Caustic solutions, austenitic steels

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. 

Austenitic stainless steels will exhibit stress-corrosion cracking in hot aqueous chloride solutions, in acid chloride containing solutions at room temperature, in hot caustic solutions and in high-temperature high-pressure oxygenated water. 

The behaviour of austenitic stainless steels in caustic solutions has received less attention than cracking in chloride environments. Transgranular cracking has been reported for low-carbon (< 0.05%) steels in caustic solutions, whereas higher carbon content alloys cracked intergranularly. Wilson and Aspen showed that resistance to cracking was not decreased by sensitisation heat treatments. Type 316 stainless steel has been shown to be more susceptible to cracking in caustic than type 304. ... 

Figure 5-6. Intergranular stress corrosion crack on austenitic stainless steel in caustic solution at 200 °C a) Optical microscopy on metallographic section b) SEM of crack faces.

Figure 5-22. Mixed cracking of an austenitic stainless steel in deaerated caustic solution at 200 °C (SEM examination) a) thick oxide layer on the free surfaces, b-c) no visible traces of dissolution on the crack faces.

Sodium hydroxide is widely used in refinery and petrochemical plant operations to neutralize acid constituents. At ambient temperature and under dry conditions, NaOH can be handled in carbon steel equipment. Carbon steel is also satisfactory for aqueous caustic solutions below 50-80 °C, depending on concentration. For caustic service above these temperatures, but below 95 °C, carbon steel can also be used if it has been post-weld heat treated to avoid SCC at welds. Austenitic stainless steels, such... 

Caustic see of carbon steel occurs at temperatures above 50 to 80 °C (120 to 180 °F), depending on caustic concentration. Welded carbon steel components that are exposed to caustic solutions above these temperatures should be post-weld heat treated at 620 °C (115 °F) for 1 h per 25 mm (1 in.) of metal thickness. Caustic SCC of austenitic stainless steels occurs between 105 and 205 °C (220 and 400 °F), depending on caustic concentration. 

The greatest problems with austenitic stainless steel piping usually arise when the unit is off stream rather than when it is operating. Such problems must be anticipated. The use of stainless steels requires that the necessary steps be taken to avoid them. Chlorides and caustics can cause any austenitic stainless steel pipe to crack trans-granularly under some conditions. Plain chromium stainless steels do not crack in chloride solutions, but they usually pit badly enough to be only moderately satisfactory. 

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