Lecture 13: Basic Principles of Corrosion Control

• Basic Principles
• Materials Selection
• Design Against Corrosion
• Atmospheric Corrosion
• Corrosion Inhibitors
• Summary
• Reading Assignments

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Basic Principles of Corrosion Control

 (1) Materials selection

(2) Design

(3) Coatings

(4) Inhibitors

(5) Electrochemical protection

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Materials Selection

The minimum cost materials of low corrosion resistance require frequent maintenance and replacement.

Design Against Corrosion

Process Variables:

Relative humidity

Temperature

Pressure

pH

O₂ concentration

Solid or dissolved pollutants

Concentration

Velocity

Stress

etc

These variables have obvious effects on corrosion rate and materials of construction.

e.g. when T^oC is up , corrosion rate goes up , require more resistant materials.

Corrosion protection measures:

• do not introduce localized galvanic cells into the system.
• use of coatings, paintings and electrochemical protection.

Corrosion data: obtained under specific conditions must be treated with care when used

Design against corrosion:

(1) avoid bimetallic corrosion cell

(2) avoid differential aeration cells: crevices, debris traps

(3) avoid turbulent flow

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Atmospheric Corrosion

Major factors affecting atmospheric corrosion:

(1) Dust content

suspended salt particles, carbon compounds and metal oxides, etc. => abrasive and corrosive actions: combined with moisture initiate corrosion by galvanic or differential aeration.

(2) Gases in the atmosphere

SO₂, SO₃ and H₂S from burning of coal, oil and gasoline produce acids, increases corrosion rate.

(3) Moisture (humidity)

high R.H. results in differential aeration cells

(4) temperature

when T^oC is up, corrosion rate is up; but dissolved gases decreases , so corrosion rate decreases .

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Corrosion Control by Inhibitors

Inhibitor: a chemical when added in small quantity to the environ reduces corrosion rate.

Classification:

(1) anodic inhibitors/passivators: retard the anodic process, e.g.

• molybdates, silicates, phosphates, borates (requires dissolved O₂)
• chromates, nitrates (self-oxidizing)

(2) cathodic inhibitors: retard the cathodic process, e.g: zinc salts, magnesium salts

(3) mixed/adsorption inhibitors: retard both anodic and cathodic processes, e.g: chromate/polyphosphate/zinc

Anodic inhibitor more efficient but dangerous;
Cathodic inhibitor safer to use but less efficient;
Mixed inhibitor efficient and safe
Cathodic type to retard the overall corr rate
Anodic type to seal off the active anodic sites.

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Summary

It is easier to erase lines in a drawing than to replace failed components in a plant. Materials selection and design are most effective and economical ways of corrosion control and prevention. Relative humidity is a most important factor in atmospheric corrosion. The effect of temperature is somehow unpredictable. Coatings and electrochemical protection can also be specified at the design stage. Corrosion inhibitors if used properly can reduce the corrosion rate by 90% or more but they may induce localised corrosion such as pitting.

Reading Assignments

To reinforce learnings in this lecture read pages 256-273, 279-300 (textbook)
To prepare yourself for the next lecture read pages 302-335 (textbook)

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