Corrosion selective

Selective corrosion or selective leaching occurs in alloys in which one element is clearly less noble than the others. As a result of this form of corrosion the less noble metal is removed from the material, leading to a porous material with very low strength and ductility. However, regions that are selectively corroded are sometimes covered with corrosion products or other deposits. Thus, the component keeps exactly the same shape, making the corrosion difficult to discover.  

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Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Dezincification Dezincification is corrosion of a brass alloy containing zinc in which the principal product of corrosion is metallic copper. This may occur as plugs rilling pits (plug type) or as continuous layers surrounding an unattacked core of brass (general type). The mechanism may involve overall corrosion of the alloy followed by redeposition of the copper from the corrosion products or selective corrosion of zinc or a high-zinc phase to leave copper residue. This form of corrosion is commonly encountered in brasses that contain more than 15 percent zinc and can be either eliminated or reduced by the addition ox small amounts of arsenic, antimony, or ph osphorus to the alloy. 

There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

To discover the effective potential ranges for electrochemical protection, the dependence of the relevant corrosion quantities on the potential is ascertained in the laboratory. These include not only weight loss, but also the number and depth of pits, the penetration rate in selective corrosion, and service life as well as crack growth rate in mechanically stressed specimens, etc. Section 2.4 contains a summarized survey of the potential ranges for different systems and types of corrosion. Four groups can be distinguished ... 

Dezincification of brasses When dezincihcation occurs, regions of the brass become replaced by a porous mass of copper which, though retaining the shape of the original article, has virtually no strength. There has long been discussion as to whether there is selective corrosion of the zinc in the brass, which leaves the copper behind, or whether complete dissolution of the brass occurs, followed by re-deposition of copper. Possibly both processes occur in different circumstances. The mechanism has been investigated and discussed by Evans, Fink", Lucey , Feller" and Heidersbach , and is referred to in many other papers. ... 

Selective corrosion in the heat-affected zone of a weld occurs most commonly when unstabilised stainless steels are used in certain environments. The obvious answer is to use an extra-low-carbon grade of stainless steel, e.g. types 304L, 316L or a stabilised grade of steel, e.g. types 321 and 347. Knifeline attack at the edge of a weld is not commonly encountered and is seldom predictable, and it must be hoped that it is revealed during preliminary corrosion testing. 

The Ni-28Mo alloy provides a special case of selective corrosion analogous to the weld-decay type of attack it may be removed by solution treatment or using an alloy containing 2%... 

In condensed moisture, there is sufficient corrosion of zinc to give protection at pores in a coating on steel without the formation of as much zinc corrosion product as would develop on a wholly zinc surface. In solder-ability the coating is tin-like when new or stored dry, but the selective corrosion of zinc in humid conditions may produce a layer obstructive to easy soldering. 

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

Selective Corrosion Problems Affecting Pre-Boiler Section Equipment... 

In a subsequent work [182], it was shown that the photoelectrochemical performance of InSe can be considerably improved by means of selective (photo)electrochemical etching. Interestingly, whereas the cleavage vdW plane showed little improvement, the photocurrent in the face parallel to the c-axis was doubled. Note that, in contrast to InSe crystals cleaved in the plane perpendicular to the c-axis that are almost defect free, the crystals cut in the plane parallel to the c-axis contain a high density of defects on their surface which leads to a high rate of electron-hole recombinations and inferior quantum efficiency. The asymmetry in the role of electrons and holes, as manifested, e.g., in the fact that surface holes carry out the selective corrosion of the semiconductor surface in both cleavage orientations, was discussed. 

Intergranular Corrosion Selective corrosion in the grain boundaries of a metal or alloy without appreciable attack on the grains or crystals themselves is called intergranular corrosion. When severe, this attack causes a loss of strength and ductility out of proportion to the amount of metal actually destroyed by corrosion. 

Effect of a selected corrosion inhibitor on the curing of the polymeric coating. 

Means for Preventing Tank Leakage. In addition to the common sense approaches, such as selecting corrosion, weather, and moisture-resistant materials of construction, providing excellent foundations, and leak-detection instrumentation, there are additional preventive measures that can be taken. Cathodic protection, for example, is described in an article on Corrosion. 

Corrosion from chlorides always tends to be more severe when the metal surface is shielded by foulants or semiporous deposits such as silt and muds, as various forms of selective corrosion can take place, with often very high concentrations of chloride forming under the deposit. 

The selective corrosion of cast iron (graphitization), the preferential corrosion of the steel welding (grooving corrosion), sensitization and knife line attack of welded stainless steels are typical examples of corrosion influenced by metallurgical parameters. 

Figure 6.25 Selective corrosion by layer type (dezincification) of a bolt in brass7...

As mentioned earlier, the samples were very heterogeneous. Either the melting process was incomplete and the higher melting solids that remained closest to the crucible walls were unevenly mixed and not representative of the completely melted portion that was decanted from the crucible, or selective corrosion or fractionation of certain elements occurred. 

Secondly, we focused on the role of oxygen impurities in element-selective corrosion of austenitic stainless steels in liquid sodium. Calculation was carried out for an oxygen impurity associated with an Na atom located at the site on top of a Cr atom at the Fe(OOl) surface in sodium. The result shows that a positively charged Cr atom will be released selectively into sodium by an 0 anion. 

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