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Beginners Guide to Corrosion - Part 3

(continued from part 2)
If two areas of a component in close proximity differ in the amount of reactive constituent available the reaction in one of the areas is speeded up. An example of this is crevice corrosion which occurs when oxygen cannot penetrate a crevice and a differential aeration cell is set up. Corrosion occurs rapidly in the area with less oxygen. The potential for crevice corrosion can be reduced by:
• Avoiding sharp corners and designing out stagnant areas
• Use of sealants
• Use welds instead of bolts or rivets
• Selection of resistant materials
Temperature changes can alter the corrosion rate of a material and a good rule of thumb is that 10 oC rise doubles the corrosion rate. If one part of component is hotter than another the difference in the corrosion rate is accentuated by the thermal gradient and local attack occurs in a zone between the maximum and minimum temperatures. The best method of prevention is to design out the thermal gradient or supply a coolant to even out the difference.
This is corrosion accelerated by the action of fluid flow sometimes with the added pressure of abrasive particles in the stream. The protective layers and corrosion products of the metal are continually removed exposing fresh metal to corrosion. Prevention can be achieved by:
• Reducing the flow rate and turbulence
• Use of replaceable or robust linings in susceptible areas
• Avoiding sudden changes of direction
• Streamlining or avoiding obstructions to the flow
The combined action of cyclic stresses and a corrosive environment reduce the life of components below that expected by the action of fatigue alone. This can be reduced or prevented by:
• Coating the material
• Good design that reduces stress concentration
• Avoiding sudden changes of section
• Removing or isolating sources of cyclic stress
Relative motion between two surfaces in contact by a stick-slip action causing breakdown of protective films or welding of the contact areas allowing other corrosion mechanisms to operate. Prevention is possible by:
• Designing out vibrations
• Lubrication of metal surfaces
• Increasing the load between the surfaces to stop the motion
• Surface treatments to reduce wear and increase friction coefficient.
The combined action of a static tensile stress and corrosion which forms cracks and eventually catastrophic failure of the component. This is specific to a metal material paired with a specific environment. Prevention can be achieved by:
• Reducing the overall stress level and designing out stress concentrations
• Selection of a suitable material not susceptible to the environment
• Design to minimise thermal and residual stresses
• Developing compressive stresses in the surface the material
• Use of a suitable protective coating
A surprising fact is that hydrogen atoms are very small and hydrogen ions even smaller and can penetrate most metals.
Hydrogen, by various mechanisms, embrittles a metal especially in areas of high hardness causing blistering or cracking especially in the presence of tensile stresses. This problem can
be prevented by:
• Using a resistant or hydrogen free material
• Avoiding sources of hydrogen such as cathodic protection, pickling processes and certain welding processes
• Removal of hydrogen in the metal by baking.
This text is "Crown Copyright" and permission to quote/reprint has been granted by the National Physical Laboratory - Teddington England.
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