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Basic Principles of Corrosion Control
(1) Materials selection
(2) Design
(3) Coatings
(4) Inhibitors
(5) Electrochemical protection
The minimum cost materials of low corrosion resistance require frequent maintenance and replacement.
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 ToC Ý , corrosion rate Ý , require more resistant materials.
Corrosion protection measures:
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
Major factors affecting atmospheric corrosion:
(1) Dust content
suspended salt particles, carbon compounds and metal oxides, etc. => abrasive and corrosiove 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, Ý corr rate.
(3) Moisture (humidity)
high R.H. results in differential aeration cells
(4) temperature
ToC Ý , corrosion rate Ý; but dissolved gases ß , so corrosion rate ß .
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.
(2) cathodic inhibitors: retard the cathodic process, e.g
zinc salts, magnesium salts
fig 13.2
(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
Ý
Cathodic type to retard the overall corr rate
Anodic type to seal off the active anodic sites.
It is earier 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 atmosperic corrosion. The effect of temperature is somehow unpredictable. Coatings and electrochemcial 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.
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)
