Transcript Document
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 1214wt.%. 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 materials. aqueous corrosion resistance of 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 is below yield point Tensile stress Corrosive environment is often specific to the alloy system Pure metals are more resistance to SCC but and susceptibility increases with strength Susceptible material Corrosive environment Stress corrosion cracking 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 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 intergranular SCC Fracture surface due to local stress has reached its tensile strength value on the remaining section Electrochemical effect pitting Zone 1 cracking zones passive Usual region for TGSCC, mostly is initiated by pitting corrosion (transgranular cracking propagation needs higher energy) Zone 2 active Usual region for IGSCC, SCC usually occurs where the passive film is relatively weak 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.