ASM Metals HandBook Volume 12 - Fractography
857 pág.

ASM Metals HandBook Volume 12 - Fractography


DisciplinaMetalurgia Mecânica151 materiais1.917 seguidores
Pré-visualização50 páginas
anode in a galvanic couple. 
If the exposed metal surface can form a new passive film (repassivate) faster than the new metal surface is created by film 
rupture, the corrosion attack will stop. However, if the repassivation process is suppressed, as in the presence of chlorides, 
or if the repassivated film is continuously ruptured by strain, as when the material creeps under stress, the localized 
corrosion attack proceeds (Ref 167, 168, 169, 170, 171, 172). The result is the formation and progressive enlargement of a 
pit or crevice and an increase in the concentration of hydrogen ions and an accompanying decrease in the pH of the 
solution within the pit. 
The hydrogen ions result from a chemical reaction between the exposed metal and the water within the cavity. The 
subsequent reduction of the hydrogen ions by the acquisition of electrons from the environment results in the formation of 
hydrogen gas and the diffusion of hydrogen into the metal. This absorption of hydrogen produces localized cracking due 
to a hydrogen embrittlement mechanism (Ref 173, 174). Because the metal exposed at the crack tip as the crack 
propagates by virtue of hydrogen embrittlement and the applied stress is anodic to the oxidized sides of the crack and the 
adjacent surface of the material, the electrochemical attack continues, as does the evolution and absorption of hydrogen. 
The triaxial state of stress and the stress concentration at the crack tip enhance hydrogen embrittlement and provide a 
driving force for crack propagation. 
In materials that are insensitive to hydrogen embrittlement, SCC can proceed by the anodic dissolution process with no 
assistance from hydrogen (Ref 149, 155, 161). Alloys are not homogeneous, and when differences in chemical 
composition or variations in internal strain occur, electrochemical potential differences arise between various areas within 
the microstructure. For example, the grain boundaries are usually anodic to the material within the grains and are 
therefore subject to preferential anodic dissolution when exposed to a corrosive environment. Inclusions and precipitates 
can exhibit potential differences with respect to the surrounding matrix, as can plastically deformed (strained) and 
undeformed regions within a material. These anode-cathode couplings can initiate and propagate dissolution cracks or 
fissures without regard to hydrogen. 
Although other mechanisms may operate (Ref 175, 176, 177, 178), including the adsorption of unspecified damaging 
species (Ref 177) and the occurrence of a strain-induced martensite transformation (Ref 178), dezincification or 
dealloying (Ref 145, 146, 147, 148) appears to be the principal SCC mechanism in brass (copper-zinc and copper-zinc-tin 
alloys). Dezincification is the preferential dissolution or loss of zinc at the fracture interface during SCC, which can result 
in the corrosion products having a higher concentration of zinc than the adjacent alloy. This dynamic loss of zinc near the 
crack aids in propagating the stress-corrosion fracture. 
Some controversy remains regarding the precise mechanics of dezincification. One mechanism assumed that both zinc 
and copper are dissolved and that the copper is subsequently redeposited, while the other process involves the diffusion of 
zinc from the alloy, resulting in a higher concentration of copper in the depleted zone (Ref 179). However, there is 
evidence that both processes may operate (Ref 180). 
Like hydrogen embrittlement, SCC can change the mode of fracture from dimple rupture to intergranular decohesion or 
cleavage, although quasi-cleavage has also been observed. The change in fracture mode is generally confined to that 
portion of the fracture that propagated by SCC, but it may extend to portions of the rapid fracture if a hydrogen 
embrittlement mechanism is involved. 
Stress-corrosion fractures that result from hydrogen embrittlement closely resemble those fractures; however, stress-
corrosion cracks usually exhibit more secondary cracking, pitting, and corrosion products. Of course, pitting and 
corrosion products could be present on a clean hydrogen embrittlement fracture exposed to a corrosive environment. 
SCC of Steels. Examples of known stress-corrosion fractures are shown in Fig. 50, 51, 52, 53, 54, 55, and 56. Steels, 
including the stainless grades, stress corrode in such environments as water, sea-water, chloride- and nitrate-containing 
solutions, and acidic as well as basic solutions, such as those containing sodium hydroxide or hydrogen sulfide. Stress-
corrosion fractures in high-strength quench-and-temper hardenable or precipitation-hardenable steels occur primarily by 
intergranular decohesion, although some transgranular fracture may also be present. 
 
Fig. 50 Stress-corrosion fractures in HY-180 steel with an ultimate strength of 1450 MPa (210 ksi). The steel 
was tested in aqueous 3.5% sodium chloride at an electrochemical potential of E = -0.36 to -0.82 VSHE (SHE, 
standard hydrogen electrode). Intergranular decohesion is more pronounced at lower values of stress intensity, 
Kl = 57 MPa m (52 ksi in .)(a), than at higher values, Kl = 66 MPa m (60 ksi in .) (b). Source: Ref 154 
 
Fig. 51 Stress-corrosion fractures in a 25% cold-worked type 316 austenitic stainless steel tested in a boiling 
(154 °C, or 309 °F) aqueous 44.7% magnesium chloride solution. At low (14 MPa m , or 12.5 ksi in .) Kl 
values, the fracture exhibits a combination of cleavage and intergranular decohesion (a). At higher (33 MPa 
m , or 30 ksi in .) values of Kl the principal made of fracture is intergranular decohesion (b). Source: Ref 181 
 
Fig. 52 Effect of electrochemical potential on the stress-corrosion fracture path in a cold-worked AISI C-1018 
low-carbon steel with a 0.2% offset yield strength of 63 MPa (9 ksi). The steel was tested in a 92- °C (198- °F) 
aqueous 33% sodium hydroxide solution. At a potential of E = -0.76 VSHE, the fracture propagates along grain 
boundaries (a) by a metal dissolution process; however, at a freely corroding potential of E = -1.00 VSHE, the 
fracture path is transgranular and occurs by a combination of hydrogen embrittlement and metal dissolution 
(b). Source: Ref 182 
 
Fig. 53 Stress-corrosion fractures from two different areas in a 7075-T6 aluminum alloy specimen exposed to 
water at ambient temperature. The fracture exhibits intergranular decohesion, although same dimple rupture is 
present near center of fracture in (a). 
 
Fig. 54 Stress-corrosion fractures in a Cu-30Zn brass tested in distilled water at a potential of E = 0 VSCE (SCE, 
saturated calomel electrode). Brass containing 0.002% As fails by predominantly intergranular decohesion (a), 
and one with 0.032% As fails by a combination of cleavage and intergranular decohesion (b). Source: Ref 176 
 
Fig. 55 Stress-corrosion fracture in a Cu-30Zn brass with 0.032% As tested in water containing 5 × 10-3% 
sulfur dioxide at a potential of E = 0.05 VSCE. The periodic marks are believed to be the result of a stepwise 
mode of crack propagation. Source: Ref 176 
 
Fig. 56 Stress-corrosion fracture in an annealed Ti-8Al-1Mo-1V alloy tested in aqueous 3.5% sodium chloride. 
The fracture surface exhibits cleavage and fluting. Source: Ref 89 
Figure 50 shows a stress-corrosion fracture in an HY-180 quench-and temper hardenable steel tested in aqueous 3.5% 
sodium chloride. The stress-corrosion fracture was believed to have occurred predominantly by hydrogen embrittlement 
(Ref 154). Increasing the stress intensity coefficient, KI, resulted in a decreased tendency for intergranular decohesion; 
however, the opposite was true for a cold-worked type 316 austenitic stainless steel tested in boiling aqueous magnesium 
chloride (Ref 181). It was shown that increasing KI or increasing the negative electrochemical potential resulted in an 
increased