lecture8 - corrosion prevention and control

7/31/2019 Lecture8 - Corrosion Prevention and Control

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Corrosion Prevention and Control

Degradation of Materials


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Primary and Secondary Mechanisms

Primary Corrosion Mechanism


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Primary and Secondary Mechanisms

Secondary Corrosion Mechanism

Contributions from people, designers,

manufacturers

Influence of a remote subsystem: effect of ac/dc Unpredictability in the environment


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Corrosion Prevention

Materials Selection

Design Factors

Life Prediction Analysis


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Materials Selection

Cost Availability

Raw materials

Quantity

Production costs

Labor requirements

Anticipated service

life

Made to order

Bought from

manufacturer

Available off the

shelf

Tooling required


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Materials Selection

Material Properties

Mechanical: hardness,creep, fatigue, stiffness,

compression, shear,impact, tensile, wear

Physical

Electrical

Magnetic

Thermal

Chemical

Corrosion resistance


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Design-Related Causes

Use of dissimilar metal

Poor drainage

Presence of crevices

Relative motion between two interacting parts orbetween a metal and the environment

Selective leaching of a metal in an alloy

Inability to clean the surface thoroughly


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Design Considerations

Environment

Stress

Shape

Compatibility Movement

Temperature


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Environment

Natural Chemical

Air

Waters

Soils

Urban and industrial

atmospheres

Environmental

pollutants

Chemical plant


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Environment


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Stress

Static Fluctuating

Stress Corrosion

Cracking

Corrosion Fatigue


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Stress


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Shape


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Compatibility

Within the metal

Within the environment

Contact with other materials

Welds


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Compatibility


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Movement

Flowing fluids

Parts moving in the fluid

Two-phase and multi-phase fluids


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Temperature

Environmental factors

Sources of heat

Condensation


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Corrosion Protection

Choice of Materials

Addition of Inhibitors

Use of Protective Coatings

Cathodic and Anodic Protection


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Addition of Inhibitors Purpose:

Suppress the corrosion reaction

Produce a competent corrosion product

Example Chemicals:

Zinc/polyphosphates Chromates

Phosphonates

Polymers

Nitrates Vanadates

Arsenates


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Addition of Inhibitors

Cathodic Inhibitors Anodic Inhibitors

Inhibit the hydrogen

evolution in acidic

solution or reductionof oxygen in neutral or

alkaline solution

Effective in the pH

range 6.5-10.5

Increase in potential

to reach passivation

region; dependent on

concentration of

inhibitor


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Protective CoatingsApplication of PAINTS as protective barriers PAINTS

Source of insulation of the metal from the corrosiveenvironment

HOW TO ACHIEVE CORROSION CONTROL BYCOATING? Barrier coating mitigates current transfer between

anode and cathode

Cathodic protection as in the case of Zn-containingpigments

Inhibitive primer which forms passive films or layers


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Composition of Paints

Vehicle: liquid that gives the paint its fluidity anddries or evaporates to form a solid film

Pigment: suspended in the vehicle, the pigment

controls the corrosion rate or the rate of diffusion

of the reactants through the dry film

Additives or Fillers: these accelerate the drying

process or better enable the dry coating to

withstand the working environment


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Protective Coatings

Barrier coatings must be: Impermeable to ionic species

Impermeable to oxygen

Able to maintain adhesion under wet conditions


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Metallic Coatings

provide enhanced corrosion resistance of metalsas either barrier coatings or sacrificial coatings

Durable, usually easy to form but sometimes

porous

Examples: Nickel, lead, zinc, copper, cadmium,

tin, chromium


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Inorganic Coatings

inorganic, nonmetallic coatings that act as abarrier between the corrosive environment and

the base material being protected

Oxide film

Useful for providing high temperature corrosion

protection

Examples: chromate films, phosphate coatings


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Cathodic Protection

Two Types: Impressed Current

Sacrificial Anode


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Cathodic Protection

Impressed Current


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Cathodic Protection

Impressed Current


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Cathodic Protection

Sacrificial Anode


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Cathodic Protection

Sacrificial Anode


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Anodic Protection

Make metal an anode and drive it into its passiveregion

Applicable only to metals which exhibit an active-

passive behavior such as Cr, N, Fe, Ti, Al and

alloys containing these elements


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Cathodic vs. Anodic Protection

lecture8 - corrosion prevention and control

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