Corrosion oxide-hydroxides

Manufacture. Fluoroborate salts are prepared commercially by several different combinations of boric acid and 70% hydrofluoric acid with oxides, hydroxides, carbonates, bicarbonates, fluorides, and bifluorides. Fluoroborate salts are substantially less corrosive than fluoroboric acid but the possible presence of HF or free fluorides cannot be overlooked. Glass vessels and equipment should not be used. 

Internal surfaces of all tubes were severely attacked (Fig. 4.29). A brown deposit layer consisting of magnetite, iron oxide hydroxide, and silica covered all surfaces. Deposition was thicker and more tenacious along the bottom of tubes. These deposits had a distinct greenish-blue cast caused by copper corrosion products beneath the deposit. Underlying corrosion products were ruby-red cuprous oxide crystals (Fig. 4.29). Areas not covered with deposits suffered only superficial attack, but below deposits wastage was severe. 

Pourbaix has evaluated all possible equilibria between a metal M and HjO (see Table 1.7) and has consolidated the data into a single potential-pH diagram, which provides a pictorial summary of the anions and cations (nature and activity) and solid oxides (hydroxides, hydrated oxides and oxides) that are at equilibrium at any given pH and potential a similar approach has been adopted for certain M-H2O-X systems where A" is a non-metal, e.g. Cr, CN , CO, SOj , POj", etc. at a defined concentration. These diagrams give the activities of the metal cations and anions at any specified E and pH, and in order to define corrosion in terms of an equilibrium activity, Pourbaix has selected the arbitrary value of 10 ° g ion/1, i.e. corrosion of a metal is defined in terms of the pH and potential that give an equilibrium activity of metal cations or anions > 10 g ion/1 conversely, passivity and immunity are defined in terms of an equilibrium activity of < 10 g ion/1. (Note that g ion/1 is used here because this is the unit used by Pourbaix in the S.I, the relative activity is dimensionless.)... 

Vernon claims that in outdoor atmospheres the corrosion product consists largely of zinc oxide, hydroxide and combined water, but also contains zinc sulphide, zinc sulphate and carbonate. The following table gives the composition of typical films formed in an industrial atmosphere. 

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

High levels of chelant or oxygen affect the redox tendencies of iron-oxygen reactions and permit the liberation of Fe2+ ions (corrosion) from a metal surface and their subsequent chelation, thus preventing the formation or repair of blanketing ferric oxides, hydroxides, or a passivated magnetite film. 

Figure 16.2 Bimetallic corrosion of iron. The immediate anode product is Fe2+(aq), which reacts with dissolved oxygen and OH- (from the cathode) in bulk solution to form iron(III) oxide/hydroxide which precipitates away from the anodic surface.

Partial blocking effect was first identified for pure iron in contact with aerated sulphuric acid medium [55]. Corrosion of carbon steel in sodium chloride media clearly showed the porous layer effect (see Section 5.2) [74]. The same effect was found for zinc corrosion in sodium sulphate [75] and the properties of the layer which was demonstrated to be formed of an oxide/hydroxide mixture were further used for building a general kinetic model of anodic dissolution [76], usable for measurement of the corrosion rate from impedance data. 

Metals are subject to electrochemical corrosion in the presence of water Metal atoms lose electrons to become positively charged metal ions that go into solution. These then react with other chemical species in the soil ground-water to form solid corrosion products (e.g., metal oxides, hydroxides, sulfates). It is these solid corrosion products that often form a colored matrix with soil particles around the corroding object (Cronyn 1990). The initial formation of the metal ions takes place at a site on the metal known as the anode, whereas the electrons produced consumed by another reaction with an electron acceptor (the cathode). Due to the electrical conductivity of metals the location of the anode and cathode can be at different locations on the metal surface. In the presence of water and oxygen the cathodic reaction is... 

Ferrous corrosion products do not have a biocidal effect however, iron corrosion tends to form a much more extensive and encapsulating matrix. This corrosion matrix, which is principally formed of iron oxides, hydroxides, and soil minerals, will encapsulate any organic materials. This usually takes the form of a negative cast (Keepax 1975), provided that the primary layer of corrosion product has been laid down prior to any extensive degradation of the organic material, then fine surface detail will be preserved in the corrosion cast. Because the iron corrosion will not inhibit degradation... 

Inevitably, corrosion is not a process that proceeds by straightforward, uncomplicated active dissolution, especially over extended exposure periods. Reactive materials (e.g., Fe, carbon steel, Cu, U02) corroding in neutral to slightly alkaline solutions (4 < pH < 9) when oxide/hydroxide solubilities are low, tend to accumulate corrosion product deposits. 

Corrosion products such as the oxides, hydroxides, carbonates, sulfates, basic sulfates hydroxy carbonates, hydroxy chlorides are formed in the various environments (marine, urban, rural, industrial) and the initially loosely bound products may become adherent in the course of time. Corrosion can occur in the pores of the corrosion product layers. The low corrosion rate observed in atmospheric corrosion, R has been expressed as ... 

We see that solid Fe(OH)2 will be oxidized by O2 in aqueous media, no matter what the pH, and, of course, bare iron metal will always be susceptible to corrosion by aqueous oxygen but may become coated with potentially protective oxide/hydroxide films at pH greater than about 9. 

For some materials, runoff and spalling losses are negligible in short-term exposures, e.g., Rj =Sj =0, so that m replaces in Equation 4. If, furthermore, the system of oxides, hydroxides, etc., which comprise the corrosion product are thermodynamically stable, i.e., relatively insensitive to changes in the environment so that the composition does not vary greatly, a plot of as a... 

Dissolved metal ions can either pass into the bulk solution or react immediately with OH to form insolnble metal oxides/hydroxides that coat the metal surface. For iron corrosion, iron hydroxide species react farther with water and oxygen to form rust (Fe(OH)3) according to the following reaction seqnence ... 

Dissolution of the chlorides from the corrosion products is an essential part of the conservation process. It is essential that the artefact is immersed in an electrolyte that will not corrode the metal any further, while this dissolution is taking place. Corrosion scientists have developed redox potential - pH diagrams from thermodynamics in order to predict the most stable form of the metal. These diagrams are divided into three zones. Where metal ions are the most stable phase, this is classed as a zone of corrosion. If the metal itself is the most stable species, this is said to be the zone of immunity. The third zone is where solid metal compounds such as oxides, hydroxides, etc, are the most stable and may form a protective layer over the metal surface. This zone is termed passivity and the metal will not corrode as long as this film forms a protective barrier. The thickness of this passive layer may only be approximately 10 nm thick but as long as it covers the entire metal surface, it will prevent further corrosion. 

The hydrogen reduction conservation process was first employed in Sweden in 1964 for ferrous artefacts recovered from the Swedish warship, the Vasa. The method was further developed at Portsmouth to treat the large number of finds recovered from the Solent and land-based archaeological sites within the Wessex region. The principle of the process is to heat the artefact in an atmosphere of hydrogen in order to sublime off the volatile chlorides and at the same time reduce the oxides, hydroxides, chlorides and eventually to the metallic state. The volume change associated with the reduction of the iron compounds is sufficiently high to enable the release of deeply-buried chlorides particularly at the metal/corrosion product interface. 

BARIUM OXIDE (1304-28-5) BaO Noncombustible solid. The aqueous solution is a medium-strong base. Violent exothermic reaction with acids. Reacts exothermically with water or steam, forming heat and corrosive barium hydroxide may cause spontaneous combustion. Reacts violently with dinitrogen tetraoxide, hydrogen sulfide. 

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