Duplex stress corrosion cracking

Duplex stainless steels (ca 4% nickel, 23% chrome) have been identified as having potential appHcation to nitric acid service (75). Because they have a lower nickel and higher chromium content than typical austenitic steels, they provide the ductabdity of austenitic SS and the stress—corrosion cracking resistance of ferritic SS. The higher strength and corrosion resistance of duplex steel offer potential cost advantages as a material of constmction for absorption columns (see CORROSION AND CORROSION CONTROL). 

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. 

Duplex stainless steels are mostly composed of alternate austenite and ferrite grains. Their structure improves resistance to chloride-induced stress corrosion cracking. In certain reducing acids, such as acetic and formic, preferential attack of the ferrite is a serious problem. 

Duplex, and super-duplex stainless steels, contain high percentages of chromium. They are called duplex because their structure is a mixture of the austenitic and ferritic phases. They have a better corrosion resistance than the austenitic stainless steels and are less susceptible to stress corrosion cracking. The chromium content of duplex stainless steels is around 20 per cent, and around 25 per cent in the super-duplex grades. The super-duplex steels where developed for use in aggressive off-shore environments. 

Duplex stainless steels (4% nickel, 23% chrome) may offer cost advantages in absorption columns. These materials provide the ductability of austenitic stainless and the stress-corrosion cracking resistance of ferritic stainless steel104. 

The duplex stainless steels are superior to other stainless steels with respect to high resistance to chloride stress corrosion cracking, high mechanical strength, lower thermal expansion than the austenitic grade steels, and good erosion and wear resistance. 

With the desired microstructure, these alloys are resistant to hydrogen stress cracking and much more resistant to chloride stress corrosion cracking than are the austenitic stainless steels. (The threshold temperature for chloride stress corrosion cracking of duplex alloys in neutral pH aqueous chlorides is about 300°F [150°C].) The chloride stress corrosion cracking resistance of the duplex alloys is similar to that of superaustenitic alloys such as Alloy AL-6XN. Because they contain about 50% ferrite, the duplex stainless steels are more susceptible to hydrogen embrittlement. 

T.M. Devine and B.J. Drummond, Use of Accelerated Intergranular Corrosion Tests and Pitting Corrosion Tests to Detect Sensitization and Susceptibility to Intergranular Stress Corrosion Cracking in High Temperature Water of Duplex 308 Stainless Steel, Corrosion, Vol 37, 1981, p 104-115... 

Duplex stainless steels offer high strength, coupled with resistance to abrasion and erosion and to stress corrosion cracking. It is claimed that Ferralium alloy... 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

A. Bhattacharya, Stress corrosion cracking of duplex stainless steels in caustic solutions (Ph.D. thesis), Georgia Institute of Technology, Atlanta, GA, 2008, pp. 1-3. 

Duplex stainless steels have a mixed microstructure of ferrite and austenite with chromium content in the range between 19% and 32%, molybdenum up to 5%, and lower nickel contents than austenitic stainless steels. They exhibit better corrosion resistance to pitting, stress corrosion cracking, and crevice corrosion than austenitic stainless steels, and are approximately twice as strong. 

H.Y. Liou, R.I. Hsieh, W.T. Tsai, Microstructure and stress corrosion cracking in simulated heat-affected zones of duplex stainless steeb, Corros. Sci. 44 (2002) 2841—2856. 

R.K. Singh Raman, W.H. Slew, Role of nitrite addition in chloride stress corrosion cracking of a super duplex stainless steel, Corros. Sci. 52 (2010) 113—117. 

H. Spaehn, Stress corrosion cracking and corrosion fatigue of martensitic, ferritic, and ferritic-austenitic (duplex) stainless steel, in P. Marcus, J. Oudar (Eds.), Corrosions Mechanisms in Theory and Practice, Marcel Dekker, Inc., New York, 1995, pp. 449-487... 

R. Javaherdashti, R.K. Raman Singh, C. Panter, E.V.P. Pereloma. Stress corrosion cracking of duplex stainless steel in mixed marine cultures containing sulphate reducing bacteria. In the Proceedings of Corrosion and Prevention 2004 (CAP04), Perth, Australia, 2004. 

In contrast to stress corrosion cracking, corrosion fatigue is not just limited to specific metal-environment systems, but affects all metals that are subjected to cyclic tensile stresses in a corrosive environment. Figure 11.46 illustrates the non-specificity of corrosion fatigue [24]. The number of cycles to failure is plotted as a function of the amplitude of the applied stress for duplex stainless steel samples exposed to air, or immersed in a solution of either 3% NaCl or 0.05M H2SO4. The specimens degrade more rapidly in sulfuric acid in spite of the fact that this solution does not cause stress corrosion cracking for this type of steel, contrary to the NaCl solution. 

In the 1970s—when strength considerations became an issue for stainless steels—duplex versions were developed, which also increased resistance to chloride stress corrosion cracking. In the 1980s stainless steels with higher molybdenum were formulated to solve problems with localized corrosion encountered in aggressive environments. 

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