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Department of Materials Engineering

Tutorial Week 7

ENG1050 & MCD4220 Engineering Materials

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The standard EMF 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 - 2.262 - 2.714 - 2.924

metal Vo Reduction Reactions

Cu2+ + 2e- Cu (V0 = 0.340V)

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Connect Cu and Fe in a standard cell. What happens? 1. What is the voltage? 2. Label the anode/cathode 3. Which direction do the electrons flow?

Electrode potentials: Standard EMF series

25C

Cu Fe

Cu2+ Fe2+1M 1M

Cu2+ + 2e- Cu Vo = 0.340V Fe2+ + 2e- Fe Vo = -0.440V Reverse Fe and add Cu2+ + Fe Fe2+ + Cu Vo=0.78V

ANODECATHODE

e-

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Galvanic Series

The Galvanic Series Platinum

Increasing inert(cathodic)

Increasingly active(anodic)

Gold Graphite Titanium Silver 316 Stainless steel (passive) 304 stainless steel (passive) Inconel (80Ni-13Cr-7Fe) (passive) Nickel (passive) Monel (70Ni-30Cu) Copper-nickel alloys Bronzes (Cu-Sn alloys) Copper Brasses (Cu-Zn alloys) Inconel (active) Nickel (active) Tin Lead 316 stainless steel (active) 304 stainless steel (active) cast iron iron and steel aluminium alloys cadmium commercially pure aluminium zinc magnesium and magnesium alloys

Metals within a bracket have similar potentials: if combined galvanic corrosion will be limited

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Galvanic Corrosion

Q. A copper pipe is connected to a stainless steel pipe in a hot water system and there is evidence of corrosion at the junction. Provide an explanation.

Two possibilities: 1) If stainless steel is in its passive state, then copper will corrode 2) If stainless steel is in its active state, then the stainless steel will corrode.

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Question

Would you expect iron to corrode in water of high purity ie. lacking oxygen and metal ions?

Why or why not?

Answer: No: needs other metal ions to complete the electrical circuit.

Also oxygen is needed for the reduction reaction ie. It accepts the electrons given up by the metal

Fe + 1/2O2 + H2O Fe2+ + 2OH- Fe(OH)2

2Fe(OH)2 +1/2O2 + H2O 2Fe(OH)3

Dont memorise these equations

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Galvanic Corrosion

The following pairs of alloys are coupled in seawater predict if corrosion is likely, and if so, identify the alloy that will corrode:

Tin & Magnesium Answer: magnesium will corrode Zinc & low carbon steel Answer: zinc will corrode Brass (60Cu-40Zn) and Monel (70Ni-30Cu) Answer: corrosion unlikely lie in the same bracket Titanium and 304 stainless steel Answer: stainless steel will corrode (both active and passive

states are below Ti Cast Iron and stainless steel Answer: cast iron will corrode (it is below both active and passive

states of stainless steel)

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Design

You are a materials engineer working for Airbus and you must select a metallic alloy to be used as a rivet for joining two aluminium sheets in the fuselage. In point form, discuss three (3) design considerations that will limit corrosion.

1) choose a metal alloy more inert to have a high ANODE-to-CATHODE surface area ie your dont want the rivet corroding!

2) electrically insulate using polymer spacer between the two dissimilar metals

3) prevent water from entering in between the Al sheets

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Question

A Cu hot water pipe is exhibiting extensive corrosion in its interior.

Describe a possible cause:

Crevice corrosion (crevice formed from a solid obstacle in the pipe). Concentration gradients of O2 or other ions.

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Corrosion Fundamentals

Q. What kind of corrosion occurs with an oxygen-concentration cell?

A. Localised anodic dissolution eg. under a bit of rust, or in a crevice formed from overlapping pieces of metal.

Q. What is galvanic corrosion?

A. Corrosion of a metal driven by electrical contact with a more noble metal eg. steel in contact with copper

Q. Why doesnt chromium rust?

A. The surface is passivated by an adherent oxide film (which is very thin and transparent. It is self-repairing if scratched off.

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Chromium Plating

chromium

Q. What happens if you scratch the surface of the chrome exposing the steel? Identify the Anode and Cathode and show the direction of the electrons.

A: A galvanic cell will form with the steel being the anode. Highly localised corrosion will occur. ANODE:CATHODE is small.

copper

nickel

anode

cathode

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H2O and O2

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Provide an explanation for this! Localised corrosion at flange of water pipe

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Crevice Corrosion

Low conc. O2

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List 7 Engineering Design Rules to Reduce Corrosion

1. When determining metal thickness for mechanical strength, allow for thinning from corrosion

2. Weld rather than rivet containers to reduce crevice corrosion. Choose a rivet more cathodic than the material.

3. Try and use galvanically similar materials. If bolting dissimilar metals separate them with non-metallic gaskets and washers.

4. Avoid sharp corners in pipes to prevent erosion corrosion. Avoid areas of high stress to reduce stress corrosion cracking.

5. Design tanks for easy drainage and cleaning 6. Design for easy removal of components expected to fail in service eg.

pumps

7. Design to avoid hot spots when heating systems are used.

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Mass loss from Corrosion

Q. If a sacrificial magnesium anode corrodes with an average current of 0.80 A for 100 days, what is the mass loss in this time period.

Assume MW(Mg)=24.31g/mol and n=2 (Mg2+), F = 96500C mol-1

Answer:

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Exercise E17.11 Ashby: Use CES Software

Pipe work is required for a food pickling plant to carry vinegar (a weak acid) at 100C from one vat to another. The liquid is under pressure, requiring a material with a strength of at least 100MPa, and for ease of installation it must be able to be bent, requiring a ductility of at least 10%. Find the four cheapest materials that meet the constraints summarised:

Function Pipework for hot acetic acid Constraints Durability in weak acid = very good Maximum operating temp >100C Yield strength > 100 MPa Elongation > 10% Objective Minimise material price Free variable Choice of material

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Stainless steel would certainly be the best choice