Stainless Steels

Stainless steels are iron base alloys that contain a minimum of approximately 12% Cr, the amount needed to prevent the formation of rust in unpolluted atmosphere.

Alloying elements of stainless steel and their effect on

corrosion resistance Other than Ni, Cr and C, the following alloying elements may

also present in stainless steel: Mo, N, Si, Mn, Cu, Ti, Nb, Ta and/or W.

Main alloying elements (Cr, Ni and C):

1. Chromium

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.

2. Nickel

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 weldability.

Nickel improves mechanical properties of stainless steels servicing at high temperatures.

Nickel increases aqueous corrosion resistance of materials.

3. Carbon

Very strong austenite forming element (30x more effective than Ni). 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).

Minor alloying elements: Manganese

Austenitic forming element. When necessary can be used to substitute Ni. Concentration of Mn in stainless steel is usually 2-3%.

Molybdenum

Ferritic forming element. Added to increase pitting corrosion resistance of stainless steel (2-4%).

Molybdenum addition has to be followed by decreasing chromium concentration and increasing nickel concentration Improves mechanical properties of stainless steel at high temperature. Increase aqueous corrosion resistance of material exposed in reducing acid.

Tungsten

Is added to increase the strength and toughness of martensitic stainless steel.

Nitrogen (up to 0.25%)

Stabilize austenitic structure. Increases strength and corrosion resistance. Increases weldability of duplex SS.

Titanium, Niobium and Tantalum

To stabilize stainless steel by reducing susceptibility of the material to intergranular corrosion.

Copper

Is added to increase corrosion resistance of stainless steel exposed in environment containing sulfuric acid.

Silicon

Reduce susceptibility of SS to pitting and crevice corrosion as well as SCC.

Influence of alloying elements on pitting corrosion resistance of stainless steels

Influence of alloying elements on crevice corrosion resistance of stainless steels

Influence of alloying elements on SCC resistance of stainless steels

Five basic types of stainless steels Austenitic: Susceptible to SCC. Can be hardened only by cold

working. Good toughness and formability, easily to be welded and high corrosion resistance. Nonmagnetic except after excess cold working due to martensitic formation.

Martensitic: Application: when high mechanical strength and wear resistance combined with some degree of corrosion resistance are required. Typical application include steam turbine blades, valves body and seats, bolts and screws, springs, knives, surgical instruments, and chemical engineering equipment.

Ferritic: Higher resistance to SCC than austenitic SS. Tend to be notch sensitive and are susceptible to embrittlement during welding. Not recommended for service above 3000C because they will loss their room temperature ductility.

Duplex (austenitic + ferritic): has enhanced resistance to SCC with corrosion resistance performance similar to AISI 316 SS. Has higher tensile strengths than the austenitic type, are slightly less easy to form and have weld ability similar to the austenitic stainless steel. Can be considered as combining many of the best features of both the austenitic and ferritic types. Suffer a loss impact strength if held for extended periods at high temperatures above 3000C.

Precipitation hardening: Have the highest strength but require proper heat-treatment to develop the correct combination of strength and corrosion resistance. To be used for specialized application where high strength together with good corrosion resistance is required.

Stress Corrosion Cracking of Stainless Steel

Stress corrosion cracking (SCC) is defined as crack nucleation and propagation in stainless steel caused by synergistic action of tensile stress, either constant or slightly changing with time, together with crack tip chemical reactions or other environment-induced crack tip effect.

SCC failure is a brittle failure at relatively low constant tensile stress of an alloy exposed in a specific corrosive environment.

However the final fracture because of overload of remaining load-bearing section is no longer SCC.

Three conditions must be present simultaneously to produce SCC:

- a critical environment

- a susceptible alloy

- some component of tensile stress

Tensile stress

Corrosive environment

Susceptible material

Stress corrosion cracking

Tensile stress is below yield point

Corrosive environment is often specific to the alloy system

Pure metals are more resistance to SCC but and susceptibility increases with strength

Typical micro cracks formed during SCC of sensitized AISI 304 SS

Surface morphology

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 transgranular SCC on

austenitic stainless steel in hot chloride solution

Fracture surface of intergranular SCC on

carbon steel in hot nitric solution

Fracture surface due to intergranular SCC

Fracture surface due to local stress has reached its tensile

strength value on the remaining section

Electrochemical effect

pitting

passive

active

cracking zones

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

Zone 1

Zone 2

Note that non-susceptible alloy-environment combinations, will not crack the alloy even if held in one of the potential zones.

Temperature and solution composition (including pH, dissolved oxidizers, aggressive ions and inhibitors or passivators) can modify the anodic polarization behavior to permit SCC.

Susceptibility to SCC cannot be predicted solely from the anodic polarization curve.