal stability in the region of 700–800 °C. In other words, they may be used to increase operating efficiency in the solar systems in which high turbine inlet temperatures must be achieved. Investigation of this chloride salt family helped to understand the involved corrosion mechanisms and suggested metal alloys for use in containment vessels, piping, pumps, valves, and tanks, as exemplified in Table 13.6.

An overview of experimental observations and results of liquid Li and LiCl corrosion at 725 °C of engineering nonferrous materials has been explained by Olson et al. (1998). It has been observed that oxygen contamination is particularly harmful for the tantalum‐ and niobium‐based refractory metal alloys, whereas nitrogen is deleterious to iron‐based alloys. Materials tested included RA333, Hastelloy X, Airesist 213, Ta–2.5W, and Nb–1Zr. The corrosion and protection mechanism of molten salt electrodeposited chromium coatings in a LiCl–KCl eutectic at 450 °C has been studied by Emsley and Hill (1987). Factors influencing the optimum coating thickness on 20Cr–25Ni–Nb‐stabilized stainless steel to achieve a satisfactory lifetime were discussed.

The corrosion behavior of Mo–Al₂O₃–Cr₂O₃ cermets in BaCl₂ molten salt has been shown to be mainly due to the electrochemical corrosion of the component Mo (Wang et al. 1991). It was also found that the other component (Cr₂O₃) is beneficial to the corrosion resistance of the cermets investigated.

The corrosion behavior of mild steel (St35.8), boiler steel 13Cr–Mo44, and stainless steel X 10Cr–Ni–Mo18 in contact with the eutectic salt mixtures AlCl₃–NaCl, LiCl–LiNO₃–NaCl, NaCl–NaNO₃, and KCl–LiCl has been investigated by Heine (1985). The test conditions were adapted to the operating conditions of latent heat storage systems. Only pure salts were used. Good corrosion resistance was observed.

Intergranular corrosion is the major corrosion morphology by molten chloride salts.

Another frequently observed corrosion morphology is internal attack by void formation. Voids tend to form at grain boundaries as well as in the grain interior. The continuing formation and growth of chromium compounds at the metal surface causes outward migration of chromium and inward migration of vacancies, thus leading to internal void formation (Koger and Pohlman 1987).

Alloying elements, specifically refractories, are thought to improve corrosion resistance in chlorides by stable spinel layer formation that tend to slow diffusion of Cr from the base alloy to the melt.

13.8 Nitrates/nitrites

Molten nitrates are commonly used for heat treatment baths; therefore, a great deal of material compatibility information exists. Plain carbon and low‐alloy steels form protective iron oxide films that effectively protect the metal surface to approximately 500 °C. Chromium additions to the melt further increase the corrosion resistance of the steel, and hydroxide additions to the melt further increase the resistance of chromium‐containing steels. Aluminum and aluminum alloys should never be used to contain nitrate melts, because of the danger of explosion.

Nitrate–nitrite mixtures are also widely used to heat treat salt baths at temperatures ranging from 160 to 590 °C, as well as a medium for heat transfer or energy storage.

Electrode potential oxygen partial pressure diagrams for the iron‐molten NaNO₃ system at 600 and 700 K and for iron, cobalt, and nickel in molten sodium nitrite have been constructed 40 years ago. In both cases, four well‐defined regions, corresponding to metal corrosion, immunity, passivity, and passivity breakdown, are observed.

The oxidation kinetics of iron in molten alkali metal nitrates has also been investigated between 350 and 470 °C. The parabolic rate law, with a temperature‐dependent constant, appears to be followed. The activation energy for corrosion is found to be greater in KNO₃ than in NaNO₃. X‐ray studies show that the oxidation product is Fe₃O₄. The results are comparable with the oxidation kinetics of iron in air or oxygen. The effects of alkali metal and alkaline earth halides on the oxidation kinetics of iron and low carbon steels in molten KNO₃–NaNO₃ at 400 °C have also been studied. The corrosion of iron in these melts appears to begin with pitting corrosion that eventually spreads to the entire surface. The rate of attack increases with halide concentration and seems to depend on both the anion and the cation, aggressiveness increasing in the order KCl < KBr < Kl for the anion and CaCl₂ < BaCl₂ < LiCl < NaCl < KCl for the cation; halide is found to be incorporated in the oxide film formed. It is found that low carbon steels are more resistant to corrosion than pure iron. It is suggested that the corrosion behavior is similar to that in atmospheres containing halogens at high temperatures.

Ishikawa and Sasaki (1981) have carried out immersion and electrical resistance tests in alkali nitrate melts of 350–450 °C to elucidate the corrosion behavior of iron wire specimens. A parabolic law was verified for the iron specimens. Moreover, the sensitive resistometry has been shown to be a useful technique for the continuous determination of the corrosion behavior in various salt systems. Nitrate–nitrite mixtures and corrosion of iron and stainless steels by these melts were extensively studied (as a function of temperature and oxoacidity) in relation to their use as a coolant and storage fluid in solar thermal electric power plants (Picard et al. 1987). In particular, passivation of iron is observed only in a narrow acidity domain where NaFeO₂ can be formed. It was also demonstrated that a nitriding process appears only as a consequence of the oxidation process.

The corrosion resistance of Al, Ni, Ti, Ta, Nb, carbon steel, and stainless steel was studied in molten LiNO₃–NaNO₃–KNO₃ eutectic for the chemical open‐circuit oxidation and for conditions of cathodic polarization. Experiments were carried out at 632 K under an argon atmosphere during 100 hours. By using X‐ray diffraction (XRD), electron spectroscopy for chemical analysis (ESCA), secondary ion mass spectroscopy (SIMS), scanning electron microscopy (SEM), and gravimetric method, the metals under study show relatively high corrosion resistance in nitrate melts. Oxide films of predominantly higher oxidation state were formed on their surfaces. The effect of cathodic polarization on their corrosion behavior was insignificant. Only in the case of Ni, a decrease in oxidation rate was observed under the conditions of cathodic polarization (Yurkinsky et al. 1998).

Molten salt corrosion behavior of heat transfer plant materials, SS41, 2.25Cr–1Mo steel, SUS304, and Inconel 625, was studied in temperatures of 450 and 550 °C. The corrosion rate in the molten salt decreased in the decreasing order of SS41, 2.25Cr–1Mo steel, SUS304, and Inconel 625. And the corrosion resistance of SS41, 2.25Cr–1Mo steel, and SUS304 strongly depended on the temperature and Cl‐exp content of the molten salt, while Inconel 625 showed high corrosion resistance in the molten salt environment. The morphology of corrosion products was examined by electron probe microanalysis (EPMA), XRD, SEM, and Auger electron spectroscopy (AES). Corrosion products of SS41 and 2.25Cr–1Mo steel consisted of porous and easy‐pearling multilayer films of α‐Fe₂O₃, KFeO₂, NO₂O–Fe₂O₃, and Fe₃O₄, while the corrosion products of SUS304 and Inconel 625 consisted of compact and well‐sticked iron oxide films that contain Ni and Cr. The materials containing much more than 10 wt% Cr showed high corrosion resistance against the molten salt (Ebara et al. 1988).

Electropolished iron spontaneously passivates in molten sodium nitrate–potassium nitrite in the temperature range of 230–310 °C at certain potentials. A magnesite (Fe₃O₄) film is formed, along with a reduction of nitrite or any trace of oxygen gas dissolved in the melt. At higher potentials, all reactions occur on the passivated iron. Above the passivation potentials, dissolution occurs with ferric ion soluble in the melt. At even higher potentials, nitrogen oxides are evolved, and nitrate ions dissolve in the nitrite melt. At higher currents, hematite (Fe₂O₃) is formed as a suspension, and NO₂ is detected. Carbon steel in molten sodium nitrate–potassium nitrate (NaNO₃–KNO₃) at temperatures ranging from 250 to 450 °C forms a passivating film consisting mainly of Fe₃O₄.

Iron anodes in molten alkali nitrates and nitrites at temperatures ranging from 240 to 320 °C acquire a passive state in both melts. In nitrate melts, the protective Fe₃O₄ oxidizes to Fe₂O₃, and the gaseous products differ for each melt.

An interesting study was conducted on the corrosion characteristics of several eutectic molten salt mixtures on such materials as carbon steel, stainless steel, and Inconel in the temperature range of 250–400 °C in a nonflowing system. As expected, the corrosion rate was much higher for carbon steel than for stainless steel in the same mixture. Low corrosion rates were found for both steels in mixtures containing large amounts of alkaline nitrate. The nitrate ions had a passivating effect.

Electrochemical studies showed high resistance to corrosion by Inconel. Again, the sulfate‐containing mixture caused less corrosion because of passivating property of the nitrate as well as the preferential adsorption of sulfate ions.

Surface analysis by AES indicated varying thicknesses of iron oxide layers and nickel and chromium layers. The Auger analysis showed that an annealed and air‐cooled stainless steel specimen exposed to molten lithium chloride (LiCl)–potassium chloride (KCl) salt had corrosion to a depth five times greater than that of an unannealed stainless steel specimen. Chromium carbide precipitation developed during slow cooling and was responsible for the increased corrosion. The mechanism of corrosion of iron and steel by these molten eutectic salts can be described by the following reactions:

13.6