ASM Metals HandBook Volume 12 - Fractography
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ASM Metals HandBook Volume 12 - Fractography

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Film rupture causes anodic dissolution of metal as a result of galvanic coupling between 
the unprotected bare metal surface at the rupture site and the adjacent passive oxidized surface film. 
In SCC, the initial film rupture occurs as a result of strain from the applied load, and when repassivation is retarded or 
arrested by some critical ion, such as chloride, that is present in the environment, the anodic dissolution is maintained and 
the crack propagates. During corrosion fatigue, the strain due to the cyclic loading continuously ruptures the protective 
film and mechanically prevents it from repassivating. The anodic dissolution at the rupture sites relieves strain hardening 
at surface slip bands during crack initiation and promotes cleavage on prismatic lattice planes and/or decohesion along 
grain boundaries in the plane strain region adjacent to the crack tip during crack propagation. The mechanism for the 
strain-hardening relief and subsequent fracture is the generation and the movement of divacancies. There is evidence from 
numerous alloy systems and corrosive environments that the mechanism for SCC and corrosion fatigue involves the relief 
of strain hardening at the crack tip by corrosion-generated divacancies (Ref 169). 
The theory also proposes that because the crack propagation rate is often increased by a lower cyclic frequency and longer 
hold times (dwell at maximum load); elevated-temperature fatigue in vacuum and lower temperature corrosion fatigue 
may also occur by the same mechanism. At elevated temperatures, thermally generated vacancies can cause dislocation 
rearrangement; in corrosion fatigue, corrosion-induced divacancies may perform the same function (Ref 169). 
Corrosion Fatigue of Martensitic Stainless Steels. The drastic effect that pitting corrosion has on the fatigue life 
of a material is illustrated by the corrosion fatigue testing of an SUS 410J1 stainless steel (Japanese equivalent of type 
410 stainless steel) that was tempered at 600 and 750 °C (1110 and 1380 °F) to an ultimate tensile strength of 930 and 
700 Mpa (135 and 102 ksi), respectively (Ref 233). The fatigue testing was performed at 25 °C (75 °F) in air and in a 3% 
sodium chloride solution (pH 7) with a stress intensity range of \u2206K = 3 to 20 Mpa m (2.5 to 18 ksi in ), a load ratio of 
R = 0, and a cyclic frequency of 60 Hz for the fatigue life tests and a frequency of 30 Hz for the crack propagation rate 
Fatigue life tests for the material tempered at 600 °C (1110 °F) showed that at a constant 107 cycles to failure fatigue life 
the 500-MPa (72.5-ksi) fatigue strength observed for specimens tested in air was reduced to about 100 MPa (14.5 ksi) in 
the brine. At a constant 500-MPa (72.5-ksi) fatigue strength, the 107 cycles to failure air endurance limit was reduced to 
105 cycles in the salt solution. This drastic reduction in fatigue life was due to early corrosion fatigue crack initiation from 
corrosion pits. However, crack growth rate tests indicated that at \u2206K values of less than about 18 MPa m (16.5 ksi in ) 
the corrosion fatigue crack growth rate in brine was slightly lower than that in air. This anomaly was believed to be due to 
the relatively high (30 Hz) testing frequency, which resulted in fluid not being able to escape from the region of the crack 
tip during the decreasing portion of the cyclic loading. The wedging action of the trapped fluid reduced the effective 
stress intensity at the crack tip and thus reduced the crack growth rate. 
Both tempers showed an increase in the amount of intergranular fracture when tested in the salt solution; however, the 
amount of intergranular fracture varied with stress intensity. The 750- °C (1380- °F) tempered material showed a 
maximum of 30 to 60% intergranular fracture (versus about 12 to 26% in air) at a maximum stress intensity of Kmax = 15 
MPa m (13.5 ksi in ). A similar effect was observed for a nickel-chromium-molybdenum-vanadium steel tested in 
moist air (see Ref 220 and the section "Effect of Gaseous Environments" in this article) and for a type 410 stainless steel 
that was corrosion fatigue tested in various environments (Ref 234). Analysis of a large number of corrosion fatigue tests 
indicated that there was little correlation between the amount of intergranular fracture and the crack propagation rate in 
this stainless steel (Ref 234). 
Corrosion Fatigue of Aluminum Alloys. A decrease in the fatigue strength was observed for a 7075-T6 high-
strength aluminum alloy tested at room temperature in dry air and in a 3% sodium chloride solution at a constant mean 
stress of 207 MPa (30 ksi), a load ratio of 0 < R < 0.7, and a cyclic frequency of 30 Hz (Ref 235). At a constant 3 × 106 
cycles to failure, the fatigue strength in air was 115 MPa (17 ksi) (cyclic stress; \u2206\u3c3= ± 115 MPa, or 17 ksi) and 50 MPa 
(7.25 ksi) in the saline solution. This difference in fatigue strength was reflected in the fracture appearance (Fig. 86). 
Fig. 86 Effect of environment on the fatigue fracture appearance in a 7075-T6 aluminum alloy tested at room 
temperature. (a) Fatigue tested in dry air; cyclic stress (\u2206\u3c3) = ±110 MPa (16 ksi), cycles to failure (Nf) @ 6 × 
105. (b) Corrosion fatigue tested in a 3% sodium chloride solution, \u2206\u3c3= ±68 MPa (9.9 ksi), Nf @ 6 × 105. Note 
that the fatigue fracture in air initiated at a grain boundary (A) but propagated by cleavagelike fracture, with 
river patterns emanating from the origin. The corrosion fatigue fracture in the salt solution initiated and 
propagated some distance by intergranular decohesion. However, as the crack depth increased (\u2206K increased), 
the fracture changed to a primarily transgranular, cleavagelike fracture with river patterns. Source: Ref 235 
The fatigue fracture in air initiated along grain boundaries, but changed immediately to a cleavagelike fracture with river 
patterns reminiscent if Stage I fatigue (Fig. 86a). In the saline solution, the crack exhibited substantial intergranular 
decohesion at the fracture origin and for some distance from the origin before the crack size had increased (\u2206K increased) 
sufficiently to result in the crack changing to a principally cleavagelike fracture with distinct river patterns (Fig. 86b). The 
cleavagelike transgranular fracture in air exhibited finer, feathery river pattern lines, and the fracture extended over a 
number of grains before changing direction. In the 3% sodium chloride solution, the river pattern lines were generally 
coarser and more distinct, and a set of lines was confined to a single grain; a new set of lines initiated at the grain 
boundary as the fracture entered the adjacent grain. In the fatigue striation forming portions of the fatigue cracks, the 
striations in air were ductile, while the striations that formed in the saline solution had a brittle character. Although the 
reduction in the fatigue life of the specimens tested in the 3% sodium chloride solution was reflected in the fracture 
appearance, the mechanism responsible for the decrease was hydrogen embrittlement. The embrittlement attack was 
enhanced by the cyclic mechanical rupture of the passive film and the prevention of repassivation by the presence of the 
chloride ion (Ref 235). 
Corrosion Fatigue of Titanium Alloys. Chloride-containing liquid environments are damaging not only to iron and 
aluminum alloys but also titanium and its alloys. This was shown by fatigue tests conducted using a basal-textured IMI 
155 (British designation for commercially pure titanium containing 0.34% oxygen alloy at room temperature in laboratory 
air and in a 3.5% sodium chloride solution (Ref 236). The fatigue tests were conducted within a stress intensity range of 
\u2206K = 5 to 25 MPa m (4.5 to 23 ksi in ), a load ratio of 0.35, and a cyclic frequency of 130 Hz. 
The fatigue tests showed that at a stress intensity range of \u2206K = 8 MPa m (7.5 ksi in ) the