Surface metallic oxide layer

Upon exposure to oxygen, all metals form surface metal oxide layers which vary in thickness and structure, depending on the identity of the base metal and the oxide formation conditions. Mercury and noble metals generally form very thin oxide films. On the other hand, most metals of primary commercial importance (i.e. aluminum, iron, zinc, etc.), tend to form oxide layers which are thick enough (40-80 A or more), so that the underlying metal atoms do not contribute in an appreciable way to the adhesion forces in metal/polymer systems U). 

Anyone who has ever painted a wrought iron fence to prevent rust appreciates the concept of a protective barrier. Aluminum and titanium are protected from corrosion due to the rapid formation of a surface metallic oxide layer, which tightly adheres to the metal surface. 

Aminoamides Polyaminoamides are used to improve bonding of PVC plastisols to metal surfaces. Used at 3-5 phr, generally dissolved in solvent or plasticizer, such products are available under the trade names Euretek (Schering) and Versamid (Henkel = Cognis). The bond formed is to the surface metal oxide layer. Combinations with urethane adhesion promoters should not be used, since the aminoamides are strong urethane catalysts. A topcoat (without adhesion promoter) is usually desirable it should be formulated to resist amine stain. Phenolic antioxidants used with aminoamides (or other amine additives) should have all ortho and para positions blocked to prevent color development on aging. Similarly, aliphatic phosphites are the best choice. 

Corrosion Control. Sihca in water exposed to various metals leads to the formation of a surface less susceptible to corrosion. A likely explanation is the formation of metahosihcate complexes at the metal—water interface after an initial dismption of the metal oxide layer and formation of an active site. This modified surface is expected to be more resistant to subsequent corrosive action via lowered surface activity or reduced diffusion. 

The corrosion resistance imparted to tantalum by the passivating surface thermal oxide layer makes the metal inert to most ha2ards associated with metals. Tantalum is noncorrosive in biological systems and consequently has a no chronic health ha2ard MSDS rating. 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

Since the formation of the Grignard compound takes place at the metal surface, a metal oxide layer deactivates the metal, and prevents the reaction from starting. Such an unreactive metal surface can be activated for instance by the addition of small amounts of iodine or bromine. 

Figure 35. Amplitude factor of the symmetrical fluctuation for anodic dissolution through a metal oxide layer with complex formation. Dm = 1.0 x 10-9 m2 s-1, Jt = 1.0 x 10"5 nr s-1 mol-1, m = 2, m = 2 1.Curves 1,2, and 3 correspond to the surface concentrations of the anion, (C (jr, yt 0)) = 10, 50, and 100 mol m-J, respectively.

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

In some cases, the oxide-coating protects the surface from further oxide buildup. One example is that of aluminum where an oxide coating appears almost instantaneously once the pristine surface is exposed to air. Yet, there are many cases where the oxide layer continues to buildup until the metal is totally consumed (One example is that of iron and "rust"). How is this possible Wagner hypothesized that both metal and oxide ions difiosed through the metal oxide layer so as to build up the layer thickness from both sides. The following diagram is one representation of such a mechanism ... 

In addition to the inorganic hydroxyl groups which are exposed on many mineral surfaces (metal oxides, phyllo-silicates and amorphous silicate minerals) we need to consider the particular features relating to charge on the silica surfaces of layer silicates. 

Fig. 12.1 Main structural models of graphene-metal oxide hybrids, (a) Anchored model oxide particles are anchored to the graphene surface, (b) Encapsulated model oxide particles are encapsulated by graphene, (c) Sandwich-like model graphene is sandwiched between the metal oxide layers, (d) Layered model a structure composed of alternating layers of oxide nanoparticles and graphene, (e) Mixed model graphene and oxide particles are mechanically mixed and graphene sheets form a conductive network among the oxide particles. Red metal oxide Blue graphene. Reprinted with permission from [41]. Copyright 2012, Elsevier B.V.

Chromate compounds have been considered by many to be the best inhibitors available. They are typically composed of mixtures of sodium bichromate and chromic acid, and art through passivation of the metal surfaces. Passivation involves formation of a tough metal oxide layer or other film on the surfaces. Chromate concentrations of 200 to 1000 ppm in the cooling water are generally required, although for environments where bimetallic influences exist, chromate levels must be much higher. For instance, when steel and copper surfaces are present in the system, chromate levels often exceed 2000 ppm (BETZ 1982, pp. 207, 212). 

The shorter-term exposure experiments show that some portion of the organically-bound chlorine, such as trichlorethane or its decomposition products, remains absorbed on a 304 stainless steel surface, even after heating at 35-40°C in a high vacuum. Conversion to an ionic species begins after a short contact period and can be detected using XPS. Formation of the ionic chloride is likely the result of hydrolysis by water also absorbed on the surface, and is perhaps catalyzed by the surface metal oxides. Further atmospheric exposure up to a few months increases the relative amount of the ionic form of chlorine. The composition of the surface oxide layer was altered, with chromium oxide replacing iron oxide as the major species. There was further evidence that chlorine was present as iron chloride, perhaps up to 5% of the surface film. The conditions under which oxidation of such surfaces occurred are quite comparable to those which could occur on steel surfaces in industrial usage. 

The results obtained for the solar cell discussed above suggest a strong interaction between the chromophore and the metal oxide layer, a large surface area, thus yielding large absorbances, and an efficient charge-separation upon injection. Several studies have indeed been carried out in an attempt to utilize the potential of nanocrystalline metal oxides as substrates for electrochromic devices. A particularly interesting approach has been reported by Fitzmaurice and co-workers [17]. These authors have constructed an electrochromic device based on the combination of... 

Surface protection A thin protective metal oxide layer can be formed on the surface of organic or inorganic substrates Priming of the substrate by solutions of titanium/zirconate or by partial hydrolyzing and heating or chemical vapor deposition... 

Another way moisture can degrade the strength of adhesive joints is through hydration or corrosion of the metal oxide layer at the interface. Common metal oxides, such as aluminum and iron, can undergo hydration. The resulting metal hydrates become gelatinous, and they act as a weak boundary layer because they exhibit very inadequate bonding to their base metals. Thus, the adhesive or sealant used for these materials must be compatible with the firmly bound layer of water attached to the surface of the metal oxide layer. 

The actual metal surface that takes part in the bonding is illustrated in Fig. 16.1. Adhesives recommended for metal bonding are in reality used for metal-oxide bonding. They must be compatible with the firmly bound layer of water attached to surface metal-oxide crystals. Even materials such as stainless steel and nickel or chromium are coated with transparent metal oxides that tenaciously bind at least one layer of water. 

Another important type of electro-organic process is that in which reaction takes place at a bulk phase oxide layer on metallic electrode surfaces. Such oxide layers can mediate the oxidation of organic molecules, as in the case of alcohols and amines at oxidized Ni [529], and passage of current may be regarded as serving to maintain the oxide layer. 

The continued oxidation of the metal substrate beneath the protective oxide layer must become a diffusion-controlled process for thick enough oxide films in which either metal atoms or oxygen atoms diffuse through the metal oxide layer to the appropriate interface where reaction proceeds. Let us assume a thick enough oxide layer on a plane metal surface where a steady state has been achieved. Then we can write for the rate of formation of metal oxide, MO, per unit area (assuming metal ion diffusion) ... 

In a CMP process, a rotating polymer-based pad is pressed against the polished metal/oxide layer surface of a wafer while slurry or a combination of slurry and other chemicals is introduced into the polish platen. In a post-CMP cleaning process, residual abrasive particles and residues of polished layers are removed via flushing of the post-CMP solution and gentle mechanical... 

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