6.4 Flashcards
Corrosion Definition
Metals returning to thermodynamically stable state
Anode
Attacked
Losses material
Cathode
Corrosive
Gains material
What is a neutral atom
Ion
Oxidation is
The combination of a Metal with Oxygen of Atmosphere
Oxide Film
Forms to protect against corrosion
What metals is Oxide film present in
Pure Aluminium
Titanium
Chromium
Oxidation Rate decreases as
Oxide film thickness increases
If Oxides react with Water
Increased Oxidation
Increases in temp Oxidation
Increases
Corrosion in Steel
Rust
Corrosion in Aluminium Alloy
White Powder
Corrosion in Magnesium Alloy
Grey Powder
Corrosion in Copper Alloys
Green Coating
Corrosion in Cadmium and Zinc
White Powder
Another Name for Galvanic Action
Electro Chemical
Dissimilar Metal Corrosion
Rate of Galvanic Action
Slow
Low Current
What is needed for Galvanic Action
2 Disimilar Metals
Electrolyte
Crack Formation in a corrosive environment
Stress
Stress is more common in
Alloys
Stress can lead to failure below UTS but especially in
Elevated Temp
Surface Corrosion
Direct Chemical Attack
EG: Battery Acid Or Waste
Pitting
Localised
Small holes act as stress risers
Caused via
Non Metallic lnclusions
Poor Coating
Damage
Crevice Corrosion
Aluminium Alloys
Localised
Bulges between fasteners
Due to changes in Local Chemistry of Crevice
Prevented via Primer and Sealent before Assy
Intergranular Corrosion
Aluminium Alloys and Stainless Steels
Cracks and Blisters
Due to Improper Heat Treatment or Heat In Aircraft
Must be scrapped
Laminar / Exfoliation Corrosion
Extruded or Rolled Parts
Extreme Intergranular
Layers of corrosion
Fretting Corrosion
2 Loaded Surfaces with Continuous Movement
Removes Coating
Causes Fatigue and Cracking
Fretting Corrosion in Steel
Cocoa Powder
Fretting Corrosion In Aluminium Alloys and Magnesium Alloys
Grey / Black Powder
What is a Anti Fretting Corrosion Agent
Molybdenum Disulphide Grease
Stress Corrosion
Caused by Tensile Stress in a Corrosion Environment
Cracks Lengthen Rapidly
Tensile Stress + Aluminium + _____
= Stress Corrosion
Hydrogen
Tensile Stress + High strength steel + _____
= Stress Corrosion
Hydrogen
Tensile Stress + Stainless + _____
= Stress Corrosion
Chlorides
Filiform Corrosion
Oil or Grease Films
Worm like
Often starts and rivets and spreads
Mercury Spillage
Aluminium Alloy
Forms a Amalgam
Soaks in and attacks grain boundaries
Aircraft cannot be moved
Cleaned
X Ray
How to Clean Mercury Spillage
Vacuum
Syringe
Nickel plated carbon fiber brush
Microbial Corrosion
Fuel tanks
Dormaint until water and temp
Feeds of hydrocarbons in fuel
Attaches and damages structure
Hydrogen Embrittlement in Ferrous Metals
Can occur at loads 30% of UTS
Hydrogen from Corrosion
Attaches to Surfaces under sustained load
Embrittlement Occurs
Harder the metal more likely for HE
Removal of Hydrogen Embrittlement in Ferrous
Bake Op at 191
Hydrogen Embrittlement in Titanium
Can be bonded
Room Temp; Acids
288 °C ; Steam Oil or Other Hydrogen
Stubstances
Hydrogen gathers at areas of high stress
HE reduces impact and tensile strength
Removal of Hydrogen Embrittlement in Ferrous
Relief Op at 524 °C for 6 hours
Corrosion Fatigue
Combination of Corrosion and Fatigue
Mechanical degradation due to cyclical loading and corrosion
Normal Fatigue
90% Nucleation 10% Propagation
Corrosion Fatigue
10% Nucleation 90% Propagation
Corrosion in Iron + Steel
Rust
Corrosion in Aluminium Alloys
White Grey Powder
Corrosion in Magnesium
White
Corrosion in Copper
Blue Green Patina
Corrosion in Chromium and Zinc
White
Corrosion in Nickel and Chromium
Blue Green
Nickel Plating is used for
Heat Resistance
Chromium Plating is used for
Wear resistance
What can improve Electrolyte Conductivity
Dirt
Salts
Acidic Gasses
Exhaust Gasses
What Battery Spillage is Acid Spillage
Lead Acid
To Neutralise Acidic Spillage
10% By weight Sodium Bicarbonate
What batteries can produce Alkaline Spillage
Nickel Cadmium (Patassium Hydroxide Spillage)
How to neutralise Alkaline Spillage
Neutralised by 5% by weight Chromic Acid Crystals
