Selective Leaching Type Corrosion

Name - Parekh Jayesh MRoll no. - 388Class - M.E(welding)Subject - Corrosion of WeldmentGuided By - Dr. Sunil Kahar

14 February 2016

Introduction Corrosion is defined as the destruction or

deterioration of a material because of reaction with environment. Corrosion is a natural process which converts a refined metal to a more stable form, such as its oxide or hydroxide.

• Types of Corrosion

1) Galvanic Corrosion2) Crevice Corrosion3) Pitting4) Intergrainnular Corrosion5) Selective leaching6) Erosion Corrosion7) Stress corrosion8) Hydrogen Damage

Selective leaching

De-alloying or selective leaching refers to the selective removal of one element from an alloy by corrosion processes.

A common example is the dezincification of brass, whereby a weakened, porous copper structure is produced, without modification of overall part dimensions.

Selective leaching is also called de-alloying, de-metalification, parting, and selective corrosion.

The selective removal of one element can proceed in uniform manner or localize (plug type) scale.

MechanismThere are three steps of mechanism for dezincification.

1) First entire brass alloy is dissolved.2) The Nobel metal is replated (e.g. Cu in Brass)3) Active metal is leached away. (e.g. Zn in Brass)

Zinc is quite reactive to corrosive environment while copper is more Nobel metal.

Zinc can corrode slowly in corrosive environment and leaching of zinc occurs in brass which makes porous structure of copper. This mechanism is called dezincification.

The analysis of dezincified area shows 90 to 95% copper with present as copper oxide.

When oxygen is present then it also enters into cathodic reaction and increase rate of attack.

The material remaining is a copper-rich sponge with poor mechanical properties, and a color changed from yellow to red.

Dezincification can be caused by water containing sulfur, carbon dioxide, and oxygen. Stagnant or low velocity waters tend to promote dezincification.

Characteristics of dezincification Common yellow brass consists of approximately 30%

zinc and 70% copper. Dezincification is readily observed with the naked

eye because the alloy color visible in red color.

There are two types of dezincificationa) Uniform type

In uniform type or layer type dezincification result in relatively uniform zone of dezincified material, with the underlying material remain unaffected.

Brass is with high zinc content in acidic environment is highly prone to uniform dezincification.

Fig.(a) Uniform or layer type selective leaching

b) Plug or localized type Plug type dezincification results in localized

penetration of dezincified areas that progress through wall thickness of the material.

Plug type corrosion is most likely to occur in basic or natural environment and at high temperatures.

The plug type corrosion generally occur in brasses in which zinc content is low.

Fig.(b) Plug Type Selective Leaching

Prevention Dezincification can be minimized by reducing

aggressiveness of the environment. Usually less susceptible material is used Addition of 1%Sn in 70-30 brass which improve

resistance towards dezincification. Addition of small amount of arsenic, antimony and

phosphorous as inhibitors. Cupronickel (70%-90% Cu, 30%-10% Ni) is used in

severely corrosive environment.

2.Graphitic Corrosion Grey cast iron shows the effect of selective leaching

particularly in mild environments. In graphitization selective leaching of iron from grey

cast iron. Affected surfaces develop a layer of graphite, rust, and metallurgical impurities.

The degree of loss depends on depth of attack. Graphitization is usually slow process.

It results from exposures of 50 years or more. Pipelines made of cast iron and buried in soil containing sulphates.

Graphitization not occurs in nodular or malleable cast iron because free graphite network not available.

White cast iron has no essential free graphite so it is not subjected to graphitization.

Fig.(c) Graphitic Corrosion

Selective leaching of material depend on,1) Alloy composition2) Environment

Alloy composition

Environment Element removed

Brass Waters, especially in stagnant condition

Zinc

Gray iron Soils, water IronAluminum

bronzeHydrofluoric

acid Aluminum

Tin bronze High temperature

steam and hot brine

Tin

Gold alloy with copper or silver

Sulfide solution and

human saliva

Copper, Silver

Material Inlet pipe of condenser made of brass

Environment

River water

Component

Inlet pipe of condenser on cooling side

Observation

The internal surface was generally covered with a layer of porous copper-based corrosion products, indicating that layer-type dezincification had occurred in addition to plug-type attack.

High salt content of the water and high temperatures likely promoted layer-type dezincification.

Water analyses indicated high conductivity and sulfate concentration in the water.

Microscopic examinations indicated that the microstructure was single-phase

Case Study

Tools

 

       

Internal surface (cooling water side) of a brass condenser tube

Micrograph showing deep, plug-type dezincification on the cooling water side of a brass Condenser tube.

  

Prevention

The addition of an adequate concentration of a corrosion-inhibiting element may have prevented the plug-type dezincification and limited layer-type dezincification.

It was recommended that tube surfaces be kept clean to minimize the possibility of concentration of aggressive species beneath occlusive material.

Thank You…