Transcript Shipwrecks, Corrosion and Conservation

Shipwrecks, Corrosion and
Conservation
Summary Slides PART 3 – Jack Dengate
• Stainless steels are favoured
corrosion resistant metals because of
a passive film of chromium (III)
oxide on the surface that resists
corrosion.
Corrosion resistant
metals
• Surface alloys can be
created by various
methods to give ordinary
steel a surface similar to
stainless steel.
• The hull of a ship would
have the corrosion
protection of an outer
layer of stainless steel
without the expense of
this material.
Surface Alloys
• Polymer paints protect against rust
by forming a film over the surface
of the steel that is impervious to
oxygen and water.
• These paints also form a layer of a
very insoluble ionic substance
called pyroaurite.
• This ionic layer bonds strongly to
the surface of the steel and well into
the polymer layer. It prevents the
movement of ions across the surface
of the steel.
Polymer Paints
Electrolysis
• Reduction occurs at the negative electrode, the cathode
• Oxidation occurs at the positive electrode, the anode
• The charge on the electrodes is different for an
electrolytic cell and a galvanic cell. The cathode is
negative in an electrolytic cell while the cathode is
positive in a galvanic cell
Electrolysis
• Anions carry charge towards the anode
• Cations carry charge towards the cathode
• Electrolysis only occurs when the voltage applied to the
cell is Greater than the calculated cell potential
• The reactions that occur at the anode and cathode can
depend on the applied voltage.
Electrolysis
An example of cathodic
protection, described in
terms of the
oxidation/reduction
chemistry:
An iron tank buried in the
ground may be protected
from corrosion by attaching
a block of zinc to it.
When someone is available to
monitor the tank and manage
the situation, this method of
cathodic protection is cheaper
than using a galvanised tank.
Cathodic Protection
• The zinc block is easily replaced when it has nearly
completely corroded.
• The zinc is more active than the iron and corrodes
preferentially.
• The zinc block and iron from the tank form an
electrochemical cell and the zinc is oxidised as the anode.
Zn 
Zn2+ + 2e–
Cathodic Protection
• The electrons flow into the iron preventing the formation
of Fe2+ ions. The electrons produced by the oxidation of
the zinc reduce any Fe2+ ions that form, back to Fe
atoms.
• The site where reduction occurs is called the cathode and
the method is called cathodic protection.
Fe2+ + 2e–  Fe
Cathodic Protection
• The rate of corrosion of the zinc block is slow because it
forms a coating of dull white zinc hydroxide carbonate
which protects the underlying zinc.
• In marine environments sacrificial anodes are usually
used. Zinc sacrificial anodes attached to outboard motors
gradually dissolve as they are “sacrificed” instead of the
metal parts of the outboard motor.
Cathodic Protection
• In wet terrestrial environments, such as the surroundings
of a metal pipeline, the impressed current method is
usually used. Insertion of a Direct Current power supply
between a metal or graphite anode and the pipeline
provides a flow of electrons to protect the pipeline metal.
•
The surroundings must be wet so that electrolytes can
flow through the ground to complete the electric circuit
between the negative pipeline and the positive anode. The
anode should last a long time as it is not sacrificed when
an impressed current is used.
Cathodic Protection