Fracture Toughness Behavior of Welded Service Aged Carbon Steels in Mildly Sour Waters

Abstract

Cracking of steels in refinery H2S service occurs in two regimes: 1) low pH and 2) high pH. Cracking in the low pH regime (pH < 6 or 7) is associated with ingress of hydrogen from metal dissolution with H2S poisoning the hydrogen recombination reaction. In the high pH regime (pH > 7), cracking requires the presence of species other than just H2S. While this has been a subject of interest since the failure of an amine absorber tower in 1984, limited laboratory testing has been conducted in either regime. The majority of test data that does exist was from studies conducted in more aggressive environments, such as NACE A or NACE B solutions. Additionally, most of the work was conducted on more modern steels rather than service aged of vintage steel. The lack of available data inhibits the ability to perform fitness for service (FFS) assessments without using overly conservative approaches. A recent test program was undertaken to evaluate the fracture toughness properties of the weld heat-affected zone (HAZ) of an ex-service steel in mildly acidic sour water environments more typical of the downstream refining industry. The results of the test program, including the test environment and approach are discussed. In addition to the test program, the current state of understanding sour water cracking is reviewed relative to both the low pH and high pH regimes. The gaps in the current knowledge are discussed.