Chromium additions

Fig. 1.94 Effect of chromium additions on the erosion corrosion of 70/30 cupro nickel in...

Mechanistically chromium additions have been shown to significantly enrich (lOX) in the magnetite oxide layer and, it has been suggested that this lowers its solubility. Additions of small amounts of oxygen to the water, increases the metal s potential and promotes the formation of haematite... 

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 ... 

Dilute binary alloys of nickel with elements such as aluminium, beryllium and manganese which form more stable sulphides than does nickel, are more resistant to attack by sulphur than nickel itself. Pfeiffer measured the rate of attack in sulphur vapour (13 Pa) at 620°C. Values around 0- 15gm s were reported for Ni and Ni-0-5Fe, compared with about 0-07-0-1 gm s for dilute alloys with 0-05% Be, 0-5% Al or 1-5% Mn. In such alloys a parabolic rate law is obeyed the rate-determining factor is most probably the diffusion of nickel ions, which is impeded by the formation of very thin surface layers of the more stable sulphides of the solute elements. Iron additions have little effect on the resistance to attack of nickel as both metals have similar affinities for sulphur. Alloying with other elements, of which silver is an example, produced decreased resistance to sulphur attack. In the case of dilute chromium additions Mrowec reported that at low levels (<2%) rates of attack were increased, whereas at a level of 4% a reduction in the parabolic rate constant was observed. The increased rates were attributed to Wagner doping effects, while the reduction was believed to result from the... 

Very recently Nicholls and Stephenson" confirmed the beneficial effects of chromium additions in a comprehensive series of laboratory tests using synthetic ash deposits in which 75 different alloys were examined. They concluded that nickel- or iron-based material with chromium contents in excess of 25% offered the best resistance to attack in their tests. 

A minimum chromium concentration of approximately 11% is typical for stainless steel. As more chromium is added, corrosion resistance improves. Concentrations of chromium >20% are found in some alloys. Chromium addition leads to the formation of a tight-forming oxide film on the surface of the metal. This stable film is self-healing, which means that the film will reform if scratched or broken. This oxide is quite resistant to attack by acids, bases, organic compounds, and inorganic salts. 

Slainless steels haxe a large degree of resistance In chemical attack. This properly sometimes is referred to as /hissn its This property results when iron is alloyed with at least I I i chromium. The corrosion resistance is lurther enhanced hy higher chromium additions and by the addition of nickel. A steel with I2 4 chromium will slain, but will not exhibit progressive rusting in normal atmospheres. Under normal circumstances, a steel with I N T chromium w ill mil slain, hut may discolor, particularly in heavy industrial areas. When ST nickel is added to an IS < chromium steel, the metal will be stain-resistant m all but the very worst of atmospheres. F.ven further enhancement of corrosion and heat resistance results w ith the addition of molybdenum. 

Effect of chromium additives on hydrogenation activity was rather good the initial specific activity improved with addition of Cr (x = 0.07), but further increase of Cr (x = 0.11) did not change significantly the initial... 

The chief ore1 is chromite (FeCr2C>4), which is a spinel with Crm on octahedral sites and Fe11 on the tetrahedral ones. If pure chromium is not required—as for use in ferrous alloys such as the chromium additive used to make stainless steel—the chromite is reduced with carbon in a furnace, affording the carbon-containing alloy ferrochromium ... 

Mass spectrometric studies were made by Farber and Srivastava (7) on the reactions Involved with vanadium and chromium additives in potassium-seeded Equilibrium values at an average flame temperature of 2250 K for the reaction for... 

Mass spectrometric studies were made by Farber and Srivastava (6) on the reactions involved with vanadium and chromium additives in potassium-seeded H /Og flames. Equilibrium values at an average flame temperature of 2250 K for the reaction CrOg(g) + HgO(g) = H2CrO (g) were analyzed by Farber and Srivastava (6). They calculated AjH (298.15 K) = -64.6 7 kcal mol for CrOg(g). The experimental determination of AjH (298.15 K) for H2CrO (g) also required the A H (298.15 K) value for CrO(g). Adjusting the calculations of Farber and Srivastava (6) to be consistent with the JANAF AjH (298.15 K) value for CrO(g) (2), we recalculate A H (289.15 K) = -72.6 kcal mol for CrOg(g). 

High temp. Hydrogen attack a. Steels without chromium additives. 

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

The present contribution focuses on the effect of various elements added by ion implantation on the isothermal and cyclic oxidation behaviour of the 7-Ti A1 based inter-metallic alloys Ti-48Al-2Cr and Ti-48Al-2Cr-2Nb at 800°C in air.These particular materials were selected since the ternary chromium addition improves the mechanical properties especially room temperature ductility [9-11] and the quaternary niobium addition improves the oxidation resistance [8, 14, 15]. Comparison will be made between materials modified by ion implantation and alloys in which elements were added by alloying techniques. 

If the operation is clean, as it usually is when hydrogen is present, the iron sulfide corrosion product is not protective. Under these conditions, the corrosion rate is linear and does not decrease with time, and chromium additions are not beneficial. Carbon steel and 9% chromium steel corrode at substantially the same rate and a 5% chromium steel may corrode even faster than the other two. So don t waste your money by picking a chromium content higher than you need to resist hydrogen attack. 

For a well designed membrane module of reasonably large scale, the initial cost should be dominated by the cost of the palladium content of the thin metal membrane (assuming a palladium alloy comprises the permselective layer). The module itself will be made of steel, most likely a nickel alloy, with or without significant chromium addition. The cost of the steel and the assembly labor should not exceed the cost of the palladium alloy membrane. 

The addition of Cr to Ni-Al alloys results in a remarkable synergistic effect which is of great technological importance. For example, chromium additions of about 10 wt% can enable external AI2O3 formation on alloys having aluminium levels as low as 5 wt%. This phenomenon has allowed the design of more ductile alloys and coatings. 

Dzh] Dzhavadov, D.M., Tyapkin, Yu.D., The Structure of Iron-Cobalt with Chromium Additions , Russ. Metall., (6), 152-154 (1977), translated from Izv. Akad. Nauk SSSR, Met., (6), 188-191 (1977) (Experimental, Phase Relations, 7)... 

Numerous tests - including natural seawater exposure tests - have demonstrated that a small chromium addition reduces the corrosion rates considerably without rendering steels more susceptible to pitting corrosion. In the upper part of Figure 15, the influence of chromium on seawater corrosion of a structural steel is presented [47]. Accordingly, only 0.5% and 1% Cr have a significant effect and reduce mass losses by 35%/65% compared to chromium-free steel. Improvements from higher chromium contents above this level are then relatively small. 

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