Corrosion in aerospace environments — stress corrosion cracking, hydrogen embrittlement: Question bank

Recall Before you start — the 3 words that unlock everything

SCC needs three things at once: a susceptible material, a sustained tensile stress, and a specific corrosive species (the "tripod"). Almost every trap below is really just checking whether you remembered that all three must be present simultaneously — and that hydrogen embrittlement is the fourth face lurking at the crack tip.

Definition Symbols this page uses — read once before you start

$K$ — stress-intensity factor. A single number that measures how severely a crack tip is "loaded," combining the applied stress and the crack length: $K = Y σ π a$ (here $σ$ = applied tensile stress, $a$ = crack length, $Y$ = geometry factor $\approx 1$ ). Its units are $MPa m$ (stress $\times$ square-root of length). Bigger $K$ = harder push on the crack tip. See Fracture mechanics — stress intensity factor and $K_{IC}$.
$K_{I C}$ — fracture toughness. The value of $K$ at which the crack runs instantly (fast, purely mechanical fracture). The material's ceiling.
$K_{I S C C}$ — the SCC threshold. A much lower value of $K$ , above which a crack can grow slowly by stress corrosion. Rule: $K_{I S C C} ≪ K_{I C}$ ; keep the operating $K$ below $K_{I S C C}$ .
$σ_{co h}$ — cohesive strength. The stress needed to pull the atomic bonds apart at a plane — i.e. the "true" bond-breaking strength of the lattice. Hydrogen lowers it.
Faraday's law $m = \frac{M Q}{z F}$ . Mass $m$ of metal dissolved when charge $Q$ passes; $M$ = molar mass (g/mol), $z$ = electrons per metal atom, $F$ = Faraday constant ( $\approx 96485 C/mol$ ).
$Q_{tip}$ — the charge passed at the freshly bared crack tip before its protective film re-heals; it is this charge (not the total available) that drives crack advance.
$H$ vs $H_{2}$ — READ THIS CAREFULLY. Throughout this page, a lone $H$ (sometimes written $H_{a d s}$ or $H_{ab s}$ ) means a single atomic hydrogen atom — a tiny, mobile, reactive radical small enough to squeeze between metal atoms and diffuse into the lattice. $H_{2}$ is the molecule (two H atoms bonded), a fat gas bubble that mostly stays outside the metal. It is the atomic $H$ , never $H_{2}$ , that causes embrittlement. So the cathodic step is properly written $2 H^{+} + 2 e^{-} \to 2 H_{a d s}$ (atomic H sticking to the surface), which only later pairs up as $2 H_{a d s} \to H_{2}$ if it doesn't first dive into the metal.
HEDE / HELP — the two hydrogen-cracking mechanisms: HEDE = Hydrogen-Enhanced DEcohesion (hydrogen weakens the atomic bonds directly, lowering $σ_{co h}$ ); HELP = Hydrogen-Enhanced Localized Plasticity (hydrogen makes slip easier, concentrating damage). Both are routes of hydrogen embrittlement.
Hydrogen relief bake — a mandatory heat treatment (typically ~190 °C for ≥8 h) applied after plating/before loading; it warms the part so trapped atomic hydrogen diffuses back out of the metal before stress can concentrate it at crack tips.

Intuition Picture the crack tip first (three views you'll keep referring to)

The single scene behind almost every question on this page is a sharp crack tip doing three things at once. Study these before you start — the reveals below point back at them.

View 1 — the stress field: why $K$ blows up and why $K_{I S C C} ≪ K_{I C}$ . The stress near a sharp tip rises steeply as you approach it (roughly as $1/ r$ , $r$ = distance from the tip). A longer crack $a$ raises the whole field, and $K = Y σ π a$ is the one number scaling it. Because the corrosive environment does part of the bond-breaking chemically, the crack can creep forward at a stress-intensity ( $K_{I S C C}$ ) far below the value needed for a purely mechanical snap ( $K_{I C}$ ).

View 2 — the occluded cell: why the tight crack turns acidic. The narrow crack seals off its own pocket of liquid. Metal ions $M^{n +}$ dissolve at the tip, can't escape, and hydrolyse with water to spit out $H^{+}$ — pulling the local pH down to 2–3 even when the outside is neutral seawater.

View 3 — the stress gradient pumps hydrogen to the tip. Atomic $H$ is generated by that acid, then drifts up the stress gradient toward the stretched (dilated) lattice right at the tip — exactly where it lowers $σ_{co h}$ and cracks the bonds. This is the geometric reason production and damage coincide at one point.

True or false — justify

SCC can occur at an applied stress far below the material's yield strength.

True — SCC is a sub-critical mechanism; the corrosion at the tip does the "extra work," so cracks grow at stresses that would never cause failure by load alone.

A part that shows zero visible general corrosion is therefore safe from SCC.

False — SCC is highly localised at the crack tip; the bulk surface can look pristine while a hairline crack quietly runs. Absence of general corrosion is not absence of SCC.

Removing the sustained tensile stress alone can stop an SCC crack.

True — kick out any one leg of the tripod and it collapses; with no tensile stress there is no crack-tip film rupture and no crack opening, so SCC halts even with material and environment still present.

Compressive residual stress at the surface promotes SCC.

False — SCC needs tensile stress to open and advance the crack; putting the surface in compression (e.g. shot peening) is a deliberate SCC defence.

Austenitic stainless steel 304/316 is immune to SCC because its passive film resists corrosion.

False — it is notoriously prone to chloride SCC above ~60 °C; stress locally ruptures the very film that stops general corrosion, and chlorides attack the bared metal underneath.

Hydrogen embrittlement requires molecular

H_{2}

gas to be present in the metal.

False — it is atomic hydrogen (

H

, a single reactive atom) diffusing into the lattice that embrittles; the molecule

H_{2}

is too large to enter and does little harm on its own (see View 3).

If the operating stress-intensity factor

K

is kept below the fracture toughness

K_{I C}

, the part is safe indefinitely under corrosive sustained load.

False — SCC propagates once

K

exceeds the much lower threshold

K_{I S C C}

; the crack grows over time until

a

is large enough that

K

finally reaches

K_{I C}

and the part snaps (the two thresholds are drawn in View 1).

Cathodic protection is always beneficial, so more of it is always better.

False — over-protection drives the potential so negative that the cathodic reaction

2 H^{+} + 2 e^{-} \to 2 H_{a d s}

generates atomic hydrogen at the surface, causing hydrogen embrittlement in high-strength steels. See Cathodic protection.

The interior of a tight crack is at the same pH as the bulk solution.

False — the crack is an occluded cell; metal ions hydrolyse (

M^{n +} + H_{2} O \to M (O H)^{(n - 1) +} + H^{+}

), generating

H^{+}

and dropping local pH to 2–3 even in neutral bulk (View 2).

Baking a Cd-plated bolt after plating restores full ductility if done before loading.

True — a hydrogen relief bake (~190 °C, ≥8 h) exploits the mobility of atomic hydrogen: heat lets it diffuse back out of the metal before stress traps it at crack tips. This bake is mandatory in aerospace specs.

Corrosion fatigue and SCC are the same failure mode.

False — SCC needs a sustained (static) tensile stress, whereas corrosion fatigue needs cyclic stress; corrosion fatigue has no strict threshold like

K_{I S C C}

and attacks many alloy/environment pairs that are SCC-immune.

Spot the error

"7075-T6 aluminium is a high-strength alloy, so it must have the best SCC resistance."

The error reverses the truth — high strength usually means worse SCC/HE susceptibility, because the cohesive-strength margin is small and the microstructure (grain-boundary precipitates in 7075) offers easy crack paths.

"During slip-dissolution SCC, the whole crack surface dissolves uniformly."

Only the tip dissolves — the crack walls stay covered by the protective oxide film, so dissolution is concentrated at the freshly ruptured tip, keeping the crack sharp.

"Hydrogen embrittlement and SCC are unrelated failure modes with no overlap."

At an acidified crack tip anodic dissolution produces the atomic hydrogen that then embrittles the metal ahead of the tip — they are coupled faces of environment-assisted cracking, not separate.

K = Y σ π a

shows that a longer crack always reduces the stress a part can bear."

The equation is fine, but the reasoning skips the SCC threshold

K_{I S C C}

(the value of the stress-intensity factor above which slow SCC growth begins) — below it the crack doesn't grow at all; the danger is that once

K

exceeds

K_{I S C C}

the crack grows further until

K

finally reaches the fracture toughness

K_{I C}

and it snaps (View 1). See Fracture mechanics — stress intensity factor and $K_{IC}$.

"Since Faraday's law gives mass

= \frac{M Q}{z F}

(with

M

the molar mass,

z

the electrons per atom,

F

the Faraday constant), a higher charge always means a faster crack."

Only the charge passed before the film re-heals (repassivates) — call it

Q_{tip}

, the charge at the freshly bared crack tip — drives crack advance; a film that heals fast limits

Q_{tip}

regardless of total available charge. See Passivation and oxide films.

"Cadmium plating protects the bolt, so it can only make things safer."

The acid plating bath co-deposits atomic hydrogen (

H_{a d s}

, single reactive atoms) at the bolt (cathode), charging the steel with hydrogen — protection against corrosion but a fresh route to embrittlement unless baked out. Connects to Electrochemistry — galvanic cells & Faraday's law.

"A crack tip is just a stress concentrator — chemistry happens the same everywhere along the crack."

The tip is a chemically distinct micro-reactor (View 2): sealed off, acidified, and at peak triaxial stress, so both dissolution and hydrogen uptake are massively amplified there compared to the open crack mouth.

Why questions

Why does a crack tip specifically become the site where atomic hydrogen accumulates?

The tip is both the most acidic (occluded-cell

H^{+}

generation, View 2) and the highest triaxial-stress region; atomic

H

is driven up the stress gradient toward the dilated lattice at the tip (View 3), so production and accumulation geometrically coincide there.

Why is the SCC threshold

K_{I S C C}

so much smaller than the fracture toughness

K_{I C}

K_{I C}

measures resistance to instantaneous mechanical fracture, whereas

K_{I S C C}

only needs enough driving force for slow chemically-assisted advance — the environment does much of the bond-breaking work, so a far lower stress intensity suffices (the two levels are marked on the field in View 1).

Why does fixing "the cheapest tripod leg" defeat SCC completely rather than just slowing it?

SCC needs all three legs simultaneously; the failure is multiplicative, not additive, so nulling any single factor (stress, environment, or susceptibility) stops propagation entirely rather than merely reducing its rate.

Why does dividing the Faraday mass-rate by density give a crack velocity?

Mass per area per time divided by mass per volume leaves length per time — the dissolution eats material off the tip face, so material-removed-depth-per-time is the crack advance speed.

Why are high-strength alloys intrinsically more vulnerable to environment-assisted cracking?

Their high working stresses sit close to the cohesive strength

σ_{co h}

(the stress that pulls the atomic bonds apart), so even a small hydrogen-induced drop in

σ_{co h}

, or a small dissolution notch, is enough to push local stress over that reduced bond-breaking limit.

Why does stress rupture the passive film rather than the film simply blocking corrosion forever?

Slip steps produced by the applied stress shear through the brittle oxide at the tip, exposing bare reactive metal faster than it can re-passivate — stress is what keeps re-opening the protective barrier. See Passivation and oxide films.

Why does the tight geometry of a crack (View 2) matter as much as the chemistry inside it?

The narrowness is what seals the pocket: it throttles diffusion so hydrolysis products (

H^{+}

M^{n +}

) build up instead of washing away, turning an ordinarily mild bulk into a local acid — geometry creates the aggressive chemistry.

Edge cases

What happens to SCC if the environment is completely dried out and stays inert?

The corrosive-species leg of the tripod is removed, so SCC cannot initiate or propagate — this is why sealed/desiccated storage and barrier coatings are legitimate SCC fixes.

At exactly

K = K_{I S C C}

(the stress-intensity factor sitting right on the SCC threshold), what is the crack doing?

It is at the borderline of arrest — below it the crack is dormant, at/above it slow SCC growth becomes possible, so design keeps

K

safely below this value with margin, never right on it.

If a metal cannot dissolve anodically at all (perfectly noble), can it still suffer environment-assisted cracking?

Yes — the hydrogen-assisted route can still operate: HEDE (Hydrogen-Enhanced Decohesion) or HELP (Hydrogen-Enhanced Localized Plasticity) proceed if atomic hydrogen enters from another source (plating, cathodic charging), so ruling out anodic dissolution does not rule out embrittlement.

What is the effect of zero applied stress but severe corrosive environment on SCC?

No SCC — without tensile stress there is no crack opening or film rupture at a tip; you may get pitting or crevice attack, but not stress corrosion cracking.

For a very low-strength, very ductile metal in a chloride environment, how does SCC susceptibility compare to a high-strength grade?

Generally much lower — the large cohesion/plasticity margin blunts crack tips and tolerates hydrogen, so SCC and HE risk rise sharply as strength (and hardness) climb.

At the limit of a perfectly self-healing (instantly repassivating) film, what happens to slip-dissolution crack growth?

The tip charge

Q_{tip} \to 0

because the bare metal is covered before appreciable charge passes, so the anodic-dissolution contribution to crack advance vanishes — repassivation kinetics set the crack speed.

If a part sees cyclic rather than sustained load in a corrosive medium, which environment-assisted mode dominates?

Corrosion fatigue — cyclic stress repeatedly ruptures the film and pumps fresh solution to the tip, so cracks grow even below the fatigue limit in air and even in alloy/environment pairs that show no static SCC.

For an inert-environment fatigue test versus the same cyclic load in salt water, what happens to component life?

Life drops sharply in salt water — corrosion fatigue has no true endurance limit, so there is no "safe" stress amplitude below which the part lasts forever, unlike dry fatigue.