Surface iron impurities

Cathodic protection applications in fresh water include use of ferrite-coated niobium , and the more usual platinum-coated niobium . Platinised niobium anodes have been used in seawater, underground and in deep wells " and niobium connectors have been used for joining current leads Excellent service has been reported in open-seawater, where anodic potentials of up to 120V are not deleterious, but crevice corrosion can occur at 20 to 40V due to local surface damage, impurities such as copper and iron, and under deposits or in mud ... 

When it consists of only silica and water, flint is basically colorless. Impurities within the stone, however, render colored varieties. Thus, much flint is tan, beige, or jet black with a brown homy appearance (the latter color is caused mainly by iron impurities), but there are also gray, pink, and even red varieties. If dark flint is heated to temperatures above 450°C, the outer layer changes to white. Flint that underwent such a heating process is known as burned or calcinated flint, a misnomer probably used because the white surface appears to have changed into lime (Luedtke 1992 Sieveking and Bart 1986). Burned flint is easier to work than is the natural mineral, a property... 

The EDX spectrum (Fig. 11.8) shows the main surface scale impurity peaks of silica, aluminium, sodium, chloride and iron. If this EDX is compared to that of a new, clean membrane surface (Fig. 11.9), the clean surface shows sulphur, carbon and oxygen, which is typical of a porous polysulphone support. It was concluded that the scale is amorphous, composed of aluminosilicate and silicate. These compounds are normally found in trace amounts in brine solutions. Analysis showed that the surface could be cleaned with hydrochloric acid and analysis of the dissolved scale was similar to the EDX spectrum analysis. Review of the plant operation determined that the precipitation was the result of high pH in combination with high silica concentrations in the brine. 

The balance of evidence suggests that the catalyst in ammonia oxidation has a clean surface free from S and C and that aging is a consequence of the formation of gaseous Pt02. The situation for the catalyst used in the Andrussow process is less clear. Despite much lower oxygen pressures and consequent surface contamination by C and S species, the surface restructures due, perhaps, to surface energy differences aging is linked to build-up of carbonaceous films and the presence of iron impurities. 

It has been recently revealed that ZSM-5 zeolite exhibits an extremely high catalytic selectivity for the oxidation of benzene to phenol. The high reactivity of the zeolite should be ascribed to iron impurity arising in the intermediary steps in the zeolite synthesis . A surface oxygen (O) or a-oxygen, generated on Fe-ZSM-5 zeolite during N2O decomposition (equation 2)... 

Single metals and alloys such as wrought iron, bronze and lead will corrode even though they are not joined to a different metal or alloy. In reality, anodes and cathodes are set up on the surface of the metal. This will be due to the different phases present in the alloy, e.g. iron (ferrite) will be the anode and graphite the cathode in a cast iron, impurities such as sulfides present in wrought iron and variation in the copper/zinc ratio in adjacent grains in a brass alloy. The reactions taking place on the anode and cathode will be the same except the anode may be a different metal if the object was not iron. With a copper artefact, for example, the anode reaction will be ... 

Third, in both studies significant amounts of surface carbon impurities have been observed. Ertl and Thiele attributed the presence of the carbon to contamination, which occurred during handling of the samples. Analysis of the carbon Is spectra in the present samples led to the conclusion that at least some of the carbon is an intrinsic impurity of the iron compounds. On the other hand, the carbonate species found in the catalyst precursor is attributed to uptake of carbon dioxide by the alkaline promoter phases during storage of the catalyst. Therefore, the... 

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Despite its electrode potential (p. 98), very pure zinc has little or no reaction with dilute acids. If impurities are present, local electrochemical cells are set up (cf the rusting of iron. p. 398) and the zinc reacts readily evolving hydrogen. Amalgamation of zinc with mercury reduces the reactivity by giving uniformity to the surface. Very pure zinc reacts readily with dilute acids if previously coated with copper by adding copper(II) sulphate ... 

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