Salt melts corrosion

There are two cases in which a metal can be attacked by a salt melt if it is soluble in the melt, or if it is oxidised to metal ions. In the first case, attack occurs by direct dissolution without oxidation of the metal and the mechanism is likely to be closely similar to attack by liquid metals (Section 2.9). If the solubility is appreciable, excessive corrosion can be expected, but with few exceptions metals appear to be appreciably soluble only in their own salts. Most of the metals of the first and second groups of the periodic... 

It is usual to choose a container metal for fused salts sufficiently noble for the displacement reaction (2.16) to be negligible, and the most important aspects of corrosion are, as in aqueous solutions, those which involve reducible impurities, although in a salt melt there is also the additional possibility of a reducible anion (see above). All such factors can be described as controlling the oxidising power of the melt, which can be defined in terms of a redox potential just as in aqueous solutions The redox potential is expressed by relationships of the form... 

Electrochemical corrosion of metals Since the aggressiveness of salt melts is governed by redox equilibria, and is often controlled by composition of the external atmosphere, effects analogous to electrochemical or oxygen-concentration corrosion in aqueous systems can occur in salt melts. Tomashov and Tugarinov determined cathodic polarisation curves in fused chlorides and concluded that the cathodic reactions of impurities could be represented as ... 

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

The release of alkaline compounds is also an important factor in coal combustion. Most high-temperature corrosion problems in fossil fuel boilers are the result of salt melts (54,55). Alkali metal compounds volatilize from coal at the high temperatures of conventional combustion and subsequently condense on heat transfer surfaces. The lower temperatures of FBC are expected to reduce salt volatility, a fact that has been confirmed by Vogel et al. (16). 

Ionic liquids (IL) are salts melting at low temperatures, and represent a novel class of solvents with non-molecular ionic character. In contrast to a classical molten salt, which is a high-melting, highly viscous, and very corrosive medium, an ionic liquid is already liquid at temperatures below 100 °C and is of relatively low viscosity [4]. In most cases, ionic liquids consist of combinations of cations such as ammonium, phosphonium, imidazolium, or pyridinium with anions such as halides, phosphates, borates, sulfonates, or sulfates. The combination of cation and anion has a great influence on the physical properties of the resulting ionic liquid. By careful choice of cation and anion it is possible to fine tune the properties of the ionic liquid and provide a tailor-made solution for each task (Fig. 1), and this is why ionic liquids are often referred to as designer solvents or materials. 

After anion metathesis, halides have to be completely removed, not only because of their corrosive nature, but also because they can deactivate catalysts dissolved in the ionic liquid. Any trace of impurity (including Li+, K+, Na+ and Ag+) can have a considerable influence on the physical properties of the salts (melting point, viscosity, density). Furthermore, if PFe salts are not completely... 

One example is the hot corrosion of a preoxidized nickel specimen by a thin Na2S04 melt film in a 0.1 wt. % SO2-O2 gas mixture at 1200 K [29]. By variation of the oxide scale thickness and the purity of the material, different regimes of corrosion were investigated passive state, pseudopassive state, and active state. The passive state of 99.9975% of pure nickel, preoxidized in pure O2 for 2 h at 1200 K is controlled by diffusion of 8207 in the salt melt. The corresponding Nyquist plot of impedance data shows linear behavior in the low-frequency range withaslope of45° (Fig. 16).The semicircle at higher frequencies was attributed to the resistance of the NiO layer itself The active state was established on less pure nickel... 

FIGURE 20.53 High-temperature corrosion in salt melts (a) schematic of corrosion (b) dependence of corrosion in salt melts on the potential at 760°C for 23 h. 

The more common classification scheme is to divide the corrosive media into their state of aggregation, that is to subdivide into corrosion by solids, liquids and gases. While solid state corrosion is rarely dealt with, we have vast amount on literature on hot gas corrosion. The case of corrosion by liquids is commonly further subdivided into more specific cases, such as aqueous corrosion (e.g. acids and water), corrosion by glasses, metal melts and salt melts. The last case is for historic reasons known in the form of a rather misleading expression hot corrosion. A special case, which spans from the liquid into the gaseous state is given by the corrosion in hot water systems hydrothermal corrosion. 

Corrosion by Salt Melts (Hot Corrosion) Data and reviews of the mechanisms of the hot corrosion behavior of SiC have been presented in a number of papers by Jacobson and coworkers [65-68]. 

It is very difficult to quantify hot corrosion in laboratory experiments with pre-loaded samples, because the salt melts propagate rapidly both by spreading and gas phase transport, resulting in uneven coverage of the sample and changes with time. 

Corrosion by Salt Melts Hot corrosion of Si3N4 has been extensively studied and reviewed by researchers at NASA labs [67,68,77,125,126]. The basic attack is that on the protective silica layer, which makes the behavior similar to that of SiC and basic mechanisms can be taken from the chapter on SiC above. 

Fused-salt type corrosion (Corrosion by the peroxide ion that melted into NaOH]... 

Electrochemical cells can also be used to attempt to obtain data on the mechanisms of the salt-induced corrosion processes. Cyclic voltammetry has been used [78-22] to obtain information on the oxidation and reduction reactions that may occur during molten salt corrosion. Chronopotentio-metric investigations with platinum as the working electrode in cells can also be used to determine and control the compositions of molten salts, as well as to measure the solubilities of various oxidation products in melts [25-28,30]. 

Reaction (2-48) is shifted to the left, leading to a salt melt of low 0 activity which corresponds to low Na20 activity. This form of hot corrosion is usually encountered at lower temperatures of between 600 and 800 °C. It is also often called type II hot corrosion or low temperature hot corrosion. Type I hot corrosion has been assigned to the situation at high temperatures, which means above the melting point of Na2S04 of 884 °C. In particular, low temperature hot corrosion can lead to complex surface layers consisting of solid and liquid phases. This... 

A large body of literature exists on the corrosion of metal alloys by molten fluorides. The major impurities that must be removed to prevent severe corrosion of the container metal are moisture/oxide contaminants. A great deal of effort has stiU to be devoted to develop analytical and purification methods able to (1) identify the oxygen-containing species (oxide type, hydroxyl, etc.) (2) purify the molten—salt mixture, and (3) determine accurately the oxygen content in salt melts. 

Model for salt melt-induced corrosion, showing iron dissolution, transport of iron chloride and oxide precipitation [9. ... 

A. Ruh and M. Spiegel. Kinetic investigations on salt melt induced high-temperature corrosion of pure iron. EUROCORR 2003, 28 Sept.-Oct. 2003, Budapest, Hungary. 

Corrosion phenomena beneath molten salts were initially investigated for sulphate systems [e.g. 2]. A review of fundamental work on sulphate melt-induced corrosion is given by Rapp [3]. Increased corrosion is excited by dissolution of the passivating oxide layer in the salt melt. Two major dissolution mechanisms have to be considered basic dissolution and acidic dissolution. Basic dissolution is caused by a basic melt, meaning a high activity of 0 , for example in the form of Na20. By reaction with the oxide, this leads to the formation of a complex oxide ion (e.g. FeO ) according to Eq. 30.1. 

A model for hot corrosion beneath molten chloride is shown in Fig. 30.15 [8, 13]. In that model corrosion starts with the dissolution of Fe at the metal/salt melt interface and the formation of iron chloride. If the gas phase contains no chlorine species at the beginning, the chlorine source must be the molten chlorides. In this case chlorine can be released by the oxidation of the chloride. 

---