The use of 0.05 psi H2S partial pressure as the threshold determining when sulfide stress cracking (SSC) resistant materials are needed was codified in NACE MR 0175 (1975) nearly a half century ago and has served the industry well since implementation. The origins of this pressure are empirical and largely forgotten but there is significant history showing that it is a safe, conservative limit. As the oil and gas industry continues to innovate and mature, it is imperative to maintain knowledge of the basis of the sour limit and how it may be applied.
For the past 70 years, H2S and its associated cracking mechanisms have been investigated, limits developed, and material properties identified. Prior to 1975 there were no standards, but there were standardized requirements outlined in various NACE (1) regional and national documents, in particular NACE 1F163 and 1F166, which were the precursors to NACE MR0175 (Wellheads and Trees). Some materials properties, notably the 22 HRC hardness limit had been notionally recommended in these early reports but were not formally standardized until NACE MR 0175 was issued in 1975.1-3 When NACE MR 0175 (1975) was issued, the materials requirement focused on acceptable material properties for wellhead and tree materials since they involved the largest perceived safety risks.
Setting a threshold when an environment should be considered sour or not was not included in NACE MR 0175 (1975) but was under prior industry consideration based on Texas Statewide Rule No. 36 which used 100 ppm H2S in the flow stream as a limit. API (2) 14E (1975) noted a limit as 0.05 psia H2S partial pressure or greater than 15 ppm (mg/L) H2S in the aqueous solution as a sour/non-sour threshold.4 The table from API RP 14E (1975) with this information is provided in Table 1. The 0.05 psia (0.345 kPa) H2S partial pressure was adopted by NACE MR 0175 in the 1978 edition as the sour/non-sour threshold. The 15 mg/L H2S limit was not carried forward in MR 0175 and is considered more permissive for H2S content as 0.05 psia (0.345 kPa) H2S would translate into approximately 10 mg/L H2S at ambient pressure in a low salinity aqueous phase; higher pressures or salinity would result in even less dissolved H2S.