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DESCRIPTIONSelective leaching mechanism and control measures.
Selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the material.
The less noble metal is removed from the alloy by microscopic-scale galvanic corrosion mechanism.
The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, eg. copper and zinc in brass.
The elements most typically undergoing selective removal are zinc, aluminum, iron, cobalt, chromium, and others.
The most common example is selective leaching of zinc from some brasses with less than 85% content of copper in presence of oxygen and moisture.
The Above process of zinc removal is called the Dezincification.
Countermeasures Use Alloys which are not susceptible to leach.
Using suitable heat treatment.
Altering the environment (Lowering oxygen content) .
Use cathodic protection.
Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making it work as a cathode of an electrochemical cell.
This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell.
Cathodic protection systems are most commonly used to protect steel, water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms and onshore oil well casings.
Types Galvanic CP .
This is achieved by joining the sacrificial anodes with the metal to be protected from corrosion.
Galvanic or sacrificial anodes are made in various shapes using alloys of zinc, magnesium and aluminum. The electrochemical potential, current capacity, and consumption rate of these alloys are superior for CP than iron.
Impressed current CP .For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection.
Impressed current cathodic protection (ICCP) systems use anodes connected to a DC power source (a cathodic protection rectifier).
A typical ICCP system for a pipeline would include an AC powered rectifier with a maximum rated DC output of between 10 and 50 amperes and 50 volts.
Galvanized steel .Galvanizing (or galvanizing, outside of the USA) generally refers to hot-dip galvanizing which is a way of coating steel with a layer of metallic zinc .
Galvanized coatings are quite durable in most environments because they combine the barrier properties of a coating with some of the benefits of cathodic protection.
If the zinc coating is scratched or otherwise locally damaged and steel is exposed, the surrounding areas of zinc coating form a galvanic cell with the exposed steel and protect it from corrosion.