Corrosion in aerospace environments — stress corrosion cracking, hydrogen embrittlement

1. What are we actually talking about?

2. The mechanism — derived from first principles

2.1 Electrochemistry behind the crack

Corrosion is an electrochemical cell. At an anode the metal dissolves: $\text{M} \longrightarrow \text{M}^{n+} + n e^-$

The electrons are consumed at a cathode. In near-neutral aerated salt water: $O_2 + 2H_2O + 4e^- \longrightarrow 4OH^-$

In acidic or occluded (crack-tip) conditions instead: $2H^+ + 2e^- \longrightarrow 2H_{ads} \longrightarrow H_2$

2.2 Two competing SCC mechanisms

Derivation of the dissolution rate (Faraday from scratch):

Charge to dissolve $N$ atoms of charge $z$ : $Q = N z e$ .
Moles dissolved $= N/N_A$ , mass $= (N/N_A)M$ .
Substitute $N = Q/(ze)$ and $F = N_A e$ : $\boxed{\;m = \frac{M\,Q}{zF}\;}$ So crack velocity $\propto$ tip current density — the faster the bare metal corrodes before repassivating, the faster the crack runs.

2.3 Fracture-mechanics threshold

Figure — Corrosion in aerospace environments — stress corrosion cracking, hydrogen embrittlement

3. Worked examples

Worked example Example 1 — Will this crack grow? (threshold check)

A 7075-T6 aluminium wing fitting carries $\sigma = 150\ \text{MPa}$ . A surface flaw $a = 2\ \text{mm}$ is found. For this alloy in salt-laden air $K_{ISCC}=12\ \text{MPa}\sqrt{\text{m}}$ , $Y=1.12$ . Step 1 — compute $K$ . Why? Compare driving force to threshold. $K = 1.12 \times 150 \times \sqrt{\pi \times 0.002} = 1.12\times150\times0.0793 = 13.3\ \text{MPa}\sqrt{\text{m}}$ Step 2 — compare. Why? $K > K_{ISCC}$ . $13.3 > 12 \Rightarrow$ SCC will propagate. The part is unsafe even though 150 MPa is far below yield (~500 MPa). Step 3 — fix. Why pick the cheapest tripod leg? Reduce stress, shot-peen to put surface in compression, or apply a barrier coat to remove the environment.

Worked example Example 3 — Faraday crack-velocity estimate

A crack tip passes current density $i = 50\ \text{A/m}^2$ over bare iron ( $M=56\ \text{g/mol}$ , $z=2$ , $\rho = 7.87\ \text{g/cm}^3 = 7.87\times10^6\ \text{g/m}^3$ ). Step 1. Mass dissolution rate per area $= \dfrac{Mi}{zF} = \dfrac{56\times50}{2\times96485} = 0.0145\ \text{g/m}^2\text{s}$ . Why? Faraday per second. Step 2. Velocity $= \dfrac{\text{mass rate}}{\rho} = \dfrac{0.0145}{7.87\times10^6} = 1.8\times10^{-9}\ \text{m/s}$ . Why divide by density? mass/area/time ÷ (mass/volume) = length/time. $\Rightarrow \approx 0.16\ \text{mm/day}$ — slow per second but catastrophic over a flight-hour budget.

4. Common mistakes (Steel-man + fix)

5. The 80/20 — what actually carries the marks

Flashcards

What three conditions must co-exist for stress corrosion cracking?

A susceptible material + sustained tensile stress + a specific corrosive environment (the SCC "tripod").

Why is

K_{ISCC}

important for design rather than

K_{IC}

SCC grows cracks at

K_{ISCC}

, which is far below the fast-fracture toughness

K_{IC}

; staying under

K_{IC}

alone does NOT prevent slow time-dependent SCC.

Write the stress-intensity factor and define each term.

K = Y\sigma\sqrt{\pi a}

;

Y

=geometry factor,

\sigma

=tensile stress,

a

=crack length.

Why does a crack tip become acidic even in neutral water?

Dissolved metal ions hydrolyse (

M^{n+}+H_2O\to M(OH)^{(n-1)+}+H^+

) in the occluded crack, dropping local pH to ~2–3.

What is hydrogen embrittlement at the atomic level?

Atomic H diffuses into the lattice and collects at high-stress regions, lowering the cohesive bond strength so cracks form below yield.

Name 4 sources of hydrogen that cause HE.

Corrosion (cathodic reaction), acid pickling, electroplating, welding/service environment.

Why are high-strength alloys most vulnerable to HE?

High working stresses leave a small cohesion margin, so even small hydrogen content reduces cohesion below the applied stress.

What is the standard cure for hydrogen embrittlement after plating?

A "hydrogen-relief bake" (e.g. ~190 °C for ≥8 h) to diffuse H out before loading.

Why can over-aggressive cathodic protection cause failure?

Too-negative potential drives

2H^++2e^-\to H

, injecting hydrogen and embrittling high-strength steel.

State the slip-dissolution mechanism of SCC.

Stress ruptures the passive film at the tip, bare metal dissolves anodically (Faraday), film re-heals on walls → sharp advancing crack.

Faraday's law for mass dissolved at a crack tip.

m = MQ/(zF)

; crack velocity ∝ tip current density.

Why is 304/316 austenitic stainless steel risky in hot chloride?

It suffers chloride stress-corrosion cracking despite excellent general-corrosion resistance.

Cheapest engineering fix to put a surface into compression against SCC.

Shot peening (induces compressive residual stress that closes/retards surface cracks).

Recall Feynman: explain it to a 12-year-old

Imagine a metal bar that's perfectly strong, but you keep it slightly bent (stressed) and splash salty water on it. Tiny pinholes of rust start eating into it — but here's the trick: the bend pulls the pinhole open, so it eats deeper instead of spreading wide, like a paper cut that keeps splitting when you stretch the paper. That's stress corrosion cracking. Now, sometimes the rusting also makes tiny hydrogen "ghosts" that sneak between the metal's atoms and act like grease, so the atoms slip apart more easily — that's hydrogen embrittlement. Both make the bar snap with way less force than it should — so engineers either un-bend it, dry it, swap the metal, or bake the ghosts out.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, aerospace mein sabse khatarnaak failure wo hota hai jo dikhta hi nahi — part bilkul healthy lagta hai, strength test paas kar leta hai, par yield se bahut kam stress par hi crack ho jaata hai. Iska reason hai do "silent killers": Stress Corrosion Cracking (SCC) aur Hydrogen Embrittlement (HE). SCC ke liye teen cheezein ek saath chahiye — ek susceptible material, ek sustained tensile stress, aur ek specific corrosive environment (jaise salt water ya chloride). Isko tripod samjho: koi ek leg hata do, stool gir jaata hai. Yahi engineering ka best trick hai — jo leg sabse sasti hai usko hata do (jaise shot-peening se surface ko compression mein daal do, ya coating laga do).

Crack tip par ek interesting cheez hoti hai: wahan solution "trapped" ho jaata hai (occluded cell). Metal ions paani ke saath hydrolyse karke $H^+$ banate hain, isliye pH 2-3 tak gir jaata hai bhale bulk neutral ho. Yeh acid wahan atomic hydrogen paida karta hai. Yahi hydrogen lattice ke andar ghus jaata hai aur atoms ke beech ka bond weak kar deta hai — yeh hai hydrogen embrittlement. Isiliye SCC aur HE ko saath padhate hain: high-strength steel mein crack tip dono kaam ek saath karta hai.

Maths simple hai par powerful: $K = Y\sigma\sqrt{\pi a}$ . Jab tak $K$ threshold $K_{ISCC}$ se neeche hai, crack badhta nahi. Par yaad rakho $K_{ISCC}$ , $K_{IC