Module 4.5 — A distinct, serious failure mode where the corrosion happens beneath the surface, and the tube can fail with almost no visible external warning.
Click any stage to see what's happening. Atomic hydrogen diffuses into steel and reacts with carbon at grain boundaries, building internal pressure that embrittles and eventually fractures the metal from the inside.
Every mechanism covered so far in this track — oxygen pitting, caustic gouging, acid phosphate corrosion — involves metal being directly dissolved away at a surface. Hydrogen damage is different in kind: the damage occurs beneath the surface, inside the metal's internal structure, and can produce a sudden, brittle failure with comparatively little visible external evidence beforehand. This makes it one of the more concerning mechanisms in this track precisely because it doesn't announce itself the way surface-visible corrosion often does.
Hydrogen damage typically starts with localized acid corrosion at a tube surface, often beneath a porous deposit — the same kind of concentrating condition responsible for both caustic gouging and acid phosphate corrosion. Acid corrosion of carbon steel generates atomic hydrogen as a byproduct. Under normal circumstances, much of this hydrogen combines into hydrogen gas (H₂) at the surface and simply escapes into the boiler water. But some fraction of the atomic hydrogen, being extremely small, diffuses directly into the steel itself before it can combine into the larger H₂ molecule.
Once inside the steel, atomic hydrogen can react with carbon present in the steel's microstructure — carbon steel, as the name implies, contains carbon — forming methane gas (CH₄) at grain boundaries within the metal. Methane molecules are far too large to diffuse back out of the steel the way atomic hydrogen diffused in. As this reaction continues, methane accumulates at grain boundaries, building internal pressure and progressively weakening the bonds between grains. This process, called decarburization when it consumes the steel's carbon content and hydrogen embrittlement more broadly, leaves the affected steel brittle and prone to sudden fracture under stresses the material would normally withstand easily.
Because hydrogen damage originates from acid corrosion at the surface, everything that prevents acid phosphate corrosion (Module 4.4) and, more broadly, any localized acidic attack on carbon steel, is also hydrogen damage prevention. There's no separate "hydrogen damage chemistry program" — it's a downstream consequence of allowing acid corrosion conditions to occur and persist. This is a useful way to think about the relationship between the mechanisms in this track: several distinct-looking failure modes trace back to a shared handful of preventable root causes.