Traditionally, in the selection of Oil Country Tubular Goods (OCTG) for sour gas service, Corrosion Resistant Alloys (CRA?s) are screened first by their pitting resistance equivalent number (PREN) and then by environmental cracking data generated in sour brine environments. The theory is that a pit occurs first, which provides a stress-riser for initiation of anodic chloride stress corrosion cracking (KC). Among the primary CRA?s currently used in the cold worked condition for OCTG in sour gas wells are alloy 825 (UNS N08825) and alloy 28 (UNS N08028). While alloy 28 has a somewhat higher PREN than alloy 825, alloy 825 has a significantly higher nickel content. Slow strain rate (SSR) tests conducted in severe sour brine environments showed that the higher nickel content of alloy 825 results in better stress corrosion cracking resistance than that exhibited by alloy 28. The effect of nickel content on chloride SCC resistance of austenitic alloys was originally reported by H.R.Copson in 1959. This suggests that in some cases for austenitic alloys, the nickel content of the CRA may be more important than the PREN in OCTG selection.
CRA?s have historically been screened in the selection of OCTG applications by a required minimum PREN based on environmental severity and maximum operating temperature. ?Typically, the Pitting Resistance Equivalent Number? is calculated using the following equation PREN = %Cr + (3.3 x %Mo) + (11 x %N) + 1.5 (%W + %Nb) (1)
The most commonly accepted theory in Oil Patch is that pitting occurs first in the sour brine environments, which provides stress risers for anodic chloride SCC to occur. This logic suggests that the elimination of pitting, by use of an alloy with sufficient PREN for the environmental severity, will prevent the initiation of SCC. However, the effect of alloying elements such as nickel on anodic chloride SCC resistance is not addressed.
For quite some time, INCOLOYB alloy 825 and alloy 28 have been successfully used as cold worked OCTG in sour gas well$. While alloy 825 has a PREN of 31 compared to a PREN of 38 for alloy 28, alloy 825 has a significantly higher nickel content of 42% compared to only 31% for alloy 28. The beneficial effect of nickel content on chloride SCC resistance of austenitic alloys has been well established?. In the case of alloys 825 and 28, the nickel content of the CRA may be more important than the PREN in OCTG selection.
