Corrosion layer structure

Corrosion layer and its interfaces with PbSnCa grid and paste Corrosion layer structure... 

Feitknecht has examined the corrosion products of zinc in sodium chloride solutions in detail. The compound on the inactive areas was found to be mainly zinc oxide. When the concentration of sodium chloride was greater than 0-1 M, basic zinc chlorides were found on the corroded parts. At lower concentrations a loose powdery form of a crystalline zinc hydroxide appeared. A close examination of the corroded areas revealed craters which appeared to contain alternate layers and concentric rings of basic chlorides and hydroxides. Two basic zinc chlorides were identified, namely 6Zn(OH)2 -ZnClj and 4Zn(OH)2 ZnCl. These basic salts, and the crystalline zinc hydroxides, were found to have layer structures similar in general to the layer structure attributed to the basic zinc carbonate which forms dense adherent films and appears to play such an important role in the corrosion resistance of zinc against the atmosphere. The presence of different reaction products in the actual corroded areas leads to the view that, in addition to action between the major anodic and cathodic areas as a whole, there is also a local interaction between smaller anodic and cathodic elements. 

Figure 8. Structure of the corrosion layer at the grid surface. Penetration and corrosion rates are approximated for room temperature and normal float voltage (2.23-2.25 V/cell) (see text).

The corrosion of metals and alloys generally starts at the surface with the formation of an outer layer, which may develop into a crust of corrosion products. If a crust is formed, it generally has a layered structure comprising two or more compounds (1) an outer, rather stable, mineralized layer that often covers entirely the surface of the objects, and underneath, (2) a less mineralized, unstable, and chemically active layer. Some corrosion layers may also bind ugly and disfiguring earthy accretions. 

At present the iron-based alloys diffusion saturation by nitrogen is widely used in industry for the increase of strength, hardness, corrosion resistance of metal production. Inexhaustible and unrealized potentialities of nitriding are opened when applying it in combination with cold working [1-3], It is connected with one of important factors, which affects diffusion processes and phase formation and determines surface layer structure, mechanical and corrosion properties, like crystal defects and stresses [4, 5], The topical question in this direction is clarification of mechanisms of interstitial atoms diffusion and phase formation in cold worked iron and iron-based alloys under nitriding. 

Thermal transmission testing is an excellent way of detecting various types of anomalies such as surface corrosion under paint before the corrosion becomes visually evident. Thin, single-layer structures, such as aircraft skin panels, can be inspected for surface and subsurface discontinuities. This test is simple and inexpensive, although materials with poor heat-transfer properties are difficult to test, and the joint must be accessible from both sides. For nonmetallic materials, the defect diameter must be on the order of 4 times its depth below the surface to obtain a reliable thermal indication. For metals, the defect diameter must be approximately 8 times its depth. Some bright surfaces such as bare copper and aluminum do not emit sufficient infrared radiation and may require a darkening coating on their surface. 

The adsorption of an inhibitor affects the dielectric constant since the double layer structure is affected. This effect is exemplified by the corrosion of iron in 10% HC1 by quinolinium compounds.49 The inhibition by quinolinuim compounds has been attributed to the ordering effect by the re-electron system of the inhibitor molecules on the water molecules located at the metal interface. 

Although the rate of corrosion may be regarded as high at the initiation of the process, for instance Mott and Bott [1993] noticed that corrosion sites on mild steel in contact with a simulated cooling water appeared within 3-4 hours exposure, it is likely that as the corrosion layer thickens the rate of reaction will decrease due to the barrier imposed by the deposit. Much will depend on the structure of the corrosion products, i.e. whether they are porous or compact. 

In the case of metallic adsorbates (metal deposits, underpotentially deposited upd-layers, catalytically active metal deposits), the type of coordination to surface sites (one-, two- or three-fold) and the distance to these sites may be of interest. Vice versa the same type of data may be of importance in the case of adsorbed ions on metal electrodes or about the atomic environment of a given atom/ion in an interphase. Analysis of the fine structure of X-ray absorption (EXAFS, XANES) close to the X-ray absorption edge of the species (atom) of interest will yield this data provided the sample can be prepared in a very thin layer in order to exclude unwanted bulk interference. Otherwise the experiment can be done in reflection (SEXAFS). Information about the distance between the atom of interest and its first and sometimes even second shell of surrounding species can be derived from the spectra [95]. Availability of a suitable light source, generally a synchrotron (for details see p. 15), is an experimental prerequisite. The method has been applied in studies of passive and corrosion layers on various metals [96-102] and of molecular and ionic adsorbates on single crystal surfaces [103]. 

Dowling [53] proposed a mechanism for corrosion of structural steel exposed to wet soKd elemental sulfur (Fig. 8). Freestanding moisture and steel/sulfur contact are requisites for corrosion of structural steel by solid elemental sulfur. The principal form of attack in S/H2O media is not due to secondary acid generation resulting from hydrolysis of the sulfur. Instead, the author demonstrated that steel oxidation was coupled to sulfur reduction through an electron conductive iron sulfide layer. Evidence is also presented for the direct electrochemical reduction of solid elemental sulfur in the presence of FeS, supporting the role of this process as the partial cathodic step in the mechanism of the sulfur corrosion reaction. 

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