Avoiding Caustic Stress Corrosion Cracking of Refinery Equipment and Piping
|Publication Date:||14 March 2015|
This standard practice establishes guidelines to avoid caustic SCC of equipment and piping. It addresses applications that use "fresh" caustic.
In some services that involve contaminated caustic, SCC has been observed at conditions within area "A" of the Caustic Service Chart shown in Figure 1 (particularly when contaminated with sulfide compounds). The need for thermal stress relief should be considered when the service contains contaminated caustic.
The practices detailed below are specifically intended for handling aqueous solutions containing sodium hydroxide (NaOH). However, several companies extend these practices to include aqueous solutions of other strong alkali compounds such as potassium hydroxide (KOH) and lithium hydroxide (LiOH). Although amines form a high pH solution and may cause SCC, it is considered a different mechanism for the purposes of this standard and amine SCC is not addressed. A good reference on amine SCC is API(1) 945.3 Carbonate SCC is another "different" SCC mechanism occurring at high pH, which is not included in the scope of this standard. It is covered by NACE Publication 34108.4
Some proprietary caustic solutions are used in the industry,
e.g., potassium carbonate/bicarbonat
Other terms for caustic SCC used in past literature include caustic embrittlement, caustic cracking, and alkaline cracking.
(1) American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005-4070.