Nickel alloys carburisation

In the gas-cooled reactor, reaction.between the coolant and the moderator results in formation of a proportion of carbon monoxide in the atmosphere. This gas can be carburising to nickel-base alloys but the results of tests in which CO2 was allowed to react with graphite in the furnace indicate that the attack on high-nickel alloys is slight, even at moderately high temperatures and is still mainly due to simple oxidation. 

Table 7.27 Effect of lime and lemperaiure on ihe visible penetration of carburisation in nickel alloys under contact pressure (l-79N/mm ) with graphite ...

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

Green Rot carburisation and oxidation of certain nickel alloys at around 1000°C resulting in a green corrosion product. 

While carburisation itself is not a normal corrosion process, in that there is no metal wastage, absorption and diffusion of carbon can lead to significant changes in the mechanical properties of the affected material and in particular to marked embrittlement. Furthermore, initial carburisation can produce an acceleration of the normal oxidation process, a phenomenon that is notable in nickel-chromium alloys. 

Tests on a wide range of alloys at temperatures varying from 704 to 927°C have been made by Bernsen et al." to determine the temperature limits beyond which engineering materials carburise when held in contact with graphite. Table 7.27 lists the maximum penetrations of the carburised zones while nickel in general showed no visible evidence of carburisation the associated hardness measurements indicated solution of carbon even at 704°C. At this temperature the chromium-containing alloys showed little tendency to carburisation, but at 816°C carburisation leading to the formation of chromium carbide was rapid. 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

As expected, no carburisation attack at all was detected on iron-aluminium-chromium alloys after 1000 hours exposure in CH4/H2 environments at 850°C, 1000°C and 1100°C. Since the formation of chromia and iron requires relatively high oxygen partial pressures, alumina is the only stable phase at the low partial pressure of the used gas. If once formed, alumina is impervious to carbon, provided the scale remains intact [20], Excellent resistance to carburisation was also found for other alumina forming alloys like nickel aluminides [21] and Ni-Al-Cr alloys [22], The results of the present work show that 10 wt% aluminium are sufficient to prevent carburisation. It is expected, that the minimum aluminium concentration is even lower than 10 wt%. 

The effect of copper alloying on the carburisation of nickel is significant, as shown in Fig. 2.1. The rate of carburisation decreases markedly with... 

---