Passivation ferrous

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron. Surface Sd. 68 459—468 Bruno, J. Sturam, J.A. Wersin, P. Brand-berg, E. (1992) On the influence of carbonate on mineral dissolutions I. The thermodynamics and kinetics of hematite dissolution in bicarbonate solutions at T = 25°C. Geo-chim. Cosmochim. Acta 56 1139—1147 Brusic.V. (1979) Ferrous passivation. In Corrosion Chemistry, 153—184 Bruun Hansen, H.C. Raben-Lange, R. Rau-lund-Rasmussen, K. Borggaard, O.K. 

Hydrogenis prevented from forming a passivating layer on the surface by an oxidant additive which also oxidizes ferrous iron to ferric iron. Ferric phosphate then precipitates as sludge away from the metal surface. Depending on bath parameters, tertiary iron phosphate may also deposit and ferrous iron can be incorporated into the crystal lattice. When other metals are included in the bath, these are also incorporated at distinct levels to generate species that can be written as Zn2Me(P0 2> where Me can represent Ni, Mn, Ca, Mg, or Fe. 

Surface films are formed by corrosion on practically all commercial metals and consist of solid corrosion products (see area II in Fig. 2-2). It is essential for the protective action of these surface films that they be sufficiently thick and homogeneous to sustain the transport of the reaction products between metal and medium. With ferrous materials and many other metals, the surface films have a considerably higher conductivity for electrons than for ions. Thus the cathodic redox reaction according to Eq. (2-9) is considerably less restricted than it is by the transport of metal ions. The location of the cathodic partial reaction is not only the interface between the metal and the medium but also the interface between the film and medium, in which the reaction product OH is formed on the surface film and raises the pH. With most metals this reduces the solubility of the surface film (i.e., the passive state is stabilized). 

Items of plant fabricated from stainless steels should be inspected before first use and after any maintenance work or unplanned shutdown. All materials that rely for their corrosion resistance on the presence of an oxide or similar passive layer are susceptible to localized attack where that layer is absent or damaged. Damage is most commonly caused by scratching, metallic contamination (nearby grinding or touching with ferrous tools), embedding of grit and weld spatter. 

Steels and stainless steels show preferential nucleation of pits at inclusions, most notably sulphide inclusions ". Other sulphur-rich regions in ferrous and nickel-based alloys may also lead to premature failure. It has been shown that accumulation of sulphur on the surface of these materials retards passivity and enhances dissolution of the metal. These effects occur in any solution in which the metal shows an active region and they are also preferential pitting sites in the presence of chloride. A recent notion for... 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Experimental installations have been established from time to time to demonstrate the possibility of using ferrous metals in anolytes " selected to minimise polarisation and to reduce metal ionisation by making the metal passive. 

Temperatures reached during phosphate-based cleaning programs are only modest, and under these lower temperature conditions the passivated film consists of gamma iron oxide (yFe203) together with primary ferrous phosphate [Fe2H2(P04)2] and tertiary ferrous phosphate [Fe3(P04)2]. 

Erythorbic acid and sodium erythorbate are very safe products, widely used in the food industry as antioxidants and alternatives to vitamin C. In the water treatment industry, they are strong reducing agents that reduce metal oxides and hydroxides to their more soluble ferrous forms and promote the passivation of boiler waterside surfaces (magnetite formation). 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

In iron-deficient rats, nickel enhanced the absorption of iron (Nielsen 1980 Nielsen et al. 1980, 1984). This effect of nickel was only observed when ferric sulfate was given. No interaction was observed when iron was given as a 60% ferric/40% ferrous sulfate mixture. It has been proposed that nickel facilitates the passive diffusion of ferric ions by stabilizing the transport ligand (Nielsen 1980). 

The usual chemical employed is sodium molybdate (dihydrate), which acts as an anodic inhibitor and forms, in the presence of some oxygen, a passivating film composed of a ferrous-ferric-Mo-oxides complex. 

In order to make the walls of the iron pot passive a small amount of alkali chlorate (some 20 kg per 8 000 kg of caustic) is added to the melt. The iron which in spite of this measure dissolves in the caustic in the form of sodium ferrate Fe(ONa)s, is reduced by an admixture of sulphur to give ferrous sulphide which is separated by settling together with other impurities, especially sodium car-bonate. The melt is left standing and cooled to 350 °C. It is then siphoned or pumped by an immersion pump directly to iron drums where it solidifies. Flaked hydroxide can be also obtained if a water cooled rotating drum is dipped into the melt. A thin film of hydroxide will adhere to the rotating drum. The film solidifies and is scraped off with a thin knife. (See Fig. 108.)... 

---