tech forum 2009 - basic corrosion

7/31/2019 Tech Forum 2009 - Basic Corrosion

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Differentiating Between Several

Types of Basic Corrosion

Presented ByBernardo Duran

Tech Forum 2009


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Sources

NACE Basic Corrosion Class

Forms of Corrosion Recognition and

Prevention Volume 1

Google and Yahoo for Pictures


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Goals of This Presentation

Highlight Relevant Information from the


Provide some basic corrosion information

for your general knowledge


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Types of Corrosion

General

Localized

Galvanic

Environmental Cracking


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General Corrosion


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General Corrosion

Definition and Description

Corrosion that occurs over most of the surface

of a structure, without significant

localization of attack. General corrosion

usually results in relatively uniform

corrosion on the structure.


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General Corrosion

The Three Categories of General Corrosion

Category I

Category II

Category III


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General Corrosion

Category I

The electrode potential of the metal in its

environment, and the kinetics of the

reaction between the metal and the

environment, determine the corrosion rate

of the metal. For example, black steel inoxygen-rich salt water.


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General Corrosion

Category II

A passive barrier coating develops on the surface of

the metal, which affects the rate of corrosion ofthe metal. Corrosion of the metal does not stop

completely, but it is usually decreased

considerably. The protection offered by the

passive barrier coating is different for differentenvironments.


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General Corrosion

Category III

The corrosion rate of the metal in an

environment is small enough for the metal

to be considered immune to attack in that

environment. Examples include gold

exposed to the atmosphere, or tin in sealedfood containers.


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General Corrosion


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General Corrosion


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General Corrosion


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General Corrosion


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General Corrosion


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General Corrosion


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General Corrosion


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General Corrosion

Methods of Control

Barrier Coatings

Inhibitors, pH Control, Deaeration

Cathodic Protection

Proper Materials Selection


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General Corrosion

Effect of Using Inhibitor


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Localized Corrosion


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Localized Corrosion


Corrosion that occurs at distinct locations,

usually relatively small when compared tothe entire surface area of the metal. Often

caused by chemical or physical differences

of local cells on metal.


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Localized Corrosion

Types of Localized Corrosion

Pitting Corrosion

Crevice Corrosion


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Localized Corrosion

Pitting Corrosion

Pitting corrosion takes the form of deep and

narrow corrosive attack, which often causesrapid penetration of the wall thickness.

(Forms of Corrosion)

Pitting can be limited to breaks or holidays inbarrier coating systems.


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Localized Corrosion

Pitting Shapes


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Localized Corrosion - Pitting


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Localized Corrosion - Crevice


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Localized Corrosion - Crevice


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Galvanic Corrosion


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Galvanic Corrosion


When two metals with differing electrical

potentials are electrically coupled to oneanother in an electrolyte. One metal will act

as the cathode and the other will act as the

anode. The corrosion rate of the anode canincrease and that of the cathode can

decrease depending on several variables.


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Galvanic Corrosion

Variables That Affect Galvanic Corrosion

Difference in electrical potential between

the dissimilar metals (or nonmetal as in thecase of carbon or mill scale)

Ratio of the surface areas

The electrolyte

Corrosion product films

Distance


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Galvanic Corrosion


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Galvanic Corrosion


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Galvanic Corrosion


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Galvanic Corrosion

Methods of Control

Barrier Coatings

Electrical Techniques

Design


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Galvanic Corrosion

Electrical Techniques


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Stress

The higher the stress, the greater the

possibility of cracking, both in frequency ofcracks, and the amount of time in which

they occur.


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Temperature

Increasing temperatures increase the

possibility of stress corrosion crackingoccurring. In contrast, hydrogen induced

cracking is most severe at ambient

temperatures, and the possibility forcracking decreases at higher temperatures.


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Stress Corrosion Cracking

A brittle failure mode in an otherwise ductile metal,

resulting from the combined action of a tensilestress and a specific corrosive environment. Only

specific corrosive media promote stress corrosion

cracking of an alloy system (e.g., caustic/carbon

steel, chlorides/stainless steel and ammonia/copperalloys).


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Environmental Cracking -

Hydrogen Induced Cracking


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Liquid Metal Cracking

A form of embrittlement that results from the

combined action of a tensile stress and aliquid metal in contact with the alloy

surface. Metals with low melting

temperatures, such as mercury, cadmiumand zinc, can cause liquid metal cracking.


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Methods of Control

Change the material

Electrochemical Techniques

Change the Design


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Summary

Types of Basic Corrosion

General

Localized

Galvanic



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Further Study


Forms of Corrosion Recognition and

Prevention Volume 1 AGA Technical Department and Library

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