dealloying corrosion

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.

Dealloying Corrosion: It is the removal of one solid element from alloy by corrosion

processes. Dealloying corrosion is also called Dezincification Selective Leaching Parting

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.

Types of Dealloying:

Dezincification

Graphitization

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

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

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

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.

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.

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

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