How to Prevent Metals from Corroding
Prevent uniform attack corrosion by protecting the metal surface., Prevent galvanic corrosion by halting ion flow from one metal to another., Prevent pitting corrosion by protecting the metal surface, avoiding environmental chloride sources, and...
Step-by-Step Guide
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Step 1: Prevent uniform attack corrosion by protecting the metal surface.
Uniform attack corrosion (sometimes shortened to "uniform" corrosion) is a type of corrosion that occurs, appropriately, in a uniform fashion over an exposed metal surface.
In this type of corrosion, the entire surface of the metal is under attack from corrosion and, thus, the corrosion proceeds at a uniform rate.
For example, if an unprotected iron roof is regularly exposed to rain, the entire roof surface will come into contact with roughly the same amount of water and thus will corrode at a uniform rate.
The easiest way to protect against uniform attack corrosion is usually to put a protective barrier between the metal and the corroding agents.This can be a wide variety of things
- paint, an oil sealant, or an electrochemical solution like a galvanized zinc coating.
In underground or immersion situations, cathodic protection is also a good choice. -
Step 2: Prevent galvanic corrosion by halting ion flow from one metal to another.
One important form of corrosion that can occur regardless of the physical strength of the metals involved is galvanic corrosion.
Galvanic corrosion occurs when two metals with differing electrode potentials are in contact with one another in the presence of an electrolyte (like saltwater) that creates an electrical conducting path between the two.
When this happens, metal ions flow from the more-active metal to the less-active metal, causing the more-active metal to corrode at an accelerated rate and the less-active metal to corrode at a slower rate.
In practical terms, this means that corrosion will develop on the more-active metal at the point of contact between the two metals.
Any method of protection that prevents ion flow between the metals can potentially halt galvanic corrosion.
Giving the metals a protective coating can help prevent electrolytes from the environment from creating an electrical conducting path between the two metals, while electrochemical protection processes like galvanization and anodizing also work well.
It's also possible to thwart galvanic corrosion by electrically insulating the areas of the metals that come into contact with each other.
Additionally, the use of cathodic protection or a sacrificial anode can protect important metals from galvanic corrosion.
See below for more information. , Pitting is a form of corrosion that takes place at the microscopic scale but can have large-scale consequences.
Pitting is of great concern for metals that derive their corrosion resistance from a thin layer of passive compounds on their surface, as this form of corrosion can lead to structural failures in situations where the protective layer would normally prevent them.
Pitting occurs when a small part of the metal loses its protective passive layer.
When this happens, galvanic corrosion occurs at a microscopic scale, leading to the formation of a tiny hole in the metal.
Within this hole, the local environment becomes highly acidic, which accelerates the process.
Pitting is usually prevented by applying a protective coating to the metal surface and/or using cathodic protection.Exposure to an environment high in chlorides (like, for example, salt water) is known to accelerate the pitting process. , Crevice corrosion occurs in spaces of a metal object where access to the surrounding fluid (air or a liquid) is poor
- for instance, under screws, under washers, under barnacles, or between the joints of a hinge.
Crevice corrosion occurs where the gap near a metal surface is wide enough to allow fluid to enter but narrow enough that the fluid has difficulty leaving and becomes stagnant.
The local environment in these small spaces becomes corrosive and the metal begins to corrode in a process similar to pitting corrosion.
Preventing crevice corrosion is generally a design issue.
By minimizing the occurrence of tight gaps in a metal object's construction through closing these gaps or allowing circulation, it's possible to minimize crevice corrosion.
Crevice corrosion is of special concern when dealing with metals like aluminum which have a protective, passive outer layer, as the mechanism of crevice corrosion can contribute to the breakdown of this layer., Stress corrosion cracking (SCC) is a rare form of corrosion-related structural failure that is of particular concern to engineers charged with building structures intended to support important loads.
In the event of SCC, a load-bearing metal forms cracks and fractures below its specified load limit
- in severe cases, at a fraction of the limit.
In the presence of corrosive ions, tiny, microscopic cracks in the metal caused by tensile stress from a heavy load propagate as the corrosive ions reach the tip of the crack.
This causes the crack to gradually grow and potentially cause eventual structural failure.
SCC is especially dangerous because it can occur even in the presence of substances that are naturally only very mildly corrosive to the metal.
This means that the dangerous corrosion occurs while the rest of the metal surface superficially appears unaffected.Preventing SCC is partly a design issue.
For instance, by choosing a material that is SCC-resistant in the environment in which the metal will operate and ensuring that the metal material is properly stress-tested can help prevent SCC.
Additionally, the process of annealing a metal can eliminate residual stresses from its manufacture.
SCC is known to be exacerbated by high temperatures and the presence of liquid containing dissolved chlorides. -
Step 3: Prevent pitting corrosion by protecting the metal surface
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Step 4: avoiding environmental chloride sources
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Step 5: and avoiding nicks and scratches.
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Step 6: Prevent crevice corrosion by minimizing tight spaces in the design of the object.
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Step 7: Prevent stress corrosion cracking by using only safe loads and/or annealing.
Detailed Guide
Uniform attack corrosion (sometimes shortened to "uniform" corrosion) is a type of corrosion that occurs, appropriately, in a uniform fashion over an exposed metal surface.
In this type of corrosion, the entire surface of the metal is under attack from corrosion and, thus, the corrosion proceeds at a uniform rate.
For example, if an unprotected iron roof is regularly exposed to rain, the entire roof surface will come into contact with roughly the same amount of water and thus will corrode at a uniform rate.
The easiest way to protect against uniform attack corrosion is usually to put a protective barrier between the metal and the corroding agents.This can be a wide variety of things
- paint, an oil sealant, or an electrochemical solution like a galvanized zinc coating.
In underground or immersion situations, cathodic protection is also a good choice.
One important form of corrosion that can occur regardless of the physical strength of the metals involved is galvanic corrosion.
Galvanic corrosion occurs when two metals with differing electrode potentials are in contact with one another in the presence of an electrolyte (like saltwater) that creates an electrical conducting path between the two.
When this happens, metal ions flow from the more-active metal to the less-active metal, causing the more-active metal to corrode at an accelerated rate and the less-active metal to corrode at a slower rate.
In practical terms, this means that corrosion will develop on the more-active metal at the point of contact between the two metals.
Any method of protection that prevents ion flow between the metals can potentially halt galvanic corrosion.
Giving the metals a protective coating can help prevent electrolytes from the environment from creating an electrical conducting path between the two metals, while electrochemical protection processes like galvanization and anodizing also work well.
It's also possible to thwart galvanic corrosion by electrically insulating the areas of the metals that come into contact with each other.
Additionally, the use of cathodic protection or a sacrificial anode can protect important metals from galvanic corrosion.
See below for more information. , Pitting is a form of corrosion that takes place at the microscopic scale but can have large-scale consequences.
Pitting is of great concern for metals that derive their corrosion resistance from a thin layer of passive compounds on their surface, as this form of corrosion can lead to structural failures in situations where the protective layer would normally prevent them.
Pitting occurs when a small part of the metal loses its protective passive layer.
When this happens, galvanic corrosion occurs at a microscopic scale, leading to the formation of a tiny hole in the metal.
Within this hole, the local environment becomes highly acidic, which accelerates the process.
Pitting is usually prevented by applying a protective coating to the metal surface and/or using cathodic protection.Exposure to an environment high in chlorides (like, for example, salt water) is known to accelerate the pitting process. , Crevice corrosion occurs in spaces of a metal object where access to the surrounding fluid (air or a liquid) is poor
- for instance, under screws, under washers, under barnacles, or between the joints of a hinge.
Crevice corrosion occurs where the gap near a metal surface is wide enough to allow fluid to enter but narrow enough that the fluid has difficulty leaving and becomes stagnant.
The local environment in these small spaces becomes corrosive and the metal begins to corrode in a process similar to pitting corrosion.
Preventing crevice corrosion is generally a design issue.
By minimizing the occurrence of tight gaps in a metal object's construction through closing these gaps or allowing circulation, it's possible to minimize crevice corrosion.
Crevice corrosion is of special concern when dealing with metals like aluminum which have a protective, passive outer layer, as the mechanism of crevice corrosion can contribute to the breakdown of this layer., Stress corrosion cracking (SCC) is a rare form of corrosion-related structural failure that is of particular concern to engineers charged with building structures intended to support important loads.
In the event of SCC, a load-bearing metal forms cracks and fractures below its specified load limit
- in severe cases, at a fraction of the limit.
In the presence of corrosive ions, tiny, microscopic cracks in the metal caused by tensile stress from a heavy load propagate as the corrosive ions reach the tip of the crack.
This causes the crack to gradually grow and potentially cause eventual structural failure.
SCC is especially dangerous because it can occur even in the presence of substances that are naturally only very mildly corrosive to the metal.
This means that the dangerous corrosion occurs while the rest of the metal surface superficially appears unaffected.Preventing SCC is partly a design issue.
For instance, by choosing a material that is SCC-resistant in the environment in which the metal will operate and ensuring that the metal material is properly stress-tested can help prevent SCC.
Additionally, the process of annealing a metal can eliminate residual stresses from its manufacture.
SCC is known to be exacerbated by high temperatures and the presence of liquid containing dissolved chlorides.
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