Chromium oxidation potential

To appreciate the potential changes in food cans, it is necessary to describe briefly the steel-based materials used in modern can manufacturing operations. The tin can is made from a special grade of thin gage, low carbon, cold-rolled steel, which is generally referred to as a tin mill product. The base steel is coated with either tin, a chromium-chromium oxide system, or it is just cleaned and oiled. It may also be coated with organic coatings. 

Attempts of Sanko and Stefanovskii to make direct measurements of the oxidation potentials of couples involving these chromium species were not successful. An estimate of the potential of the Cr(VI)/Cr(V) couple was given by West-heimer, based on the equilibrium between chromium(VI) and iron(II), viz. 

Since the reaction is markedly displaced to the left, the standard potential of Fe(III)/Fe(ir) couple (0.77 V) must exceed that of the Cr(VI)/Cr(V) couple therefore Fft/s niust be < 0.6 V. This estimate seems reasonable. Regarding the oxidation potential of chromium(IV) there is an estimate, also by Westheimer, that the Cr(IV)/Cr(III) couple has a value of 4/3 1.5 V. This was based on the fact that... 

The corrosion of stainless steel in 0.1 mol-1 NaCl solutions at open circuit potential was studied in detail by Bruesch et al. [106] using XPS in combination with a controlled sample transfer system [38]. It was verified by XPS analysis that the passivating film contains chromium oxide. The position and the height of the Cr concentration maximum depends critically on the bulk chromium content of the steel. Significant variations in the electrode passivation properties were observed at a Cr concentration of 12%, while the film behaviour was found to be rather independent of the other components like Mo, Ni, Cu. From the fact that the film structures and... 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Figure 9.1 compares the synthesis of acetophenone by classic oxidation of 1-phenylethanol with stoichiometric amounts of chromium oxide and sulphuric acid, with an atom efficiency of 42%, with the heterogeneous catalytic oxidation with O2, with an atom efficiency of 87%, and with water as the only by-product. This is especially important if we consider the environmental unfriendliness of chromium salts the potential environmental impact of reactions can be expressed by the environmental quotient (EQ), where E is the E-factor (kg waste/kg product) and Q is the environmental unfriendliness quotient of the waste. If Q is... 

Gadolinium is a strong reducing agent. It reduces oxides of several metals such as iron, chromium, lead, manganese, tin, and zirconium into their elements. The standard oxidation potential for the reaction... 

The standard oxidation potential for the reaction Cr(s)— Cr3+(aq) + 3e is 0.74 V. Despite the large, positive oxidation potential, chromium is used as a protective coating on steel automobile bumpers. Why doesn t the chromium corrode ... 

The hydrogenation of fatty acids or fatty esters is of industrial importance for the production of fatty alcohols. Usually, the hydrogenation is performed in slurry-phase or fixed-bed reactors over copper-chromium oxide catalyst at elevated temperature and pressure.37 Rieke et al. investigated the hydrogenation of methyl dodecanoate over copper-chromium oxide at 280°C and 13.8 MPa H2, in order to study the side reactions that occur during hydrogenation.37 On the basis of the potential reaction routes described by Rieke et al., the pathways leading to C12 alcohol and various byproducts are summarized in Scheme 10.2, with exclusion of the formation and reactions of acetals. It has been found that both catalytic activity and selectivity correlated well with the crystallinity of the copper-chromium ox-... 

It can be seen that, on complexation, either the ferrocene or the chromium oxidations become slightly more difficult, but the difference in redox potential decreases with increasing number of SiMe2 units. This suggests that they tend to prevent interactions between the two metal centers. 

Passivation of iron under critical conditions is predicted. Hematite (Fe203) may still be the main corrosion product in the neutral water pH (pH 7.2) region, but the passivation potential range is narrower and shifts to negative potentials, compared with regions on the diagram for ambient conditions. For chromium, no solid chromium oxide stable species is predicted within the stable region of neutral water. This indicates chromium oxidation without any passivation oxide film formation. 

There is a problem as to the effect of the contaminants upon the development of catalytic activity during heating. We assume that most of the nitrogen is present as potential ammonium nitrate and we note that chromium oxide is an effective catalyst for the decomposition of ammonium nitrate at about 200° (J02). We have heated the catalyst in a tube separated from a mass spectrometer by a valve. After heating to 100° for 30 minutes, the valve was opened and the gases evolved were analyzed. The tube was then pumped out and the temperature increased 50°, held for 30 minutes, analyzed, pumped out, and the temperature raised 50° again. This was continued to 400°. Except at 150- 00°,... 

Parbhu, A.N. et al.. Specific ion binding influences on surfaces on surface potential of chromium oxide, Curr. Appl. Phys. 4, 152, 2004. 

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