CORROSION CONTROL. MATERIAL SELECTION ALTERATION OF ENVIRONMENT PROPER DESIGN CATHODIC PROTECTION ANODIC PROTECTION COATINGS & WRAPPING. (1) MATERIAL SELECTION (selection of proper material for a particular corrosive service). Metallic [metal and alloy]
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ALTERATION OF ENVIRONMENT
COATINGS & WRAPPING
(selection of proper material for a particular corrosive service)
Stainless steels are iron base alloys that contain a minimum of approximately 11% Cr, the amount needed to prevent the formation of rust in unpolluted atmosphere.
Dissolution rate, cm/sec
Minimum concentration of Cr in a
stainless steel is 12-14wt.%
Structure : BCC (ferrite forming element)
* Note that the affinity of Cr to form Cr-carbides is very
high. Chromium carbide formation along grain
boundaries may induce intergranular corrosion.
Sigma phase formation which is initially formed at grain boundaries has to be avoided because it will increase hardness, decrease ductility and notch toughness as well as reduce corrosion resistance.
Structure: FCC (austenite forming element/stabilize austenitic structure)
Added to produce austenitic or duplex stainless steels. These materials possess excellent ductility, formability and toughness as well as weld-ability.
Nickel improves mechanical properties of stainless steels servicing at high temperatures.
Nickel increases aqueous corrosion resistance of materials.
AISI : American Iron and Steel Institute
Very strong austenite forming element (30x more effective than Ni). I.e. if austenitic stainless steel 18Cr-8Ni contains ≤0.007%C, its structure will convert to ferritic structure. However the concentration of carbon is usually limited to ≤ 0.08%C (normal stainless steels) and ≤0.03%C (low carbon stainless steels to avoid sensitization during welding).
Austenitic forming element. When necessary can be used to substitute Ni. Concentration of Mn in stainless steel is usually 2-3%.
Ferritic forming element. Added to increase pitting corrosion resistance of stainless steel (2-4%).
Molybdenum addition has to be followed by decreasing chromium concentration (i.e. in 18-8SS has to be decreased down to 16-18%) and increasing nickel concentration (i.e. has to be increased up to 10-14%).
Improves mechanical properties of stainless steel at high temperature. Increase aqueous corrosion resistance of material exposed in reducing acid.
Is added to increase the strength and toughness of martensitic stainless steel.
Stabilize austenitic structure. Increases strength and corrosion resistance. Increases weld ability of duplex SS.
To stabilize stainless steel by reducing susceptibility of the material to intergranular corrosion. Ti addition > 5x%C. Ta+Nb addition > 10x%C.
Is added to increase corrosion resistance of stainless steel exposed in environment containing sulfuric acid.
Reduce susceptibility of SS to pitting and crevice corrosion as well as SCC.
- a critical environment
- a susceptible alloy
- some component of tensile stress
Pure metals are more resistance to SCC but not immune and susceptibility increases with strength
Tensile stress is below yield point
Stress corrosion cracking
Corrosive environment is often specific to the alloy system
Example of crack propagation during transgranular stress corrosion cracking (TGSCC) brass
Example of crack propagation during intergranular stress corrosion cracking (IGSCC) ASTM A245 carbon steel
Fracture surface of intergranular SCC on carbon steel in hot nitric solution
Fracture surface of transgranular SCC on austenitic stainless steel in hot chloride solution
Fracture surface due to local stress has reached its tensile strength value on the remaining section
Fracture surface due to intergranular SCC
Usual region for TGSCC, mostly is initiated by pitting corrosion(transgranular cracking propagation needs higher energy)
Usual region for IGSCC, SCC usually occurs where the passive film is relatively weak