Selective leaching corrosion

Crevice corrosion Stress corrosion cracking Intergranular corrosion Selective leaching Erosion corrosion Flydrogen damage Dilon [12] grouped corrosion into two categories. 

Localized corrosion is either macroscopic or microscopic. The major forms of macroscopic corrosion in addition to uniform are galvanic, erosion, crevice, pitting, and exfoliation. Microscopic corrosion includes intergranular corrosion, selective leaching, and stress-corrosion cracking. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

The various types of localised corrosion have been enumerated in Table 1.2 in Section 1.1, and many of them are dealt with in some detail in other sections of this volume. For this reason this section will be confined to a consideration of the factors that give rise to crevice corrosion, filiform corrosion, pitting, selective leaching and erosion-corrosion and of the mechanisms of these forms of localised attack. 

Dezincification As either plug dezincification (localized corrosion) or layer dezincification (general corrosion) and refers to zinc (Zn) being selectively leached out of brass. It can be prevented either by reducing the Zn content to below 15% or by the addition of trace amounts of inhibiting elements such as arsenic (As)... 

NOTE Do not confuse graphitization with graphitic corrosion, which is different. Graphitic corrosion causes the iron in cast iron to selectively leach out, leaving behind a porous graphite structure. 

Studies performed so far using Monel (nickel-copper alloys) have shown its low corrosion-resistance due to problems associated to selective leaching in the SCWO processes. Normally it is not to be considered in the materials selection. 

Form of corrosion of brasses, whereby the zinc is selectively leached out of the brass. Prevented by either reducing zinc content to below... 

Figure 2. Effect of various stages of corrosion on changes of glass surfaces. Conditions (b) type 2, a = 0, selective leaching (c) type 4, 0 < a < 1, selective leaching and network dissolution and (d) type 5, a = 1, network dissolution. (Reproduced, with permission, from Ref. 7. Copyright 1980, North-Holland Publishing...

Pitting corrosion Intergranular corrosion Stress-corrosion cracking Selectively leaching Erosion-corrosion... 

Selective leaching is the process whereby a specific element is removed from an alloy because of an electrochemical interaction with the environment. Dezinci-fication of brass alloys is the most familiar example of this type of corrosion. It occurs most commonly when there is exposure to soft waters and can be accelerated by high carbon dioxide concentrations and the presence of chloride ions. 

Acidic solutions of Fe(III) and a mixed culture of thiobacilli and leptospirilli produced characteristically different pits on a pyrite/arse-nopyrite flotation concentrate, suggesting direct bacterial attack on the surface. In both cases, the arsenopyrite was selectively leached (92). Using X-ray spectrometry and X-ray mapping on thin sections of chalcopyrite attacked by a variety of chemolithotrophic bacteria, it was shown that attachment of the bacteria was specific to exposed pyrite and chalcopyrite regions (12). Different corrosion patterns were seen in the presence and absence of bacteria. 

Corrosion sometimes removes one element from a solid solution of alloy and this process is described as selective leaching. This can easily be seen with brass where the surface changes to a copper colour from the original yellow. 

Single-component corrosion types, important for heat exchanger design and operation, are as follows (1) uniform attack corrosion, (2) galvanic corrosion, (3) pitting corrosion, (4) stress corrosion cracking, (5) erosion corrosion, (6) deposit corrosion, and (7) selective leaching [153],... 

CORROSION, GRAPHITIC - Corrosion of grey iron in which the iron matrix is selectively leached away, leaving a porous mass of graphite behind. This type of corrosion occurs in relativel mild aquous solutions and on buried piping. 

A variation on selective leaching is exfoliation, which is corrosion spreading below the surface. It differs from pitting in that the attack has a laminated appearance with whole layers of material peeled away in the form of a flaky or blistered surface. It is well known in certain aluminum alloys and is combated by proper heat treatment and alloying. 

Dezincification, or selective leaching or parting, was first observed on brasses. The zinc is selectively leached out of the alloy, leaving a brittle, weak, and porous mass. It consists predominantly of copper plus copper oxides. The obvious mechanism is solution or corrosion of the brass. The zinc stays in solution, and the copper plates back onto the surface. 

This form of corrosion corresponds to a process where one constituent of an alloy is preferentially removed, leaving behind an altered structure. The most common example for this type of corrosion is the selective removal of zinc from brass (Cu-Zn alloys), also referred to as dezincification. Selective leaching has been observed in other alloy systems containing Al, Fe, Co, and Cr. 

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