Nickel passive layer thickness

Using the unique four-electrode STM described above, Bard and coworkers (Lev, 0. Fan, F-R.F. Bard, A.J. J. Electroanal. Chem.. submitted) have obtained the first images of electrode surfaces under potentiostatic control. The current-bias relationships obtained for reduced and anodically passivated nickel surfaces revealed that the exponential current-distance relationship expected for a tunneling-dominated current was not observed at the oxide-covered surfaces. On this basis, the authors concluded that the nickel oxide layer was electrically insulating, and was greater than ca. 10 A in thickness. Because accurate potential control of the substrate surface is difficult in a conventional, two-electrode STM configuration, the ability to decouple the tip-substrate bias from... 

The lag between the time nitinol was first produced and the time it was used commercially in medical devices was due in part to the fear that nickel would leach from the metal and not be tolerable as a human implant. As it turns out, with the correct understanding of its surface electrochemistry a passivating layer on the surface of the nitiol can be induced by an anodizing process. This layer is comprised of titanium oxide approximately 20-nm thick which acts as a barrier to prevent the electrochemical corrosion of the nitinol itself. Without an appreciation for the electrochemistry at its surface, nitinol would not be an FDA approved biocompatible metal and a whole generation of medical devices would not have evolved. This is really a tribute to the understanding of surface electrochemistry within the context of implanted medical devices. 

The electrochemical oxidation of the nickel is of special interest since it is a typical passivation metal in which very thin passive oxide films of a few nm thickness on the surface can cover the substrate metals efficiently. The passive oxide layer on the nickel was studied by Sikora and Mac Donald [118] who claimed that the passive film consisted of the inner nickel oxide of a barrier layer and an outer Ni(OH)2 porous or hydrated layer, in which the inner layer behaves as a p-type oxide with a cation vacancy. Oblonsky and Devine measured the surface enhanced Raman spectra of the nickel passivized in a neutral borate solution and estimated the amorphous Ni(OH)2 in the passive potential region and the NiOOH in the higher transpassive region [119]. Further, the passive films formed in the acidic and neutral solutions were assumed as partially hydrated nickel oxide [120,121]. The anodic film formed in the alkaline solution was assumed to be Ni(OH)2 in the... 

At high potentials in the passive region, the imaging of nickel surfaces proves difficult owing to the formation of thick oxide layers." It was shown by Bhardwaj et that on polycrystalline iron in a borate electrolyte, oxide formation starts as patches on the surface that gradually fuse together to establish a surface oxide fihn. Also, clusters of the hydroxide were seen" on a polycrystalline iron surface obsaved by in situ STM and after potential cycles in an NaOH electrolyte. 

Oxidation can be viewed as the chemisorption of oxygen. For example, nickel and silicon are oxidized at ambient conditions. The resulting oxide layer is thermodynamically more stable and passivates the pure material below it. Another important example is the oxidation of aluminum which provides the metal with a very hard roughly 100 nm thick aluminum oxide (AI2O3) layer. To stabilize the aluminum surface even more and to passivate it against reactive chemicals the thickness of the oxide layer can be increased electrochemically. This procedure is called the eloxal process (efectrolytical oxidation of a/uminum). 

Another process of physical protection is the formation of an oxide layer that makes the metal passive. This procedure is used for aluminium. Aluminium is normally anodized in 10 per cent sulphuric acid with steel or copper cathodes until an oxide thickness of 10-100 pm is obtained. As the more superficial part of the oxide layer has a fairly open structure it is possible to deposit metals (cobalt, nickel, etc.) or organic pigments in the pores and seal with boiling water or with an alkaline solution. The colours after metallic deposition are due to interference effects. Chromic and oxalic acids are also used significantly as electrolyte. 

One example is the hot corrosion of a preoxidized nickel specimen by a thin Na2S04 melt film in a 0.1 wt. % SO2-O2 gas mixture at 1200 K [29]. By variation of the oxide scale thickness and the purity of the material, different regimes of corrosion were investigated passive state, pseudopassive state, and active state. The passive state of 99.9975% of pure nickel, preoxidized in pure O2 for 2 h at 1200 K is controlled by diffusion of 8207 in the salt melt. The corresponding Nyquist plot of impedance data shows linear behavior in the low-frequency range withaslope of45° (Fig. 16).The semicircle at higher frequencies was attributed to the resistance of the NiO layer itself The active state was established on less pure nickel... 

Metals in the passive state (passive metals) have a thin oxide layer on their surface, the passive film, which separates the metal from its environment. Metals in the active state (active metals) are film free. Most metals and alloys that resist well against corrosion are in the passive state stainless steel, nickel-chromium based superalloys, titanium, tantalum, aluminum, etc. Typically, the thickness of passive films formed on these metals is about 1-3 nm. 

The passive film on nickel can be formed quite readily in contrast to the formation of the passive film on iron. Differences in the nature of the oxide film on iron and nickel are responsible for this phenomenom. The film thickness on nickel is between 0.9 and 1.2 mm, whereas the iron oxide film is between 1 and 4 mm. There are two theories as to what the passive film on nickel is. It is entirely NiO with a small amoimt of nonstoichiometry, giving rise to Ni cation vacancies, or it consists of an inner layer of NiO and an outer layer of anhydrous Ni(OH)2. The passive oxide film on nickel, once formed, cannot be easily removed by either cathodic treatment or chemical dissolution. 

The oxidation of the working electrode results in the formation of an oxide layer comparable to passivation in wet electrochemistry. For the oxidation reaction to continue, oxygen or nickel ions will have to diffuse through an oxide layer of increasing thickness. The thickness of this layer can be calculated from the total anodic charge. For the Ni/NiO electrode at 750°C and a scan rate of 20 mV/s, this yields a thickness of about 1 nm of NiO. The reduction reaction will be affected in a similar way, but now a thin Ni layer will impede the transport of oxygen (see Figure 15.15). 

The combined ellipsometry-reflectance method—three-parameter ellipsometry—has been subject to error analysis. Cahan showed that, while it is possible in principle to obtain an unambiguous solution for the optical constants and thickness of a film by three-parameter ellipsometry, the method does not guarantee that a solution can be obtained in practice. He also pointed out, by working with sample data from electrochemically produced films, that the numbers obtained as the solution are not necessarily physically real when the three-layer model is inadequate for the particular system. Chung, Lee, and Paik" studied the forward and reverse sensitivity analyses for three-parameter ellipsometry to obtain the forward sensitivity coefficients (dMldoi) > and the reverse sensitivity coefficients for a passive film on nickel (here... 

Passivity has been attributed, since the time of Faraday, to an oxide film. In a few cases the presence of this has been directly demonstrated, and the response of a passive electrode to pH changes is very much that of a metal-metal oxide electrode. Uncertainties still exist, however, as to the nature and thickness of the surface film. On platinum, nickel and iron the onset of passivation may only require a monolayer of oxide, or of adsorbed oxygen atoms, although a layer several Angstroms thick may be produced in time. Evidence for these very thin layers, which are quite undetectable visibly, comes from the very small cathodic pulse of electricity that is sufficient to remove them. In other cases, e.g. lead, quite thick films are needed to preserve the passive state. Partial protection is obtained for many metals in a variety of electrolytes so long as conditions favour the precipitation of a film of insoluble hydroxide or salt (see Anodising),... 

To examine the development of the solidified layer and the effect of passive heating on the development of the solidified layer, tiie flow of the polymer melt with passive heating was analyzed [11]. The mold cavity for simulation had a thickness of 1.2 mm and diameter of 86 mm, the same as those of our patterned magnetic media substrates. The initial melt temperature and mold temperature were 300°C and 100°C, respectively. Polycarbonate was used as the polymer material, and nickel was used as the stamper material. For the mold material and insulation layer, tool steel and polyimide were chosen, respectively. The flow rate and packing pressure are 14.15 cmVs and 3920 N/cm, respectively. For flow analysis of injection molding with passive heating, a mathematical model was constructed in the cavity and in the passively heated mold, consisting of the stamper the insulation layer and the... 

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