Corrosion in aerospace environments — stress corrosion cracking, hydrogen embrittlement

5.4.9 · Chemistry › Materials Chemistry (Aerospace)

1. Hum actually kisi baat kar rahe hain?

2. Mechanism — first principles se derive karke

2.1 Crack ke peeche electrochemistry

Corrosion ek electrochemical cell hai. Anode par metal dissolve hota hai: $M ⟶ M^{n +} + n e^{-}$

Electrons ek cathode par consume hote hain. Near-neutral aerated salt water mein: $O_{2} + 2 H_{2} O + 4 e^{-} ⟶ 4 O H^{-}$

Acidic ya occluded (crack-tip) conditions mein iski jagah: $2 H^{+} + 2 e^{-} ⟶ 2 H_{a d s} ⟶ H_{2}$

2.2 Do competing SCC mechanisms

Dissolution rate ki derivation (Faraday from scratch):

$N$ atoms of charge $z$ dissolve karne ka charge: $Q = N z e$ .
Moles dissolved $= N / N_{A}$ , mass $= (N / N_{A}) M$ .
$N = Q / (z e)$ aur $F = N_{A} e$ substitute karo: $m = \frac{M Q}{z F}$ Toh crack velocity $\propto$ tip current density — bare metal jitna tez corrode hoga repassivating se pehle, utna tez crack chhalega.

2.3 Fracture-mechanics threshold

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

4. Common mistakes (Steel-man + fix)

5. 80/20 — jo cheez actually marks laati hai

Flashcards

Stress corrosion cracking ke liye kaun si teen conditions ek saath honi chahiye?

Ek susceptible material + sustained tensile stress + ek specific corrosive environment (SCC "tripod").

Design ke liye

K_{I S C C}

K_{I C}

se zyada important kyun hai?

SCC

K_{I S C C}

par cracks grow karta hai, jo fast-fracture toughness

K_{I C}

se kaafi neeche hai; sirf

K_{I C}

ke under rehna slow time-dependent SCC nahi rokta.

Stress-intensity factor likho aur har term define karo.

K = Y σ π a

;

Y

=geometry factor,

σ

=tensile stress,

a

=crack length.

Neutral water mein bhi crack tip acidic kyun ho jaata hai?

Occluded crack mein dissolved metal ions hydrolyse karte hain (

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

), local pH ~2–3 tak gira dete hain.

Atomic level par hydrogen embrittlement kya hai?

Atomic H lattice mein diffuse hota hai aur high-stress regions par collect hota hai, cohesive bond strength kam karta hai toh cracks yield se neeche form hoti hain.

HE cause karne wale hydrogen ke 4 sources batao.

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

High-strength alloys HE ke liye sabse zyada vulnerable kyun hain?

High working stresses ek chhota cohesion margin chhodte hain, toh thoda sa hydrogen content bhi cohesion ko applied stress se neeche gira deta hai.

Plating ke baad hydrogen embrittlement ka standard cure kya hai?

Ek "hydrogen-relief bake" (e.g. ~190 °C for ≥8 h) taaki loading se pehle H diffuse ho sake.

Over-aggressive cathodic protection failure kyun cause kar sakta hai?

Bahut-negative potential

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

drive karta hai, hydrogen inject karta hai aur high-strength steel ko embrittle kar deta hai.

SCC ka slip-dissolution mechanism batao.

Stress tip par passive film rupt karta hai, bare metal anodically dissolve hota hai (Faraday), film walls par re-heal hoti hai → sharp advancing crack.

Crack tip par mass dissolve hone ka Faraday's law.

m = M Q / (z F)

; crack velocity ∝ tip current density.

Hot chloride mein 304/316 austenitic stainless steel risky kyun hai?

Yeh excellent general-corrosion resistance ke bawajood chloride stress-corrosion cracking suffer karta hai.

SCC ke against surface ko compression mein daalne ke liye sabse sasta engineering fix kya hai?

Shot peening (compressive residual stress induce karta hai jo surface cracks ko close/retard karta hai).

Recall Feynman: 12-saal ke bachche ko samjhao

Ek metal bar socho jo perfectly strong hai, lekin use thoda sa moda rakha hai (stressed) aur iske upar namkeen paani daala jaata hai. Rust ke tiny pinholes isme khaana shuru kar dete hain — lekin trick yeh hai: moda hona pinhole ko kholke kheenchta hai, toh woh wide failne ki jagah andar khaata hai, jaise ek paper cut jo tab split hota rehta hai jab aap kaagaz kheencho. Yahi stress corrosion cracking hai. Ab, kabhi kabhi rusting tiny hydrogen "ghosts" bhi banata hai jo metal ke atoms ke beech ghus jaate hain aur grease ki tarah kaam karte hain, toh atoms aasaani se alag ho jaate hain — yahi hydrogen embrittlement hai. Dono bar ko kaafi kam force se snap karte hain — toh engineers ya toh use seedha karte hain, sukhate hain, metal badal dete hain, ya ghosts ko bake out karte hain.