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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