Alloying chromium

The basic corrosion behaviour of stainless steels is dependent upon the type and quantity of alloying. Chromium is the universally present element but nickel, molybdenum, copper, nitrogen, vanadium, tungsten, titanium and niobium are also used for a variety of reasons. However, all elements can affect metallurgy, and thus mechanical and physical properties, so sometimes desirable corrosion resisting aspects may involve acceptance of less than ideal mechanical properties and vice versa. 

Fig. 3.67 Changes in corrosion rates of amorphous Fe-, Co- and Ni-base alloys measured in I N HCl at room temperature as a function of alloy chromium content "...

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. 

Carbon monoxide under pressure attacks unalloyed and low-alloyed steels at 130 to 140°C with formation of ironpentacarbonyl. Attack virtually ceases above 350°C. Where high-alloy chromium and chromium nickel steels are used the danger of damage is significantly reduced. Chromium steels with 30% Cr, and austenitic steels with 25% Cr and 20% Ni are completely... 

Chromium 50d X X The leather tanning industry manufacture of catalysts pigments and paints fungicides the ceramics and glass industries photography chrome alloy chromium metal production chrome plating corrosion control 111, 23,3231, 3512,3521,3522, 361,362,372, 38, 94... 

An aliquot part of the Be (tfa)2-containing benzene layer is submitted to gas-chro-matographic analysis. In ferrous alloys chromium also is determined as Cr(tfa)3. In the presence of catalytic amounts of nitric acid, the reaction with trifluoracetyl-acetone is stimulated with microwave radiofrequency energy without organic sol-vent ° ... 

Welding Problems With Cr-Mo Pipe. Do not use low-chromium steel pipe unless you are willing to pay for more careful welding and post-weld heat treatments. At a given hardness, low-alloy chromium-molybdenum steels have somewhat more ductility than carbon steels. However, because they air harden so much, they usually require post-weld heat treatments to toughen the weld metal and heat-affected zone. This heat treatment complicates field welding. 

Soil redox conditions or Eh status governs the oxidation and reduction of some trace metals found in wetlands. Trace metals are present in various oxidation states, for example, chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms Cr(ll), Cr(lll), and Cr(Vl). The trivalent Cr(lll) and the hexavalent Cr(Vl) are the most important forms in the environment. 

An amoimt up to 5% chromium (0.08% C) was reported to decrease weight losses in seawater at the Panama Canal [53] at the end of one year. A sharp increase in rates was observed between 2 and 4 years after 16 years, the chromium steels lost 22-45% more weight than did 0.24% C steel. Depth of pitting was less for the chromium steels after one year, but comparable to pit depth in carbon steel after 16 years. Hence, for long exposures to seawater, low-chronaium steels apparently offer no advantage over carbon steel. By comparison, however, low-alloy chromium steels (<5% Cr) have improved resistance to corrosion fatigue in oil-well brines free of hydrogen sulfide. 

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