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Beginners Guide to Corrosion - Part 2


(continued from part 1)
 
3 LOCALISED CORROSION: 70% of failures
 
The consequences of localised corrosion can be a great deal more severe than uniform corrosion generally because the failure occurs without warning and after a surprisingly short period of use or exposure. Application of the five basic facts needs greater thought and insight.
 
3.1 GALVANIC CORROSION
 
This can occur when two different metals are placed in contact with each other and is caused by the greater willingness of one to give up electrons than the other. Three special features of this mechanism need to operate for corrosion to occur:
 
• The metals need to be in contact electrically
• One metal needs to be significantly better at giving up electrons than the other
• An additional path for ion and electron movement is necessary.
 
Prevention of this problem is based on ensuring that one or more of the three features do not exist.
 
Break the electrical contact using plastic insulators or coatings between the metals. Select metals close together in the galvanic series.
Prevent ion movement by coating the junction with an impermeable material, or ensure environment is dry and liquids cannot be trapped.
 
 
3.2 PITTING CORROSION
 
Pitting corrosion occurs in materials that have a protective film such as a corrosion product or when a coating breaks down. The exposed metal gives up electrons easily and the reaction initiates tiny pits with localised chemistry supporting rapid attack. Control can be ensured by:
 
• Selecting a resistant material
• Ensuring a high enough flow velocity of fluids in contact with the material or frequent washing
• Control of the chemistry of fluids and use of inhibitors
• Use of a protective coating
• Maintaining the material’s own protective film.
 
Note: Pits can be crack initiators in stressed components or those with residual stresses resulting from forming operations. This can lead to stress corrosion cracking.
 
 
3.3 SELECTIVE ATTACK
 
This occurs in alloys such as brass when one component or phase is more susceptible to attacke than another and corrodes preferentially leaving a porous material that crumbles. It is best avoided by selection of a resistant material but other means can be effective such as:
 
• Coating the material
• Reducing the aggressiveness of the environment
• Use of cathodic protection
 
 
3.4 STRAY CURRENT CORROSION
 
When a direct current flows through an unintended path and the flow of electrons supports corrosion. This can occur in soils and flowing or stationary fluids. The most effective remedies involve controlling the current by:
 
• Insulating the structure to be protected or the source of current
• Earthing sources and/or the structure to be protected.
• Applying cathodic protection
• Using sacrificial targets.
 
 
3.5 MICROBIAL CORROSION
 
This general class covers the degradation of materials by bacteria, moulds and fungi or their by-products. It can occur by a range of actions such as:
 
• Attack of the metal or protective coating by acid by-products, sulphur, hydrogen sulphide or ammonia
• Direct interaction between the microbes and metal which sustains attack. Prevention can be achieved by:
• Selection of resistant materials
• Frequent cleaning
• Control of chemistry of surrounding media and removal of nutrients
• Use of biocides
• Cathodic protection.
 
 
3.6 INTERGRANULAR CORROSION
 
This is preferential attack of the grain boundaries of the crystals that form the metal. It is caused by the physical and chemical differences between the centres and edges of the grain. It can be avoided by:
 
• Selection of stabilised materials
• Control of heat treatments and processing to avoid susceptible temperature range.
 
(to be continued in part 3)
 
This text is "Crown Copyright" and permission to quote/reprint has been granted by the National Physical Laboratory - Teddington England.
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