Stress corrosion, 3.4

Stress corrosion occurs when a part is under mechanical stress and at the same time is being exposed to a corrosive environment, for instance, a steel he rod or bolt connecting the two flanges of a tank that is immersed in corrosive fluid. Stress corrosion failure is brittle in nature, and its fracture surface 

The fine crack induced by stress corrosion often penetrates into the part and is difficult to detect from the outside surface. However, the resulting damage can be catastrophic. A disastrous failure may occur unexpectedly with minimal warning. The experimentally tested SCC data are notoriously scattering. To demonstrate an equivalent SCC resistance is both technically challenging and financially costly. Nonetheless, it is a necessary test for many load-bearing parts used in a corrosive environment. 

Systems with lifetime-potential curves of type (a) in Fig. 2-17 can be cathodi-cally protected against stress corrosion. The following metals belong to these systems  

Cases (e), (g), and (h) are of interest in the cathodic protection of warm objects (e.g., district heating schemes [89] and high-pressure gas lines downstream from compressor stations [82]) because the media of concern can arise as products of cathodic polarization. The use of cathodic protection can be limited according to the temperature and the level of the mechanical stressing. The media in cases (a) and (f) are constituents of fertilizer salts in soil. Cathodic protection for group I is very effective [80]. 

During the rainy season, the British Army in India did not often go out and stored weapons and ammunition in the stables. At the end of the monsoon, when taking up arms again, the soldiers saw that many cartridges in brass 70/30 were unusable because of cracks. Since these problems were related to the season and did not occur during the dry season, corrosion experts in the UK coined the term season cracking for this type of corrosion. It was not until 1921 that an explanation could be given for this phenomenon  

Until the early 20th century, steel steam boilers would explode because of rivet rupture. Corrosion experts found anomalous traces of sodium hydroxide in hidden recesses under the rivets. It was only in 1927 that an explanation was given for these cases of rupture due to stress corrosion which had been the cause of many fatal accidents all over the world. 

Only much later did English-speaking corrosion experts adopt the term stress corrosion . In France, until the 1970s, the term corrosion sous tension (strain corrosion) 

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A process involving combined corrosion and straining of the metal due to residual or applied stresses. The occurrence of stress corrosion cracking is highly specific only particular metal/environment systems will crack. 

The appearance of stress corrosion cracking may be either intergranular or transgranular in nature. 

Gibala R and Hehemann R F (eds) 1984 Hydrogen Embrittlement and Stress Corrosion Craoking (Metals Park, OH American Soceity of Metals)... 

Staehle R W, Forty A J and Rooyen D v (eds) 1969 Fundamental Aspeots of Stress Corrosion Craoking (Houston, TX NACE)... 

Jones R H (ed) 1993 Stress-Corrosion Craok/ng (Materials Park, OH ASM)... 

Speidel M O, Denk J and Scarlin B 1991 Stress Corrosion Craoking and Corrosion Fatigue of Steam-Turbine Rotor and Blade Materials (Luxembourg Commission of the European Communities)... 

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). 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

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). 

Titanium is resistant to nitric acid from 65 to 90 wt % and ddute acid below 10 wt %. It is subject to stress—corrosion cracking for concentrations above 90 wt % and, because of the potential for a pyrophoric reaction, is not used in red filming acid service. Tantalum exhibits good corrosion resistance to nitric acid over a wide range of concentrations and temperatures. It is expensive and typically not used in conditions where other materials provide acceptable service. Tantalum is most commonly used in appHcations where the nitric acid is close to or above its normal boiling point. 

Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Stress Corrosion Cracking of Ahoy 600," Report NP-2114-SR, Electric Power Research Institute, Palo Alto, Calif., Nov. 1981. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Ammonia is corrosive to akoys of copper and zinc and these materials must not be used in ammonia service. Iron or steel should usuaky be the only metal in ammonia storage tanks, piping, and fittings. It is recommended that ammonia should contain at least 0.2% water to prevent steel stress corrosion. Mercury thermometers should be avoided. 

R. J. I In dinger and R. M. Curran, Experience with Stress Corrosion Cracking in Earge Steam Turbines, Corrosion 81, National Association of Corrosion Engineers, Toronto, Ontario, Canada, 1981. 

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