Chromium-nickel alloys, anodic

Anodic Polarization of Iron-Chromium-Nickel Alloys... 

Anodic polarization curves for chromium-nickel alloys in 1 N H2S04. Redrawn from Ref 1 3... 

Nickel has a very small effect on the anodic polarization behavior of iron, and hence, iron-nickel alloys are of minor significance as corrosion-resistant alloys. However, the addition of nickel to iron-chromium alloys (AISI 200 series) permits conversion of the latter as ferritic alloys to austenitic iron-chromium-nickel alloys (AISI 300 series). In... 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

Fis. 5.38 Anodic polarization of chromium-nickel binary alloys in 1 N H2S04 + 1 N NaCl. Redrawn from Ref 13... 

F.G. Hodge and B.E. Wilde, Effect of Chloride Ion on the Anodic Dissolution Kinetics of Chromium-Nickel Binary Alloys in Dilute Sulfuric Acid, Corrosion, Vol 26, 1970, p 146-150... 

Porous metallic gas diffusion electrodes are used in these fuel cells. The anode consists of a nickel alloy with 2% of chromium. Chromium that is added prevents recrystallization and sintering of the porous nickel though it works as an electrode. This action is based on chromium forming a thin layer of chromium oxide at the nickel grain boundaries, which interferes with the surface diffusion of the nickel atoms. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

The choice must be made between the use of more ejq)en-sive corrosion-resistant alloys, and the use of a less expensive, less corrosion-resistant alloys protected by chemical treatment. Chsuiges in the raw material costs of chromium, nickel, and other constituents in corrosion-resistant alloys affect the initial construction costs. A less expensive material, that would otherwise be unacceptable due to low corrosion resistance, can still be the best choice when used with chemical corrosion inhibitors, or other protection methods such as anodic or cathodic protection. 

Anodic protection of high alloy steels (chromium-nickel and chromium-nickel-molybdenum) is possible in all sulfuric acid concentrations and necessary especially at elevated temperatures (up to approx. 120 °C). For example, the protection of steel containing 18% Cr, 10% Ni, and 2% Mo in sulfuric acid of 20-60% concentration (at 47 °C) causes a decrease of the corrosion rate of over 1000 times. For a given acid concentration, the dissolution rate during anodic protection depends on the composition of the steel and the temperature of the solution. 

Another electrochemical way to separate the analyte from the matrix that has been described is electrolytic deposition of the matrix elements on a mercury cathode in the presence of low sulfuric acid concentrations. Once the matrix is separated, the solution is carried to the spectrometer and elements are determined. This procedure is especially useful for the analysis of alloys, because elements such as rare earth elements can be easily separated from iron, chromium, nickel, copper, etc. For the analysis of steel samples, this method provides limits of quantification 10 times better than direct analysis, without the need for matrix matching. Recently, an FI electrolytic dissolution procedure has been reported for the treatment of metallic (i.e. high-purity copper) samples, the specimen acting as the anode of an electrodissolution ceU. ... 

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]. 

Anodes. Lead—antimony (6—10 wt %) alloys containing 0.5—1.0 wt % arsenic have been used widely as anodes in copper, nickel, and chromium electrowinning and metal plating processes. Lead—antimony anodes have high strength and develop a corrosion-resistant protective layer of lead dioxide during use. Lead—antimony anodes are resistant to passivation when the current is frequendy intermpted. 

Amorphous Fe-3Cr-13P-7C alloys containing 2 at% molybdenum, tungsten or other metallic elements are passivated by anodic polarisation in 1 N HCl at ambient temperature". Chromium addition is also effective in improving the corrosion resistance of amorphous cobalt-metalloid and nickel-metalloid alloys (Fig. 3.67). The combined addition of chromium and molybdenum is further effective. Some amorphous Fe-Cr-Mo-metalloid alloys passivate spontaneously even in 12 N HCl at 60° C. Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acids of various concentrations and different temperatures are shown in Fig. 3.68 ... 

The alloying elements molybdenum and copper do not, by themselves, enhance passivity of nickel in acid solutions, but instead ennoble the metal. This means that, in practice, these alloying elements confer benefit in precisely those circumstances where chromium does not, viz. hydrogen-evolving acidic solutions, by reducing the rate of anodic dissolution. In more oxidising media the anodic activity increases, and, since binary Ni-Mo and Ni-Cu alloys do not passivate in acidic solutions, they are generally unsuitable in such media. 

Because these variables have a very pronounced effect on the current density required to produce and also maintain passivity, it is necessary to know the exact operating conditions of the electrolyte before designing a system of anodic protection. In the paper and pulp industry a current of 4(KX) A was required for 3 min to passivate the steel surfaces after passivation with thiosulphates etc. in the black liquor the current was reduced to 2 7(X) A for 12 min and then only 600 A was necessary for the remainder of the process . From an economic aspect, it is normal, in the first instance, to consider anodically protecting a cheap metal or alloy, such as mild steel. If this is not satisfactory, the alloying of mild steel with a small percentage of a more passive metal, such as chromium, molybdenum or nickel, may decrease both the critical and passivation current densities to a sufficiently low value. It is fortunate that the effect of these alloying additions can be determined by laboratory experiments before application on an industrial scale is undertaken. 

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