Sour service produces four distinct kinds of damage that get lumped together and should not be. They start the same way and end very differently, and the inspection that finds one will not find the others.
All four start identically. Water plus hydrogen sulphide corrodes steel, and the sulphide poisons the reaction that would normally combine hydrogen atoms into gas at the surface. Unable to combine and leave, atomic hydrogen diffuses into the steel instead.
What happens next depends on where the hydrogen ends up and what the steel is like.
Hydrogen atoms reach an internal discontinuity — a lamination, a large non-metallic inclusion — and recombine into molecular hydrogen inside the cavity. Molecular hydrogen cannot diffuse back out. Pressure builds until the steel deforms, and a blister appears.
Recognising it: a rounded bulge on the surface, often several centimetres across. Visible, and the least ambiguous of the four.
Finding it: visual inspection, or ultrasonic scanning which shows the separation mid-wall. Blisters are rarely the real problem in themselves; they matter because they tell you hydrogen is being charged into the steel, which means the other three are credible too.
The same accumulation, but instead of simply bulging, cracks grow from the cavities and link up between adjacent inclusion planes. The result is a stepwise, ladder-like pattern of cracking running roughly parallel to the plate surface, entirely inside the wall.
The distinguishing feature: HIC needs no applied stress at all. It is driven purely by internal hydrogen pressure and the steel's own cleanliness. A vessel sitting at atmospheric pressure can develop it.
Recognising it: mid-wall stepwise cracking, parallel to the surface, associated with the rolling direction of the plate. Nothing visible externally.
Finding it: ultrasonics. Straight-beam scanning shows the mid-wall reflectors; PAUT images the pattern. Nothing on the surface will find it, and thickness readings will report full wall.
The same stepwise cracking, but under an applied tensile stress the small cracks stack through the wall instead of spreading along it, producing a ladder climbing from the process surface towards the outside. Because it is stress driven, it concentrates at welds where residual stress is highest.
Why it is the dangerous one: ordinary HIC grows parallel to the surface and does not directly threaten containment. SOHIC grows through the wall.
Finding it: PAUT or TOFD, with a procedure written for near-surface stacked cracking at the weld. This is not a scan you get by accident.
Different again. Here the hydrogen embrittles a hard microstructure, and under tensile stress the steel cracks. It needs three things together: hydrogen charging, tensile stress, and hardness.
That third condition is why SSC is the one you can genuinely design out. Keep the hardness of the weld and heat-affected zone below the accepted limit — commonly 22 HRC, with the detail set out in NACE MR0175 / ISO 15156 — and susceptibility falls away.
Where it appears: weld heat-affected zones, hard weld metal, cold-worked areas, and anywhere a repair was made without post-weld heat treatment.
Finding it: wet fluorescent magnetic particle on the surface. And, more usefully, a hardness survey — because hardness identifies the susceptible zone before it cracks, which no crack-detection method can do.
| Needs stress? | Where | Direction | Found by | |
|---|---|---|---|---|
| Blistering | No | At inclusions, mid-wall | Bulges outward | Visual, UT |
| HIC | No | Mid-wall, at inclusion planes | Parallel to surface | UT, PAUT |
| SOHIC | Yes | At welds | Through wall | PAUT, TOFD |
| SSC | Yes | Hard HAZ and weld metal | Through wall | WFMT + hardness |
A WFMT survey of the welds finds SSC and finds nothing else. A UT scan of the plate finds HIC and blistering and will not reliably find SSC at a weld toe. A vessel in sour service that has had only one of those inspections has been inspected for one of four mechanisms.
The four have different controls, which is another reason to keep them separate:
Blistering and HIC are assessed under API 579 Part 7, which has procedures specifically for blisters and for HIC damage — including how to treat a blister depending on whether it has vented and whether it is near a weld or a discontinuity.
SOHIC and SSC are crack-like flaws and go to Part 9. And Part 9 will ask whether the mechanism is still active. In sour service, unless the environment has actually changed, the answer is yes — which means the flaw will grow and accepting it as it stands is not valid without a growth analysis and a re-inspection interval.
Blistering, HIC and hydrogen attack in the library, each with the giveaway that identifies it and the method that finds it.
Open the mechanism libraryWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide