Corrosion and degradation of materials

• Corrosion

• : the destructive electrochemical attack of a material.

• Oxidation reaction

• Reduction reaction

Electrochemical Corrosion

• Corrosion: Metal is the anode (-), V_{metal} < 0

• Electrodeposition: Metal is the cathode (+), V_{metal} > 0

Standard Hydrogen Electrode

• Corrosion

• Metal is the anode (-), V_{metal} < 0

• Electrodeposition

• Metal is the cathode (+),

Galvanic Series

• Ranking of the reactivity of metals/alloys in seawater

• Uniform Attack:

• Oxidation & reduction reactions occur uniformly over surfaces.

• Selective Leaching

• : Preferred corrosion of one element.

• Galvanic:

• Dissimilar metals are physically joined in the presence of an electrolyte.

• Crevice:

• Narrow and confined spaces

• Pitting:

• Downward propagation of small pits and holes.

• Pitting:

• Downward propagation of small pits and holes.

• Stress corrosion:

• Corrosion at crack tips when a tensile stress is present.

• Erosion-corrosion:

• Combined chemical attack and mechanical wear.

Corrosion Prevention

• Materials Selection:

• Environmental Alteration:

• Apply physical barriers.

• Add inhibitors:

• Cathodic (or sacrificial) protection:

• Materials Selection:

• use metals that are relatively unreactive in the corrosion environment.

• Use metals that passivate.

• Environmental Alteration:

• Lower the temperature.

• Add inhibitors:

• Substances added to a solution that decrease its reactivity.

• Cathodic (or sacrificial) protection:

• Attach a more anodic material to the one to be protected.

Degradation of polymers

• Polymeric materials deteriorate by physio-chemical means

• Mechanisms for degradation are not well understood due to the chemical complexity of polymers

Types of Deterioration

• Swelling and dissolution

• Bond Rupture (‘scission’)

• Weathering

• Swelling

• the liquid or solute diffuses into and is absorbed within the polymer

• Dissolution:

• occurs when the polymer is completely soluble

• Radiation Effects:

• Chemical Reaction Effects

• Thermal Effects:

• Bond Rupture (‘scission’)

• Radiation Effects:

• certain types of radiation can penetrate a polymer and cause the removal of an orbital electron (ionization) causing the breaking of a bond

• Chemical Reaction Effects:

• Ozone, oxygen etc. can cause scission as a result of chemical reaction

• Thermal Effects:

• degradation happens when polymers are subjected to high temperatures resulting in scission of molecular chains

• Weathering

• Exposure to outdoor conditions which may be a combination of several processes - usually by means of oxidation