Nickel penetration mechanism

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

Chromium-bearing nickel alloys may also contain iron or molybdenum. Inconel 600 is a low-iron (<10%) alloy. Its resistance is similar to that of nickel, and it is mechanically stronger. It is favored for such items as heating coils and exchanger plates. It is subject to stress cracking under severe conditions and so should be stress-relieved. Alloy 600 actually is superior to nickel in the presence of sulfur bodies. The formation of a nickel-nickel sulfide eutectic results in intergranular penetration of ordinary nickel. 

All full-penetration, pressure-containing welds are 100% radiographed to the standards of Section III of the ASME Boiler and Pressure Vessel Code. Weld preparation areas, back-chip areas, and final weld surfaces are magnetic-particle or dye-penetrant examined. Other pressure-containing welds, such as used for the attachments of nonferrous nickel-chromium-iron mechanism housings, vents, and instrument housings to the reactor vessel and head, are inspected by liquid-penetrant tests of the root pass, the lesser of one-half of the thickness or each 1/2-inch of weld deposit, and the final surface. Additionally, the base metal weld preparation area is magnetic-particle examined prior to overlay with nickel-chromium-iron weld metal. 

PTFE is used as an organic binder to link the catalyst particles and to produce a multifunctional electrode. For G D Es, hydrophobic and hydrophilic pore systems are required. For porous catalysts, for example, the Raney nickel catalyst, hydrophilic materials allow the penetration of the electrolyte into the electrode and the transport of the ions between the reaction zones the hydrophobic pore system - outside the catalyst - is required for the transport of gases to the reaction zones. In addition, to provide mechanical stability, the PTFE in the electrodes forms a hydrophobic pore network. 

Penetration of chloride ions this mechanism [first discussed by Hoar et al. (1965)] involves, following the adsorption of Cl" on the passive film surface, the entry of Cl" into the film and its transport through the passive film to the metal/oxide interface, where it causes breakdown of the passive film. The accumulation of Cl" at the interface or the formation of metal chloride may cause the film breakdown. Support of this mechanism is provided by the observation of chlorides in the inner oxide part of the passive film on nickel (Marcus and Herbelin, 1993), Fe-Cr (Yang et al., 1994), and aluminum (Natishan etal., 1997). 

A Materials Analysis Company electron beam microprobe was used for the analysis of nickel. Characteristic radiation emitted by the nickel in the specimen was resolved by a properly positioned lithium fluoride crystal and the intensity was measured with a proportional detector. A motor-driven gear mechanism moved the sample in a step-wise fashion relative to the electron beam. Integrated counts were taken at various intervals along the radius of an oxidized nickel sphere and into the glass sufficient to give a smooth curve for the concentration as a function of distance. The electron beam diameter was 1 ji and the depth of penetration was a maximum of 3 fi. These values are small compared with the 100 // oxidized nickel particle size. The electron beam microprobe was equipped to take photographs of the X-ray image for any element from an oscilloscope. In order to minimize the... 

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