Iron surface, pretreatment

In recent work related to the electrodeposition of PPy from an aqueous pyrrole-oxalic acid solution, the influence of the iron surface pretreatment on the corrosion properties was reported by Van Shaftinghen, Deslouis et al. [79]. The performances of PPy-coated iron samples, obtained through three different electropolymerization techniques were tested (i) electropolymerization at constant current (1 mA cm ) (denoted galstat), (ii) potentiostatic electropolymerization at IV vs. Ag/AgCl in oxalic acid for 600 s followed by addition of pyrrole into the solution (denoted one-step), (iii) potentiostatic polarization in a two-step process identical to the previous one, except that a prehminary polarization at 0 V vs. Ag/ AgCl for 600 s was carried out in the electrolyte alone (denoted two-step) (Figure 16.11). From the anodic curves of iron in oxalic acid alone, a first zone of potential ranging... 

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

Figures 33.13 shows the topography of the two plasma polymer layers deposited under different conditions on a polished iron surface. Both films show a similar topography as observed by atomic force microscopy, but the film deposited on O2 plasma-pretreated polished iron showed a little more grainy surface than (Ar + H2) plasma-pretreated sample. In Figure 33.14 the root mean square value is plotted against the film thickness. The grainy surface (O2 plasma pretreated), which showed a higher deposition rate, increased the roughness as the thickness increased as expected.

The macro-scale investigations showed that pretreatment of an iron surface with steam at 700°C itduces a dramatic increase in the catalytic activity for carbon deposition from hydrocarbons. Spectroscopic analysis (Auger and Mossbauer) combined with weight increase measurements prove that treatment of iron with steam at 700°C results in the conversion of the surface of the iron to FeO. At 800°C, this process is not just limited to the uppermost surface layers but penetrates to an appreciable depth of the material after a three hour treatment. Indeed Mossbauer spectroscopy data shows that nearly all of a 0.013 cm Fe foil is transformed to FeO in this time at 800°C. It should be mentioned that the reaction of steam with iron to produce FeO may be possible at temperatures above 570°C (3). The nonstoichiometric nature of FeO has been the subject of "a considerable number of papers. It is known, however, that the defects present in this material are vacant cation sites and trapped positive holes (26). 

Figures 32(a and b) show typical microscopic pictures of FFC on polymer-coated iron, and aluminum. FFC develops in the presence of pores, mechanical defects, unprotected cut edges, or residual salt crystals underneath the organic coating. The corrosion filaments start growing perpendicular from a defect into the polymer-coated area. FFC occurs only at moderate humidity (60-95%) and therefore, not under full immersion conditions. FFC has been found to be triggered by anions such as chloride, bromide, and sulfate. The filament growth rate increases with temperature. Like for cathodic delamination on iron and zinc the corrosion kinetics depend strongly on the surface pretreatment and coating composition.

Undoubtedly, electrodeposition of conducting polymers on iron is the process most studied. The first PPy coating of iron in aqueous media was carried out by Schirmeisen and Beck [85] in the presence of nitrate salts, then, by Lacaze etal. [12,13] who improved the process and achieved good protection after the iron surface had been pretreated by dilute nitric acid in order to improve the adhesion [86,87]. Just as in the nitrate process, this procedure provided a passivation of iron, but also involved an increase of the roughness of the surface resulting from a slight attack on the surface by the acid, the effect of which was to improve the mechanical adherence [88]. 

T. Van Schaftinghen, C. Deslouis, A. Hubin, and H. Terryn, Influence of the surface pretreatment prior to the film synthesis on the corrosion protection of iron with pol3fpyrrole films, Electrochim. Acta, 51, 1695-1703 (2006). 

For the investigation of the effect of surface pretreatment, two different mild steel samples were prepared, and their O Is and Fe 2p3/2 XPS spectra were measured prior to the inhibitor adsorption procedure. The dependence of the adsorption procedure on sample pretreatment, i.e., how the chemical state (the oxide content) of the iron surface affects the adsorption of the inhibitor can be followed in Figs. 9-37 to 9-40. 

Figure 4.17. SEM of the restructured AI ,Oj,/Fe(110) surface (a) after a 0.05 torr water-vapor treatment and reduction in nitrogen and hydrogen (b) after a 20 torr water-vapor pretreatment followed by reduction. The aluminum oxide is located underneath the iron surface, so it does not block active catalytic iron sites.

The same coverages of potassium coadsorbed with two monolayers of aluminum oxide on the Fe(llO), Fe(lOO), and Fe(lll) surfaces hindered the restructuring process in water vapor (see Section 4.6). As increasing amounts of potassium were coadsorbed, more aluminum oxide was detected by AES after water pretreatments of 20 torr, and less restructuring of the iron occurred (rates of ammonia synthesis over these surfaces were less than the rates on those surfaces which were restructured with aluminum oxide alone). There is a one-to-one ratio between aluminum oxide and potassium on the surface and, in the case where one monolayer of potassium was deposited on two monolayers of aluminum oxide, AES showed that no aluminum oxide or potassium migrated from the iron surface after a 20 torr water-vapor pretreatment and restructuring of the surface failed to occur. 

Fe(lll) surfaces. Also, the effects of the ammonia pretreatment on iron surfaces with coadsorbed potassium and aluminum oxide are presented. 

When nitrogen is the restructuring agent it is not thermodynamically favorable for iron nitride to cover aluminum oxide, probably because of the absence of a strong chemical interaction between iron nitride and aluminum oxide. Hence, AES finds aluminum oxide on the iron surface after Al O /Fe surfaces have been pretreated in ammonia. Contrary to this, in the case of restructuring in water vapor, AES finds that Al O leaves the iron surface region, residing underneath the active iron surface. 

Pretreating iron single crystals in high pressures of ammonia prior to ammonia synthesis have been shown to induce a surface restructuring. Both the Fe(llO) and Fe( 100) surfaces are found to approach the Fe( 111) activity after ammonia pretreatment. Treatment of the Fe(l 11) surface in ammonia causes a surface transformation to Fe(211). The presence of aluminum oxide on the iron surface inhibits ammonia-induced restructuring and potassium shows no observable effect. 

Eor the cover-coat direct-on process, a ferric sulfate [10028-22-5] Ee2(S0 2> etch is included in the metal pretreatment for rapid metal removal. It is designed to remove ca 20 g/m (2 g/ft ) of iron from the sheet metal surface. Hydrogen peroxide [7722-84-1/, H2O2, is added intermittently to a 1% ferric sulfate solution to reoxidize ferrous sulfate [7720-78-7] EeSO, to ferric sulfate. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

First comes the pretreatment stage. After rust removal and alkaline degreasing, a zinc phosphate formulated pretreatment (see Section 15.2) is applied by dip or spray-dip. Crystalline iron-rich zinc phosphate forms on the metal surface at a coating weight of 0.5-4.5 g/m. ... 

The effect of oxidation pretreatment and oxidative reaction on the graphitic structure of all CNF or CNF based catalysts has been studied by XRD and HRTEM. From the diffraction patterns as shown in Fig. 2(a), it can be observed the subsequent treatment do not affect the integrity of graphite-like structure. TEM examination on the tested K(0.5)-Fe(5)/CNF catalysts as presented in Fig.2(b), also indicates that the graphitic structure of CNF is still intact. The XRD and TEM results are in agreement with TGA profiles of fi-esh and tested catalyst there is no obviously different stability in the carbon dioxide atmosphere (profiles are not shown). Moreover, TEM image as shown in Fig. 2(b) indicates that the iron oxide particle deposited on the surface of carbon nanofibcr are mostly less than less than 10 nm. 

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