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Forms of Corrosion (Part 2) - Coggle Diagram
Forms of Corrosion (Part 2)
Localised Corrosion
7 Forms
Environmentally induced cracking
Hydrogen induced cracking
:check:Brittle failure caused by penetration and diffusion
of atomic hydrogen into the crystal structure of a metal
:check:Atomic hydrogen may be produced from corrosion of steel
in acidic, non oxygenated environments
Theories
Pressure theory
Part of the diffusing atomic hydrogen gets trapped into microscopic voids around non-metallic inclusions in the metal, thus allowing atomic hydrogen to recombine and form hydrogen gas
Hydrogen gas pressure buildup generates very high localized stresses which initiates cracking along lines of weakness in the steel, leading to brittle fracture
Decohesion theory
Decohesion theory states that dissolved hydrogen migrates into a triaxially stressed region and embrittles the lattice by lowering the cohesive strength between metal atoms
Surface energy theory
This theory states that the absorption of hydrogen decreases the surface free energy of the metal, enhancing propagation of the crack tip
The theory explain the crack propagation of highstrength steels in low-pressure hydrogen environments
Prevention
Reducing the residual (internal) stresses
reversed by heat treatment
Minimising hydrogen content
Corrosion fatigue
Forms of Corrosion Fatigue
Initiation of small crack
Propagation of the crack, leading to failure
Formation of slip bands leading to intrusions and extrusions
Prevention
Cathodic protection
Inhibitors
Select a more resistant material
Stress corrosion cracking
brittle failure of a metal under the combined effects of a static
tensile stress and a specific chemical environmen
Proceed in 2 ways
May run through the individual grains
Cracks may propagate along the grain boundaries
Mechanism
Film – induced cleavage
:check:The sides and tip of the crack are covered by a brittle film
:check:The crack growing in the brittle film may propagate further into the metal
:check:Anodic dissolution and corrosion are not necessary to propagate the
crack, but only to form the required brittle surface film
:check:The crack is then blunted by the plastic deformation
:check:For the crack to grow further, the surface film must reform at the crack
tip surface
Hydrogen embrittlement
:check:H2 atoms produced by the cathodic reaction diffuse to
regions of tri-axial stress at the crack tip
:check:These H2 atoms weaken the inter-atomic bonds at the
crack tip
Source of Hydrogen
Electroplating
Corrosion
Welding
Contact with gaseous hydrogen
Anodic dissolution
If the stress absence, the metal is unreactive to the environment because of the existing of a protective passive film at the crack tip
In the presence of stress, it leads to failure of this passive film
by plastic strain and active corrosion occurs
If the repassivation rate is low
The crack widens and blunts leading to slow crack growth rates
Excessive metal dissolution can occur at the crack tip and crack
sides
If the repassivation rate is rapid
It will lead to slow propagation rates because the protective film is
restored fast and no metal dissolution occurs
Testing methods
Slow strain rate test
:check:Subject tensile test specimens to constant strain rates while
immersed in a test solution (susceptible solution) until failure
:check:Measure the % reduction in area or elongation
:check:Examine the fracture surface
Fracture Mechanics
:check:Subject a pre-cracked specimen to a constant load
:check:Measure the crack growth rate
Prevention
Avoid the necessary environment
Apply electrochemical protection where possible
Remove stress
Use a different material
Erosion-corrosion/Flow induced Corrosion
Type of flow
Cavitation corrosion
:CHECK:caused by formation and collapse of bubbles of vapour.
:CHECK:Vapour bubbles form because of pressure changes (which falls < 0) across surfaces exposed to high velocity liquid flow
:CHECK:When the pressure increases again the collapse of the vapour bubbles creates an intense shockwave that removes metal or oxide from the metal surface
Prevention
Careful design to minimise pressure drops across the metal surface
Optimum material selection
Environmental modifications:
:check:Remove abrasive particle
:check:Remove air
Cathodic protection
Use protective coating
Erosion-corrosion
:check:Erosion - corrosion attack in metals occurs due to the
relative motion of a corrosive fluid and a metal surface
:check:Corrosion is accelerated by impact of solid particles
:check:These particles may remove metal, or they may just
remove oxide and allow metal to corrode more quickly
Prevention
Design
Abrupt changes in flow direction should be avoided
Designs creating turbulence, flow restrictions and obstructions
are undesirable
Rough surfaces
increased pipe diameters
Materials selection
Fretting corrosion
:check:damage that occurs at the
asperities of contact metal surfaces
:check:occurs at the interface of two highly stressed surfaces in
the presence of repeated relative surface motion
:check:The most common type of fretting is caused by vibration
Minimised
design the parts
to exhibit less or relative motion
Use of lubrication on contacting
surfaces
Use insulation material between
surfaces
Reduce load between surfaces
Select a more resistant material
Flow can...
Increase the rate of dissolution of corrosion product films
Mechanically remove oxides
Increase transport of oxygen to the metal surface
Pitting corrosion
(Part 1)
Intergranular corrosion
:check:Microstructure of metals and alloys is made up of grains
:check:associated with chemical segregation effects
:check:associated with Precipitate of compounds
Prevention
Use a stabilised grade of SS, which contain strong carbide forming elements such as Nb (type 347 SS) or Ti (type 321 SS)
Heat treatment to redissolve the carbides (post welding heat treatment)
Use low carbon content
Weak corrosive conditions does not effect IGC
Low acidity (high pH) will generally reduce the susceptibility to IGC
Exfoliation Corrosion
-IGC that Found in Alloy
Prevention
Removal of atmospheric pollutants such as H2S and
SO2
Selection of resistant but lower strength Al alloys
Heat treatment
Crevice corrosion
(Part 1)
Selective leaching(Dealloying)
:check:The removal of one element from the alloy to
the electrolyte
:check:The entire exposed surface of the metal may be attacked
:check:The cause of dealloying is a “galvanic effect”
Example
Dezincification
:check:Removal of zinc in chloride water from brass
:check:occurs in two-phase α + β alloys especially in
the high Zn β phase
:check:Removal of Zn leaves behind a porous
Mechanism
The more active alloying element, zinc,selectively dissolves or “leaches” out of the brass leaving behind a porous, weak copper structure
Both Zn and Cu dissolve, and the more noble Cu than
redeposits as a porous layer
Prevention
Select low-zinc red brasses (<15%) which is generally
immune to dezincification
Sn addition to (α + β) brasses which inhibit attack in the αphase in the marine environment.
Use cupronickles
Addition of P, As or Sb in single phase α admiralty brass
Graphitic corrosion of grey cast
iron
:check:occurs exclusively in grey cast iron which has a
continuous graphite network in its microstructure
:check:The graphite act as cathode accelerates anodic dissolution of nearby iron, leaving behind the graphite network
:check:observed in buried cast iron pipe after many years
exposure in soil
Prevention
Coating
Cathodic protection either by ICCP or sacrificial anodes
Use more resistant material such as other grades of cast
iron
Galvanic or two-metal corrosion
(Part 1)
