As a result of a Carbonate Stress Corrosion Cracking (CSCC) event at one refinery an investigation was made into the cause and mitigation of CSCC. At the specific refinery, the immediate cause was attributed to a change in catalyst type in the upstream cat cracker feed hydrotreater (CFHT) which reduced the feed sulfur/feed nitrogen (S/N) ratio in the feed, resulting in an increased carbonate concentration, and an increased likelihood of cracking. This paper outlines the information that was developed during the investigation and also outlines the development of tools that could be utilized by other refinery fluidized catalytic cracker units (FCCU's) to better assess risk of CSCC.
Industry guidelines (API 581) provide a method for estimating CSCC risk based on [CO3=] and pH, but no immediately available method was found to measure [CO3=] directly. Therefore ionic modeling was used to predict pH and [CO3=] based on feed S, feed N. Also developed was a method to assess risk based on the information more typically available and that could be trended and used predictively. These included pH, and feed S/N ratio. A method was developed to measure CO2 in sour water using ion specific electrodes (ISE's) that could be converted to [CO3=] using either equilibrium constants from literature and pH or using ionic modeling tools. In this way, the likelihood on the pH x [CO3=] risk space could be assessed.
Based on this review it is believed that: CSCC may be episodic in nature; understanding of the process trends (pH, and S/N) are critical in assessing the risk of CSCC; traditional alkalinity measurements (P, M alkalinity) are not useful in measuring [CO3=]; mitigation of cracking can include PWHT, changing water wash variables (corrosion inhibitors, de-emulsifiers, make-up rate, etc.); establishing operating envelopes that include feed S/N and a link to the likelihood of CSCC can be used to prevent CSCC.
As a result of a CSCC event at a refinery in 2005, an investigation was made into the causes and possible mitigation of CSCC. This paper summarizes that experience and learning's to date that are transferable to other refineries.
The petroleum refining industry has experienced CSCC in FCCU light ends recovery systems for several years. One of the best documented and extensive reports is by Kmetz and Truax who reported in 1990 that CSCC in FCCU 's was experienced as early as 19831. Other units in the refinery industry have experienced similar alkaline stress corrosion cracking including amine units, sour water stripper units, catacarb CO2 recovery process units. The pipe line transportation industry has experienced elevated pH external stress corrosion cracking in bicarbonate/carbonate environments with high cyclic stress2.
. In 1972, a paper by Sutcliff, Fessler and Parkins related that the type of inter-granular cracking experienced with ferritic steels in alkaline pH across multiple industries were very closely related3.
