Nickel alloys, environment-alloy stress-corrosion cracking

As with alloys of other metals, nickel alloys may suffer stress-corrosion cracking in certain corrosive environments, although the number of alloy environment combinations in which nickel alloys have been reported to undergo cracking is relatively small. In addition, intergranular attack due to grain boundary precipitates may be intensified by tensile stress in the metal in certain environments and develop into cracking. Table 4.28 lists the major circumstances in which stress corrosion or stress-assisted corrosion of nickel and its alloys have been recorded in service and also shows the preventive and remedial measures that have been adopted, usually with success, in each case. 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

The fracture mode of stress-corrosion cracks in austenitic stainless steels can be transgranular, intergranular or a mixture of both. One of the earliest environments found to cause problems was solutions containing chlorides or other halides and the data due to Copson (Fig. 8.30) is very informative. The test solution for that data was magnesium chloride at 154°C the alloys contained 18-20alloy with a composition of approximately 18Cr-8Ni has the least resistance to cracking in this environment. 

Kane, R. D., Greer, J. B., Hanson, J. R., et tJ., Stress Corrosion Cracking of Nickel Base Alloys in Chloride Containing Environment, Paper 174, CORROSION/79, NACE, Atlanta, GA, April 1979. 

ASTM G 37 (Practice for Use of Mattsson s Solution of pH 7.2 to Evaluate the Stress Corrosion Cracking Susceptibility of Copper-Zinc AUoys) is an accelerated stress corrosion cracking test environment for brasses (copper-zinc alloys). The use of this test environment is not recommended for other copper alloys since the results may be erroneous, providing completely misleading rankings. This is particularly true of alloys containing aluminum or nickel as deliberate alloying additions. 

The austenitic alloys are particularly susceptible to stress corrosion cracking (SCC) in hot chloride environments. This form of corrosion can be minimized by increasing the amount of nickel in the alloy. 

Several environments and specimen designs are available for SCC evaluation [74]. ASTM G 36, Practice for Evaluating Stress-Corrosion Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution, provides a more severe environment than generally found in service. Resistance in this test depends largely on the nickel content and most austenitic and duplex stainless steels are readily cracked. 

This is a chromium-nickel-molybdenum alloy, with its composition shown in Table 8.4. It has excellent resistance to chloride pitting and stress corrosion cracking environments. It finds use in the chemical processing and utility industries. 

While general corrosion resistance is important, one of the major reasons that nickel-based alloys are specified for many applications is their excellent resistance to localized corrosion, such as pitting, crevice corrosion, and stress corrosion cracking. In many environments, austenitic stainless steels do not exhibit general attack but suffer from significant localized attack, resulting in excessive downtime and/or expensive repair and replacement. 

Because alloy B-2 is nickel rich (approximately 70%), it is resistant to chloride-induced stress corrosion cracking. Because of its high molybdenum content, it is highly resistant to pitting attack in most acid chloride environments. 

Many of the alloys of copper are more resistant to corrosion than is copper itself, owing to the incorporation either of relatively corrosion-resistant metals such as nickel or tin, or of metals such as aluminium or beryllium that would be expected to assist in the formation of protective oxide films. Several of the copper alloys are liable to undergo a selective type of corrosion in certain circumstances, the most notable example being the dezincification of brasses. Some alloys again are liable to suffer stress corrosion by the combined effects of internal or applied stresses and the corrosive effects of certain specific environments. The most widely known example of this is the season cracking of brasses. In general brasses are the least corrosion-resistant of the commonly used copper-base alloys. 

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