Intuition The ONE core ideaA metal part can break at a stress far below the load it was designed for — but only when a pulling force, a susceptible metal, and a corrosive liquid act together at the same tiny spot . Everything on this page is just the vocabulary you need to describe that one spot: what pulls it, what dissolves it, and how sharp the crack there has grown.
This is the toolbox page for the parent topic . Before you can follow stress corrosion cracking or hydrogen embrittlement , you must own every symbol they use. We build each one from nothing — plain words first, then a picture, then why the topic can't live without it .
A formula is a sentence. Letters are nouns, operators are verbs. If even one noun is a stranger, the whole sentence is noise. So we introduce the nouns in the order the topic needs them: first the pieces of an atom, then how atoms trade charge, then how charge becomes dissolved metal, then how force concentrates at a crack, and finally how hydrogen sneaks in.
Definition Atom, electron, ionAn atom is the smallest piece of an element that is still that element. Around its core sit tiny negatively-charged particles called electrons . If an atom loses electrons it is left positive; if it gains them it becomes negative. A charged atom is an ion .
Picture a metal atom as a coin sitting in a wall of identical coins (the solid metal). To "corrode" is for that coin to hand over one or more of its electrons, become a positive ion, and float away into the water — leaving the electrons behind in the metal wall.
Symbols you now own:
Why the topic needs it: corrosion is atoms turning into ions by shedding electrons. Without M e − n M → M n + + n e −
Definition Reaction, the arrow⟶ before → after statement. Whatever is on the left (reactants ) turns into whatever is on the right (products ). The arrow ⟶ same number of atoms and the same total charge must appear on both sides — nothing is created or destroyed, only rearranged.
Fe → Fe 2 + + 2 e − + 2 − 2 = 0 = ( + 2 ) + ( − 2 ) = 0 Balanced. In words: "iron gives up two electrons and becomes a + 2
Definition Anode and cathode
The anode is the spot where metal loses electrons (dissolves). It is the place damage happens.
The cathode is the spot where those electrons are used up by something in the water.
The two are wired together through the metal itself — electrons flow from anode to cathode inside the solid.
Why the topic needs it: every corrosion equation on the parent page is an anode half or a cathode half. Knowing which side eats metal (anode) and which side may make hydrogen (cathode) is the whole plot.
H + H + H + acidic it is. pH is just a number that measures this: low pH (2–3) = very acidic, pH 7 = neutral, high pH = basic. Each step down in pH means ten times more H +
Intuition The picture — why crack tips go sourImagine the inside of a hairline crack as a sealed cave. Fresh water can't flush it. Metal ions pile up and react with water to spit out H + hydrolysis ). H + occluded cell .
Why the topic needs it: acidic crack-tip water is the factory that makes atomic hydrogen — the raw material of hydrogen embrittlement. No pH concept, no understanding of why the tip and only the tip breeds hydrogen.
Definition Stress, symbolσ Stress = how hard you pull, spread over the area you pull on:
σ = area force megapascals (MPa tensile stress — pulling that tries to open a crack, not squeezing that closes it.
Two people pulling on the ends of a rubber band put it in tension. Cut a small nick in the band while it's stretched and the nick opens — tensile stress is what pries crack faces apart. Squeeze the band instead (compression) and the nick clamps shut. That is exactly why engineers deliberately push part surfaces into compression (see Shot peening and residual stress engineering ).
Definition Yield strengthσ y far below σ y
Why the topic needs it: stress is one leg of the SCC tripod, and it is the "driving" quantity inside the crack-severity formula in §6.
a a a same pulling stress. Measured in metres inside formulas — always convert millimetres to metres first (2 mm = 0.002 m
Common mistake Forgetting to converta Why it feels right: cracks are naturally described in mm. The trap: the formula π a a 2 0.002 K 1000 ≈ 31 Fix: convert first, every time.
Intuition WHY we can't just use stress aloneTwo parts under the same stress σ not equally close to breaking if one has a longer crack. We need a single number that blends stress and crack length . That number is the stress intensity factor K
Intuition The picture — whyr r 1/ r K size knob on that whole curve — turn K
Definition The two thresholdsK I S C C K I C
K I C fracture toughness : the value of K instantly (fast fracture). The "I C K I S C C K slowly grows by SCC in a corrosive environment. It is much smaller than K I C K K I S C C K I C Fracture mechanics — stress intensity factor and $K_{IC}$ .
Why the topic needs it: K K I S C C
Q i F
Charge Q total amount of electricity that has flowed (units: coulombs, C). One electron carries charge e Current i rate of charge flow — charge per second (amps, A). Current density is current spread over area (A/m 2 F Faraday constant , F ≈ 96485 C/mol M N A
Intuition Why the topic needs FaradayThe anodic-dissolution model of SCC says the crack advances by dissolving metal at its tip. Faraday's law is the only tool that converts the electrical current there into an actual crack velocity (see Example 3 on the parent page). Full background: Electrochemistry — galvanic cells & Faraday's law .
H a d s H 2 H + H a d s H 2 atomic H slip between the metal atoms and travel deep into the solid.
Definition Cohesive strengthσ co h C H
σ co h C H as C H σ co h — hydrogen weakens the bonds it wedges into. The crack advances when local stress exceeds this reduced σ co h
Why the topic needs it: this is the entire mechanism of hydrogen embrittlement — atomic H σ co h
Definition Passive (oxide) filmMany alloys (aluminium, stainless steel) grow a thin, invisible oxide skin that seals the metal from the water — this is passivation . It is what makes stainless "stainless". When scratched, it usually re-heals in milliseconds.
Intuition Why this matters for SCCStress can rip this film open at a crack tip faster than it heals, exposing bare metal that dissolves — the slip-dissolution model. So the very shield that stops general corrosion becomes the weak point under stress. Depth in Passivation and oxide films .
Atoms electrons ions M and e-
Reaction arrow anode and cathode
pH and H+ occluded crack tip
Faraday law charge to mass
Atomic hydrogen and cohesion
Environment assisted cracking
Cover the answer and test yourself — you are ready for the parent page only when every line is easy.
What does M n + n A metal atom that has lost n + n n z
Which electrode dissolves the metal — anode or cathode? The anode; the cathode consumes the electrons.
Why does a crack tip become acidic even in neutral seawater? It is an occluded (sealed) cell; trapped metal ions hydrolyse and release H +
What is stress σ Force per unit area; tensile (pulling) stress opens cracks, compression closes them.
Write the stress intensity formula and name every symbol. K = Y σ π a :
Y geometry factor,
σ tensile stress,
a crack length in metres.
Why is there a square root of a Because crack-tip stress rises like
1/ r , so the crack's driving force scales with
a .
Difference between K I S C C K I C K I C K I S C C K I S C C ≪ K I C
State Faraday's law and what it converts. m = M Q / ( z F )
What is the Faraday constant F About 96485 C/mol
Why is atomic H H 2 A lone H σ co h H 2
What happens to σ co h C H It falls — dissolved hydrogen weakens atomic bonds, so cracking happens at lower stress.
Why must you convert a K The formula's units demand metres; using mm inflates
K by
1000 ≈ 31 .
