1. THE CORROSION PROCESS (Five facts that help with control)
series of steps tells us a lot about the corrosion process.
(1) Ions are involved and need a medium to move in (usually water)
(2) Oxygen is involved and needs to be supplied
(3) The metal has to be willing to give up electrons to start the process
(4) A new material is formed and this may react again or could be protective of the original metal
(5) A series of simple steps are involved and a driving force is needed to achieve them
Interfering with these steps allows the corrosion reaction to be stopped or slowed to a manageable rate.
2 UNIFORM CORROSION 30% of failures
Uniform corrosion, as the name suggests, occurs over the majority of the surface of a metal at a steady and often predictable rate. Although it is unsightly its predictability facilitates easy control, the most basic method being to make the material thick enough to function for the lifetime of the component. Uniform corrosion can be slowed or stopped by using the five basic facts;
down or stop the movement of electrons
(a) Coat the surface with a non-conducting medium such as paint, lacquer or oil
(b) Reduce the conductivity of the solution in contact with the metal an extreme case being to keep it dry. Wash away conductive pollutants regularly.
(c) Apply a current to the material (see cathodic protection).
(2) Slow down or stop oxygen from reaching the surface. Difficult to do completely but coatings can help.
(3) Prevent the metal from giving up electrons by using a more corrosion resistant metal higher in the electrochemical series. Use a sacrificial coating which gives up its electrons more easily than the metal being protected. Apply cathodic protection. Use inhibitors.
(4) Select a metal that forms an oxide that is protective and stops the reaction.
Control and consideration of environmental and thermal factors is also essential.
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.
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.
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 materialís 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 attack 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.
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.
CONCENTRATION CELL CORROSION (CREVICE)
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 that 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
3.8 THERMOGALVANIC CORROSION
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.
CORROSION CAUSED BY COMBINED ACTION
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, hydrogen ions are 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 and certain welding processes
∑ Removal of hydrogen in the metal by baking.
Note: The above aims to give an introduction to aspects of aqueous corrosion. It does not constitute a complete detailed analysis. Neither the Marine Corrosion Forum, nor any of its members can be held liable for any problems that arise from acting on these statements. Companies are advised to take specific advice for their corrosion problems, and not to rely on this overview.
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