ASM Metals HandBook Volume 12 - Fractography
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than those containing sulfur dioxide. Atmospheres that produced a significant increase in the fatigue crack growth rate often showed no appreciable corrosion attack on unstressed material. The increase in the fatigue crack growth rate in air was probably due to the diffusion of oxygen along the grain boundaries and to the addition of oxide growth stresses at the crack tip. The mechanism of sulfur attack and acceleration of the fatigue crack growth rate could involve the formation of low-melting (eutectic) liquids at grain boundaries, or it could be due to the formation and subsequent fracture of brittle sulfur compounds (Ref 227). A similar accelerating effect of sulfur-containing atmospheres was observed when an annealed Incoloy 800 was fatigue tested in air, dry helium gas, helium with 0.35% hydrogen sulfide, and helium with 0.75 to 9.3% sulfur dioxide (all helium-containing gases were at slightly above atmospheric pressure) in the range of 316 to 650 °C (600 to 1200 °F) (Ref 228). The fatigue tests were conducted in the approximate stress intensity range of \u2206K = 28 to 44 MPa m (25.5 to 40 ksi in ), at load ratios of R = 0.1 to 0.33, and at a cyclic frequency of 0.1 Hz. At equivalent concentrations, the atmospheres containing hydrogen sulfide were found to be a far more aggressive environment than those containing sulfur dioxide, although above 400 °C (750 °F) all of the sulfur-containing atmospheres, when compared to helium gas, produced a four- to fivefold increase in the fatigue crack growth rate. The atmospheres containing sulfur dioxide were apparently less aggressive because of the inhibiting effect of oxygen, which is produced by the reaction of sulfur dioxide with the alloy. The mechanism for the general increase in the fatigue crack growth rate in sulfur-containing atmospheres is believed to be sulfur enrichment of crack tip region (Ref 228), and this seems to be supported by fractographic evidence. In the 316- to 427- °C (600- to 800- °F) temperature range (the fracture surfaces were obscured at higher temperatures), the fatigue fracture surfaces produced in sulfur-containing atmospheres exhibited ridges and fatigue striations (Fig. 84). The ridges and stretched zones were probably formed by a localized, rapid advance of the crack front. This effect is believed to be due to the periodic local arrest of the crack by the formation of sulfide particles in the sulfur enrichment zone at the crack tip. Although the crack front as a whole is accelerating, the crack may slow down locally. When this occurs, sulfide particles have time to form in the enrichment zone. The sulfide particles (Fig. 85) appear to blunt and stop the growth of the crack, and when it finally propagates past the particle, it does so by sudden fracture, which results in a ridge and a stretched zone and locally accelerated fatigue growth, as indicated by the increase in the striation spacing immediately adjacent to the ridge-type structures. Unlike Inconel 718, no significant intergranular fracture was observed, and testing in air had no appreciable effect on the fatigue crack growth rate. Fig. 84 Typical ridges and fatigue striations on the fracture surface of an annealed Incoloy 800 specimen tested in a sulfidizing atmosphere in the 316- to 427- °C (600- to 800- °F) temperature range. (b) Higher- magnification fractograph of the area indicated by arrow in (a). Note the width of the smooth stretched areas at the ridges. Source: Ref 228 Fig. 85 Discrete sulfide particles on fracture surfaces of an Incoloy 800 tested in a sulfidizing atmosphere. Energy-dispersive x-ray analysis indicated that sulfur was present at particles 1 and 3 and that no sulfur was detected on surfaces 2 and 4. The sulfur-containing particles are believed to be responsible for locally arresting the fatigue crack. Source: Ref 228. Effect of Liquid Environments. Corrosive liquid environments, such as water, brines, organic fluids, basic or acid media, and molten salts, can affect the rate at which fatigue cracks propagate and the fracture appearance. Fatigue that occurs in environments that are corrosive to the material is referred to as corrosion fatigue. An overview of the basic mechanisms involved in fractures due to corrosive attack is provided in the section "Mechanisms of SCC" in this article. In general, any environment that promotes the initiation of fatigue cracks, such as pitting, enhanced corrosion, or oxidation; that allows an embrittling species to enter the material; that promotes strain-hardening relief (which lowers the fracture stress) at the crack tip; or that interferes with crack tip slip reversal will accelerate the corrosion fatigue crack growth rate and decrease the fatigue life of the material. Mechanism of Corrosion Fatigue. In order to understand the mechanism of corrosion fatigue crack initiation, the process has been approached from a purely theoretical standpoint by using basic corrosion kinetics and fracture mechanics (Ref 229). The theoretical analyses have been applied to three specific corrosion conditions; general, pitting, and passive corrosion. The basic mathematics describing the kinetics and fracture mechanics involved for each of the three conditions is complex; however, the analyses yielded the following conclusions. First, under conditions of general corrosion, corrosion fatigue initiation is controlled by the corrosion rate and the applied alternating stress range. General corrosion is defined as fairly uniform attack that results in loss of material and a reduction in the load-bearing cross section. Second, in pitting corrosion, in which the corrosion attack is localized, corrosion fatigue initiation is controlled by the rate at which pits nucleate and grow to a critical depth. The critical depth of a pit is a function of the applied alternating stress range. Under pitting conditions, no corrosion fatigue limit exists. Third, for passive corrosion, the corrosion fatigue crack initiates at a site where unprotected new metal exposed by slip steps penetrating the passive surface film is dissolved during repassivation. As these local slip emergence and repassivation dissolution cycles proceed, a small, sharp notch can form along the slip band. A corrosion fatigue crack initiates when the notch reaches a critical depth determined by the applied cyclic stress range. For passive corrosion, therefore, the corrosion fatigue initiation is controlled by the repassivation kinetics and the critical notch depth. It was also concluded that under conditions of passive corrosion, there is a critical repassivation current density below which corrosion fatigue crack initiation cannot occur and crack initiation is controlled by air fatigue behavior (Ref 229). In some cases, cracks propagating under a static load in a corrosive environment, such as a type 304 austenitic stainless steel tested in a boiling magnesium chloride solution, exhibit periodic, parallel crack-arrest marks on the fracture surface that strongly resemble brittle fatigue striations (Ref 230, 231, 232). The crack can proceed by very short bursts of cleavage fracture with about a 1-\u3bcm spacing between the crack-arrest lines (Ref 232). This type of crack propagation is believed to be due to localized embrittlement by hydrogen at the crack interface (Ref 230, 231). Also, when tested under similar stress intensity range, \u2206K, conditions, cleavage fracture due to corrosion attack and cleavagelike corrosion fatigue cracks have been shown to exhibit similar crack growth rates and fracture appearances (Ref 232). In view of the similarities, a unified theory, developed using the principles of corrosion kinetics and fracture mechanics, has been proposed linking SCC and corrosion fatigue (Ref 169). The theory states that passive film rupture is a critical event in both fracture processes.