Wet H2S Cracking

Chapter from the Metallurgy & Corrosion Design Guide.

WET H2S CRACKING GUIDELINES

Wet H2S Cracking Mechanisms

Mechanism	Abbreviation	Description	Where It Occurs	Prevention	Inspection Method	Standard
Sulfide Stress Cracking	SSC	Hydrogen-induced cracking of hard, high-strength steel in H2S	Welds, HAZ, high-hardness areas	Max 22 HRC, PWHT, proper WPS	MPI, hardness testing	NACE MR0175/ISO 15156
Hydrogen Induced Cracking	HIC	Stepwise internal cracking from H2 blistering	Base metal (mid-wall)	HIC-tested steel, low S/P content	UT shear wave, WFMT	NACE TM0284
Stress-Oriented HIC	SOHIC	HIC cracks linking in through-wall direction under stress	HAZ, high residual stress areas	PWHT + HIC-resistant steel	UT, TOFD, phased array	NACE SP0472
Hydrogen Blistering	N/A	Surface blisters from H2 gas accumulation at inclusions	Vessel shell, any exposed surface	Clean steel (low inclusions)	Visual, UT mapping	NACE SP0296

Wet H2S Service Definition

A system is considered to be in wet H2S service when ALL of the following apply:
• Free water is present (or may condense)
• H2S is present at any concentration >50 ppm in the water phase (some specs use >1 ppm)
• Operating temperature is below 300°F (above 300°F, water is typically vapor and not aggressive)

NACE MR0175/ISO 15156 applies when:
• H2S partial pressure >0.05 psia (region 1 boundary)
• Sour service classifications: SSC Region 0 (non-sour), Region 1 (mildly sour), Region 2 (sour), Region 3 (severely sour)
• Material requirements become more stringent with higher H2S and lower pH

Key Material Requirements for Wet H2S:
1. Carbon steel: killed, fine-grain practice, max 22 HRC (200 BHN) including HAZ
2. PWHT mandatory for all welds, including field welds and repairs
3. HIC testing per NACE TM0284 for plate/pipe in critical service
4. Low sulfur (<0.002%) and phosphorus (<0.015%) for HIC resistance
5. Restrict CE (carbon equivalent) for weldability

Source: Metallurgy_Corrosion_Design_Guide_v1.xlsx · sheet “Wet H2S Cracking”

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