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عوامل تخریب و زوال
قطعات و سازه هاي مهندسی
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Corrosionخوردگی
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تعريف خوردگي
خوردگي عبارتند است انهدام و فساد يا تغيير و دگرگوني در خواص و مشخصات مواد
به علت واکنش آنها با محيط اطراف( عمومأ فلزات)
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.......خوردگي
ممکنامادارندنالکتروشيمياييخوردگيمعموالهاسراميکمانندفلزيغيرمواد
.گيرندقرارشيمياييحملهتحتاست
تغييرودکننجذبآباستممکنشوند،ميتخريبخورندهمحلولباپليمرها
بعضيتخريبموجبنيزبنفشماورايپرتوواکسيژنهمزماناثردهند،ابعاد
.شودميپليمرها
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What is the….
Cost of Corrosion?
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The Cost of Corrosion
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• Cost:
--4 to 5% of the Gross National Product (GNP)*
--this amounts to just over $400 billion/yr**
THE COST OF CORROSION
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COST OF CORROSIONAnnual loss due to corrosion is estimated to be 3 to 5 % of GNP, about Rs.700000 crores
Direct & Indirect losses
Direct loss: Material cost, maintenance cost, over-design, use of costly material
Indirect losses: Plant shutdown & loss ofproduction, contamination of products, loss ofvaluable products due to leakage etc, liability inaccidents
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Engineer finds corrosion in collapsed bridge at North Carolina speedway (2000)
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اثرات خوردگي
کاهش استحکام
کاهش عمر کاري
تخريب خصوصيات مکانيکي
ضايعات فلز
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عوامل موثر بر واکنش هاي خوردگي
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واحد هاي اندازه گيري خوردگي
mm/y
mpy
ipy
gmd
mmd
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واحد هاي اندازه گيري خوردگي
mm/ympy
mpy =534 WDAT
Wmg
Dg/cm3
Ain2
Tsec
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5 mpy Good corrosion
resistant material
5 to 50 mpy Low corrosion
resistant material
50 mpy Unsuitable as
constructional
material
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Fundamental Components
Corrosion occurs because of the natural tendency for most metals to return to their natural state; e.g., iron in the presence of moist air will revert to its natural state, iron oxide.
4 required components in an electrochemical corrosion cell:1) An anode; 2) A cathode; 3) A conducting environment for ionicmovement (electrolyte); 4) An electrical connection between theanode and cathode for the flow of electron current.
If any of the above components is missing or disabled, theelectrochemical corrosion process will be stopped.
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واکنش هاي الکتروشيمايي
: واکنش هاي الکتروشيميايي
Zn Zn2 2e
eeFe 2F 2
2H 2e H2(gas)
O2 4H 4e 2H2O
O2 2H2O 4e 4(OH)
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• Two reactions are necessary:-- oxidation reaction:
-- reduction reaction:
Zn Zn2 2e
2H 2e H2(gas)
• Other reduction reactions in solutions with dissolved oxygen:
-- acidic solution -- neutral or basic solution
O2 4H 4e 2H2O
O2 2H2O 4e 4(OH)
Electrochemical Corrosion
Zinc
Oxidation reactionZn Zn2+
2e-Acid solution
reduction reaction
H+H+
H2(gas)
H+
H+
H+
H+
H+
flow of e-
in the metal
Corrosion of zinc in an acid solution
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Standard EMF Series
metalo
• Metal with smaller
V corrodes.• EMF series
Au
Cu
Pb
Sn
Ni
Co
Cd
Fe
Cr
Zn
Al
Mg
Na
K
+1.420 V
+0.340
- 0.126
- 0.136
- 0.250
- 0.277
- 0.403
- 0.440
- 0.744
- 0.763
- 1.662
- 2.363
- 2.714
- 2.924
metal Vmetalo
mo
re a
nodic
more
cath
odic
DV =
0.153V
o
-
1.0 M
Ni2+ solution
1.0 M
Cd2+ solution
+
25°C NiCd
Cdo
Nio
• Ex: Cd-Ni cell
V < V Cd corrodes
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دسته بندي خوردگي
ود همچنان الگوي مشخص و معيني براي اين دسته بندي وج.ندارد
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FORMS OF CORROSION
Corrosion may be classified in different ways
Wet / Aqueous corrosion & Dry Corrosion
Room Temperature/ High Temperature Corrosion
CORROSION
WET CORROSION
DRY CORROSION
CORROSION
ROOM TEMPERATURE
CORROSION
HIGH TEMPERATURE
CORROSION
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Differences between dry and wet corrosion
Dry corrosion• Corrosion occurs in the absence of moisture.
• It involves direct attack of chemicals onthe metal surface.• The process is slow.• Corrosion products are produced at thesite of corrosion.• The process of corrosion is uniform.
Wet corrosion• Corrosion occurs in presence of conductingmedium.• It involves formation of electrochemicalcells.• It is a rapid process.• Corrosion occurs at anode but rust is depositedat cathode.• It depends on the size of the anodic partof metal.
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• Uniform AttackOxidation & reduction
occur uniformly over
surface.
• Selective LeachingPreferred corrosion of
one element/constituent
(e.g., Zn from brass (Cu-Zn)).
• IntergranularCorrosion along
grain boundaries,
often where special
phases exist.
• Stress corrosionStress & corrosion
work together
at crack tips.
• GalvanicDissimilar metals are
physically joined. The
more anodic one
corrodes.(see Table
17.2) Zn & Mg
very anodic.
• Erosion-corrosionBreak down of passivating
layer by erosion (pipe
elbows).
• PittingDownward propagation
of small pits & holes.
• Crevice Between two
pieces of the same metal.
Rivet holes
FORMS OF CORROSION
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Uniform Corrosion: Rust!
Prevention:
استفاده از انواع پوشش ها
کاربرد مواد شيميايي کند کننده
محافظت کاتدي
محافظت آندي
تغيير نوع آلياژ
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Galvanic Corrosion
(فلز غير فعال)کاتد
(فلز فعال تر)آند
(هآب، خاک، محلول هادي الکتريسيت) الکتروليت
(تماس فيزيکي بين فلزات) مسير الکترون ها
Solutions:
تي گاهي يکي از اين روش ها و گاهي ترکيبي از آنها بايس.اجرا گردد
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خوردگي گالوانيک
.نقش مهمي در سرعت خوردگي دارد( کاتد و آند) در خوردگي گالوانيک نسبت مساحت ها
𝑰𝒄𝒂𝒕𝒉𝒐𝒅𝒆 = 𝑰𝒂𝒏𝒐𝒅𝒆
𝒊𝒄 ∗ 𝑨𝒄 = 𝒊𝒂 ∗ 𝑨𝒄
Pitting and Creviced Corrosion
Prevention:
Weld – don’t rivet
Use non-absorbing gaskets
Polish surfaces
Add drains – avoid stagnant water
Adjust composition; e.g., add Mo to SS
Causes: concentration gradients in electrolyte cause some areas high in ion concentrations that accelerate oxidation
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:شدت خوردگي حفر دار شدن با چند شاخص بيان مي گرددقطر حفره ها
حداکثر عمق حفره هاتعداد حفره ها در واحد سطح
Pitting Factor = Pmax/Pave
پسيوکسيدافيلمکهفلزاتيدر.افتدمياتفاقفلزاتازبسياريدرخوردگيايناکسيداليهپتانسيلايندر.گرددميآغازخاصيپتانسيلدرشدندارحفرهدارند،.آيدميپديدحفرهوشدهشکسته
نقاططحسدرشيمياييوساختاريهايآلياژ،ناهمگنيدرفلزيغيرهايناخالصي.استخوردگينوعاينشروع
خوردگي حفره اي
Intergranular Corrosion
يطمحدرودانهمرزراستايدرنمونهخوردگیآندرکهشدهایجادخاصیآلياژهايدر.دهدمیرخمشخصی
ناحيهلتشکيونزنزنگفوالدهايدرهادانهمرزدرکرومکاربيدرسوبمثالعنوانبه.کرومازعاري
Solutions:
دانهمرزدرکرومهايرسوبکردنحلمنظوربهباالدمايدرحراتیعمليات
ماستفاده از فوالدهاي زنگ نزن با حداقل کربن به منظور کاهش تشکيل کاربيد کرو
Niو Tiاستفاده از آلياژهایی با عناصر کاربيد زاي قوي مانند
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Erosion Corrosion
Causes:
پرتاب شدید سيال با مواد ساینده به سطح آلياژ
Commonly found in piping, propellers, turbine, valves and pumps
Solutions:
در سيالتغيير طراحی به منظور حداقل کردن اغتشاش•
تنداستفاده از آلياژهایی که مقاوم به فرسایش هس•
حذف ذرات ساینده از سيال•
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Selective Leaching
ماند.گرددميحذفمحيطدرعنصريکآلياژهاناحيهوشودميحذفرويآندرکهبرنجآلياژ
.گذاردميجايبهمتخلخلنيز ديده شده Al, Fe, Co, Crدر آلياژهاي پايه
.است
Solutions:
استفاده از پوشش محافظ•تغيير نوع آلياژ•
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Stress Corrosion
ترک در راستاي مرز دانه پيش می رود در اثر تنش اعمالی و یا تنش باقيمانده و یا گاز درون فلز
e.g., brasses are sensitive to ammonia
Stress levels may be very low
Solutions: Reduce stress levels
Heat treatment
Atmosphere control
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تردي هيدروژنیMetals loose strength when Hydrogen is absorbed through surface, especially along grain boundaries and dislocations
Often occurs as a result of decorative plating
High strength steels particularly susceptible
Can be removed by “heat treatment” the alloy
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Types of stress
◦ Residual stresses are due to the manufacturing processes that leave stresses in a material. Welding leaves residual stresses in the metals welded.
◦ Structural stresses are stresses produced in structural members because of the weights they support.
◦ Pressure stresses are stresses induced in vessels containing pressurized materials.
◦ Flow stresses occur when a mass of flowing fluid induces a dynamic pressure on a conduit wall. The force of the fluid striking the wall acts as the load.
◦ Thermal stresses exist whenever temperature gradients are present in a material. Different temperatures produce different expansions and subject materials to internal stress.
◦ Fatigue stresses are due to cyclic application of a stress. The stresses could be due to vibration or thermal cycling.
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Types of stressTensile stress is that type of stress in which the two sections of material on either side of a stress plane tend to pull apart or elongate.
Compressive stress is the reverse of tensile stress.
Shear stress exists when two parts of a material tend to slide across each other in any typical plane of shear upon application of force parallel to that plane.
LIQUID METAL EMBRITTLEMENT
Certain metals like Al and stainless steels undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb Cd etc.
Molten metal atoms penetrate the grain boundaries and fracture the metal
Fig. Shows brittle IG fracture in Al alloy by Pb
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CORROSION FATIGUE S-N DIAGRAM
Synergistic action of corrosion & cyclic stress. Both crack nucleation and propagation are accelerated by corrodent and the S-N diagram is shifted to the left
AirAir
CorrosionCorrosion
log (cycles to failure, Nf)
Str
ess
Am
pli
tud
e
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نماييد؟بررسيراهيدروژنيترديازناشيتخريبمکانيزم
راآنگيخوردمکانيزمونمودهشناساييراهستندخوردگيپديدهتحتعموماٌکهصنعتيقطعهدوحداقلنماييد؟ارائهپيشنهاداتيآنرفعجهتهمچنينکنيد،مطرح