Shipwrecks, Corrosion and ConservationSummary Slides PART 3 – Jack Dengate

Corrosion resistant metals

• Stainless steels are favoured corrosion resistant metals because of a passive film of chromium (III) oxide on the surface that resists corrosion.

Surface Alloys

• 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.

Polymer Paints

• 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.

Electrolysis

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.

Cathodic Protection

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.