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CATHODIC PROTECTION

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					         CATHODIC PROTECTION

First example – Humphrey Davey – 1824 – British naval ships


The standard corrosion reaction:

                     M→Mn+ + ne-             2H+ + 2e- → H2

In cathodic protection:

                     - supplying extra electrons to the system
                           - suppresses metal dissolution

2 ways      -        external power supply
                     or
                     appropriate galvanic coupling

                     ___________________________


Impressed current technique


Insulated external power supply – connected to circuit

-ve terminal connected to object
+ve terminal connected to inert graphite electrode


Supplies electrons
to the tank and
suppresses
corrosion

Very common in
underground
storage vessels and pipelines
Galvanic coupling technique

i.e. ‘sacrificial anode’

e.g. Magnesium in a hot water tank

Mg anode corrodes preferentially when
galvanically coupled

Eventually – it needs replacing

CAUTION!
      all that Mg in solution……
a problem?


Also – used to protect pipelines




      - placed at regular intervals
            - hard to replace – so make them big enough!

Magnitude of protective current
– several standard values:

the more aggressive the
environment – the higher
the required current
If an appropriate value is not known,

measure the potential using a reference electrode

common reference:
copper sulphate
(see earlier notes)

reasonably cheap set-up
and quite accurate

can also be used to determine how many
sacrificial anodes are required

choice of anode material is important

Mg is not very efficient – but generates a
high potential




Also – for underground systems – type of ‘ground’ important
      i.e. stray current
aspects

e.g. tanks and pipelines
in close proximity!

Big problem in high
density areas


One novel initiative:

Cathodic protection of metal bridges using solar panels to generate
current (USA, Japan)
             ANODIC PROTECTION

Fairly new concept
      - based on advanced electrochemistry technologies
Basics – formation of protective film through externally applied
currents

in theory – anodic currents should increase corrosion rate

However – reverse is true for active-passive metals

      e.g. Ni, Fe, Cr, Ti etc…

Careful application of currents → passivation occurs

            -      must use a potentiostat!         (previous notes)

e.g. anodic protection of steel tank
containing sulphuric acid

3 terminals – tank, AUX, REF

Potentiostat – maintains constant
potential between tank and REF




Anodic protection is very effective if done right

				
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