corrosion in deep look
TRANSCRIPT
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• 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
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
Mechanism of corrosion
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
Mechanism of corrosion
• ) ( 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
Mechanism of corrosion
• In this case, The two metals are
different so, There is corrosion due to change of potential “E”
Mechanism of corrosion
• 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
Mechanism of corrosion
• 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
Mechanism of corrosion
السطح • على األوكسجين تركيز يكون الحًالة هذه فىالجزء ويكون جلفًانية خلية فيتكون العمق من أعلى
العمق فى الذى والجزء كًاثود السطح من anodeالقريب
Mechanism of corrosion
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
Mechanism of corrosion
• Primary distribution: There is random distribution of violet and blue color in the drop due to formation of Fe+2 Ion and OH- ion
Mechanism of corrosion
• 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)
Mechanism of corrosion
• 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)
Mechanism of corrosion
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
Mechanism of corrosion
• Active anode Ex: Fe,by increasing Potential the current Increase and corrosion propagated
Mechanism of corrosion
• Inert Anode: Ex: Pb, by increasing potential the metal didn’t corroded
and current still constant until O2 evaporated
Mechanism of corrosion
• 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
Mechanism of corrosion
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
Mechanism of 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
Mechanism of corrosion
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
Mechanism of corrosion
• 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 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.
Mechanism of corrosion
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
Mechanism of corrosion
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