Corrosion

Question 0

define surface corrosion

Answer 0

-> caused by direct or indirect chemical attack

Question 1

Types of corrosion

Answer 1

• > chemical

- > galvanic

Question 2

define pitting corrosion

Answer 2

localised form of corrosion

• > hard to see becomes stress raiser
• > caused by chemicals or mechanical damage

Question 3

define Crevice corrosion

Answer 3

• > most common
• > caused by stagnant solution (micro-environmental)
• > occurs in fastener heads, thread etc
• > caused by absorption of oxygen into the liquid. surface more O2 creates cathode.

Question 4

How is crevice corrosion detected

Answer 4

detected by pillowing or opened joints

Question 5

define intergranular corrosion

Answer 5

also known as intercrystaline corrosion

only seen on aluminium alloy or stainless steel

Question 6

intergranular corrosion caused and detect by

Answer 6

caused due to incorrect manufacture or heat treatment

• > grain growth in the boundary area creates dissimilar metal corrosion
• > only detected with X-Rays or when item falls off

Question 7

define exfoliation corrosion

Answer 7

AKA laminar corrosion

• > only in extruded or rolled metals
• > mostly aluminium alloys
• > follows the grain

Question 8

Define Fretting Corrosion

Answer 8

• > corrosion takes place between two loaded surfaces

- > surfaces are moving

Question 9

detection and treatment of Fretting corrosion

Answer 9

• > recognised by cocoa powder

- > prevented by sealant

Question 10

Define Stress corrosion

Answer 10

• > only occurs with metal under tensile stress
• > the stress can be natural or created
• > cracks develop and lengthen quickly

Question 11

Define Filiform Corrosion

Answer 11

• > only occurs under grease, oil or varnish

- > can occur due to incorrect painting

Question 12

Define Mercury contamination

Answer 12

-> only occurs with mercury spillage

Question 13

how is mercury contamination removed

Answer 13

• > vacuum cleaner with mercury trap
• > syringe
• > damaged area cut out and repaired

Question 14

Define Microbial corrosion

Answer 14

-> hormoconis resinae ( cladosporium resinae)

Question 15

Define Hydrogen embrittlement

Answer 15

• > occurs in ferrous, aluminium and titanium alloys
• > must have sustained loads (UTS)
• > hydrogen must be present in the metal

Question 16

Define anodising

Answer 16

• > protection from corrosion
• > electrolyte bath
• > oxygen combines with the component corroding it a little but ultimately protecting it

Question 17

Define Chromic acid anodising

Answer 17

• > good corrosion resistance

- > coating is thin and relatively soft

Question 18

Define sulphuric acid anodising

Answer 18

-> various colours

Question 19

Define Chemical treatment

Answer 19

• > immersion or painting of a component with a specific chemical for a specific time
• > one type Alocrom
• > different chemicals produce different colours

Question 20

Define Pasivation

Answer 20

chemically passive layer

Question 21

what are the different types of metal coating

Answer 21

• > cladding
• > metal spray
• > electroplating

Question 22

Define cladding

Answer 22

-> sheet aluminium has 99.9% pure aluminium layer rolled onto both sides

Question 23

Define metal spray

Answer 23

-> a metallic coating of aluminium, zinc, tin, copper, lead, brass and bronze
-> two types , metallising = zinc or aluminium
Aluminising = aluminium alloys

Question 24

Define electroplating

Answer 24

• > carried out in a bath with an electrolyte, component and plating material
• > coating of codmium or zinc = less noble
• > coating of nickel or chromium = more noble

Question 25

Define partial plating

Answer 25

• > area not being plated has a wax applied onto it
• > and lacquer
• > masked with lead foil