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forms of corrosion part 2 - Coggle Diagram
forms of corrosion part 2
INTERGRANULAR CORROSION
Intergranular corrosion is a localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected
Microstructure of metals and alloys is made up of grains
Exfoliation
Corrosion
Corrosion products building up along the grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect
Prevention of Exfoliation Corrosion (IGC)
Removal of atmospheric pollutants
Selection of resistant but lower strength Al alloys
Heat treatment.
form of IGC found in high strength aluminium alloys
Prevention of IGC/Sensitisation of SS/Weld Decay
Heat treatment to redissolve the carbides
Weak corrosive conditions
Use a stabilised grade of SS, which contain strong carbide forming elements such as Nb
Low acidity (high pH) will generally reduce the susceptibility to IGC
Use low carbon content
IGC is associated:
With chemical segregation effects
Precipitate of compounds
IGC then occurs at the GB phase that has lost an element necessary for adequate corrosion resistance
IGS in normally occurs if the steel is heated within 425 degree celcius to 815 degree celcius
SELECTIVE LEACHING
The entire exposed surface of the metal may be attacked
Dealloying is the removal of one element from the alloy to the electrolyte.
The cause of dealloying is a “galvanic effect”
examples of dealloying
The removal of zinc in chloride waters from brass (Cu Zn alloy) called dezincification
Graphitic corrosion of grey cast iron
Dealloying causes loss in mechanical strength without changes in shape of structure or component
Examples of Selective Leaching
Dezincification
Mechanism of Dezincification
Both Zn and Cu dissolve and the more noble Cu than redeposits as a porous layer
The more active alloying element. zinc, selectively dissolves or 'leaches' out of the brass leaving behind a porous, weak copper structure
Prevention of
Dezincification:
Use cupronickles
Select low zinc red brasses 15 which is generally immune to dezincification However this alloy is expensive due to difficulty to die cast or forge
Sn addition to (α β) brasses which inhibit attack in the α
phase in the marine environment
Addition of P, As or Sb in single phase α admiralty brass
Selective removal or dissolution of Zn occurs in brasses (Cu Zn alloys) with 15 Zn in prolonged exposure to aerated water high in CO 2 or chlorides
Graphitic Corrosion
GR occurs exclusively in grey cast iron which has a
continuous graphite network in its microstructure
the graphite act as cathode accelerates anodic dissolution of nearby iron, leaving behind the graphite network. maintain structural shape but losses mechanical strength
GR is observed in buried cast iron pipe after many years
exposure in soil
Ductile and malleable cast iron do not suffer from graphite corrosion because they do not have continuous graphite network
Prevention of
Graphitic Corrosion:
Coating
Cathodic protection either by ICCP or sacrificial anodes
Use more resistant material such as other grades of cast iron.
EROSION-CORROSION/FLOW INDUCED CORROSION
Erosion
Corrosion
These particles may remove metal, or they may just
remove oxide and allow metal to corrode more quickly
occurs in valves, heat exchanger tubes, pumps and at elbows and tees in pipelines and any structural features that change the flow direction or velocity and increases turbulence
Corrosion is accelerated by impact of solid particles.
Prevention of Erosion
Material selection plays an important role in minimizing erosion corrosion damage
Design is also important:
It is generally desirable to reduce the fluid velocity and promote laminar flow; increased pipe diameters are useful.
Rough surfaces are generally undesirable.
Designs creating turbulence flow restrictions and obstructions are undesirable.
Abrupt changes in flow direction should be avoided.
Erosion corrosion attack in metals occurs due to the relative motion of a corrosive fluid and a metal surface.
Cavitation
Corrosion
Cavitation corrosion is a form of erosion corrosion and is caused by formation and collapse of bubbles of vapour
Vapour bubbles form because of pressure changes (which falls 0 across surfaces exposed to high velocity liquid flow
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 of Cavitation
Corrosion
Environmental modifications:
Abrasive particle in fluids can be removed
Removal of air
cathodioc protection
Optimum material selection
Use protective coating
Careful design to minimize pressure drops across the metal surface
Types of flow induced corrosion include:
Cavitation corrosion
Fretting corrosion
Erosion corrosion
Fretting Corrosion
It occurs at the interface of two highly stressed surfaces in the presence of repeated relative surface motion, for example vibration .
Austenitic stainless steels, Ti and Al
alloys are the most susceptible to fretting corrosion because they are relatively soft materials
Fretting corrosion is a damage that occurs a t the asperities of contact metal surfaces.
Fretting corrosion can be minimised
Careful design: 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
Many engineering structures operate with electrolytes
flowing either through or around them. Flow can:
Increase the rate of dissolution of corrosion product films
Mechanically remove oxides
Increase transport of oxygen to the metal surface
ENVIRONMENTALLY INDUCED CRACKING
Many stress related failures are associated with:
Hydrogen induced cracking
Corrosion fatigue (CFC)
Stress corrosion cracking(SCC)
EIC results in brittle fracture.
design
Transport: shock, impact damage and vibration
Operation: fault conditions
Fabrication: residual and thermal stresses
Stress corrosion cracking
SCC can proceed in either of two ways:
Cracks may propagate along the grain boundaries
May run through the individual grains
Mechanisms of
SCC
Film induced cleavage
The sides and tip of the crack are covered by a brittle film
The crack growing in the brittle film may propagate further into the metal
The crack is then blunted by the plastic deformation
For the crack to grow further, the surface film must reform at the crack tip surface.
Hydrogen embrittlement
Hydrogen atoms weaken the inter atomic bonds at the crack tip
Hydrogen atoms produced by the cathodic reaction diffuse to regions of tri axial stress at the crack tip
Sources of hydrogen
Electroplating
Corrosion
Welding
Contact with gaseous hydrogen
Anodic dissolution
The presence of stress leads to failure of this passive film by plastic strain and active corrosion occurs
The crack tip repassivates and the process restarts again.
In the absence of stress, the metal is nonreactive to the environment because of the existing of a protective passive film at the crack tip
The passivation rate is an important factor in this
mechanism
Three conditions must be present simultaneously for SCC to occur:
Specific
environment
Tensile stress
Susceptible
alloy
SCC is very common and is the cause of major industrial costs and safety hazards
SCC is brittle failure of a metal under the combined effects of a static tensile stress and a specific chemical environment
Two main methods are used
Slow strain rate test
Fracture mechanics
Prevention of
SCC
Avoid the necessary environment
Apply electrochemical protection where possible
Remove stress
Use a different material
When these stresses are being applied in a corrosive environment, new types of corrosion may occur, classified as EIC
Corrosion
Fatigue
Unlike SCC, corrosion fatigue is non specific
Corrosion fatigue cracks develop in 3 stages:
Initiation of small crack
Propagation of the crack, leading to failure
Formation of slip bands leading to intrusions and extrusions
At high stress levels, the mechanism is dominated by the mechanical stress.
Corrosion fatigue is similar to SCC, except that the stresses are cyclic
Prevention of Corrosion
fatigue
Inhibitors
Select a more resistant material
Cathodic protection
Engineering materials are frequently required to withstand loads
Hydrogen
Attack
Theories of Hydrogen Induced Cracking
Surface energy theory
Decohesion theory.
Pressure theory
Atomic hydrogen may be produced from corrosion of steel in acidic, non oxygenated environments
Prevention of
HIC
HIC can be reversed by heat treatment
Minimising hydrogen content & Reducing the residual (internal)
HIC is a brittle failure caused by penetration and diffusion of atomic hydrogen into the crystal structure of a metal