chapter five week 14

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Diploma in Mechanical Engineering (Material)

JF 302 MATERIAL TECHNOLOGY 1

www.pis.edu.my

peneraju ilmu sejagat

Week 14


CLO 3 : Show various principles of material testing and

material defects.


Describe the corrosion process

Describe non-ferrous metals


definition types of corrosion corrosion control oxidation

Think critically 5.5

Compare the emf and galvanic series.




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 V metal o

more

anodic

m

ore

cath

odic

Platinum

Gold

Graphite

Titanium

Silver

316 Stainless Steel (passive)

Nickel (passive)

Copper

Nickel (active)

Tin

Lead

316 Stainless Steel (active)

Iron/Steel

Aluminum Alloys

Cadmium

Zinc

Magnesium

more

anodic

(active)

more

cath

odic

(inert

)

Galvanic series




List TWO (2) corrosion test methods.

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List TWO (2) corrosion test methods.

1. Polarization methods

2. Electrochemical impedance spectroscopy

(EIS)




Polarization methods.

1. Polarization methods:

a) Potential control

b) Linear polarization resistance





1. Objectives:

determine corrosion current density under steady-state conditions using polarization methods

2. Measuring corrosion rates using polarization methods

provide several advantages:

quick results high sensitivity non-destructive





a) Potential control

Potentiostat control

Potentiostat: RE

AUX

WE

Reference connection for

potential measurement

Connect electrodes to

corresponding terminals

on potentiostat





b) Linear polarization resistance Three electrodes:

i) conventional counter

ii) reference electrode

iii) working electrode

Provides lower solution resistance,

therefore is better for low conductivity and

more complex instrumentation.




Electrochemical impedance spectroscopy (EIS)

Advantages:

1. Useful on high resistance materials such as paints and

coatings

2. Time dependent data is available

3. Non- destructive

4. Quantitative data available

5. Use service environments

Disadvantages:

1. Expensive

2. Complex data analysis for quantification

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Electrochemical impedance spectroscopy (EIS)

Current phase shift due to impedance. Through a

capacitor this phase shift is 90o

Applied

Voltage

Resulting

Current



Methods of Corrosion Control

1. Barrier Protection

Provided by a protective coating that acts as a barrier between

corrosive elements and the metal substrate.

2. Cathodic Protection Employs protecting one metal by connecting it to another metal that is

more anodic, according to the galvanic series.

3. Corrosion Resistant Materials

Materials inherently resistant to corrosion in certain environments.



Barrier Protection paint

powder coatings

galvanizing

Steel section

Protective primer

Secondary coating

Decorative coating



Cathodic Protection impressed current

galvanic sacrificial anode

galvanic zinc application

zinc metallizing zinc-rich paints

hot-dip galvanizing



Impressed Current

External source of direct current power is connected (or impressed) between the structure to be protected and the ground bed (anode).

Ideal impressed current systems use ground bed material that can

discharge large amounts of current and yet still have a long life

expectancy.



Impressed Current

Figure shows the CP cable connection

across the insulation joint of the

pipeline.

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Galvanic Sacrificial Anode

Pieces of an active metal such as magnesium or zinc are placed in contact with the corrosive environment and are electrically connected

to the structure to be protected.



Galvanic Sacrificial Anode

Aluminium anodes mounted on a steel-

jacketed structure.



Galvanic Zinc Application zinc metallizing (plating)

Feeding zinc into a heated gun, where it is melted and sprayed on a

structure or part using combustion gases and/or auxiliary compressed

air.

zinc-rich paints

Zinc-rich paints contain various amounts of metallic zinc dust and are

applied by brush or spray to properly prepared steel.

hot-dip galvanizing

Complete immersion of steel into a kettle/vessel of molten zinc.



Hot-dip Galvanizing Process Surface Preparation

Galvanizing

Inspection



Hot-dip Galvanizing Process Surface Preparation

Degreasing

Removes dirt, oils, organic residue

Pickling

Removes mill scale and oxides

Fluxing

Mild cleaning, provides protective layer



Hot-dip Galvanizing Process Galvanizing

a) Steel articles are immersed in a bath of molten zinc ( 830 F) b) > 98% pure zinc, minor elements added for coating properties (Al, Bi, Ni)

c) Zinc reacts with iron in the steel to form galvanized coating.

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Hot-dip Galvanizing Process Inspection

a) Steel articles are inspected after galvanizing to verify conformance to

appropriate specs.

b) Surface defects easily identified through visual inspection.

c) Coating thickness verified through magnetic thickness gauge

readings.



Design of a Corrosion Protection System

Steel troughs are located in a field to provide drinking water for a herd of cattle. The

troughs frequently rust through and must be replaced. Design a system to prevent or

delay this problem.

SOLUTION

The troughs are likely a low-carbon, unalloyed steel containing ferrite and cementite, producing a

composition cell. The waterline in the trough, which is partially filled with water, provides a concentration

cell. The trough is also exposed to the environment, and the water is contaminated with impurities.

Consequently, corrosion of the unprotected steel tank is to be expected. Several approaches might be used

to prevent or delay corrosion. We might, for example, fabricate the trough using stainless steel or

aluminium. Either would provide better corrosion resistance than the plain carbon steel, but both are

considerably more expensive than the current material. We might suggest using cathodic protection; a

small magnesium anode could be attached to the inside of the trough. The anode corrodes sacrificially and

prevents corrosion of the steel. This would require that the farm operator regularly check the tank to be

sure that the anode is not completely consumed. We also want to be sure that magnesium ions introduced

into the water are not a health hazard. Another approach would be to protect the steel trough using a

suitable coating. Painting the steel (that is, introducing a protective polymer coating) or using a tinplated

steel provides protection as long as the coating is not disrupted. The most likely approach is to use a

galvanized steel, taking advantage of the protective coating and the sacrificial behaviour of the zinc.

Corrosion is very slow due to the large anode area, even if the coating is disrupted. Furthermore, the

galvanized steel is relatively inexpensive, readily available, and does not require frequent inspection.



Discuss the oxidation of steel.

chapter five week 14

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