One of the biggest problems caused by improper exposure of steel to the environment is corrosion. This chemical phenomenon degrades the metal, even severely damaging its mechanical properties. In work involving storage and use of iron and steel, therefore, it is the designer's duty to analyze the environment to which the metal will be exposed, as well as the exposure time.
In general, it's more advantageous to use resistant materials or methods of protection against the adverse effects of the environment. However, often, mainly due to financial reasons, unprepared metals are used, even knowing that deterioration will occur.
In this post, we'll explain what steel corrosion is and how it occurs, and we'll present some ways to overcome this problem. Check it out!
Corrosion is a process of metal deterioration resulting from the interaction between natural agents, such as oxygen gas present in the air, and atoms on the metal's surface. When the material contains iron, we call the product of this reaction "rust" (iron oxide). All metals can corrode, producing different "types of rust."
Some damage resulting from this wear is merely cosmetic. Other, more serious, damage involves parts failing, which lose their mechanical properties and become more fragile, potentially causing leaks, ruptures, and process interruptions.
These structural changes can occur in different ways: affecting the entire material or just part of it, in cracks or in the grains (internal structures) of the metal.
Corrosion is caused by two electrochemical processes: oxidation and reduction. The first is the removal of free electrons from an atom. The second occurs when the atom takes free electrons for itself.
For corrosion to occur, there must be free electrons, that is, atoms that need electrons to stabilize themselves, and a medium through which this will occur (such as water, for metals, present even in atmospheric air).
Specific paints serve as protection, creating a kind of barrier or layer against external agents on the metal surface. When applied this way, they are called inhibitors. The application of inhibitors eliminates or significantly reduces the corrosion process.
The coating can also be temporary, for transportation or storage purposes, using oils or emulsions. In this case, subsequent cleaning is an important step in the process. Remember to also prevent the material from coming into contact with water before it is protected. The material must be clean and dry before applying inhibitors.
Another way to prevent corrosion is to choose materials specifically designed for situations where the problem can be anticipated—even if it can't be avoided. After all, if the designer already knows where a particular metal will be used, why not prevent corrosion at its source, avoiding future problems?
For this, there are the alloy steels (steel alloys) with stainless properties. These metals are obtained by adding nickel and chromium. The latter element, when reacting with oxygen, creates a very thin layer of corrosion, preventing deeper corrosion.
One of the best-known and most effective materials for preventing corrosion is stainless steel, which is highly durable, especially when used in more aggressive applications. There are limitations to its use when chlorine and salinity are present in the environment, but overall, this is an effective solution.
In addition to stainless steel, there are galvanized and galvalume steels. Galvanized steel has a layer of zinc deposited on its surface. This coating, like stainless steel, serves as a sacrificial layer.
Galvalume is an alloy of aluminum, zinc, and silicon. Its major advantage is its resistance to corrosion in open atmospheric environments, making it ideal for roofing. It can also undergo surface treatments, further improving its durability.
Simpler corrosion prevention methods involve preventing metal joints from accumulating water, acting as a barrier to atmospheric air, or allowing water to flow between the plates. If complete water retention is not possible, the suggestion is to create drainage gaps in the joints.
Controlling humidity and light in the storage area is also important for preventing corrosion. In an atmosphere of low relative humidity, iron practically does not corrode. The higher the humidity, the faster the corrosion process.
For those willing to invest, air conditioning devices that manage humidity levels are available on the market. These include controllers and thermostats that maintain the display environment within the required parameters to improve the material's performance.
If rust still appears on your parts after taking the aforementioned precautions, there are ways to eliminate it, minimizing the negative impacts on the metal structure.
It's important to emphasize that the appearance of corrosion on an object indicates that, at least in the affected area, its mechanical properties have been altered. Removing rust doesn't restore the material's internal structures, but it does prevent the deterioration process from occurring.
Therefore, if the corroded area is large or located in a highly stressed area of the mechanical system, it's important to consider replacing the part with a new, rust-free one. A good preventive maintenance program is an effective way to prevent parts from becoming unusable due to mechanical corrosion.
The corroded part can be placed in a solution of water and vinegar overnight and scraped with aluminum foil in the morning to remove rust residue. Other acidic solutions can also be used for this purpose: lemon water, water with phosphoric acid, or hydrochloric acid (found in cleaning products).
There are also commercial products specifically for removing rust, which can be purchased at hardware, iron and steel stores.
Mechanical abrasion is the process of scraping the metal surface with sandpaper or a grinder, usually recommended for larger areas. Start removing residue with coarser sandpaper, then use finer sandpaper to erase scratches caused by previous sandpaper and finish.
It is recommended that treated surfaces be coated with corrosion-inhibiting paint. But be careful: do not apply any paint directly to the rust! It must first be removed from the part to ensure a good result. Covering rust with unsuitable paints can cause it to spread too quickly, worsening the situation.
As we can see, there are several ways to prevent steel corrosion. Analyze the metal's application site to determine which solution you can use to prevent wear on metal items.
And you, have you thought of any other ways to prevent corrosion in steel and iron? Tell us in the comments and share your knowledge!
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