Corrosion is natural and unavoidable. All on-site machinists should know how to diagnose and treat all types, depending upon the environment, equipment and material being used.
In this blog, we will outline how to treat some of the most common varities known to on-site machinists. Please note that this is a follow-on blog from 15 of the most common forms of corrosion on-site machinists will see.
It is recommended you read this first to gain content into the treatment processes. Click here to view the article >>
Uniform & Localised
As compiled by the Beginners Guide to Corrosion, we start with uniform. Uniform can be slowed or stopped by using these 4 basic steps:
- 1 - Slow down or stop the movement of electrons by coating the surface with a non-conducting medium such as paint, lacquer or oil. Reduce the conductivity of the solution in contact with the metal and wash away conductive pollutants regularly.
- 2 - Slow down or stop oxygen from giving up electrons from reaching the surface. Difficult to do completely but coating can help.
- 3 - Prevent the metal from giving up electrons by using a more resistant metal higher in the electrochemical series. Use a sacrificial coating which gives up its electrons more easily then the metal being protected
- 4 - Select a metal that forms an oxide that is protective and stops the reaction.
For localised versions, application of the same basic steps needs to be undertaken with more thought and insight required into the machine, materials, etc.
Preventing this problem is based on ensuring that one or more of the following three features do not co-exist together. These are:
- Breaking the electrical contact using plastic insulators or coatings between the metals
- Selecting metals close together in the galvanic series
- Preventing ion movement by coating the junction with an impermeable material, or ensure environment is dry and liquids cannot be trapped
Pitting can be controlled by:
- Selecting a resistant material
- Ensuring a high enough flow velocity of fluids in contact with the material
- Control of the chemistry of fluids and use of inhibitors
- Use of protective coating
- Maintaining the material's own protective film
Note that pits can crack initiators in stressed components or those with residual stresses resulting from forming operations.
Selective attack is best avoided by selecting a resistant material, but other means can be effective such as:
- Coating the material
- Reducing the environment aggressiveness
- Using a cathodic protection
The most effective remedies for stray currents require you to control the current. This can be done by:
- Insulating the structure to be protected
- Earthing the sources and the structure to be protected
- Applying cathodic protection
- Using sacrificial targets
Microbial such as bacteria, moulds or fungi can be difficult to remove completely. Prevention can be achieved by:
- Selection of a resistant material and frequent cleaning
- Controlling chemistry of surrounding media and removal of nutrients
- Use of biocides and cathodic protection.
Caused by the physical and chemical differences between the centres and edges of the grain. It can be avoided by using:
- A selection of stabilised materials
- A control of heat treatments and processing to avoid susceptible temperature range
Also known as crevice, the potential for concentration cell can be reduced by:
- Avoiding sharp corners designing out stagnant areas
- Use of sealants
- Use welds instead of bolts or rivets
- Applying a selection of resistant materials
Components that experience changes of temperature 10 deg C or over has a chance of doubling the corroding rate. The best method of prevention is to design out the thermal gradient or supply a coolant to even out the difference.
Accelerated by the action of fluid flow with added pressure can corrode materials too. Prevention can be achieved by regularly:
- Reducing the flow rate and turbulence
- Using replaceable or robust linings in susceptible areas
- Avoiding sudden changes of direction
- Streamlining or avoiding obstructions to the flow
Fatigue can be reduced by:
- Coating the material
- Good design that reduces stress concentration
- Avoiding sudden changes of section
- Removing or isolating sources of cyclic stress
A breakdown of protective films or welding can cause fretting. Prevention is possible by:
- Designing out vibrations
- Lubrication of metal surfaces
- Increasing the load between surfaces to stop the motion
- Surface treatments to reduce the wear and increase friction coefficient.
The combination of a static tensile and corrosion will eventually create stress cracking. Prevention can be achieved by:
- Reducing overall stress level and designing out concentrated stress
- Selecting a suitable material not susceptible to the environment
- Design to minimise thermal and residual stresses
- Developing compressive stresses in the surfaces of the material
- Use of a suitable protective coating
Hydrogen can penetrate most metals, the best way to prevent the problem as best as possible is to:
- Use a resistant or hydrogen free material
- Avoid hydrogen sources such as cathodic protection
- Removal of hydrogen in the metal by backing.
Of course, these are just 15 ways in which corrosion can occur, and there will be many more ways of solving or preventing problems like the above, ever happening. Feel free to share your problems and solutions below in the comments area.
Photo Credit: Wikipedia