Monel, stress corrosion cracking

Stainless steel alloys show exceUent corrosion resistance to HCl gas up to a temperature of 400°C. However, these are normally not recommended for process equipment owing to stress corrosion cracking during periods of cooling and shut down. The corrosion rate of Monel is similar to that of mild steel. Pure (99.6%) nickel and high nickel alloys such as Inconel 600 can be used for operation at temperatures up to 525°C where the corrosion rate is reported to be about 0.08 cm/yr (see Nickel and nickel alloys). 

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

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. 

Copson, H.R., C.F. Cheng. Stress Corrosion Cracking of Monel in Hydrofluoric Acid. Corrosion 12, 12(1956) p. 647t. 

There are some differences in the behavior of alloys due to the variations in caustic composition among the three cell processes. These differences occur mostly in lower-grade applications using materials less robust than nickel. Monel, for example, is subject to liquid-metal cracking by mercury and its salts. Stainless steels seem to be equally affected by diaphragm- and mercury-cell caustic, but if the caustic is consumed in some application, the residual chloride from diaphragm-cell NaOH can cause stress corrosion cracking [146]. 

Cupronickels (Ni-Cu) In this category the most common cupronickel alloys are the Monel 400 and Monel K-500. The Ni-Cu alloys differ from nickel 200 and 201 because their strength and hardness can be increased by age hardening. Ni-Cu alloys exhibit higher corrosion resistance than commercially pure nickel, especially to sulfuric and hydrofluoric acids, and chloride brines. Handling of waters, including seawater and brackish water, is the major application of these two alloys in the CPI (e.g., desaUnation plants). In addition, Monel 400 and K-500 are immune to chloride-ion stress-corrosion cracking, which is often considered in their selection. 

Alloy 400 exhibits excellent resistance to hydrofluoric acid solutions at all concentrations and temperatures, as shown in Figure 15.1. Again, aeration or the presence of oxidizing salts increases the corrosion rate. This alloy is widely used in HF alkylation, is comparatively insensitive to velocity effects, and is widely used for critical parts such as bubble caps or valves that are in contact with flowing acid. Monel 400 is subject to stress corrosion cracking in moist, aerated hydrofluoric or hydrofluorosilicic acid vapor. However, cracking is imlikely if the metal is completely immersed in the acid. 

Monel 400 exhibits stress corrosion cracking in high temperatures, concentrated caustic, and in mercury. Refer to Table 15.4. A more detailed compilation will be foimd in Reference [6]. 

These alloys are well-known for their excellent corrosion resistance to seawater. They have been used as propellers, pump shafts, impellers and condenser tube materials. The best known is Monel (Alloy 400). It is resistant to brine and immune to stress corrosion cracking and pitting in chloride and caustic alkaline solutions. It is also resistant to HF and fluorine containing media. 

Hydrofluoric acid causes two types of corrosion, direct corrosion, in which iron fluoride is formed, and stress corrosion, in which the metal cracks. Electrolytic corrosion can also take place when the acid becomes diluted with water and two dissimilar metals or carbon are used. Because of the possibility of stress corrosion, all vessels in acid service must be stress-relieved. Monel is also subject to stress corrosion and must be stress-relieved. Care must be taken in stress-relieving Monel that sulfur or sulfur compounds do not come in contact with the Monel either before or during the heat treatment. 

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