week 6 to 10 lectures corrosion

7/26/2019 Week 6 to 10 Lectures Corrosion

http://slidepdf.com/reader/full/week-6-to-10-lectures-corrosion 1/54

MSE 424 Corrosion and Materials Protection(Room 323, Block 3)Week-6-10 Lectures

Abdul Wadood (PhD), Assistant Professor

Department of Materials Science and

EngineeringRoom# 211 (MSE)

Visiting Hours: 13:00 – 15:00

Attendance: 80% as per Institute policy

1



• Grading Policy:

Quizes: 5 (15%); Next Quiz = Wed; 9/3/2016

Presentation: 1 (5%)

Assignment: 1 (3%)

3 OHTs: 27%

Final: 50%

MSE-424 Corrosion and Materials Protection

2



• Presentation:

• Select some incident due to corrosion.

Find out the literature survey/investigationreport related to this incident. Remedies toovoid such incidents.

• Your own comments/critical analysis isalso required.

Presentations in Week-8



Reference Book



Text Book



• The most important rule is that the designer,fabricator, and galvanizer should work

together before the product is manufactured.

• This three-way communication can

eliminate most galvanizing problems.

• The designer can better appreciate hot dip

galvanizing design requirements if the basicsteps of the galvanizing process areunderstood.



Zero Resistance Ammeter (ZRA)

A zero resistance ammeter (ZRA) is a current to voltage converter

that produces a voltage output proportional to the current flowing

between its input terminals while imposing a 'zero' voltage drop tothe external circuit.

In corrosion test a ZRA is typically used to measure the galvanic

coupling current between two dissimilar electrodes.

An interesting application is when the coupling current between

two nominally identical electrodes is measured. If both electrodes

were identical then very little coupling current would flow.

In real situations these electrodes will be slightly different, one

being more anodic or cathodic than the other and a small coupling

current will exist.



• Increasing the total reduction rate increases the rate of

corrosion. Thus, it is possible to reduce corrosion byreducing the rate of either reaction:- removing oxidizer- coating the surface of metal with paint or non-conducting film.

- addition of corrosion inhibitors

Anodic reactions in corrosion : • Metal Corrosion : M M+n + ne-

• Ferrous ion oxidation : Fe2+ Fe3+ + e-

• Oxygen evolution : 2H2O O2+ 4H+ + 4e-

Cathodic reactions in corrosion :• hydrogen evolution in acid sol. : 2H+ + 2e H2

• oxygen reduction in acid sol. : O2 + 4H+ + 4e 2H2O • oxygen reduction in neutral or basic sol. : O2+ 2H2O +4e 4OH-

• metal ion reduction : M+3 + e M+2

• metal deposition : M+ + e M



-- Cost: 4 to 5% of the Gross National Product (GNP)*-- this amounts to just over $400 billion/yr**

* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to

Corrosion Science and Engineering , 3rd ed., John Wiley and Sons, Inc., 1985.

**Economic Report of the President (1998).



• The term 'polarization' derives from the early 19th-century

discovery that electrolysis causes the elements in an electrolyte to

be attracted towards one or the other pole.

• Thus, initially 'polarization' was a description of electrolysis

itself.

• In time, as more electrochemical processes were invented, theterm 'polarization' evolved to denote any (potentially

undesirable/desireable) side-effects that occur at the interface

between electrolyte and electrodes.

• Polarization isolate the electrode from the electrolyte,

impeding/delaying reaction and charge transfer between the two.



• In electrochemistry, polarization is a collective term forcertain mechanical side-effects (of an electrochemicalprocess) by which isolating barriers develop at the

interface between electrode and electrolyte.

• These side-effects influence the reaction mechanisms, aswell as the chemical kinetics of corrosion and metal

deposition.

• These mechanical side-effects are:

• activation polarization: the accumulation of gasses (or

other products) at the interface between electrode andelectrolyte.

• concentration polarization: uneven depletion of reagents inthe electrolyte cause concentration gradients in boundary

layers.



Passivity



PresentationMarks Distribution

1. Team Work 20%

2. Formal dress on presentation day 10%

3. Literature Survey/Reports about Incident 20%

4. Technical Discussion15%

5. Critical Analysis 15%

6. Time Management (In time Presentation)10%

7. Answers to Questions10%

Total Marks 100

31



P t ti



• Presentation:• Select some incident due to corrosion. Find

out the literature survey/investigation report

related to this incident. Remedies to ovoidsuch incidents.

• Your own comments/critical analysis is also

required.

Presentations in Week-8



Reference Book



Text Book



• The Pilling –Bedworth ratio (P –B ratio), in corrosion ofmetals, is the ratio of the volume of the metal oxide to thevolume of the corresponding metal (from which the oxide is

created).• On the basis of the P –B ratio, it can be judged if the metal

is likely to passivate in dry air by creation of a protectiveoxide layer.

N B Pilling and R E Bedworth suggested in 1923 that metals can be classed into two categories:



• N.B. Pilling and R.E. Bedworth suggested in 1923 that metals can be classed into two categories:those that form protective oxides, and those that cannot.

• They ascribed the protectiveness of the oxide to the volume the oxide takes in comparison to thevolume of the metal used to produce this oxide in a corrosion process in dry air.

• The oxide layer would be un-protective if the ratio is less than unity because the film that forms onthe metal surface is porous and/or cracked.

• Conversely, the metals with the ratio higher than 1 tend to be protective because they form aneffective barrier that prevents the gas from further oxidizing the metal.

• RPB < 1: the oxide coating layer is too thin, likely broken and provides no protective effect (forexample magnesium)

• RPB > 2: the oxide coating chips off and provides no protective effect (example iron)• 1 < RPB < 2: the oxide coating is passivating and provides a protecting effect against further

surface oxidation (examples aluminium, titanium, chromium-containing steels).• However, the exceptions to the above P-B ratio rules are numerous. Many of the exceptions can

be attributed to the mechanism of the oxide growth: the underlying assumption in the P-B ratio isthat oxygen needs to diffuse through the oxide layer to the metal surface; in reality, it is often themetal ion that diffuses to the air-oxide interface.



Metal Metal oxide RPB

Zinc Zinc oxide 1.58

Calcium Calcium oxide 0.64

Magnesium Magnesium oxide 0.81

Aluminium Aluminium oxide 1.28

Lead Lead(II) oxide 1.28

Platinum Platinum(II) oxide 1.56

Zirconium Zirconium(IV) oxide 1.56

Hafnium Hafnium(IV) oxide 1.62Nickel Nickel(II) oxide 1.65

Iron Iron(II) oxide 1.7

Titanium Titanium(IV) oxide 1.73

Chromium Chromium(III) oxide 2.07

Iron Iron(II,III) oxide 2.10Iron Iron(III) oxide 2.14

Silicon Silicon dioxide 2.15

Tantalum Tantalum(V) oxide 2.47

Vanadium Vanadium(V) oxide 3.25



Selective leaching• Removal of one element from a solid alloy by corrosion process is called

selective leaching, e.g. dezincification, graphitizing, de-aluminizing, de-cobaltification etc.

• Dezincification – Selective removal of zinc from brass, leaving a red copper color appearance

is an example of dezincification. – Dezincification can be either uniform (<15% Zn) and localized (low brasses) – Uniform attacked is common in acidic environments while localized is found

in neutral and alkaline environments (preferably under stagnant conditions)

• Mechanism: – Zinc dissolve and cause vacancy in lattice structure – Under deficient condition of oxygen in a corrosion system

(metal/environment), there is race to get oxygen. More reactive species willact preferentially

• Prevention

– Reducing environment aggressiveness (oxygen removing) – Cathodic protection is economical – Selecting proper material for certain environment – Alloying additions (Sn, As etc.)



How can galvanic cells form?

Anodic/cathodic phases at themicrostructural level

Differences in the concentration of the

Metal ion

Anodic/cathodic electrodes

Differences in the concentration of

oxygen

Difference in the residual stress levels



Different phases (even of the same metal) can form a galvanic couple at the

microstructural level (In steel Cementite is noble as compared to Ferrite)

Galvanic cell may be set up due to concentration differences of the metal ion in the

electrolyte A concentration cell Metal ion deficient anodic

Metal ion excess cathodic

A concentration cell can form due to differences in oxygen concentration

Oxygen deficient region anodic

Oxygen rich region cathodic

A galvanic cell can form due to different residual stresses in the same metal

Stressed region more active anodic

Stress free region cathodic

O2 + 2H 2O + 4e

4OH



• De-aluminization is a type of corrosion that consists of theselective loss of aluminum in aluminum bronzes (Cu-Sn)and nickel-aluminum bronzes.

• Adverse effects on the ultimate tensile strength ofcomponents

• Potential for non-ductile failure

• Loss of leak tightness

• It is a concern for nuclear power generation facilities thatutilize aluminum-bronze alloy piping and pipingcomponents for the transportation.

• Some aluminum alloys are susceptible to this form ofcorrosion, particularly in seawater.



De-cobaltification



De-cobaltification

• De-cobaltification is a corrosion process in which cobalt isselectively leached from cobalt-base alloys.

• It creates problems in the tooling/machining industrieswhere cobalt is leached by many of the amino alcoholsand amine-based additives found in almost all water-

miscible machining fluids. Cobalt leaching is hazardous tocarbide tools as well.

De-cobaltification problems:

• Reduced performance and life of the tool• Health problems in some workers, causing dermatitis andrespiratory problems

• Disposal of contaminated wastewater

MICROSTRUCTURAL



MICROSTRUCTURAL

EFFECTS A mechanically deformed metal or alloy

can experience galvanic corrosion due to

differences in atomic plane distortion and a

high dislocation density

I h t t t t



• Improper heat treatment can causenonuniform microstructure and therefore,galvanic-phase corrosion is enhanced in

corrosive media.

• Galvanic corrosion can occur in a

polycrystalline alloys, such as pearlitic steels,due to differences in microstructural phases.

• Pearlite consists of ferrite and cementite -----galvanic microcells between ferrite (anode )and cementite (cathode) are generated.

Standard emf series



5

• EMF series • Metal with smaller

V corrodes.• Ex: Cd-Ni cellmetal

o

Au

Cu

Pb

Sn

NiCo

Cd

Fe

Cr

ZnAl

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

- 2.262

- 2.714

- 2.924

metal Vmetalo

DV =

0.153V

o

Standard emf series



• Ranks the reactivity of metals/alloys in seawater

Platinum

Gold

Graphite

Titanium

Silver

316 Stainless SteelNickel (passive)

Copper

Nickel (active)

Tin

Lead

316 Stainless SteelIron/Steel

Aluminum Alloys

Cadmium

Zinc

Magnesium

8

Galvanic series



Inconel (super alloy): an alloy of nickelcontaining chromium and iron, resistantto corrosion at high temperatures

Galvanic Cell e flow



Anode

Zn(0.76)

Cathode

Cu(+0.34)

f

Zn Zn2+ + 2e

oxidation

Cu2+ + 2e Cu

Reduction

Zn will corrode at the expense of Cu

week 6 to 10 lectures corrosion

Documents