Chromium current density, effect

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

Table 10.31 Effect on critical current density and Flade potential of chromium content for iron-chromium alloys in lOwt.% sulphuric acid (after West )...

Table 10.32 Effect on critical current density and passivation potential on alloying nickel with chromium in In and IOn H2SO4 both containing 0-5N K2SO4 (after Myers, Beck and Fontana")...

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. 

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

Tab. 6.5. Effects of ultrasound in Chromium(VI) plating (a) effect of concentration on plating efficiency (b) effect of concentration on plating hardness (c) effect of current density on plating efficiency (d) effect of current density on mist emission (e) effect of ultrasonic power on emissions.

Chromium passivates very effectively down to very negative potentials even in strongly acidic electrolytes (Fig. 5). The cathodic current density of hydrogen evolution is followed by a small potential range of E = —0.4 to O V of anodic metal dissolution where Cr dissolves as Cr2+. At E > 0 V Cr passivates with a drop of the current density to less than 0.1 pA cm 2. In this potential range Cr3+ is the corrosion product. RRD studies have been applied to determine quantitatively the formation of Cr3+ ions. In principle the dissolution of Cr3+ at a Cr disc may be studied with two concentric analytical rings with their reduction to Cr2+ at the inner ring and its... 

The behavior along CPE varies according to the metal. The current density along CD remains fairly constant for metals such as iron, chromium and stainless steel as the potential increases the metal continues to dissolve at 100 percent efficiency. Along DE the metals dissolve in their higher valence state but at low current efficiency, since evolution of oxygen, or other anodic reaction where possible, now occurs. The film which forms on these metals is apparently very thin and readily allows the transitions of electrons from the anion to the metal by conduction or the tunnel effect. 

Figure 6 displays plots of alloy dissolution productivity (P) versus chromium content for two different electrolytes at a current density of 8 A cm-2. The maximum in Curve 2 is associated with an effect of chromium content of the alloy on the current efficiency in Na2SC>4 and NaNC>3 solutions. The variations in the electrochemical equivalent of the alloy and in the current efficiency with an increase in the Cr content have the strongest effect on the dissolution productivity. An increase in the oxidation state of chromium at Cr 15% has a weak effect on the productivity (a small inflection in the Curve 1 for NaCl). The shape of Curve 2 in Fig. 6 for the NaNC>3 solution depends on the current density in the range of low current densities, the productivity (P) of ECM of nickel and nickel-rich alloys increases with the current density, owing to an increase in the current efficiency. 

The effect of pH on the polarization of iron is shown in Fig. 5.6. The effect ofpH on the polarization of type 304 stainless steel (nominally 18 to 20 wt% Cr, 8 to 10.5 wt%Ni, 0.08 wt% C maximum) in environments based on 1 M Na2SC>4 with additions of H2SO4 and NaOH to control the pH is shown in Fig. 5.31 (Ref 28). The influence of chromium and nickel in moving the anodic polarization curve of iron to lower current densities persists over the indicated pH range with the corrosion rates being very low for pH >4.0. 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

From the analyses in Figure 17.17, it was possible to draw some conclusions concerning the interaction of chromium species with SOFC cathodes. First, it is obvious that a time dependence exists, but additionally there seems to be some saturation effect see, in particular, the results obtained at a current density of 0.5 A cm. After 3000 h of operation there is no additional Cr incorporation into the cathode. Another result is that LSCF cathodes incorporate more Cr than LSM cathodes and that there is little difference in the amount of Cr after stack operation times of 1900 h and more than 17 000 h. The results obtained by these analyses were the basis for further R D in the field of Cr-related degradation [11, 13] in order to understand better the basic interactions, chemical/electrochemical... 

Chromium poisoning of the porous composite cathode. Effect of cathode thickness and current density. 

Konysheva E, Mertens J, Penkalla H, Singheiser L, Hilpert K (2007) Chromium poisoning of the porous composite cathode effect of cathode thickness and current density. J Electrochem Soc 154(12) B1252-B1264... 

The phenomena ofbreakdown ofpassive films at more noble values of potential leads to an accelerated rate of corrosion (transpassive corrosion). The potential at which the breakdown or rupture of protective film takes place and the current density rises sharply is called transpassive potential (Iitranspassive)- In Fig. 3.26, the effect of chromium... 

Those alloying additions which decrease icritical are effective in increasing the passivating tendency. Consider alloying additions of Mo, Ni, Ta and Cb to Ti and Cr. The critical current density of Ti and Cr is reduced on addition of Mo, Ni, Ta or Cb. The potential of the above elements is active and their rate of corrosion is low. Generally, those alloying elements are useful which show low corrosion rates at the active potentials. Alloying with metals which passivate more readily than the base metal reduces icritical and induces passivity. Elements, like chromium and nickel, which have a lower (critical and Epassive than iron, reduce the... 

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