Nickel-chloride solution susceptibility

Calibration. The mass susceptibility of an aqueous nickel chloride solution is... 

Fig. 8.30 Effect of nickel content on the susceptibility to stress-corrosion cracking of stainless steel wires containing 18-20% chromium in a magnesium chloride solution boiling at 154°C...

Monel, the classic nickel-copper alloy with the metals in the ratio 2 1, is probably, after the stainless steels, the most commonly used alloy for chemical plant. It is easily worked and has good mechanical properties up to 500°C. It is more expensive than stainless steel but is not susceptible to stress-corrosion cracking in chloride solutions. Monel has good resistance to dilute mineral acids and can be used in reducing conditions, where the stainless steels would be unsuitable. It may be used for equipment handling, alkalies, organic acids and salts, and sea water. 

Information of various kinds can be obtained on complexes, including solvates, from the dependence of the magnetic susceptibility on the temperature. The change in magnetic susceptibility caused by the temperature change of a solution frequently points to different reactions occurring at the different temperatures. For example, studies by Meek [Me 60] showed that the affinity of the chloride ion for the nickel ion increased when the temperature of a DMSO solution of nickel chloride was raised. Accordingly, with increase in temperature the chloride ion, previously situated in the outer sphere, replaces one of the DMSO molecules in the inner sphere [Wa 76],... 

In tests lasting for 14 days, Copson found that the susceptibility of steel to stress-corrosion cracking in hot caustic soda solutions increased with increase in nickel content up to at least 8-5%. Alloys containing 28% and more of nickel did not fail in this period. In boiling 42% magnesium chloride the 9% nickel-iron alloy was the most susceptible of those tested to cracking (Table 3.38). Alloys containing 28 and 42% nickel did not fail within 7 days. 

Alloy 400 is known as Monel and its age-hardenable version is K-500. The corrosion resistance of alloy 400 is better than pure nickel in nonoxidizing mineral acids. The alloy shows poor corrosion resistance in HNO3, FeCl3, CuCl2, moist Cl2, chromic acid, S02 and NH3 shows corrosion resistance to HF in the absence of air at all temperatures and is susceptible to SCC in moist aerated HF and H2SiF6. Alloy 400 is unaffected by alkaline salt solutions such as chlorides, carbonates, sulfates and acetates. 

In the case of the nickel alloys, the stability of the passive layer is a problem. The alloys depend on the oxide films or the passive layers for corrosion resistance and are susceptible to crevice corrosion. The conventional mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects such as the acidification or the migration of the aggressive ions into the crevice solution [146]. These solution composition changes can cause the breakdown of the passive film and promote the acceleration and the autocatalysis of the crevice corrosion. In some cases, the classic theory does not explain the crevice corrosion where no acidification or chloride ion build up occurs [147]. 

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. 

Austenitic stainless steels suffer from SCC in solutions containing chloride ions. Accelerated tests conducted in boiling 42% MgCl2 solutions show the time-to-failure to be a function of the nickel content in Fe-Cr-Ni alloys (Fig. 14.18). The shadowed area represents the region where the steels are prone to SCC. Type 304 stainless steel is susceptible to SCC in solutions containing Cl , the maximum allowable level being ca.0.1 mgL . ... 

Because of their lack of susceptibility to chloride-induced stress corrosion cracking, nickel-copper alloys are used in seawater desalination plants as pipes for evaporators or heat exchangers. An evaporator made of alloy 400 (NiCu 30 Fe, 2.4360) exhibited a corrosion rate of > 0.01 mm/a (> 0.4 mpy) after an exposure time of 225 days to a CaCl2 concentration of up to 35 % and a temperature of 433 K (160 C). The corrosion rate was 0.07 mm/a (2.76 mpy) for a NaCl solution saturated with water vapor and air at 366 K (93 °C) [73]. 

Ferritic steels. Types 430 and 434, are resistant to SCC in MgCh at 140°C. High purity ferritic stainless steels are subjected to SCC in boiling 30% sodium hydroxide in tests exceeding 1000 h and in 42% MgCl2 in a sensitized condition. Types 430 and 446 stainless steels are subject to chloride SCC in the welded conditions. In the presence of high residual levels of copper (0.37%) and nickel (1.5%), the alloys become susceptible to SCC in 42% MgCU solution. 

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