Iron surface

Figure A3.10.1 (a) A schematic illustration of the corrosion process for an oxygen-rich water droplet on an iron surface, (b) The process can be viewed as a short-circuited electrochemical cell [4],...

Paal Z, ErtI G and Lee S B 1981 Interactions of potassium, nitrogen, and oxygen with polycrystalline iron surfaces Appl. Surf. Sc/. 8 231... 

Both UPS and XPS of solids are useful techniques. So far as studies of adsorption by surfaces are concerned we would expect UPS, involving only valence orbitals, to be more sensitive. For example, if we wish to determine whether nitrogen molecules are adsorbed onto an iron surface with the axis of the molecule perpendicular or parallel to the surface it would seem that the valence orbitals would be most affected. This is generally the case but, because ultraviolet photoelectron spectra of solids are considerably broadened, it is the X-ray photoelectron spectra that are usually the most informative. 

Figure 8.20 Nitrogen Is and oxygen Is X-ray photoelectron spectra of nitric oxide (NO) adsorbed on an iron surface. 1, Fe surface at 85 K 2, exposed at 85 K to NO at 2.65 x 10 Pa for 80 s 3, as for 2 but exposed for 200 s 4, as for 2 but exposed for 480 s 5, after warming to 280 K. (Reproduced, with permission, from Kishi, K. and Roberts, M. W., Proc. R. Soc. Land., A352, 289, 1976)...

This reaction is catalyzed by iron, and extensive research, including surface science experiments, has led to an understanding of many of the details (72). The adsorption of H2 on iron is fast, and the adsorption of N2 is slow and characterized by a substantial activation energy. N2 and H2 are both dis so datively adsorbed. Adsorption of N2 leads to reconstmction of the iron surface and formation of stmctures called iron nitrides that have depths of several atomic layers with compositions of approximately Fe N. There is a bulk compound Fe N, but it is thermodynamically unstable when the surface stmcture is stable. Adsorbed species such as the intermediates NH and NH2 have been identified spectroscopically. 

The industrial catalysts for ammonia synthesis consist of far more than the catalyticaHy active iron (74). There are textural promoters, alumina and calcium oxide, that minimise sintering of the iron and a chemical promoter, potassium (about 1 wt % of the catalyst), and possibly present as K2O the potassium is beheved to be present on the iron surface and to donate electrons to the iron, increasing its activity for the dissociative adsorption of N2. The primary iron particles are about 30 nm in size, and the surface area is about 15 m /g. These catalysts last for years. 

When water pH is between about 4 and 10 near room temperature, iron corrosion rates are nearly constant (Fig. 5.5). Below a pH of 4, protective corrosion products are dissolved. A bare iron surface contacts water, and acid can react directly with steel. Hydrogen evolution (Reaction 5.3) becomes pronounced below a pH of 4. In conjunction with oxygen depolarization, the corrosion rate increases sharply (Fig. 5.5). 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials No reactions Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Polymerization is accelerated by heat and exposure to oxygen, as well as the presence of contamination such as iron rust. Iron surfaces should be treated with an appropriate reducing agent such as sodium nitrate, before being placed into isoprene service Inhibitor of Polymerization Tertiary butyl catechol (0.06 %). Di-n-butylamine, phenyl-beta-naphthylamine andphenyl-alpha-naphthylamine are also recommended. 

Hot spot formation witliin tlie reactor can result in catalyst breakdown or physical deterioration of tlie reactor vessel." If tlie endothermic cyanide reaction has ceased (e.g., because of poor catalyst performance), the reactor is likely to overheat. Iron is a decomposition catalyst for hydrogen cyanide and ammonia under the conditions present in the cyanide reactor, and e. posed iron surfaces in the reactor or reactor feed system can result in uncontrolled decomposition, which could in turn lead to an accidaital release by overheating and overpressure. 

Passivation According to Fig. 1.15 (top) all the Fe will be converted to Fe2 03, whilst the rust originally present will be unaffected. According to Fig. 1.15 (bottom) the rust will be unaffected, whilst the iron surface exposed to the solution through pores in the rust will be passivated by a protective film of Fe203. Water will be stable except at high potentials where it will be oxidised to O2. 

The incorporation of anions from the electrolyte, such as borate and carbonate, into the oxide has also been shown to occur on iron and cobalt, such anions being restricted to the outer layers of the film. Attempts to find incorporation of chloride into passive iron surfaces from... 

Very rapid and highly localised pitting is sometimes observed on components exposed to very turbulent flow conditions leading to cavitation in the stream. In general, these conditions appear to induce corrosion rather than erosion on cast iron surfaces, in contradistinction to what usually happens with other metals, apparently because the erosive component of the liquid flow scours away corrosion-stifling films and allows the development of very active electrochemical cells on the exposed metal surfaces . 

All conventional phosphate coating processes are based on dilute phosphoric acid solutions of iron, manganese and zinc primary phosphates either separately or in combination. The free phosphoric acid in these solutions reacts with the iron surface undergoing treatment in the following manner ... 

Nature of the metal surface The critical concentration of an anion required to inhibit the corrosion of iron may increase with increasing surface roughness. Thus, Brasher and Mercer" showed that the minimum concentration of benzoate required to protect a grit-blasted steel surface was about 100 times greater than that required to protect an abraded surface. However, surface preparation had little effect on the critical inhibitive concentrations for chromate" or nitrite " The time of exposure of the iron surface to air after preparation and before immersion may also affect the ease of inhibition by anions. There is evidence """ that the inhibition by anions occurs more readily as the time of pre-exposure to air increases. Similarly, if an iron specimen is immersed for some time in a protective solution of an inhibitive anion, it may then be transferred without loss of inhibition to a solution of the anion containing much less than the critical inhibitive concentration . ... 

Thus inhibitive anions can retard the dissolution of both the T-FejO, and the magnetite layers of the passivating oxide layer on iron. This has the dual effect of preventing breakdown of an existing oxide film and also of facilitating the formation of a passivating oxide film on an active iron surface, as discussed in the previous section. 

TNPEN ignites when heated in an open dish and explds when struck with a hammer upon an iron surface. It is a good gelatinizer for NC Properties. According to Wasmer (Ref 4) Solubility. At 20° in g/100ml of solvent acet 27.7 benz 2.1 chlf 0.6 eth 0.3 CS2 0.06 CC14 0.03 w <0.01 coned sulfuric acid, si sol coned nitric acid, sol... 

The total emissivity of the oxidised iron surface is 0.78 and both emissivity and absorptivity are independent of the surface temperature. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

When iron surfaces are exposed to ammonia at high temperatures, nitriding —the incorporation of nitrogen into the iron lattice—occurs. The atomic radius of iron is 124 pm. (a) Is the alloy interstitial or substitutional Justify your answer. 

An indication of growing interdisciplinary interest in the field is illustrated in a review on new perspectives in surface chemistry and catalysis by Roberts (.160), who discussed the interaction of N2 with iron surfaces. In so doing, he referred to the Fe (N2) , matrix Mdssbauer work of Barrett and Montano (7), which showed that molecular nitrogen only bonds to iron when the latter is present as a dimer. As the chemisorption studies (161) indicated that N2 is absorbed on singleatom sites, Roberts suggested (160), of the matrix data (7), "if this is correct, then our assignment of the N(ls) peak at 405 eV to end-on chemisorbed N2 will require further investigation. Other reviews that consider matrix-isolation techniques for chemisorption simulation are collected in footnote a. 

A Japanese group reacted iron vapor with nitric oxide at 77 K (6). Two different species were observed, with Pno at 1800 and 1720 cm , that were assigned as NO species adsorbed on oxidized and metallic iron, respectively. Although no evidence was presented as to the nu-clearity of the products, the authors considered the species to be models for the chemisorption of NO on iron surfaces. 

A water droplet on an iron surface is a miniature electrochemical cell that corrodes iron metal and generates iron oxide (rust). 

These effects profoundly influence the ammonia, as shown in Fig. 8.28 where the ammonia synthesis rate is plotted for two basal planes of iron and the same iron surfaces modified with 0.1 Ml of potassium. 

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