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The purpose of scientific thought is to predict what will happen in given experimental circumstances. All the philosophical discussion is an evasion of the point. – Feynman. Last class - More corrosion - micro cells Welding HAZ Sensitization Ol\' Harold\'s story. Today -

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Presentation Transcript
slide1

The purpose of scientific thought is to predict what will happen in given experimental circumstances. All the philosophical discussion is an evasion of the point.

– Feynman

slide2

Last class -

More corrosion - micro cells

Welding HAZ

Sensitization

Ol\' Harold\'s story

Today -

Suppressing corrosion

Corrosion examples

slide3

Suppressing corrosion

Components of a cell:

anode

cathode

electrolyte

connection

slide4

Suppressing corrosion

Occlude electrolyte - coatings

asphalt

greases, oils

paints

polymers

ceramic coatings

"noble" metals

slide5

Suppressing corrosion

Occlude electrolyte - coatings

"noble" metals:

unless scratch

heals, a galvanic

cell is set up

slide6

Suppressing corrosion

Cathodic protection

galvanizing:

Zn coating applied by

hot-dipping

electroplating

slide7

Suppressing corrosion

Cathodic protection

galvanizing:

presence of

scratch does not

promote corrosion

of protected metal

but steel helps Zn corrode

slide8

Suppressing corrosion

Cathodic protection

set up cell by adding

metal which is

more anodic than

metal or alloy to

be protected

slide9

Suppressing corrosion

Cathodic protection

example:

protecting ship plate

slide10

Suppressing corrosion

Cathodic protection

example:

protecting hot water heater tanks

slide11

Suppressing corrosion

Cathodic protection

Problem:

steel fermentation tank in brewery

sacrificial anodes add metallic ions to electrolyte

slide12

Suppressing corrosion

Cathodic protection

Different approach:

Impose a voltage across cell formed by tank (cathode) and an inert anode

choice of anode material:

very inert material like Pt

but oxygen gas produced at anode

slide13

Suppressing corrosion

Cathodic protection

Different approach:

Impose a voltage across cell formed by tank (cathode) and an inert anode

choice of anode material:

use carbon - inert

C + O2  CO2

slide14

Suppressing corrosion

Cathodic protection

"…..join in a planned discussion of whether it is best to use copper tubing coils or stainless steel tubing coils inside of a stainless steel beer fermenter, the coils being used as a chiller? There has been a discussion in another brewing forum about competing concerns between the two, i.e., copper conducts heat better and is easier to bend, versus copper will slowly dissolve in the acidic environment of beer"

slide15

Suppressing corrosion

Use passivated materials

Ni - NiO

Ti - TiO2

Al - Al2O3

COR-TEN:

0.19% C

0.30% Cu

0.40% Ni

0.50% Cr

0.05% V

slide16

Suppressing corrosion

Anodizing

Al & other materials can be anodized

very thick oxide coating formed (8-50 microns)

slide17

Suppressing corrosion

2300x

540x

Thinking again of noble metal coatings, what about Cr plating?

Cr plating usually filled w/ microcracks due to stresses in plating

slide18

Suppressing corrosion

Cr plating frequently a series of layers

slide24

Corrosion area effect

Area effect:

large cathode & small anode -

corrosion accelerated

electrochemical cell current density high

try to keep potential anodic areas large relative to cathode

slide25

Polymers - the nightmare begins

I am inclined to think that the development of polymerization is, perhaps, the biggest thing that chemistry has done, where

it has the biggest effect on

everyday life

LORD TODD

President of the Royal

Society of London

slide26

What are polymers?

Why important ?

  • Long chain molecules
  • Extraordinary range of
  • physical properties
  • Many (not all) are cheap
  • Low densities
slide28

Assignment:

Review today\'s classnotes

a. coatings

b. cathodic protection

c. passivation

d. anodizing

e. Cr plating

f. corrosion examples

g. area effect

h. what is a polymer?

Read: pp 569-585, 589-591, chapter 14

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