4. Anodic Protection
❖Useful in strongly acidic/alkaline conditions.
❖Useful for metals showing active –passive
behaviour.
❖Iron,steel,aluminium,stainless steel etc show
passivity in certain range of potential applied
on them.
❖Sample Electrode-Anode (which is the
reactor,water tank,industrial water coolers etc)
❖Reference electrode-Cathode eg:Platinum
❖Counter electrode/Auxillary
electrode-Graphite.
❖Potentiostat-to apply desired voltage
5. Comparison of Anodic and Cathodic
Protection
Anodic protection Cathodic protection
Applicability
Metals Active-passive metals
only
All metals
Corrosives Weak to aggressive Weak to moderate
Relative cost
Installation High Low
Operation Very low Medium to high
Throwing power Very high Low
Significance of applied
current
Often a direct measure
of protected corrosion
rate
Complex-does not
indicate corrosion rate
Operating conditions Can be accurately and
rapidly determined by
Electrochemica
Measurements
Must usually be
Determined
by empirical testing
7. Organic Coatings
Paints:-
⚫ Paint is any colored liquid which on
drying, form a thin surface coat
and has specific functions to perform.
⚫ They are used to protect metals,
timber, or plastered surfaces from the
corrosive effects of weather, heat,
moisture or gases etc and to improve
their appearance.
⚫ Composition:
Vehicle- Non-volatile liquid
Thinner- turpentine
Drier
Fillers
Pigment- for colour
8. Qualities of a Good Paint
⚫ There are a number of ways to judge paint quality:
Wearability
Covering ability
Ease of cleaning
Protection of the substrate
Environmentally Friendly
Aesthetic
Practical and Cost Effective
9.
10.
11.
12.
13.
14.
15.
16.
17.
18. Weight Loss Method
R=KW/ATD
⚫ R= Rate of Corrosion
⚫ W= Weight Loss
⚫ A= Area of the metal piece to the nearest sq.m
⚫ T=Time of exposure to the nearest 100th
hour
⚫ D=Density of the metal g/cc
19. Varnishes
Varnish is a transparent, hard,
protective finish or film primarily
used in wood finishing but also
for other materials.
Composition:
drying oil- linseed oil
resin-Kauri gum
thinner or solvent-turpentine.
20. Qualities of a Good Varnish
A good varnish should possess the following properties
⚫ It should not hide the natural grains of the surface.
⚫ It should not shrink or show cracks after drying.
⚫ It should make the surface glossy.
⚫ It should dry rapidly.
⚫ It should impart the finished surface uniform colour
and pleasing appearance.
21. PourbaixDiagram( Eg:Iron-Water at 25⁰C)
Pourbaix diagram gives valuable information
regarding pH-potential conditions where
corrosion expected and how it can be
minimized.
Diagram/plot-plotting pH against values of
electrode potential.
Types of Zones:
❖Immunity-Upto -0.8V iron stable ,no
corrosion
,achieved by cathodic protection.
❖Corrosion -Dot- represents the point where
pH -7 and electrode potential 0.4V ,normally
iron rusts. Area –pH more than 12.
❖Passivity: Area above 0 V ,where the metal
becomes passive towards corrosion ,achieved
by Anodic protection.
