Crossion Flashcards
What is crossion
A chemical reaction
metal is converted into a metallic compound such as an oxide, hydroxide or sulphate
Corrosive attacks
The corrosion process involves two chemical changes. The metal that is attacked or oxidised undergoes an anodic change, whilst the corrosive agent is being reduced and undergoing a cathodic change.
Alclad
Highly resistance to chemical attacks
Steel corrosion
Red rust on the surface
Aluminium crosion
White / grey powder
Magnesium crosion
Grey powder with sever pitting
Copper crosion
Green powder product
How to stop copper crosion
Coated with nickel and tin
Fill form corrosion
Worm like Lead to intergranular Occurs in high humidity And when surface is slightly acid Removed using glass bead blasting material
What metals don’t corrode
Nobel metals
Gold
Platinum
What is a passive film
When a metal is tightly bound to the corroding metal that they form an invisible oxide film
Active metal
Lose electrons easily
Magnesium and aluminium
Corrode easily
Stop aluminium crosion
Anodised
Primed
Top coated with paint
Stop steel brass and bronze crosion
Cadmium plating
How to stop magnesium crosion
Chemical treatments and paint finishes
Electrochemical attraction
More commonly known as galvanic action, the essential ingredients for this
type of corrosion are two dissimilar metals in the presence of an electrolyte.
Microbiological corrosion
Microbial attack includes actions of bacteria, fungi or moulds
Stress corrosion
Stress corrosion is an intergranular cracking of the metal which is caused by a combination of stress and corrosion.
Stresses cause my internal or external loads
Influence factors of corrosion
Type of metal
Presence of a dissimilar, less corrodible metal (galvanic corrosion) Anode and cathode surface areas (in galvanic corrosion) Temperature
Heat treatment and grain direction
Presence of electrolytes (hard water, salt water, battery fluids etc.) Availability of oxygen
Presence of different concentrations of the same electrolyte Presence of biological organisms
Mechanical stress on the corroding metal
Time of exposure to a corrosive environment
What environment cause stress crosion
Salt solutions Cause aluminium
Methyl alcohol hydrochloric acid cause titanium
Air magnesium
Electrochemical reaction
When a chemical reaction is caused by an externally supplied current, as in electrolysis, or if an electrical current is produced by a spontaneous chemical reaction as in a battery
Pitting crosion
Most common for aluminium and magnesium White powdery Starts on surface extends vertically Decrease strength Starting point for intergranular corrosion
Intergranular corrosion
attack along the grain boundaries of the metal with little or no indication on the surface
High-strength aluminium alloys like 2024 and 7075 can develop intergranular corrosion if they have been improperly heat-treated and are then exposed to corrosive environments
Exfoliation corrosion
Advanced form of intergranular corrosion where the surface gains are lifted up by the force of expanding corrosion
Prone in wrought products
Getting corrosion
vibration cause the surfaces to rub together, resulting in an abrasive wear known as fretting.
The protective film on the metallic surfaces is removed by the rubbing action. The continued rubbing of protective oxide film exposes fresh active metal to the atmosphere.
Can cause server pitting
Black ring around faster heads
Microbiology corrosion
Fungl and mould
Aerobic or anergalnic corrosion
Can produce corrosive chemicals such as hydrogen sulfide Ammonia
Area must be completely removed
Crevice corrosion
This is corrosion of metal in a metal-to-metal joint, and corrosion at the edge of a joint. Sealing of joints and gaps and use of protective coatings in the fraying surface area can reduce crevice corrosion.
