Stress corrosion tubes

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

Brasses are susceptible to dezincification in aqueous solutions when they contain >15 wt% zinc. Stress corrosion cracking susceptibiUty is also significant above 15 wt % zinc. Over the years, other elements have been added to the Cu—Zn base alloys to improve corrosion resistance. For example, a small addition of arsenic or phosphoms helps prevent dezincification to make brasses more usefiil in tubing appHcations. 

Admiralty Brass and Naval Brass are 30 and 40% zinc alloys, respectively, to which a 1% tin addition has been added. Resistance to dezincification of Cu—Zn alloys is increased by tin additions. Therefore, these alloys are important for thein corrosion resistance in condenser tube appHcations. In these, as weU as the other higher zinc compositions, it is common to use other alloying additives to enhance corrosion resistance. In particular, a small amount (0.02—0.10 wt %) of arsenic (C443), antimony (C444), or phosphoms (C445) is added to control dezincification. When any of these elements are used, the alloy is referred as being "inhibited." For good stress corrosion resistance, it is recommended that these alloys be used in the fiiUy annealed condition or in the cold worked plus stress reHef annealed condition. 

Fabrication Expanding the tube into the tube sheet reduces the tube wall thickness and work-hardens the metal. The induced stresses can lead to stress corrosion. Differential expansion between tubes and shell in fixed-tube-sheet exchangers can develop stresses, which lead to stress corrosion. 

Finally, any living organism dies. Decomposition may generate ammonia at local concentrations high enough to produce stress-corrosion cracking of brass condenser tubes (Fig. 6.1). 

Figure 6.1 Stress-corrosion cracking of a brass condenser tube caused by ammonia from decomposing slime masses lodged on internal surfaces.

Figure 9.2 Longitudinal stress-corrosion cracks in a heat exchanger tnbe the broad gap between the crack faces reveals that high-level residual hoop (circumferential) stresses from the tube-forming operation provided the stress component required for SCC.

Figure 9.3 Transverse stress-corrosion cracks in a condenser tube the presence of the cracks along just one side indicates that a bending moment provided the stress.

Figure 9.4 Both longitudinal and transverse stress-corrosion cracks on a brass heat exchanger tube that had been exposed to ammonia. Note the branching of the cracks.

Cracks of the type illustrated in Fig. 9.12 occurred in many tubes of this new exchanger. All cracks occurred at the air-entry end of the cooler (Fig. 9.13) and had the brittle appearance typical of stress-corrosion cracks. 

Other examples of metallurgy decisions are red brass versus admiralty tubes with fresh water on the tubeside and suspected stress corrosion cracking conditions on the shellside, and stainless steel versus carbon steel with chlorides present. A good metallurgist should be brought in when these kinds of decisions are needed. 

Chul Kim, U. R. and van Rooyen, D., Strain rate and temperature effects on the stress corrosion cracking of Inconel 600 steam generator tubing in the (PWR) primary water conditions , Proc. 2nd Int. Conf. on Environmental Degradation of Materials in Nuclear Power Systems-VIalet Reactors, Monterey, USA, 9-12 Sept. 1985, American Nuclear Society, pp. 448-55 (1986)... 

Stainless steels Thin-walled stainless steel (Fe-18Cr-8Ni) tubes are now frequently used for domestic installations in place of copper pipe . Care is required, however, in the design of stainless steel equipment for use in waters with a high chloride content, or where the concentration can increase, since pitting attack may occur. It may also be susceptible to failure by stress-corrosion cracking under certain conditions. 

Plate 1 illustrates what can be the consequences of a stress corrosion failure in a turbine and Plate 2 the consequences of two forms of boiler tube corrosion tube thinning and hydrogen embrittlement. The economic consequences of such problems and certain other areas where information on high temperature solutions is needed have been discussed elsewhere (6). 

The Primary Reformer is a steam-hydrocarbon reforming tubular furnace that is typically externally fired at 25 to 35 bar and 780°C to 820°C on the process side. The reformer tubes function under an external heat flux of 75,000 W/m2 and are subject to carburization, oxidation, over-heating, stress-corrosion cracking (SCC), sulfidation and thermal cycling. Previously SS 304, SS 310 and SS 347 were used as tube materials. However these materials developed cracks that very frequently led to premature tube failures (see Table 5.10)88. 

S. Haruyama, Stress Corrosion Cracking by Cooling Water of Stainless-Steel Shell and Tube Heat Exchangers, Materials Performance, March 1982, 14-19. 

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