Passivity ferritic steels

Generally, all types of metal foils can be used to form a monoKth. However, ferritic steels, that is, alloys containing iron, chromium, and aluminum are preferred because these develop a passivating oxide layer upon heating in air. Particularly desirable is an alumina-rich surface that is formed at temperatures above 1073 K by oxidation of bulk aluminum and migration to the surface. 

Bemeron, R. A study of passive films formed on pure ferritic steels. Corros. Sci., 20, 899-907,1980 Pergamon Press Ltd. Great Britain. 

Besides corrosion issues of metal substrates, the use of alloys as mechanical supports of the cells is subject to interdiflfusion of iron, chromium, and nickel between ferritic steel and nickel-containing anodes during cells fabrication and operation. Diffusion of nickel into FSS substrates may cause austenitization of steels, which would result in TEC mismatch with other cell components. Diffusion of iron and chromium into Ni-based anodes may cause formation of oxide scales on nickel particles. This would result in fast degradation of cell performance during operation, as the electrochemically active surface is passivated. In order to overcome these issues, one possibility investigated by MS-SOFC developers is to use protective coatings [1-6, 13]. 

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

Most descaling and passivation processes for steels were developed prior to the widespread use of electrochemical techniques. As a result, a variety of visual and chemical tests are widely used for determining the surface cleanliness. Chemical tests have also been established to verify the presence of a robust oxide film on austenitic and ferritic stainlesses (8). These methods are very simple to conduct in a manufacturing environment, but they are qualitative in nature and rely strongly on the judgment of the inspector. Outside of the laboratory, electrochemical methods have not been widely used to evaluate cleanliness of carbon and alloy steels after pickling. Nevertheless, they are well suited for this purpose and have been examined in considerable detail in laboratory studies. 

Hong et al. examined the effect of nitric acid passivation on type 430 ferritic stainless steel using potentiodynamic polarization, EIS, and Auger electron spectroscopy (AES) (18). Passivation treatments were carried out on wet polished surfaces by immersion for 60 minutes in nitric acid solutions ranging from 1 to 61% at 50°C. Pitting potential and the magnitude of the total impedance were positively correlated with surface Cr concentration. In response to this study,... 

Cr, 8.73% Ni, 2.85% Mo, 0.70%Ma, 0. 55%"SL 0.24% 11, 0., 05% C, Alter quenching from U50°C fee steel contains 6 - ferrite and y - austenite. The 8 - phase is richer in Gr and poorer in Ni than fee second phase. Consequently 8 -ferrite will, be selectively etched over a wide range of potential whereas y - austenite will remain unetched (Fig. 15). Below this potential, fee two phases will be passive over a large... 

The stainless steel powder and discs were cleaned by immersing in a boiling bath of 15% NaOH for one hour with continuous stirring. The caustic was decanted and the surfaces neutralized by the addition of hot 10% nitric acid for less than five minutes. The acid was decanted and the stainless steel powder was rinsed in the same manner as the glass powder and freeze dried. The surface after such a passivation treatment consists pmbably of ferric ferrite, Fe(Fe02)3-. The surface area was 0.14 ttr/ghy B.E.T. nitrogen adsorption. 

Ferritic stainless steels exhibit IGSCC in hot nitrate, caustic, carbonate, and other environments. The phenomenon is potential dependent as discussed above. Susceptibility has been attributed to carbon and phosphorus segregation [94, 95]. Levels as low as 2 to 3 at. % can alter the passivity of iron in hot nitrate... 

Molybdenum in combination with chromium increases the corrosion-resistant properties of ferritic stainless steel in chloride electrolytes and is effective in increasing the resistance to pitting and crevice corrosion. Cr-Ni-Mo-Cu alloys increase the passivity in sulfuric acid concentrations with concentrations between 20% and 70%. Nickel... 

As dealt with in previous ehapters, the eorrosion resistanee of stainless steel is due to passivation by a surface film of chromium oxide. The chromium content is higher than about 11%, and the low-temperature corrosion resistance as well as the resistance to oxidation and mill scale formation at high temperature increase with increasing content of chromium. Pure chromium steels are either ferritic (low C-content, non-hardenable by heat treatment) or martensitic (traditionally higher C-content for most grades, hardenable by heat treatment). Wifli sufficient content of Ni the structure becomes austenitic, which gives increased formability, weldability. 

Chromium (Cr). Chromium increases the overall corrosion resistance of steel. Stainless steels contain in excess of 12% by weight of chromium. The corrosion resistance increases with increase in chromium content. The presence of chromium leads to the formation of a regenerative passive protective layer of chromium oxide that prevents further corrosion of steel. Chromium also contributes to increasing the hardenability of steel. It is a ferrite stabilizer, which means it promotes the formation of ferrite. Ferrite is resistant to the propagation of cracks. Presence of chromium increases the resistance of steel to pitting attacks. 

In ferritic and austenitic stainless steels, crevice corrosion is almost always initiated by local activation. This can be induced in a crevice by oxidant depletion, if necessary supplemented by halides. The passivity then breaks down. The access of oxidants to the material surface, and hence the passivity, may also be hindered by local deposits. 

The technology base for the LFR is primarily derived from the Pb-Bi liquid alloy-cooled reactors employed by the Russian Alpha class submarines. Technologies developed from the integral fast reactor metal alloy fuel recycle and refabrication development, and from the advanced liquid metal reactor (ALMR) passive safety and modular design approach, may also be applicable to the LFR. The ferritic stainless steel and metal alloy fuel developed for sodium fast reactors may also be adaptable to the LFR for those concepts with reactor outlet temperatures in the range of BSO C. 

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