Sulfide stress cracking (SSC) is defined as cracking of metal under the combined action of tensile stress and corrosion in the presence of _________ and _________.

Sulfide Stress Cracking (SSC) is a form of hydrogen stress cracking that occurs in high-strength steels or hard weld zones exposed to sour environments. According to industry standards, SSC specifically requires the concurrent presence of liquid water, hydrogen sulfide (H2S), and tensile stress (either residual or applied). Free water acts as the electrolyte allowing corrosion to proceed, producing atomic hydrogen via the anodic iron dissolution and hydrogen evolution reactions. The presence of H2S acts as a potent hydrogen poison, inhibiting hydrogen atoms from recombining into hydrogen gas molecules, thereby forcing atomic hydrogen into the steel matrix, which ultimately induces hydrogen embrittlement and catastrophic cracking.

Option A: Elemental sulfur and oxide species do not constitute the primary aqueous sour corrosion system responsible for atomic hydrogen generation in SSC.
Option B: Molecular hydrogen gas alone does not form the corrosive aqueous environment; liquid water and H2S are both necessary to drive the cracking mechanism.
Option C: Dissolved oxygen is not required for SSC; in fact, SSC occurs under anaerobic aqueous sour conditions containing hydrogen sulfide.

1. American Petroleum Institute (API), API Recommended Practice 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd Edition, Section 3.67 "Wet H2S Damage (Sulfides Stress Cracking)", Paragraph 3.67.1.