Decreased with chromium concentration steels

It is clear from the data in Table 4.6 that the corrosion rates increase with increase in chromium in sulfuric acid solution. The presence of 10% Ni in the Fe-Cr alloy results in decreasing corrosion rate with chromium concentration. The corrosion rates of Fe-Cr alloys in ferric sulfate decrease with increasing concentration of chromium in the alloy. These observations are supported by the data on corrosion potentials of stainless steels in boiling acids and chlorides measured against a saturated calomel electrode. 

Several years ago, it was shown that, during food preparation, chromium could leach from stainless steel cookware and raise the chromium concentration of the foodstuffs (Offenbacher and Pi-Sunyer 1983). An increase in chromium concentration of foods was also noted for foods cooked in stainless steel compared to glass saucepans (Accominotti et al. 1998). Food chromium concentrations may also decrease with processing such as with milling of grains for flour (Schroeder 1971)... 

The above-mentioned sensitization effect and subsequent IGC can explain the changing chromium concentration in the melt. Due to sensitization, chromium is initially consumed by the carbide phases formed and the concentration of this element in the surface layer of grains decreases. Prolonged contact of steel samples with... 

Example 5. Chromium steel (Fe-17Cr) in phosphoric acid at low concentrations shows a decreasing rate with increasing temperature (see Fig. 2.26) presumably due to surface coverage by metal phosphates. 

As high-alloy steels are used, the surface compositions change with use such changes occur in a matter of hours or even minutes. First, major increases occur in the chromium and manganese content. At or near the surface, the chromium content often increases from about 20% to 50-70 /o. The manganese content increases from <1% to 15-20 /o. Meanwhile the iron and nickel concentrations decrease from 40-50%i and 25-35% to less than 5 /o. Significant increases of... 

Aluminum and chromium are both passive metals in the true sense. Zinc exhibits good corrosion resistance, which is attributed to the formation of an adherent protective layer of corrosion products (this may also be considered as a form of passivity). Chromium, at concentrations >12%, confers passivity to its alloys with iron, and these alloys are cathodic to steel. Under most conditions, both aluminum and zinc are anodic to steel, as are iron-zinc alloys. Iron-aluminum alloys, however, are cathodic and in this respect are similar to iron-chromium alloys. The corrosion resistance of steel is increased by alloying with aluminum or chromium, such increase being marked with the latter. The addition of zinc decreases the corrosion resistance of steel, although, in many cases, a protective layer of corrosion products leads to an apparent decrease in the corrosion rate. 

In liquid sodium, dissolution kinetics of austenitic steels is also heterogeneous. Indeed, selective dissolution of chromium, nickel, and manganese occurs. After reaching steady-state corrosion, the concentrations of austenite stabilizing components are decreased to such values that the stmcture is changed to the ferritic one. The thickness of the corrosion layer remains more or less constant, since dissolution at the surface removes the ferrite, while at the interface with the austenitic matrix, diffusion removes nickel, chromium, and manganese so that ferrite becomes stable [27]. 

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