Metal surfaces nascent surface

When surfaces of tribological systems are involved in the mechanical activity of rubbing, direct reactions of surface adsorbed films with solid surfaces take place. The mechanically activated clean surface (nascent surface) of the metals and alloys is extremely reactive. Tribofilm formation is caused by the interaction between the metal (M, substrate) nascent surface under high energy and chemisorbed molecules of additive (adsorbate) (Buckley, 1981). 

Nascent atomic hydrogen released at metal surfaces by chemical reactions between the process environment and the metal (corrosion or cathodic protection reactions)... 

The thermal film made of long-chain zinc polyphosphates is formed on the surface. When friction increases, the process of transformation of phosphorus compounds into short-chain phosphate glasses is observed and iron sulfide abrasive particles are eliminated by tribochemical acid-base reactions. Under very severe wear conditions (nascent metal surface creation), an iron sulfide is formed, which will be mixed with the phosphate glasses tribofilm. 

Table 5.1. Adsorption of organic compounds on metal surfaces under static conditions and on the nascent steel surfaces...

The effect of the particular functional group on adsorption, however, is not always the same as seen on oxide surfaces. For example, carboxylic acids adsorb strongly on metal oxide surfaces, and the heat of adsorption of stearic acid is higher than that of the corresponding ester, methyl stearate (Hironaka et al., 1978). For adsorption on the nascent surface, however, propionic acid is a poor adsorbate, and the adsorption activity of propionic acid is lower than that of methyl propionate. Although propyl amine adsorbs easily on metal oxide surfaces, the adsorption activity of propyl amine is low on the fresh steel surfaces. Also, the heat of adsorption of organic sulfides on iron oxide is less than that of esters (Forbes et al., 1970b), but the results of adsorption activity on the nascent surface were the opposite. It is thus noteworthy that the chemical nature of the nascent surface of steel is often opposite to that of oxide-covered metal surfaces, with respect to adsorption. 

The adsorption of organic compounds on nascent surfaces can be considered as an acid-base reaction. According to the hard-soft acid and bases HSAB principle (Ho, 1977), polar compounds such as carboxylic acid and amine (with lone pair electrons on oxygen or nitrogen) are classified as "hard bases". A hard base reacts more easily with a hard acid than with a soft acid. Metals are classified as soft acids which react much more easily with soft bases than hard bases. The results in Table 5.1 can be explained with this concept. The soft bases (benzene, 1-hexene, diethyl disulfide) react easily with the nascent surface as a soft acid. On the other hand, the hard bases such as propionic acid, stearic acid, propyl amine and trimethyl phosphate exhibit a very low activity (Fischer et al., 1997a and 1997b Mori and Imazumi, 1988). 

Adsorption Using data from Table 5.1 Adsorption of organic compounds on the metal surface under static conditions and on the nascent steel surfaces , discuss the adsorption activity for each of the following (a) saturated hydrocarbons (n-hexane, cyclohexane), (b) the compounds which have rr-electrons (benzene, 1-hexene), (c) compounds with functional polar groups (propylamine, propionic acid). 

As described in Sec. 3.1, an energetic ion will produce a collision cascade upon surface impact. Recoiled surface atoms within the nascent hot spot may escape into the vacuum. As the sputtered atoms emerge from the surface, they can capture or lose an electron to the surface. On metal surfaces where conduction band electrons are delocalized, secondary ions are formed when sputtered atoms undergo resonant electron transfer along their outgoing trajectory. The relations expressed by Eqs. (12)-(14) in Sec. 3.2 approximate the charge transfer probability more sophisticated models have also been developed. ... 

In contrast to the conventional BLM system just described, a novel yet extremely simple method for formation of a stable BLM was recently developed in our laboratory (19-23). The technique involves the formation of self-assembled lipid bilayers on solid supports. The supported BLM (s-BLM) has a greatly improved mechanical stability (lasting indefinitely) and has desirable dynamic properties. One of the methods of formation of a s-BLM consists of two distinct steps. In the first step, the tip of a PTFE-coated platinum wire is cut off. To provide the best cut of the platinum wire, we constructed a miniature guillotine (Figure 1) where the sharp knife moves vertically onto the wire placed on the flat base. The cut is performed while the wire is immersed in a drop of lipid solution so that the initial contact of the newly exposed wire surface is with the lipid solution. In the second step, this newly cut lipid-coated PTFE-covered platinum wire is transferred into an aqueous bathing solution. This two-step self-assembled lipid bilayer works because the freshly cut metal surface is hydrophilic and attracts the polar groups of the lipid molecules. Thus, a lipid monolayer is tenaciously formed on the nascent metallic surface. Immersion of the lipid-coated wire into an aqueous solution spontaneously thins the lipid layer to a BLM that is anchored on one side to the solid support and is exposed to water on the other side. Further details of this method are available in the literature (19-23, 41). 

However, Huckel6 returned to the nascent hydrogen hypothesis in order to explain the reduction of isolated aromatic nuclei by the system alkali-metal-liquid ammonia-alcohol.5 7 Since this system contains no metal surface to catalyse the recombination of H atoms (2H -> H2), it may be the resulting concentration of free hydrogen atoms that triggers the primary process. 

Figure 5 portrays the two most essential steps of oiu- experimental procedure. In the first step as shown in Fig. 5, the tip of a Teflonstainless steel) is cut with a sharp knife tmder a lipid solution, for example, a 1% glycerol dioleate in squalene. When the nascent metal surface is exposed in a lipid solution, a monolayer of hpid molecules is irreversibly bound onto its surface. The adsorbed lipid monolayer with unattached... 

Some metals absorb hydrogen when they are placed in a hydrogen atmosphere, or particularly when they are polarized cathodically in a protonic solution, presumably due to the formation of H(a) on the metal surface. It was also known for a long time that cathodes exhibit a strong ability to reduce (hydrogenate) substances, through a species sometimes referred to by the term nascent hydrogen. 

Figure 27.3 Relationship between the distribution of nascent photon-excited electrons and the DOS of a metal. The excited electrons are classified as sub-vacuum electrons Ek < OeV) and photoelectrons, or free electrons > OeV). Adapted from Zhou et al, in Laser Spectroscopy and Photo-Chemistry on Metal Surfaces, II, 1995, with permission of World Scientific Publishing Co...

Thus the expression L = d /6D gives a value of D which is more nearly equal to the real value the more the phase-boundaiy processes are accelerated. The method can be applied equally to the diffusion of gases through hot metals, or to the diffusion of nascent gases liberated at room temperatures by electrolysis or by chemical reaction at the metal surface. 

S. Mori, Trihochemkal Activity of Nascent Metal Surfaces. Proc. Int. Tribology Conf., Yokohama (Japan), 1995, Satellite Forum on Tribochemistry, p. 37. To be published in Tribol. Lett. 

An effective surface treatment requires a clean surface. Metal surfaces are cleaned with an alkaline, neutral, or weakly acidic cleaner, an organic solvent, or by pickling with molten-salt baths [5,87]. Fluorinated surfactants in a pickling and descaling bath disperse scum, speed runoff of acid when metal is removed from the bath, and increase bath life [206,207]. The fluorinated surfactant inhibits nascent hydrogen formation and, therefore, prevents embrittlement by hydrogen [208]. 

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