corrosion in Deep look

Deep look to corrosion

By: ahmed ali

content• What is corrosion• Galvanic cell• Typical & convention current• Nernst equation• Mechanism of corrosion• Passive layer• Area Effect• Types of corrosion• Pitting corrosion• Crevice corrosion• Stray current corrosion• Intergranular corrosion• Intra- granular corrosion• Erosion corrosion• Corrosion Environment

corrosion

• Corrosion is the tendency of the metal to return to its ore

• Metal react with its environment and return to its original formula (cation) by oxidation

Mechanism of corrosion

• How corrosion occur to Iron (steel)?

By electrochemical

process :

Mechanism of corrosion• There are oxidation and reduction process

occur • The anodic oxidation and cathodic reduction are simultaneous على تحدث واإلختزال األكسدة عمليتى

) ( تزامن � تمًامًا التوازى• The rate of Oxi= rate of Redu=rate of corrosion• Anode : It’s the more active metal which have -ve

potential• Cathode : It’s the less active metal which have +ve

potential

galvanic cell

Typical & Conventional current flow

Conventional Current :current flows out of the positive terminal, through the circuit and into the negative terminal of the source. خالل البطًارية موجب من يسير التيًار أن افتراض

البطًارية لسًالب الدائرة typical Flow: is what actually happens and electrons flow out of the negative terminal, through the circuit and into the positive terminal of the source. من يسير الحقيقة فى التيًار

البطًارية لموجب السًالب

Mechanism of corrosion• galvanic cell

يترسب ثم محلوله فى للنحًاس وإختزال للزنك أكسدة هو يحدث مًاالكًاثود على النحًاس

الكًاثود وزن ويزداد اآلنود وزن يقل لذلك


هذا • فى الكًاثود على ترسيب يحدث ال الحًالة هذه فى ولكنالذائب لألوكسجين حدث اإلختزال ألن المتعًادل الوسط

ذلك من التأكد ويتم الوقت بمرور قًاعدى ليكون الوسط ويتجهللون Ph.Phبإضًافة المحلول Violetفيتلون

يحدث األوكسجين استنفًاذ وبعدللمًاء إختزال


• ) ( فى الوسط لإللكترواليت احتمًاالن هنًاك أصبحللـ � طبقًا اإلختزال pHحًالة

According to pH• In acidic medium pH<7

• In neutral or alkaline pH>7

In absence of O2

pH increase due to consumption

of H+

pH increase

pH increase

Mechanism of corrosion• Corrosion occur due to change in Potential according to Nernst equation,

Where E is potential of the cell, the more positive (cathode) and the more negative (anode)

E0 is standard potential

R is universal gas constant T is temperature

C is concentration


• In this case, The two metals are

different so, There is corrosion due to change of potential “E”


• In this case, the 2 electrodes are of the same metal but

T1>T2so,E1>E2

the half cell T2 is anode and the T1 is cathode


• In this case, the 2 electrodes are of the same metal but

C1>C2 so, E1>E2 the half cell C2 is anode

and the C1 is cathode

Mechanism of corrosion• Before adding of O2

there is no current

دفع بعد تيًار O2ولكن يمرتركيز إختالف O2نتيجة

لون عند Violetويتكونالـ إختزال نتيجة O2الكًاثود

تنعكس # الدفع اتجًاه بتغييرالعملية


السطح • على األوكسجين تركيز يكون الحًالة هذه فىالجزء ويكون جلفًانية خلية فيتكون العمق من أعلى

العمق فى الذى والجزء كًاثود السطح من anodeالقريب


المًاء • سطح عند الحديد تآكل معدل يكون الحًالة هذه فىالحديد سطح على األوكسجين توزيع انتظًام لعدم عًالى

والكًاثود اآلنود من العديد فتتكونقليل • األكسجين تركيز يكون العمق فى الموجود الحديد

أبطء التآكل معدل فيكون المعدن سطح على بإنتظًام وموزعالسطح على الذى من

Mechanism of corrosion• Let us Imagine what

happen when drop of water is on a clean Fe surface?

• We add 2 indicators to the water ph.ph & pot-ferricyanide and prepared in viscous medium

First, primary distribution 2nd, secondary distribution


• Primary distribution: There is random distribution of violet and blue color in the drop due to formation of Fe+2 Ion and OH- ion


• Secondary distribution: after 20min there is arrangement in the cell and conc of O2 in the edges(الحواف) (cathode) is higher than the central region (anode)


• After that there is rust formation in the cell has reddish-brown color, and the overall view is like this

Mechanism of corrosion• Rate of corrosion increase

due to formation of two galvanic cells:1/Iron(in the center) anode & Iron(in the edges) cathode 2/Iron(in the center) anode & Rust (cathode)

Mechanism of corrosion• Passive layer: a

spontaneous formed thin layer of metal oxide formed on the surface to protect metal from corrosion

• Iron: its porous brown film Fe2O3

• Aluminum: non-visible, very thin, rabidly formed(Al2O3)


Evans diagrampotentiodynamic polarizationit’s relation between Pot (E) and Current (i)

• It appears the Active anode& Inert anode & Passive Action

• Help to Know the potential at which there is corrosion


• Active anode Ex: Fe,by increasing Potential the current Increase and corrosion propagated


• Inert Anode: Ex: Pb, by increasing potential the metal didn’t corroded

and current still constant until O2 evaporated


• Passive Anode:in case of formation of passive layer on the metal, by increasing potential the current constant due to the passive layer until O2 evaporated


Area Effect• cathode area>>anode area

the rate of corrosion is higher

• Anode area>>cathode areathe rate of corrosion is lower

Types of corrosion and their mechanisms

• Pitting corrosion• Crevice corrosion• Stray current corrosion• Inter-granular corrosion• Intra-granular corrosion “Stress

corrosion cracking” SCC• Erosion corrosion


Pitting corrosion• Occur in surface which

have passive layer or protective coating

• Requirments:A.Passive layerB.Aggressive

anion,cl-,Br-,so4--C.Suitable condition


Steps of pitting1. Adsorption on passive layer (weakness of passive layer)2. Break down the passive layer3. Migration of aggressive anion to the metal and pitting

initiated.

Mechanism of corrosion• There is adsorption of Cl- on the

passive layer and the passive layer breaked down

• A galvanic cell formed between Fe metal & the passive layer

• The S.A of metal(anode) is very small so corrosion rate is very high

• There is hydrolysis to the corrosion product Fe++and give H+ which increase corrosion rate and prevent formation of passive layer


• Pitting potential: it is the potential at which there is corrosion due to pitting corrosion.

• At Epitt, the aggressive anion react with metal, at this point there is no evaluation of O2, but by increasing potential O2 evaluated

Mechanism of corrosion• Shapes of

pitting corrosion

Grades of pitting

according to ASTM G 46

Mechanism of corrosionCrevice corrosion :it’s kind of

corrosion occur in tight areasCrevices can be:• The gap between plates welded

together. قطع بين المسًافة أو الشقالملحومة الحديد

• The space under a sticky label.• The space between a gasket (الجوان)and

the metal in the man hole or Nozzle (especially if the gasket is absorbent).

• Any other tight gap.


Mech:• Oxy conc in crevice is smaller than bulk, this lead to formation of galvanic cell• Due to capillary action of crevice, there is hydrolysis of metal ion and H+ formed

so rate of corrosion increase• Area of crevice(anode) is smaller than bulk(cathode),so rate of corrosion increase• Conc of metal ion M+ in crevice is high so this facilitate migration of negative

aggressive anion from bulk to crevice and rate of corrosion increase• There are two(2) galvanic cell formed, first due to difference between oxi conc in

crevice and bulk. And the second between metal and rust formed from the corrosion so rate of corrosion is very high

Mechanism of corrosionStray current corrosion

• It’s corrosion from stray( (مشتتD.C current from railway systems or Other D.C source like induced current cathodic protection, These currents flow through steel structures or piping systems, causing corrosion.

• There are two galvanic cell formed:1) First Rail (anode) / Soil / Pipe (cathode)2) Second Pipe (anode) / Soil / Rail (cathode)

Mechanism of corrosionIntergranular Corrosion

“ intercrystalline corrosion” It’s type of corrosion take place

in alloy especially stainless steel {Fe+ Cr(>11%)+ C(0.05-1.5%)}

And corrosion attack the grain boundary in the steel and not appear on the metal surfaceso, it requires microstructure examination under a microscopy but it may recognized in the surface as in weld decay

Mechanism of corrosion• When heating stainless steal

there is reaction between carbon and chrome forming chromium carbides Cr23C6 so there is decreasing in conc. of Cr in the grain boundary compared with bulk so, there is galvanic cell formed and grain boundary is anode and the bulk is cathode and there is cracking in the boundary.

Mechanism of corrosion• Weld decay:

as there is high temp in welding process so, there is proper conditions to Intergranular Corrosion type to occur as the area which directly near to the weld (HAZ) “heat-affected zone” is anode and the bulk of metal is cathode

• To know more about weldment corrosion

Mechanism of corrosionPrevention of Intergranular Corrosion:• Use low carbon conc of stainless

steels (<0.05%)• Adding titanium (Ti ) or niobium

(Nb) . Titanium and niobium are strong carbide- formers. They react with the carbon instead of chromium to form the corresponding carbides preventing chromium depletion.

• Use post-weld heat treatment.

Mechanism of corrosionIntra-granular Corrosion “Stress corrosion cracking” SCC: it’s corrosion

obtained from tensile stress + corrosive environment.

• This type of corrosion may have intercrystalline corrosion and propagated to microscopic cracks in the material and corrosion increase by galvanic cell formation leading to complete cracking

Mechanism of corrosionErosion corrosion:it’s kind of corrosion obtained

from mechanical action of particles suspended in fast flowing liquid or gas or abrasion التأثير من ينتج

والغًازات للسوائل الميكًانيكىتآكل من أو األنًابيب فىخراطة فى كمًا المعًادنالتآكل نًاتج ويكون المعًادن “ “ وليس رايش المعدن نفس

وأكًاسيد صدأ

Corrosion Environment

1. Fresh water: contain oxygen which is oxidizing agent so should removed by

I. Adding scavengers like N2H4

II. By boiling

2. Sea water: contain Oxygen and salts which responsible for forming of galvanic cells and aggressive anions

3. micro-organisms: like fouling and anaerobic bacteria

Corrosion Environment

micro-organisms:A. Anaerobic bacteria

”SRB”(sulphate reducing bacteria )


Corrosion inside pipeline:due to Erosion corrosion type,and in case of waste water there is anaerobic bacteria which produce sulfuric acid and forming of biofilm which increase

rate of corrosion

Mechanism of corrosion• Protection of pipeline internal corrosion:

By Internal coating,

corrosion in Deep look

Education

Transcript of corrosion in Deep look