Corrosion is a process by which a metal loses its properties when it comes into contact with various oxidants in the environment. Corrosion takes many forms and can have many root causes. One common example of this is the rust formation process, during which iron oxides are formed in the presence of moisture. Corrosion is a major problem for builders of buildings, boats, aircraft, automobiles and most other metal structures. For example, when metal is used as part of a bridge structure, the structural integrity of that metal, which can be compromised by corrosion, is critical to the safety of the people using the bridge. See Step 1 below to understand how to protect metals from corrosion.
Method 1 of 3: Understanding the Common Types of Metal Corrosion
With so many different types of metals in use today, builders and manufacturers must protect them from various types of corrosion. Each metal has its own unique electrochemical properties that determine what types of corrosion, if any, the metal is prone to. The table below provides a selection of metals and the types of corrosion they may undergo.
|Metal||Vulnerability to metal corrosion||General preventive methods||Galvanic activity *|
|Stainless Steel (Passive)||Continuous corrosion, galvanic, point, crevice (all types of marine)||Cleaning the protective cover or insulation||Low (initial corrosion forms a persistent oxide layer)|
|Iron||Continuous corrosion, galvanic, crevice||Cleaning, protective coating or insulation, galvanizing, anti-corrosion coating||High|
|Copper||Continuous corrosion, leaching, stress||Cleaning, protective coating or insulation (usually oil or varnish), adding tin, aluminum or arsenic to melt||Moderate|
|Aluminum||Electroplating, point, crevice||Cleaning, protective coating or insulation, anodizing, galvanizing, cathodic protection, electrical insulation||High (initial corrosion forms a persistent oxide layer)|
|Copper||Galvanic, spot, aesthetic tarnishing||Cleaning, protective coating or insulation, adding nickel for smelting (especially for marine)||Low (initial corrosion forms a permanent coating)|
Note that the "Galvanic Activity" column refers to the relative reactivity of the metal, as described in a number of galvanic tables in the reference books. The main thing that needs to be understood here is: "the higher the galvanic activity of the metal release, the faster galvanic corrosion will take place when combined with a less active metal."
Step 1. Prevent continuous corrosion by protecting the metal surface
Continuous corrosion is a type of corrosion that occurs in a uniform manner on an open metal surface. In this type of corrosion, the entire metal surface is corroded and thus corrosion proceeds at the same rate. For example, if an unprotected iron roof is systematically exposed to rain, the entire roof surface will come into contact with approximately the same amount of water and thus erode at a uniform rate. The easiest way to protect against solid corrosion is to put a protective barrier between the metal and the corrosive agent. This can be avoided by a wide variety of methods, using paint, oil sealant or electrochemical solution such as galvanized zinc coating.
On the subway or when diving, cathodic protection is also a good choice
Step 2. Prevention of galvanic corrosion by stopping the ion flow from one metal to another
This is one of the important forms of corrosion that can occur regardless of the physical strength of the metals involved in galvanic corrosion. Galvanic corrosion occurs when two metals of different electrode potential are in contact with each other in the presence of an electrolyte (such as seawater) that acts as an electrical conductor between them. When this happens, metal ions flow from the more reactive metal to the less reactive metal, causing the more reactive metal to corrode at an accelerated rate and the less reactive metal 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 the flow of ions between metals can potentially stop galvanic corrosion. Protecting the metal with a protective coating can help prevent exposure to environmental electrolytes from creating an electrical conductor between the two metals, and electrochemical protection processes such as galvanizing and anodizing will work well as well. In addition, galvanic corrosion can be prevented by insulating areas of metals that come into contact with each other.
- Also, the use of cathodic protection or a sacrificial anode can protect against galvanic corrosion. See below for more information.
Step 3. Prevent pitting corrosion by protecting the metal surface, avoiding environmental sources of chloride, and avoiding dents and scratches
Pitting corrosion is a form of corrosion that occurs at the microscopic level, but can have large-scale consequences. It has a great impact on metals, which are protected from it by a thin layer of passive compounds on the surface, as this form of corrosion can lead to structural damage in situations where the protective layer usually gets in the way. Pitting corrosion occurs when a small portion of the metal loses its protective passive layer. When this happens, galvanic corrosion occurs at a microscopic level, resulting in small holes in the metal. Around these holes, the local environment becomes very acidic, which speeds up the process. Pitting corrosion can usually be prevented by applying a protective coating to the metal surface and / or using cathodic protection.
Contact with a high chloride environment (such as salt water) is known to accelerate the pitting process
Step 4. Prevent crevice corrosion by minimizing confined space in the design of the facility
Crevice corrosion occurs in areas of metal objects that have poor access to air or liquid, such as under screws, under washers, under sinks, or between hinge joints. Crevice corrosion occurs where a rupture near a metal surface is wide enough to allow liquid to enter, but narrow enough to allow liquid to exit, so it stagnates there. The local environment of these small spaces forms corrosion and the metal erodes as is the case with pitting. Crevice corrosion prevention is usually a design issue. It is necessary to minimize the occurrence of narrow gaps in the construction of metal objects by closing these gaps, or create a good flow of air or fluid to minimize crevice corrosion.
Crevice corrosion is particularly problematic when working with metals such as aluminum, which has a protective, passive outer layer, and the crevice corrosion mechanism can penetrate this layer
Step 5. Prevent stress corrosion by safe loading and / or annealing
Stress Corrosion (SCC) is a rare form of structural failure corrosion that is of particular interest to engineers who design structural structures designed to withstand severe stress. In the case of SCC, the load-bearing metal molds generate cracks and fractures below the specified load limit. In the presence of corrosive ions, tiny, microscopic cracks in the metal, caused by tensile stress from a heavy load, propagate as corrosive ions and reach the end of the crack. This leads to the fact that the cracks gradually begin to grow and can ultimately lead to the destruction of the structure. SCC is especially dangerous as it can occur even in the presence of substances that have very little effect on metal corrosion. This means that corrosion occurs while the rest of the metal surface appears to be unaffected.
- Preventing SCC is partly a design issue. For example, choosing an SCC material that is sustainable for the environment in which it will operate and testing it for sustainability can prevent SCC. In addition, the process of annealing the metal can eliminate the residual stress from its fabrication.
- SCC is known to be aggravated by high temperatures and the presence of fluids containing dissolved chlorides.
Method 2 of 3: Preventing Corrosion at Home
Step 1. Paint the metal surface
Perhaps the most common, affordable method of protecting a metal from corrosion is to simply coat it with a coat of paint. The corrosion process is moisture and oxidant interacting with the metal surface. Thus, when the metal is covered with a protective paint barrier, neither moisture nor oxidants can come into contact with the metal itself and there is no corrosion.
- However, the paint itself is prone to degradation. It is necessary to apply a new coat of paint when chips, wear or damage appear. If the paint decomposes to the point where the metal is exposed, be sure to check it for corrosion or damage.
There are many methods for applying paint to metal surfaces. Locksmiths often use some of these techniques in combination to ensure that the entire metal object is thoroughly coated. Below is a selection of the methods with comments:
- The brush is suitable for hard-to-reach places.
- The roller is used to cover large areas. A cheap and convenient way to cover.
- The spray is used to cover large areas. A fast method, but less effective than a roller (high percentage of ink loss).
- Airless / Electrostatic Airless Spray is used to cover large areas. The quick method allows you to apply paint in different thicknesses. Less wasteful than conventional spray. The equipment is expensive.
Step 2. Use marine paint for metal that comes in contact with water
Metal objects that regularly (or constantly) come into contact with water, such as boats, need special paints to protect against increased potential for corrosion. In such situations, “normal” rust corrosion is not the only problem (although it is one of the main ones), and marine life (shells, etc.) that can build up on unprotected metal can become an additional source of wear and corrosion. Be sure to use a high-grade marine epoxy paint to protect metal objects such as boats and so on. This type of paint protects the base metal from moisture and also inhibits the growth of marine life on its surface.
Step 3. Apply protective lubricants to advance metal parts
For flat, static metal surfaces, the paint does a great job of keeping moisture and preventing corrosion without affecting the metal's usefulness. However, paint is generally not suitable for moving metal parts. For example, if you paint over door hinges when the paint is dry, it will hold the hinge in place, preventing it from moving. If you force open the door, the paint will crack, leaving holes for moisture to reach the metal. The best choice for metal parts such as hinges, seams, bearings and so on is a suitable water insoluble lubricant. The thick consistency of this type of grease will naturally repel moisture and at the same time ensure smooth, smooth movement of your metal part.
Because lubricants do not dry as well as paints, they degrade over time and sometimes require reapplication. Re-lubricate metal parts to keep them in the area of the sealant
Step 4. Thoroughly clean metal surfaces before painting or lubricating
Whether you are using regular paint, marine paint, or a protective lubricant / sealant, you need to make sure the metal surface is clean and dry before starting the painting process. Make sure the metal is completely free of dirt, grease, residual welding debris, or existing corrosion could undermine your efforts, contributing to future corrosion.
- Dirt, dust and other debris make it difficult for paint, lubricants to keep paint or grease from adhering directly to the metal surface. For example, if you paint over a steel sheet with some metal shavings on the surface, the paint will adhere to the shavings, leaving gaps on the underlying metal surface. If these shavings fall off, the site becomes vulnerable to corrosion.
- If you paint or grease a metal surface with some pre-existing corrosion, you must make the surface even and smooth to ensure the best adhesion of the sealant to the metal. Use a wire brush, sandpaper and / or chemical rust remover to remove as much existing corrosion as possible.
Step 5. Keep unprotected metal items away from moisture
As noted above, moisture aggravates most forms of corrosion. If you cannot provide a metal surface with paint or sealant, you must take care that it is not exposed to moisture. Store unprotected metal tools in a dry place. This can improve their utility and lengthen their effective life. If your metal objects are exposed to water or moisture, you must clean and dry them immediately after use to prevent corrosion.
In addition to being careful about moisture exposure, be sure to store metal items in a clean, dry place. For large items that do not fit into the closet, tarp or cloth can be used. This will help trap moisture from the air and prevent dust from accumulating on the surface
Step 6. Keep metal surfaces as clean as possible
After each use of a metal part, whether painted or not, it is necessary to clean the functional surfaces by removing dirt or dust. The accumulation of dirt and debris on a metal surface can contribute to wear and tear and create holes in the metal and / or its protective coating, leading to corrosion over time.
Method 3 of 3: Electrochemical Corrosion Prevention
Step 1. Use the galvanizing process
Galvanize the metal with a thin layer to protect it from corrosion. Zinc is more reactive than the base metal, so it oxidizes on contact with air. Once the zinc layer is oxidized, it forms a protective coating, preventing further corrosion of the metal underneath. The most common type of galvanizing today is the hot-dip galvanizing process, in which metal parts (usually steel) are dipped into a vat of hot molten zinc to obtain a uniform coating.
This process involves processing with industrial chemicals, some of which are hazardous at room temperature and at extremely high temperatures, so only trained professionals can do this. The following are the main steps in the hot-dip galvanizing process on steel:
- The steel is cleaned with a caustic soda solution to remove dirt, grease, paint, etc., and then rinsed thoroughly.
- The steel is soaked in acid to remove scale and then rinsed.
- A material called "flow" is then applied and allowed to dry. This helps the zinc to adhere tightly to the steel.
- The steel is immersed in a molten zinc bath and left to warm to the zinc temperature.
- The steel is cooled in a quenching tank containing water.
Step 2. Use an anode
One way to protect metal objects from corrosion is to attach a small reactive piece of metal called a sacrificial anode to the metal. Due to the electrochemical relationship between a large metal object and a small reactive object, only a small, reactive lump of metal will corrode, leaving the large, important metal object intact. When the "sacrificial anode" is completely corroded by corrosion, it must be replaced or the corrosion will spread to a large metal object. This corrosion protection method is often used for underground structures such as underground tanks, or for objects in constant contact with water (boats).
- Anodes are made from several different types of reactive metals. Zinc, aluminum, magnesium are the most common ones used for this purpose. Due to the chemical properties of these materials, zinc and aluminum are often used for metal objects in salt water, while magnesium is more suitable for fresh water.
- The reason for the operation of the "sacrificial anode" is related to the chemistry of the corrosion process itself. When a metal object is corroded, regions are formed that are chemically similar to the anodes and cathodes in an electrochemical cell. Electrons leak from most of the anode portions of the metal surface into the surrounding electrolytes. Since the anodes are very reactive compared to the metal to be protected, the object itself becomes very cathodic, thus the flow of electrons from the “sacrificial anode” corrodes, but conserves the rest of the metal.
Step 3. Use the action of the current
Since the chemical process of metal corrosion involves electric current in the form of electrons flowing out of the metal, an external source of electric current can be used to counter current corrosion and prevent corrosion. Essentially, this process delivers a continuous negative electrical charge to the metal to be protected. This discharge overpowers the flowing electrons flowing out of the metal, stopping corrosion. This type of protection is often used for underground metal structures such as storage tanks and pipelines.
- Please note that the type of current used for modern protection systems is usually direct current (DC).
- Typically, electrical corrosion prevention is generated by burying two metal anodes in the soil near the metal object to be protected. The flow is directed through insulated wire for the anodes, which then pass through the soil into the metal object. The flow passes through a metal object and returns to the current source (generator, rectifier, etc.) through an insulated wire.
Step 4. Use anodizing
Anodizing is a special type of surface coating protection, and also uses stamps and so on. If you've ever seen a brightly colored metal carabiner, then you've seen an anodized metal surface. Rather than physically applying a protective coating, anodizing uses an electric current to provide the metal with a protective coating that prevents almost all forms of corrosion.
- The chemical anodizing process involves the fact that many metals, such as aluminum, naturally form chemical products called oxides when they come into contact with oxygen in the air. This leads to the fact that the metal usually has a thin outer layer of oxide, which protects (to varying degrees, depending on the metal) from further corrosion. The electrical current used in the anodizing process essentially creates a much thicker oxide layer on the metal surface, thereby providing greater protection against corrosion.
There are several different ways to anodize metal. The following are the main steps in this process: See How to Anodize Aluminum for more information.
- Aluminum is cleaned and degreased.
- Surface impurities of aluminum are removed with a special degreasing solution.
- Aluminum is immersed in an acid bath at a constant current and temperature (for example, 12 A / m2 and 21-22 degrees C).
- The aluminum is removed and washed.
- Aluminum is randomly immersed in the dye at 38 - 60 degrees C.
- Aluminum is sealed by placing it in boiling water for 20-30 minutes.
Step 5. Use a metal that is passivated
As noted above, some metals naturally form a protective oxide coating in air. Some metals form this oxide coating so efficiently that the metal ultimately becomes chemically inactive. We say that these metals are "passive" in reference to the passivation process by which they become less active. Depending on its use, a passivated object may not require additional corrosion protection.
One well-known example of such a metal that exhibits passivation is stainless steel. Stainless steel is an alloy of common steel and chromium that will not rust effectively in most conditions without requiring any other means of protection. Most stainless steel items are not corroded.
It is worth noting, however, that under certain conditions stainless steel is not 100% stainless, particularly in salt water. Likewise, many passive metals become non-passive under certain extreme conditions and therefore may not be suitable for all uses
- Be very careful with intergranular corrosion. This affects the metal's ability to form and manipulate, and it degrades the overall strength of the metal.
- Aluminum and steel boats should not have glued parts in order to best prevent metal corrosion.
- Never leave heavily corroded metal parts in a car or boat. Corrosion levels can vary significantly, but any corrosion can indicate severe structural damage. For safety reasons, replace or completely remove all signs of corrosion on the metal.
- When using "sacrificial anodes" refrain from painting the surface. This makes it impossible for the electrons to dry through the surface, and therefore cannot prevent the corrosion that occurs.