Corrosion — electrochemical mechanism; cathodic protection, galvanization

2.7.11 · Chemistry › Redox & Electrochemistry (Intro)

Corrosion Kya Hai?

YEH KYUN HOTA HAI:

Nature mein metals ores (oxides, sulfides) ke roop mein exist karte hain. Pure metal state higher energy par hoti hai.
Corrosion nature ka tarika hai metals ko unki stable oxidized state mein wapas laane ka.
Teeno cheezein chahiye: anode (metal oxidation site), cathode (reduction site), aur electrolyte (ions conduct karta hai).

Iron Corrosion ka Electrochemical Mechanism

Micro-Cells Ka Setup

Jab iron moist air ke samne aata hai:

Surface par water film banta hai (electrolyte)
Dissolved oxygen aur dissolved CO₂ thodi acidic conditions create karte hain
Surface imperfections, grain boundaries, ya stress points anodic regions ban jaate hain
Nearby areas jahan better oxygen access hoti hai cathodic regions ban jaate hain

Figure — Corrosion — electrochemical mechanism; cathodic protection, galvanization

Half-Reactions — First Principles Se Derive Karna

Release hue electrons metal ke through cathode tak travel karte hain.

Overall Cell Reaction aur Rust Formation

Half-reactions ko combine karna (electrons balance karne ke liye Fe oxidation ko 2 se multiply karo):

$2 Fe (s) + O_{2} (g) + 4 H^{+} (a q) \to 2 Fe^{2 +} (a q) + 2 H_{2} O (l)$

Cell potential: $E °_{cell} = E °_{cathode} - E °_{anode} = 1.23 - (- 0.44) = + 1.67 V$

Yeh bada positive $E °_{cell}$ matlab corrosion highly spontaneous hai ( $Δ G ° = - n F E ° < 0$ ).

$Fe^{2 +}$ ions phir aage react karte hain:

$Fe^{2 +} (a q) + 2 OH^{-} (a q) \to Fe(OH)_{2} (s)$

Oxygen ki presence mein, $Fe(OH)_{2}$ oxidize hokar $Fe(OH)_{3}$ ban jaata hai:

$4 Fe(OH)_{2} (s) + O_{2} (g) + 2 H_{2} O (l) \to 4 Fe(OH)_{3} (s)$

$Fe(OH)_{3}$ dehydrate hokar rust banaata hai:

$2 Fe(OH)_{3} (s) \to Fe_{2} O_{3} \cdot x H_{2} O (s) + (3 - x) H_{2} O (l)$

YEH KYUN MATTER KARTA HAI: Rust porous aur flaky hoti hai — yeh underlying metal ko protect nahi karti. Isse corrosion aur andar tak jaari rehti hai, unlike aluminum oxide (Al₂O₃) jo ek protective layer banata hai.

Worked example Example 2: Corrosion Cell EMF Calculate Karna

Seawater (pH = 8, dissolved O₂) mein ek iron pipe.

Given:

$E °_{Fe^{2 +} / Fe} = - 0.44 V$
$E °_{O_{2} / OH^{-}} = + 0.40 V$ (neutral/basic) Find: Standard cell potential.

Solution: $E °_{cell} = E °_{cathode} - E °_{anode} = 0.40 - (- 0.44) = + 0.84 V$

YEH STEP KYUN? Hum anode potential subtract karte hain kyunki oxidation reduction ka reverse hai. Positive $E °_{cell}$ spontaneous corrosion confirm karta hai.

Non-standard conditions ke liye, Nernst equation use karo: $E_{cell} = E °_{cell} - \frac{0.059}{n} lo g Q$ pH 8 par, $[H^{+}] = 1 0^{- 8}$ M. Agar $[Fe^{2 +}] = 1 0^{- 6}$ M, $P_{O_{2}} = 0.21$ atm: $Q = \frac{[ Fe ^{2 +} ] ^{2}}{[ H ^{+} ] ^{4} \cdot P _{O_{2}}} = \frac{( 1 0 ^{- 6} ) ^{2}}{( 1 0 ^{- 8} ) ^{4} \cdot 0.21} \approx 2.3 \times 1 0^{19}$

$E_{cell} = 0.84 - \frac{0.059}{4} lo g (2.3 \times 1 0^{19}) = 0.84 - 0.29 = + 0.55 V$

Abhi bhi positive hai — corrosion jaari rehti hai.

Corrosion Ko Accelerate Karne Wale Factors

Moisture aur electrolytes: Salt water (NaCl) conductivity badhata hai — ions anode aur cathode ke beech faster move karte hain.
Oxygen availability: Cathode par zyada O₂ = faster reduction = faster corrosion.
Acidity: Kam pH matlab cathodic reduction ke liye zyada H⁺; protective oxide films bhi dissolve ho jaati hain.
Temperature: Zyada T reaction rates badhata hai (corrosion roughly har 10°C rise par double ho jaati hai).
Impurities/Dissimilar metals se contact: Zyada potential differences wale galvanic cells create hote hain.

Corrosion Protection Methods

1. Cathodic Protection

A. Sacrificial Anode Method

Principle: Structure ke saath ek zyada active metal (zyada negative $E °$ wala) attach karo. Active metal anode ban jaata hai aur protected metal ki jagah corrode karta hai.

YEH KYUN KAAM KARTA HAI:

Active metal (jaise Mg, Zn) ka reduction potential Fe se lower hota hai.
Electrons metallic connection ke through Mg se Fe tak flow karte hain.
Fe cathode ban jaata hai (electrons receive karta hai) aur oxidize nahi ho sakta.
Mg oxidize hota hai: $Mg (s) \to Mg^{2 +} (a q) + 2 e^{-}$

Applications: Ship hulls, pipelines, underground tanks. Mg ya Zn blocks periodically replace kiye jaate hain.

Worked example Example 3: Sacrificial Anode Choose Karna

Ek iron pipeline ko protection chahiye. Available metals:

Copper: $E °_{Cu^{2 +} / Cu} = + 0.34 V$
Zinc: $E °_{Zn^{2 +} / Zn} = - 0.76 V$
Magnesium: $E °_{Mg^{2 +} / Mg} = - 2.37 V$

Kaun sa metal/metals kaam karega? Solution: Ek sacrificial anode ka $E ° < E °_{Fe} = - 0.44 V$ hona chahiye.

Cu: $+ 0.34 > - 0.44$ ✗ (Cu zyada noble hai; Fe, Cu ko protect karega!)
Zn: $- 0.76 < - 0.44$ ✓ (Zn preferentially oxidize hota hai)
Mg: $- 2.37 < - 0.44$ ✓ (Mg aur bhi aasani se oxidize hota hai)

YEH STEP KYUN? A ko B protect karne ke liye, electrons A se B tak flow karne chahiye. Iske liye A ka reduction potential zyada negative (kam positive) hona chahiye — A ko zyada aasani se oxidize hona chahiye.

Practical choice: Zn generally Mg se prefer kiya jaata hai kyunki:

Zn slower corrode karta hai (Mg "too active" hai aur jaldi khatam ho jaata hai)
Zn²⁺ salts Mg²⁺ se kam problematic hote hain

B. Impressed Current Method

Principle: Structure par electrons force karne ke liye external DC power supply use karo, jisse yeh cathode ban sake.

Setup:

Negative terminal → Protected structure (cathode)
Positive terminal → Inert anode (graphite, titanium, platinized titanium)
Electrolyte: surrounding soil, water

YEH KYUN KAAM KARTA HAI:

External voltage natural corrosion potential ko overwhelm kar deta hai.
Structure electrons se bhara hota hai — oxidation impossible hai.
Inert anode khud corrode hone ki jagah water/chlorides oxidize karta hai.

Advantages: Anodes replace karne ki zaroorat nahi; large structures protect kar sakta hai (lambi pipelines, oil rigs).

Disadvantage: Continuous power supply chahiye.

2. Galvanization

Specifically zinc kyun?

$E °_{Zn^{2 +} / Zn} = - 0.76 V < E °_{Fe^{2 +} / Fe} = - 0.44 V$
Zn preferentially oxidize hota hai: $Zn (s) \to Zn^{2 +} (a q) + 2 e^{-}$
Yeh electrons exposed Fe tak flow karte hain, use cathodic rakhte hue.

Process (hot-dip galvanizing):

Steel clean karo (degrease karo, acid mein pickle karo)
Flux treatment (oxide films hatata hai)
Molten Zn (~450°C) mein dip karo
Interface par Fe-Zn intermetallic layers bante hain (excellent adhesion)
Cool karo; outer layer pure Zn hai

Environmental advantage: Zn corrosion products (ZnO, Zn(OH)₂) rust se kam porous hote hain aur aage ki corrosion slow karte hain.

Worked example Example 4: Galvanized vs. Tin-Plated Iron

Tin (Sn) aur zinc dono iron ko coat kar sakte hain. Unke protection mechanisms compare karo.

Given:

$E °_{Sn^{2 +} / Sn} = - 0.14 V$
$E °_{Zn^{2 +} / Zn} = - 0.76 V$
$E °_{Fe^{2 +} / Fe} = - 0.44 V$ Analysis: Zinc coating (galvanized iron):
Agar scratched ho: Zn oxidize hota hai ( $- 0.76 < - 0.44$ ), Fe protected rehta hai.
Damage ke baad bhi, corrosion rokii jaati hai.

Tin coating (tin-plated iron, "tin cans"):

Agar scratched ho: Fe oxidize hota hai ( $- 0.44 < - 0.14$ ), Sn protected rehta hai!
Sn Fe se zyada noble hai; exposed Fe faster corrode karta hai (galvanic corrosion).
Tin sirf barrier protection deta hai, koi sacrificial protection nahi.

YEH STEP KYUN? Jab do metals electrolyte mein contact karte hain, kam noble (zyada negative $E °$ wala) anode ban jaata hai. Zn khud ko Fe ke liye sacrifice karta hai; Fe khud ko Sn ke liye sacrifice karta hai.

Conclusion: Galvanized iron outdoor use ke liye superior hai. Tin-plated iron food cans ke liye use hota hai kyunki:

Sn non-toxic hai
Cans sealed hote hain (electrolyte penetration nahi)
Short lifespan (ek baar use hote hain)

3. Doosre Protection Methods (Brief Overview)

Painting/Coatings: Barrier protection; maintain karna padta hai (scratches metal expose karte hain).
Alloying: Stainless steel (Fe + Cr + Ni) ek passive Cr₂O₃ layer banata hai jo self-heal karti hai.
Corrosion inhibitors: Chemicals jo metal surface par adsorb hote hain (jaise phosphates, chromates) ya dissolved O₂ consume karte hain.
Design modifications: Crevices avoid karo, drainage ensure karo, dissimilar metals ke beech insulating gaskets use karo.

Common mistake Common Mistake: "Galvanization Tabhi Kaam Karta Hai Jab Coating Intact Ho"

Galat idea: Ek baar Zn layer scratch ho jaaye, neeche iron turant rust karna shuru kar deta hai. Yeh sahi kyun lagta hai: Hum paint ya plastic coatings ke aadi hain — ek baar damage hone par, woh koi protection nahi dete.

Sacchi baat: Galvanization scratching ke baad bhi electrochemical protection deta hai: $Zn (s) \to Zn^{2 +} (a q) + 2 e^{-} [at scratch edge]$ Yeh electrons exposed Fe tak flow karte hain, use cathode ki tarah act karne ke liye force karte hue: $O_{2} (g) + 2 H_{2} O (l) + 4 e^{-} \to 4 OH^{-} (a q) [at exposed Fe]$

Iron oxidize nahi ho sakta kyunki yeh electrons receive kar raha hai. Sirf tab jab surrounding Zn completely deplete ho jaaye, iron rust karna shuru karega.

Fix: Recognize karo ki galvanization ek sacrificial protection system hai, sirf ek barrier nahi. Zn actively Fe oxidation ko redox coupling ke through rokta hai.

Common mistake Common Mistake: "Cathodic Protection Isliye Kaam Karta Hai Kyunki Yeh Oxygen Ko Metal Tak Pahunchne Se Rokta Hai"

Galat idea: Sacrificial anodes physically oxygen ya moisture ko block karke kaam karte hain.

Yeh sahi kyun lagta hai: Hum jaante hain ki corrosion ko oxygen chahiye, isliye access block karna logical lagta hai.

Sacchi baat: Cathodic protection electrochemically kaam karta hai, physically nahi:

Oxygen abhi bhi protected metal tak pahunchti hai.
Protected metal cathode ban jaata hai jahan O₂ reduce hoti hai: $O_{2} + 2 H_{2} O + 4 e^{-} \to 4 OH^{-}$
Oxidation (corrosion) sacrificial anode par hoti hai: $Mg \to Mg^{2 +} + 2 e^{-}$

Key hai electrons ka flow: Protected metal ko sacrificial anode se continuously electrons supply kiye jaate hain, jisse oxidation thermodynamically impossible ho jaati hai.

Fix: Samjho ki cathodic protection ek active electrochemical intervention hai, passive barrier protection nahi. Metal isliye protected hai kyunki use cathode hone ke liye force kiya jaata hai, chahe oxygen access ho ya na ho.

Recall Ek 12-Saal Ke Bacche Ko Samjhao

Socho tumhare iron bike mein ek superpower hai: yeh hawa se electrons grab kar sakti hai aur rust (yucky orange powder) mein badal sakti hai. Lekin rust weak aur flaky hoti hai, toh tumhari bike dheere dheere toot jaati hai.

Ab, yeh trick hai: Kya hoga agar hum tumhari bike ko ek dost — magnesium metal ka ek tukda — de dein jo electrons grab karne mein aur bhi better hai? Magnesium kehta hai, "Hey iron, main TUJHE apne electrons dunga! Tujhe aur rust nahi banana!" Toh magnesium dheere dheere dissolve ho jaata hai (yeh hero hai), aur tumhari iron bike strong rehti hai.

Yahi cathodic protection hai! Iron "receiver" (cathode) ban jaata hai aur magnesium "giver" (anode) ban jaata hai. Yeh aise hai jaise ek bodyguard ho jo tumhare liye hit leta hai.

Galvanization tab hota hai jab hum bike par zinc metal laga dete hain. Zinc bhi ek achha "electron giver" hai. Agar tum paint scratch karo aur iron expose ho, scratch ke paas wala zinc kehta hai, "Main tujhe protect karunga!" aur iron ko electrons deta hai, taaki iron rust na ho. Zinc dheere dheere khatam ho jaata hai, lekin neeche iron safe rehta hai.

Doosre Topics Se Connections

Electrochemical Cells — corrosion ek spontaneous galvanic cell hai
Standard Electrode Potentials — predict karna ki kaun sa metal preferentially corrode karega
Nernst Equation — pH, concentration, aur oxygen partial pressure corrosion rate ko kaise affect karte hain
Faraday's Laws — corrosion current ke through khoye hue metal ka mass calculate karna
Gibs Free Energy — $Δ G = - n F E_{cell}$ corrosion ki spontaneity determine karta hai
Passivation — Al, Cr, Ti protective oxide layers banate hain (self-limiting corrosion)
Concentration Cells — corrosion mein differential aeration cells (low O₂ area = anode)
Green Chemistry — corrosion resources waste karta hai; prevention se environmental impact kam hota hai

#flashcards/chemistry

Corrosion kya hai electrochemical terms mein? :: Ek metal ka apne environment (O₂, H₂O, acids) ke saath electrochemical reactions ke through spontaneous oxidation, jahan metal anode ki tarah kaam karta hai aur electrons kho deta hai.

Corrosion ke liye teeno zaroori components kya hain?

Anode (oxidation site), cathode (reduction site), aur electrolyte (ionic conductor).

Iron corrosion mein anodic reaction likho :: $Fe (s) \to Fe^{2 +} (a q) + 2 e^{-}$ (anode par oxidation)

Iron corrosion mein cathodic reaction likho (neutral/basic)

O_{2} (g) + 2 H_{2} O (l) + 4 e^{-} \to 4 OH^{-} (a q)

(cathode par reduction)

Iron corrosion ka standard cell potential calculate karo given $E °_{Fe^{2 +} / Fe} = - 0.44$ V aur $E °_{O_{2} / OH^{-}} = + 0.40$ V :: $E °_{cell} = 0.40 - (- 0.44) = + 0.84$ V (spontaneous)

Rust (Fe₂O₃·xH₂O) Al₂O₃ ki tarah protective kyun nahi hai?

Rust porous aur flaky hoti hai, jo continued oxygen/water penetration allow karti hai. Al₂O₃ dense aur adherent hai, ek self-healing barrier banata hai.

Cathodic protection kya hai?

Protected metal ko ek electrochemical cell ka cathode banana taaki yeh sirf electrons receive kare aur oxidize na ho sake.

Sacrificial anode protection kaise kaam karta hai?

Ek zyada active metal (zyada negative E°) attach karo jo preferentially oxidize hota hai, protected metal (cathode) ko electrons supply karta hua.

Magnesium, iron ke liye ek acha sacrificial anode kyun hai?

E °_{Mg^{2 +} / Mg} = - 2.37

< E °_{Fe^{2 +} / Fe} = - 0.44

V, toh Mg, Fe se bahut zyada aasani se oxidize hota hai.

Galvanization kya hai?

Iron ko zinc se coat karna barrier protection aur sacrificial protection dene ke liye agar coating damage ho jaaye.

Galvanization iron ko scratch hone ke baad bhi protect kyun karta hai?

Zn ka

E ° = - 0.76

<

Fe ka

E ° = - 0.44

V hai, toh Zn scratch par preferentially oxidize hota hai, Fe ko cathodic rakhta hua.

Galvanized iron vs tin-plated iron compare karo jab coating damage ho

Galvanized: Zn corrode karta hai (Zn, Fe ko protect karta hai). Tin-plated: Fe faster corrode karta hai (Fe, Sn ko protect karta hai) kyunki Sn zyada noble hai.

Cathodic protection ka impressed current method kya hai?

External DC power use karke structure par electrons force karna (cathode), ek inert anode circuit complete karta hua.

Saltwater corrosion kyun accelerate karta hai?

Salt ions (Na⁺, Cl⁻) electrolyte conductivity badhate hain, anode aur cathode ke beech ion movement speed up karte hue.

Char factors batao jo corrosion accelerate karte hain :: Moisture/electrolytes, oxygen availability, acidity (low pH), temperature increase, dissimilar metals se contact.