Nickel—tungsten temperature effect

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

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 ... 

Only a few results on the effect of temperature on Eq are available. The author has found that in the case of nickel the diffraction maxima do not alter in position between 20" and 350° C., but are merely broadened. It has recently l been shown that for tungsten this behaviour holds good up to 2100° C. Hence it appears that temperature has no effect exceeding 2 volts on the value of Eq. 

Substantial additions of cobalt to chromium plus molybdenum (or tungsten) alloys are detrimental to S.C.C. behavior, resembhng the effect of added iron rather than the beneficial effect of added nickel. Accordingly, the MP35N alloy resists S.C.C. in MgCb solution at 153-154°C, but by replacing most nickel with cobalt (and the incidental reduction of molybdenum to 6% and increase of chromium to 30%), as in Vitalhum, susceptibihty results [5]. Alloy 25 is also susceptible. This susceptibihty does not include Vitalhum exposed to saline solutions at 37°C (body temperature) in which the alloy is resistant. The situation is analogous to the observed resistance of 18-8 (types 304 and 316) stainless steels to S.C.C. in aerated chlorides at temperatures below 60-80°C, but not above. 

Cobalt occurs in two atomic forms a low temperature stable hexagonal close packed (hep) form and a high temperature stable face centered cubic (fee) form. The transformation temperature of pure cobalt is 417°C. Alloying elements such as nickel, iron, and carbon (within its soluble range) are known as fee stabilizers, and suppress the transformation temperature. Chromium, molybdenum, and tungsten, on the other hand, are hep stabilizers and have the opposite effect. 

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