It is the removal of one solid element from alloy by corrosion processes. Dealloying corrosion is also called Dezincification Selective Leaching Parting

1. Dealloying corrosion
BY:
KHUSHBAKHAT NIDA (MME-13-25)
MARIA IQBAL (MME-13-26)
NISHAT RIAZ (MME-13-27)
Institute of Advanced Materials (IAM), BZU, MULTAN.

2. Dealloying Corrosion:
 It is the removal of one solid element from alloy by corrosion processes.
Dealloying corrosion is also called
 Dezincification
 Selective Leaching
 Parting

3.  The most common example is Dezincification which is selective
removal of zinc in brass alloy.
 Similar processes occur in other alloy systems in
which aluminium , Iron, Cobalt, Chromium and
other elements are removed.

4. Types of Dealloying:
 Dezincification
 Graphitization

5. Dezincification:
 It is readily observed with naked eye
 Generally two types of dezincification
 Uniform or layer type dezincification
 Localized or plug type dezincification
 Dezincified material has lower strength

6. Uniform Type:
 Dark inner portion is Dezincified portion and outer layer is unaffected yellow Brass
 It seems to favour High brasses
and definately acid environments

7. Plug Type:
 Dark areas are Dezincified plugs and remainder tube is not corroded to any appreciable
extend
 It occurs more often in low brasses and in
neutral and alkaline environments

8. Mechanism:
 The commonly accepted mechanism consists of three steps:
 Brass dissolves
 Zinc ions stay in solution
 Copper plates back on
 Zinc is more reactive while Copper is noble and zinc corrodes slowly in pure water
 Dezincified area shows 90 to 95% porous copper with some of it present as copper
oxide.
 Porous nature permits easy contact between solution and Brass.

9. Prevention:
Dezincification can be minimized by
 Reducing aggressiveness of environment or cathodic protection ( but these are not
economical)
 A less susceptible alloy is used. For example addition of 1% Tin to 70-30 Brass (Admirality
Metal) and further adding arsenic antimony or phosphorous as “ Inhibitor”.
 For severely corrosive environments or critical parts, Cupronickel (70-90% Cu,,30-10% Ni)
are utilized.

10. Graphitization:
 Gray cast iron shows selective leaching in relatively mild environments
 During selective leaching of grey cast iron, iron is dissolved, leaving a
porous mass consisting of graphite, voids and rust
 Cast iron loses strength and its metallic properties. Dimensional changes
do not occur and dangerous situation may develop without detection
 The degree of lose depends upon the depth of the attack

11. Characteristics:
 It is usually a slow process
 It does not occur in nodular and malleable cast irons
 It is undesirable to use in underground pipelines because of soil
settlement or impact
 Ductile iron pipe with a cement mortar lining gives excellent performance
 White cast iron has essentially no free carbon so it is not subjected to
graphitization