Crack initiation passive alloys

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

Figure 4 (a) Schematic representation of a a-N curve for CF crack initiation under passive conditions, (b) Comparison of experimentally and theoretically derived fatigue lives for the XI Cr MoNb 182 alloy in 30 g/L NaCl at 80°C [8]. 

Some films are termed "passive," for stainless steels or aluminum alloys, for instance. These films can play an important role in environment-sensitive crack initiation and fracture. Under thermodynamic equilibrium conditions, the film stability may be inferred from E =/(pH) diagrams, where E is the electrical potential related to the chemical free energy G by G = -nEF, and F is Faraday s number. At equilibrium, one can define the electrode potential (related to AG) and the current density I (I e here AG is the activation energy of dissolution). 

The arbitrary division of behaviour has been made because of the extreme behaviour of some chemicals that initiate small areas of attack on a well-passivated metal surface. The form of attack may manifest itself as stress-corrosion cracking, crevice attack or pitting. At certain temperatures and pressures, minute quantities of certain chemicals can result in this form of attack. Chloride ions, in particular, are responsible for many of the failures observed, and it can be present as an impurity in a large number of raw materials. This has led to the development of metals and alloys that can withstand pitting and crevice corrosion, but on the whole these are comparatively expensive. It has become important, therefore, to be able to predict the conditions where more conventional materials may be used. The effect of an increase in concentration on pitting corrosion follows a similar relationship to the Freundlich equation where... 

Hydrogen initiation of stress-corrosion cracking is indeed the probable mechanism. However, what has been given here is rather overgeneral. For example, the stress corrosion of alloys shows specificities that hint at unexplained factors. Passive films form at the bottom of pits and it is the breaking of these upon stress that sometimes causes cracks to spread. 

In many of the alloy systems shovm in Table 1/ the stable configuration of the alloy surface is that it is filmed. Many of the alloys, e.g., stainless steels, Al, Ti, Zr and Mg alloys are only usable in such a condition. Such a consideration applies not only to these alloys covered with a thin passive film but also to those on which relatively thick films are formed. The possible mechcUiisms by which stress corrosion cracking occurs are concerned with reactions between unfilmed metal and the environment. Before consideration of these it is necessary to consider how these various types of film break down initially. While many of the alloys exhibit pitting, it is not necessary for pitting as such to precede crack propagation. Pitting is associated with static unstressed metals whereas cracking is associated with a metal whose surface is stressed. 

Parkins has devised a slow and rapid potentiodynamic scanning method to determine both the relative susceptibility to slip-film rupture-repassivation SCC susceptibility, as well as possible ranges of potential where SCC might occur [60], The method applies to metals and alloys whose oxide films can initially be cathodicaUy reduced. The validity of the method relies on the notion that SCC only occurs when crack walls are readily passivated while the crack tip dissolves at a high rate due to continual destabilization of the passive film. In this method, the rapid scan anodic polarization curve provides a measure of the bare dissolution kinetics over a range of anodic potentials. A slow anodic potentiodynamic scan provides a measure of the passive crack flank dissolution kinetics over the same range of potential. Alloy-electrol5 combinations that produce potential... 

Under certain special environmental conditions, the passive films, which were described earlier in this Chapter, are susceptible to localized breakdown. Passivity breakdown may result in accelerated local dissolution (localized corrosion) of the metal or alloy. There are two (related) major forms of localized corrosion following passivity breakdown localized corrosion initiated on an open surface is called pitting corrosion, and localized corrosion initiated at an occluded site is called crevice corrosion. In the presence of mechanical stress, localized dissolution may promote the initiation of cracks. 

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