Corrosion Flashcards
What is Corrosion?
Corrosion is the destructive and unintentional degradation of a material caused by its environment.
What are the effects of corrosion?
- Reduces strength
- Decreases operational life
- Material properties are lost
- Wastage of metal
- Contamination of fluids in
vessel and pipes
Name 3 common corrosion cells and define them.
- Galvanic corrosion cells
Dissimilar anodic/cathodic metals in an electrolyte - Concentration cells
Same metals in dissimilar conditions in a common electrolyte - Electrolytic cells
Formed when an external current is introduced into the system. This is often created as a means of protecting the structure - anodic protection
Name common corrosion mechanisms. (7 of them)
- Corrosion fatigue
- Stress corrosion cracking (SCC)
- Pitting corrosion
- Galvanic corrosion
- Caustic attack
- Intergranular corrosion
- Hyrogen embrittlement
What are the two types of protection coatings? Describe them
- Anodic coatings
Coating acts as a sacrificial anode. Galvanising is an example of anodic coatings,
where a layer of zinc is coated on steel/iron to provide sacrificial protection. Zinc
corrodes in preference to the steel/iron, even when the zinc is scratched.
- Cathodic coatings
Coating acts as a cathode, preventing corrosion because the metal is more
corrosion resistant. This coating is effective until the surface is broken and the
substrate then becomes the anode and corrodes. Example: tin coating on steel
cans
Describe an Electrochemical method to corrosion resistance.
Sacraficial anode
An anode is attached to the metal that is to be
protected and corrodes instead of this metal
The sacrificial anode is typically made of zinc
or magnesium and is periodically replaced
Do metals corrode faster at:
a. Higher temperature
b. Lower temperature
c. Environments with pH in the range of 3–5
a. Higher temperature
The effectivness of the oxide layer in protecting the the metal from oxidation depends on what.
The continuity of the oxide film.
How well the oxide film is bonded to the metal.
The conductivity of ions through the oxide film.
What should you do to avoid galvanic corrosion?
Choose metals that are close in the galvanic series and insulate dissimilar metals.
What does the corrosion rate depend on?
The temperature, chloride content and oxygen availability.
Why does chromium in stainless steels make them more corrosion resistant in
many environments than plain carbon steels?
The chromium in stainless steels causes a very thin and highly adherent surface coating to form over the surface of the alloy, which protects it from further corrosion. For plain carbon steels, rust, instead of this adherent coating, forms.
Briefly describe the two techniques that are used for galvanic protection.
i. A sacrificial anode is electrically coupled to the metal piece to be protected; this anode is also situated in the corrosion environment. The sacrificial anode is a metal or alloy that is chemically more reactive in the particular environment. It preferentially oxidises, and, upon giving up electrons to the other metal, protects it from electrochemical corrosion.
ii. An impressed current from an external dc power source provides excess electrons to the metallic structure to be protected.
Describe intergranular corrosion.
chemical segregation effects on the grain boundaries. Precipitation can produce zones of reduced
corrosion resistance in the immediate vicinity.
A classic example is the sensitisation of stainless steels
Chromium-rich grain boundary precipitates lead to a
local depletion of Cr immediately adjacent to these
precipitates, leaving these areas vulnerable to
corrosive attack in certain electrolytes.
Reheating a welded component for stress
relieving is a common cause of this problem
Solutions:
High temperature heat treat to re-dissolve carbides
Lower carbon content (in stainless steels) to minimise carbide formation
Alloy with an element of higher affinity for carbide formation (i.e. Ti or Nb)
What is Hydrogen Embrittlement?
Takes place during the plastic deformation of alloys in contact with hydrogen gas and is strain rate dependent.
Alloys like ferritic steels, nickel and titanium-based exhibit highest degradation where hydrogen pressure is high and strain rate is low.
Mechanism relates to interaction between atomic hydrogen and internal defects in the metal - hydrogen becomes trapped in the defects where they eventually interact to cause reduced ductility and tendency for cracking.
What is Stress corrosion cracking (SCC)
Defined as the cracking of metals under the combined effects of stress and corrosion, e.g. caustic cracking.
The cracks are branched and propagate intergranularly and/or intragranularly.
Stress involved may be applied or residual; localised corrosion on the surface acts as a stress raiser which initiates the crack.
Austenitic stainless steel are particularly sensitive to SCC in chloride solutions so ridgid control of water chemistry is essential.
Solutions:
Reduce stress levels
Heat treatment
Atmospheric control
