Selective leaching or dezincification

SELECTIVE LEACHING (DEZINCIFICATION)

MUHAMMED LABEEB13MY06

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CONTENTS

▪ Introduction

▪ Mechanism

▪ Types

▪ Prevention

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Selective LeachingPreferred corrosion ofone element/constituent[e.g., Zn from brass (Cu-Zn)]. Dezincification.

INTRODUCTION

SELECTIVE LEACHING

▪ The 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 dezincification

Localized dezincification

AuCuPbSnNiCoCdFeCrZnAlMgNaK

+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- 2.924

metal Vmetalo

mor

e an

odic

mor

e ca

thod

ic

EMF series

Alloy Composition Environment Element removed

Brasses Many waters, especially under stagnant conditions

Zinc

Gray iron Soils, many waters Iron

Aluminum bronzes Hydrofluoric acid, acids containing chloride ions

Aluminum

Silicon bronzes High-temperature steam and acidic species

Silicon

Tin bronzes Hot brines or steam Tin

Gold alloys with copper or silver

Sulfide solutions, human saliva

Copper, silver

Susceptibility to dealloying depends on• Alloy composition• Environment

Dezincification

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

Zinc atoms dissolve into electrolyte

Copper

Zinc

Generally three mechanisms are proposed

1. Dissolution and redeposition mechanism

▪ Entire alloy is dissolved first

▪ More noble metal is replated ( eg. Cu in Brass)

▪ Active metal is leached away ( eg. Zn in Brass )

2. Selective dissolution

▪ One species is selectively dissolved from alloy leaving a

porous residue of more noble metal behind

3. Combination of two methods 8

MECHANISM

Two forms of dezincification

Overall dimensions of the material do not change

Dezincified areas weakened or in some cases perforated

Uniform layer dezincification

Localized (Plug) dezincification

Dezincified layer

TYPES

Uniform dezincification

Bulk electrolyte sufficiently corrosive to promote dealloying

 Uniform layer dezincification in brass heat-exchanger tube

Dezincified metal Outer

tube wall

0.500 mm(0.020”)

Courtesy of James J. Dillon.  Permission granted by Nalco Chemical Company, 1987.

Localized dezincificationCaused by local increases in electrolyte corrosivity

Changes in electrolyte caused by• Deposit on surface• Local increase in temperature

0.110 mm(0.0042”)

Brass (Cu-30% zinc) exposed to room temperature salt solution for 80 days

Reprinted with permission of ASM International®. All rights reserved.

PREVENTION

▪ Reducing 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.

REFERENCE

Zaki Ahmad – Principles of Corrosion

Engineering and Corrosion Control

Mars G Fontanna – Corrosion Engineering