SELECTIVE LEACHING (DEALLOYING)IB.PT.PUTRA.MAHARTANA (2111100177)I GEDE BRAHMANDA ADITYA PARAMARTHA (21111000146)c16f21Selective Leaching

Preferred corrosion ofone element/constituent[e.g., Zn from brass (Cu-Zn)]. Dezincification.INTRODUCTIONSELECTIVE LEACHINGThe removal of one element from a solid solution alloy by corrosion processes.That element is more susceptible to corrosion than the rest, more active electrochemically and are anodically dissolved in galvanic contact with the more noble elements.

Uniform dezincificationLocalized dezincificationAuCuPbSnNiCoCdFeCrZnAlMgNaK+1.420 V+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924metalVmetalomore anodicmore cathodicEMF seriesAlloy CompositionEnvironmentElement removedBrassesMany waters, especially under stagnant conditionsZincGray ironSoils, many watersIronAluminum bronzesHydrofluoric acid, acids containing chloride ionsAluminumSilicon bronzesHigh-temperature steam and acidic speciesSiliconTin bronzesHot brines or steamTinGold alloys with copper or silverSulfide solutions, human salivaCopper, silverSusceptibility to dealloying depends onAlloy compositionEnvironmentDezincification

Occurs in brass alloys that contain more than 15% zincZinc in solid solution within copper crystal lattice

Zinc atoms dissolve into electrolyteCopperZincMECHANISMGenerally three mechanisms are proposedDissolution and redeposition mechanismEntire alloy is dissolved firstMore noble metal is replated ( eg. Cu in Brass) Active metal is leached away ( eg. Zn in Brass ) Selective dissolutionOne species is selectively dissolved from alloy leaving a porous residue of more noble metal behindCombination of two methods

7Two forms of dezincificationOverall dimensions of the material do not changeDezincified areas weakened or in some cases perforatedUniform layer dezincificationLocalized (Plug) dezincificationDezincified layerTYPESUniform dezincification

Bulk electrolyte sufficiently corrosive to promote dealloying

Uniform layer dezincification in brass heat-exchanger tubeDezincified metalOuter tube wall0.500 mm(0.020)Courtesy of James J. Dillon. Permission granted by Nalco Chemical Company, 1987.Localized dezincification

Caused by local increases in electrolyte corrosivityChanges in electrolyte caused byDeposit on surfaceLocal increase in temperature

0.110 mm(0.0042)Brass (Cu-30% zinc) exposed to room temperature salt solution for 80 daysReprinted with permission of ASM International. All rights reserved. PREVENTIONReducing the aggressiveness of the environment (i.e. oxygen removal)Use less susceptible material: 15% Zn (red brass) Addition 1% Sn to a 70-30 brass (admiralty metal)Addition of As, Sb, P as inhibitor (70%Cu, 29% Zn, 1% Sn, 0.04% As)Cupronickel (70-90%Cu, 30-10%Ni) for severely corrosive environments.

REFERENCEZaki Ahmad Principles of Corrosion Engineering and Corrosion ControlMars G Fontanna Corrosion Engineering