Dealloying environment

Dealloying is influenced by many factors. In general, any process that increases general corrosion will promote dealloying. However, specific acceleration factors may be further classified into one of three categories metallurgy, environment, and water chemistry. 

Mechanisms of SCC. Crack initiation of EAC is complex and not well understood till now. Most of the SCC systems exhibit short initiation times ranging from minutes to weeks and cracking often occurs due to the change in the environment rather than to a very long initiation time. Stress-corrosion crack growth rates are usually 10 11 and 10-6 m s In systems such as stainless steels in chloride solutions, localized corrosion may create the local conditions prone to crack development, but it is still difficult to explain the initiation of the crack in the absence of localized corrosion in environmental conditions different from that of the crack propagation.95 It should be mentioned that dealloyed surface layers such as certain copper alloys in ammonia-containing solutions are believed to cause SCC.54... 

Other Cu-based alloys Dealloying phenomena have also been discussed for Cu alloys from the Cu—Ni, Cu—Mn, and Cu—Sn systems [45, 84, 85]. In the case of long-term corrosion of Sn-bronze (a-Cu—Sn) in natural environments, which is obscured by complex patina formation, it has been shown that the relevant dealloying process is decuprification rather than destannification (as formerly assumed). 

Percolation concepts Percolation concepts of dealloying are based on the association of sharp parting limits with the abrupt occurrence of connected paths of the fast-dissolving component, when in a random solid solution the concentration of that component is being increased. Early approaches of this idea made use of probability calculus to determine the fraction of chains of the less noble component in dependence on the alloy composition. For infinite chain lengths, the results were sharp composition thresholds that varied with the chain multiplicity and were associated with Tammann s parting limits for environments with different oxidative... 

Nanoscopic Investigations of Dealloyed Surfaces Erom the background of competitive models of selective alloy dissolution as described above, a closer microscopic examination of this process with the ultimate objective of atomic resolution and chemical information on an atomic scale appears mandatory. Ex situ transmission electron microscopy (TEM) of thin, corroded alloy films provides lateral resolution at the nanometer scale, but suffers from poor depth resolution and from structural relaxation processes that may occur after termination of the anodic polarization and transferring the samples into high vacuum. Classical TEM investigations in this field were performed under open circuit conditions in oxidizing environments (that is, at > Eq) [51,... 

Monel 400, a nickel alloy containing 66.5% nickel, 31.5% copper and 1.25% iron, has a marked tendency for the initiation of pitting in chloride-containing environments where the passive film can be disturbed. Under stagnant conditions chlorides penetrate the passive film at weak points and cause pitting attack. Sulfides can cause either a modification of the oxide layer, as described for copper, or breakdown of the oxide film of nickel alloys. Pit initiation and propagation depend on depth of exposure, temperature and presence of surface deposits. Little and coworkers [30] reported selective dealloying of nickel in Monel 400 in the presence of SRB from an estuarine environment. 

Corrosion within pump and piping systems is another problem, and general uniform attack is common. Pitting, crevice corrosion, intergranular corrosion, dealloying, galvanic corrosion, and cavitation corrosion are also possible depending on the environment. 

The brasses (Cu-Zn alloys) have a corrosion resistance similar to that of copper. Under certain conditions, selective corrosion of zinc may lead to dealloying however (Chapter 7). Adding small amounts of Sn, As, Sb or P to brass permits to reduce this kind of attack. In amine-containing environments, brass is sensitive to stress corrosion (Chapter 11). 

One recent development in the static immersion testing from Newman et al. [116] originated with Bengough and May [JJ5]. Recent experimentation [116] involved exposing samples of alpha-brass in NaCl solutions with additions to simulate the local environment in crevices or otherwise inhibited transport. Under these severe conditions, dealloying can be shown to occur without an applied potential and may be reduced or eliminated by addition of arsenic to the alloy. The authors suggest that these results support the percolation mechanism with surface diffusion for dealloying rather than mechanisms that rely on formation of metastable phases or disproportionation of the less noble element. 

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