Hydrogen halides dissolution

The hydrogen halides, HF, HCl, HBr, and HI, are all polar covalent gases. In the gas phase the interactions among the widely separated molecules are not very strong, so solute-solute attractions are minimal, and the dissolution processes in water are exothermic. The... 

The hydrogen halides, HF, HCI, HBr, and HI, are all polar covalent gases. In the gas phase the interactions among the widely separated molecules are not very strong, so solute-solute attractions are minimal. Due to the polarity of the hydrogen halides and the hydration of their ions, the dissolution processes in water are very exothermic. The resulting solutions, called hydrohalic acids, contain predominandy ionized HX (X = Cl, Br, I). The ionization myo[ es protonation of a water molecule hy HX to form a hydrated hydrogen ion, H30, and halide ion X (also hydrated). 

The synthesis of anhydrous metal halides is often a complex process. In thermal dehydration reactions complete removal of water is often complicated by decomposition of the compound. The reactions of metals with gaseous hydrogen halides or with halogens usually takes place at high temperatures and requires apparatus that may not be avaUable routinely. For example, CrBra can be obtained by the action of Br2 on chromium metal at a temperature of 750 °C.53 An alternative method has been proposed for the preparation of this compound that involves the anodic dissolution of chromium in the presence of bromine with a platinum cadiode immersed in benzene connected by a salt bridge to a chromium anode immersed in MeOH/Br2. The electrolytic cell is represented by Eq. 7.7 ... 

For many metals and alloys the determination of /p is complex, and its magnitude is governed by many factors such as surface finish, rate of formation, alloying constituents, and the presence of those anions, such as halides, that promote localised breakdown. In many instances the attack on passive films by halide ions shows a temperature and concentration dependence similar to the effect of hydrogen ions, i.e. the rate of film dissolution increases with concentration in accordance with a Freundlich adsorption relationship... 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

As shown in Figure 6.49a, the cracks grow by slip dissolution due to diffusion of active water molecules, halide ions, etc., to the crack tip, followed by a rupture of the protective oxide film by strain concentration, fretting contact between the crack faces. This is followed by dissolution of the fresh exposed surface and growth of the oxide on the bare surface. For the alternative mechanism of hydrogen embrittlement in aqueous media, the critical steps involve diffusion of water molecules or hydrogen ions to the crack tip reduction to hydrogen atoms at the crack tip surface diffusion of adsorbed atoms to preferential surface locations absorption and diffusion to critical locations in the... 

The dissolution of ionic species (Fig. 11.3) occurs through the ion-dipole forces described in Section 10.2. Each positive ion in solution is surrounded by water molecules oriented with the negative end of their dipole moments toward the positive ion. Each S04 anion in solution is surrounded by water molecules oriented with the positive end of their dipole moments toward the anion. When a halide such as KCl is dissolved, the anion forms a hydrogen bond with one of the H atoms in a water molecule that places the atoms O —H—Cl nearly in a straight line as described in Section 10.2. 

It is generally assumed that ions which can accelerate either or both partial reactions in a corrosion process are capable of being adsorbed on the iron surface. Thus it is known that hydrogen sulfide ions which accelerate both partial reactions of acid corrosion (although predominantly the anodic one), and formic acid molecules which catalyze the cathodic partial reaction but inhibit the anodic one, as well as commercial inhibitors which reduce both partial reactions, are in fact adsorbed on the iron surface. As a consequence the mere fact that adsorption takes place cannot be used to predict an expected change in corrosion rate as it is also known that halide ions cat-alize the anodic dissolution of indium, while hydroxyl adsorption catalyzes the anodic dissolution of iron. Furthermore, it is also known that certain ions can act either as a catalyst or an inhibitor when adsorbed on the metal surface depending on the type of metal considered. Kolotyrkin (18) observed that the adsorp-... 

---