11678 corrosion [compatibility mode] 2

7/30/2019 11678 Corrosion [Compatibility Mode] 2

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Prepared by Course Coordinator -M r. Rahul M ehta 1

CHE021-Applied Chemistry

Prepared by

M r. Ra hu l M e ht a

Lecturer

Departm ent of Chem istryLovely Professional University, Phagwara, Punjab, India

E-mail: rahul [email protected]. in

Corrosion and its preve nt ions


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Corrosion

Prepared by Course Coordinator -M r. Rahul M ehta 2


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Defini t ion

The slow de stru ct ion o f a m et al licm ate rial by che m ical , e le ct roche m ical ,

or m eta llurgica l in te ract ion b et w ee n th een vironm en t and th e m ate r ia l is cal ledCorrosion.

e.g. Rust ing of i ron du e t o form at ion of

hydrat e d ferr ic oxide For m at ion o f gree n layer in Cu ve sse ls

du e to form at ion of basic cop pe r oxide s.Prepared by Course Coordinator -M r. Rahul M ehta 3


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M e h t a


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M etal lurgy (Reduct ion)

Ore + energy Pure M eta l

Corro sion (Oxidat ion )

e.g., rust ing of i ron w hen expo sed t o atm osph er ic con di t ions.Ru st is hydrat ed oxide (Fe2O3.xH2O).

Prepared by Course Coord inator-M r. Rahu l M ehta

Cause o f Corro sion

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Typ e s o f Corro sion

Dry (or) Chemical Corrosion

This type of Corrosion occurs mainly throughthe direct chemical action of atmosphericgases (such as O2, halogen, H2S,SO2,N2 ) toform corrosponding metal oxides, halides,sulphides,sulphates and nitrates respectively

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The at t ack of atm osph er ic oxygen on m etal form ing m etal oxide iscom m on ly called

Oxidat ion Corro sion.

This t ype of corro sion o ccurs in t he absence of m oist ure, i .e . w hyit is called

Dr y Corro sion.


O xidat ion Corrosion

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M echanism of Dry

Corrosion 2 M 2M n++2ne - (Oxidat ion)

nO2+2ne - 2nO2- (Reduction)

(Oxide ion ) 2M + n O

22 M n+ + 2nO2-

(M eta l ox ide)

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M eta l + Oxygen M eta l ox ide

N ature of Oxide F im :- If t he f i lm is,

STABLE, i t b ehaves has a prot ect ive coat ing in nat ur e e.g. , t heoxide f i lm s on Al,Pb,Cu,Pt etc. , are st able and t her efor efur t her ox idat ion cor rosion o f m eta l is proh ib i ted .

UNSTABLE t hat is t he ox ide layer form ed decom po ses backin to m eta l and oxygen.

So, oxidat io n Cor rosion is not po ssible.

M etal ox ide M etal + Oxygen

e.g., Ag, Au and Pt do no t un dergo oxidat ion Corro sion.



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VOLATILE t hat is oxide layer vo lat i l izes aft erfor m at ion and as such leaves t he und er ly ing m eta lsur face exposed fo r fu rt her at t ack.

This causes co nt inu ou s co rr o sion w hich is dan gero us

fo r the m etal e.g. M o lybdenum oxide (M oO3) POROUS t hat is t he ox ide layer form ed having pores

o r cracks.

In t his case t he at m ospher ic oxygen p asses t hr o ught he po res or cracks of t he un der ly ing m etal sur face.

This causes con t inuo us cor ro sion t i l l com pleteconversion o f m eta l in t o i ts oxide.

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W et Corrosion In t his case Co rr o sion occurs in pr esence o f air and

m oistu re th rou gh t he fo rm at ion o f e lect rochem ica lcel ls, i .e. w hy w et cor ro sion is a lso kn ow n asElect ro chem ical Cor ro sion .



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W et Corrosion This cor rosion occurs du e t o t he exist ence of separate anod ic

and cathodic par ts, be tw een w hich cur rent f low s th rough t hecon du ct ing solut ion.

At ano dic area, oxidat ion react ion o ccurs th ereby d est roy ing

th e anod ic m eta l e i ther by d isso lu t ion or fo rm at ion o fcompounds .

Hence cor rosion alw ays occurs at ano dic part s.

At Anode : M M

n+

+ n e - (O xid at io n )



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W et Corrosion At cat ho dic par t , redu ct ion react ion o ccurs.

At catho dic par t , t he d issolved con st i tu ent s in t he condu ct ingm edium accept s t he e lect ron s form ing ions (OH -, O2-).

The m eta ll ic ions form ed a t anod ic par t and t he no n m eta ll icions form ed a t cat hod ic par t d i f fuse to w ards each o t herth rough cond uct ing m edium and form a cor rosion prod uctsom ew here betw een anode and catho de .



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M e ch a nism o f W e t

Corrosion The cath od ic react ion consum es elect ro ns w ith

e i ther evo lut ion of hydrogen or absor pt ion o f oxygenw hich depend s on t he natur e of cor rosive

env i ronment .



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Evolut ion o f hydro ge n

This t ype o f corr osion occurs in acid ic m edium e.g. ,con siderin g th e m etal Fe, ano dic react ion is d issolu t ion

of i ro n as ferrou s ions w i th l iberat ion of e lect ron s. Fe Fe 2 + + 2 e - ( Oxidat ion )

The e lect ron s re leased f low t hro ugh t he m eta l f ro manode t o cat ho de, w hereas H+ions of acid ic solu t ion

are el im inat ed as hyd ro gen gas.

2H + + 2 e - H 2 ( Red uction )



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The overal l react ion is

Fe + 2 H + Fe 2 + + H 2

Th is t ype o f co rrosion causesd isp lacem ent o f hydrogen ions f rom t hesolut ion b y m eta l ions.

The ano des are large areas, w her eas cath o des are sm allareas.



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Absorp t ion of O xyge n

At anode : Fe Fe2+ + 2e - (Oxidat ion)

At cath ode :At cath ode :

The re leased e lect ron s f low f ro m anode t o cat ho deth rough iron m etal .

O2 + H2O + 2e- 2OH - (Reduction)

Fe2+ + 2OH - Fe(OH)2



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If oxygen is in excess, ferrous hydroxide iseasily oxidised to ferric hydroxide.

4Fe (OH)2 + O2 + 2H2O 4Fe (OH)3

The product called yellow rust corresponds toFe2O3. xH2O.

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D ifferent ial Aerat ion

CorrosionThis t ype o f corro sion is du e t o e lectro chem ical at t ackon t he m eta l sur face expo sed t o an e lect ro ly te o fvary in g con cent rat io ns or of var y ing aerat ion.

Dif feren t ial aerat ion cor rosion occurs w hen o ne parto f m eta l is expo sed t o d i f ferent a ir con cent rat ion f ro moth er pa r t .

Exper im enta l ly it has been o bserved t hat po oroxygenated p art s are ano dic.



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Differen tial Ae rat ion Corrosion

If a m etal e.g. , Zn is part ia lly im m ersed in a d i lut eso lut ion o f a n eut ral salt e.g., NaCl

th en t he par ts above and ad jacent t o t he w ater l ineare st ro ngly aerat ed and h ence becom e cath od ic.

W hereas part s im m ersed sho w a sm aller oxygencon cent rat ion and b ecom e ano dic.

Zn Zn 2+ + 2 e - (Oxidat ion)

O 2 + H 2O + 2 e- 2 O H - (Reduct ion)

Zn 2 + + 2OH - Zn(OH)2



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GalvanicCorrosion



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W hen t w o d issim i lar m eta ls are e lect r ica llycon nected and expo sed t o an e lect ro ly t e , t hem etal h igher in e lect rochem ical ser ies

un de rgoes cor ro sion , cal led Galvanic

corrosion.

e.g, Zinc (higher in elect ro ch em ical series)

form s t he anod e and is at t acked and gets

dissolved; w hereas cop per ( low er inelect ro ch em ical series) act s as cath o d e.



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M echa nism of Galvanic Corrosion

If t he solut ion is acid ic th en cor ro sion o ccurs byhydrogen evolut ion p rocess and i f t he so lu t ion isneu t ra l or sl ight ly a lkal ine in n atu re t hen cor ro sionoccurs by o xygen absor pt ion pr o cess.

Zn Zn 2+ + 2e - (Oxidat ion)

2H+ + 2e - H2 ( Redu ct ion )



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Hence, t h is can b e m inim ized by

1. Avoidin g galvanic co u ple

2.Provid in g an insulat ing m ater ia l bet w een

th e tw o m eta ls.



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W ate rline Corro sion

This is a lso kno w n as di f ferent ial oxygen concent rat io ncorrosion.

In general , w hen w ater is st or ed in a st eel tank, it isobserved tha t t he m ax im um am ount o f cor ro sion takesplace a lon g a l ine just beneath t he level o f t he w atermeniscus.

The area above t he w ater l ine (h ighly oxygenated ) act sas cath od ic and is no t af fect ed b y cor ro sion .



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PROTECTIO N FRO MCORROSION

It inc lud es coat in g of m eta l sur face to avoid d i rect con tact of a irw i th m etal . The t ypes coat in gs used fo r pro tect io n are:

1 . M et allic Co at in g

2 . N o n -M et allic Co at in g

3 . Or gan ic Co at in g

But before t he coat ing can be appl ied on t he m etal sur face, m eta lsur face sho uld b e prep ared. Sur face Prep arat ion s are im po rt antbecause i t en sur es un ifor m coat ing as w el l as t he coat in g is

f i rm ly at t ached .



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Surface Pre parat ionsThe var ious m eth od s used fo r p repar ing m etal sur face are

1. SOLVENT CLEANING

2. ALKALI CLEANING

3. ACID PICKLING

4. SAND-BLASTING5. M ECHANICAL CLEANING



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SO LV EN T CLEAN IN G :Solven t cleaning is used t o rem ove o i ls, greases, buf f in gcom po un ds and fat t y subst ances.

This involves in cleaning t he sur faces by t he ap pl icat ionof or ganic solvent s l ike t r ichloro et hylene,

te t rach loro ethy lene etc .This t reatm ent pro vides a m etal sur face, readi ly w ett edby aqueo us solu t ion , w hich is part icular ly requ ired forelectroplat ing.



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ALKALI CLEAN IN G:

Alkali clean ing age nt s are tr iso diumpho sphat e a long w ith soap s and w ett ing agent slike caust ic soda .

An alkal i t reat m ent is a lw ays to b e fol low edby a very thorough r insing w ith w ater and the nim m e rsion in a slight ly acidic solut ion o f 0 .1%

chrom ic acid or sod ium or po ta ssium chrom ateto rem ove th e last traces of a lkal is.



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SAND-BLASTING:The p ro cess con sist s in int ro du cing t he sand anabrasive int o an air st ream , un der pressure o f 25 t o 100atmospheres.

The b last is im pacted on t he m eta l sur face to be

cleaned.The sand -blast im pact r em oves any scale pr esent andalso causes a cert ain degree o f h arden ing of t he cleanedm etal sur face.



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M ECH AN ICAL CLEAN IN G:M echanical cleaning rem oves loo se rust and ot herim pur i t ies f rom t he sur face .

This is, gener al ly, do ne by h and cleanin g w it h abr ist le bru sh p lus so m e abrasive like sand pap ers



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General ly, t here are t w o t ypes o fm etall ic coat ings are p rot ect ion f romcorrosion.

1 . An od ic co at in gs

2 . Cat ho dic co at in gs

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AN O D IC CO ATIN GSAno dic coat ings are t ho se in w hich, t he m eta lused fo r i s m ore anod ic t han t he m eta l w h ich isto be p ro tected .

For exam ple, coat ing of A lum inum , Cadm iumand Zinc o n st eel su rface are ano dic, becauset he i r elect rode po t ent ials a re low er t han t ha t o ft he base m eta l iron .

Th erefo re sacri f icial ly, ano d ic co at ings p ro t ectt he und er ly ing base m eta l

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Galvanization

Galvanizat io n is a pro cess in w hich t he ir o n art icle ispro t ected f ro m corro sion by coat ing i t w i th a t h in layer o fzinc.

Then af t er, d ipped in t he b ath m ol t en zinc w hich is at425-4500C. So m et ho d used fo r Galvanizat io n isHOT D IPPIN G

W hen t he i ron p iece is taken ou t i t is coated w i th a th in

layer o f zinc.



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CATH O D IC CO ATIN GS

TINNING:The pr ocess of coat in g t in over t he i ro n o r st eelar t icles to pro tect i t f ro m cor rosion is know n ast inn ing.

Tin is a nob ler m eta l t han i ron , t herefore, i t is m oreresist ance to chem ical att ack. M eth od used is again Hot Dipp ing,A cleaned Ironsheet is passed t hro ugh a b ath m olt en f lux. Like Zincch lor ide , th en t h rough m ol ten t in and f inal ly th ro ugh

a suit able veget able oi l .



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CATHODIC COATINGS:

M ETAL CLAD IN GThe sur face t o b e pro t ect ed is sandw ichedbet w een t w o t h in layers o f coat m eta l and pressed

bet w een ro l lers.

The coat m eta l has t o b e anod ic t o t he base

m etal and on ly p la in sur faces can b e cladd ed.

Th is m et h od is used fo r coat ing Al, Cr, Ni ,

Duralum in ium , etc .



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ELECTOPLATING

Elect rop lat ing is t he e lect ro-depo si t ion of m eta l ,by m eans elect ro lysis over su rface of m etals, al loysor non -m eta ls.

In t his pr ocess

M etal t o be p ro tected i s m ade CATHODE

Anode is general ly o f t hat m eta l w ho se coat ing isto be done .

Bot h t he e lect rod es are d ipp ed in e lect ro ly tecon t ain in g sam e ion s , w ho se coat ing is t o b e

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ELECTOPLATING

Elect rop lat ing is t he d epo si t ion of a m eta l liccoat i ng on to th e su r face o f an ob ject by pu t t ing

a negat ive charge o nt o t he o b ject andim m ersing i t in t o a so lu t ion w hich cont a ins a

sal t o f t he m eta l t o b e deposi ted .

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M ETAL SPRAYIN G:

In t h is pro cess, t he coat in g m etal in t he

m ol t en st at e is sprayed o n t he ro ughened

sur face of t he b ase m eta l . The m eta l th ussprayed ad heres t o t he b ase m etal sur face.



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Sprayed -coat ings can beapp lied by t he fo llow ing 2

techniques.

1 . W ire -gun m et hod

2 . Po w d e r m e t a l m e t h o d



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1 . W ire -gu n m e th od

W ire-gun m ethod is m ore w ide lyused for com m on m eta ls. In t h is, the

w ire of the coat ing m eta l is m el tedby an oxy-acet y len e f lam e andato m ize d by a b last o f com pre sse d

air.


2 P d l


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2 . Po w d e r m e t alm e t h o d

In t h is, f ine ly d iv ided p ow dered m eta l is

sucked f rom t he pow der cham ber and t henheated as i t p asses t hro ugh t he f lam e of t he

b low-p ipe .The b low -p ipe d isin t egrat es t he m eta l in t o a

cloud o f m ol ten globu les, w h ich are t hen

adsorb ed o n t he b ase m eta l sur face.This m et ho d is, how ever, l im i ted t o low -

m elt ing m et als l ike Zn , Pb , Sn , et c.



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CEM EN TATIO N

Cem ent at ion is t he pr ocess in w hich base m etal ispacked in the po w dered coat ing m eta l and heated t o

the t em peratu re just be low the m e lt ing po in t .Result ing in t he fo rm at ion of layers of a lloys ofvary ing com po si t ion .

It is also suit able fo r coat ing sm all art icles o f

un even sur faces and shap es such as screw s, bo lt s,valves and gauge to o ls.



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Cat ho dic pro te ct ion In t h is m eth od , t he m eta l st ructu re can be prot ected

f rom corro sion by conn ect ing i t w i th w i re to a m oreanodic m eta l.

As t h is m or e act ive m etal is sacri f iced in t he pro cess ofsaving m etal f ro m cor ro sion , i t is kno w n as sacri f ic ialanode.

The m etals w hich are com m on ly used as sacri f ic ia lanod es are M g, Zn, Al and t heir a lloys.

The im po rt ant appl icat ions of t h is m eth od is t hepro tect ion o f un derground cables and p ipe lines f ro mso il cor ro sion .


11678 corrosion [compatibility mode] 2

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