As an investment casting foundry with over two decades of experience working with metal castings, we’ve found that metal rust is one of the most common concerns among our customers. Rust can damage the internal structure of iron or iron-containing castings, compromising their performance. So what exactly is metal rusting? What causes it? And how can we prevent it? If these questions interest you, this article will provide the answers you seek.
What is Rust?
Rust, generally speaking, is a brownish-red, flaky or powdery, porous, and brittle coating formed when iron and steel have oxidation reactions with water and oxygen in the air. Under normal conditions, iron exposed to air produces various compounds, while rust does not refer to any single one of these but is the collective term for all iron oxides.
Rust is considered a classic example of corrosion, which encompasses reactions between other metals and corrosive media. This includes the formation of patina on copper when exposed to air and the gradual loss of luster in silver, known as passivation—both of which are forms of corrosion.
The Causes of Rust Formation
Metal rusting is an electrochemical reaction. This occurs because iron or iron-containing metals do not maintain their most stable molecular structure during our daily use. Consequently, they are prone to go through redox reactions with molecules in the air to achieve a more stable state. The general process involves iron losing electrons, while oxygen in the air captures these electrons to form new substances, such as rust (iron oxide).
It is important to note that water, salt, acids, and impurities within the metal can accelerate this process. Acidic environments and the conductive particles formed when salt dissolves in water quicken the flow of electrons from the metal (iron) to oxygen. Impurities within the metal create more favorable sites for the reaction to occur.
Rust Formation During the Casting Process
Beyond the rusting that occurs naturally during use, the risk of metal rust exists throughout the entire manufacturing process of castings. From production to packaging, each step may become the culprit of metal rust due to the numerous variables in the manufacturing environment.
First, if components are not thoroughly dried during cleaning, residual moisture on the surface will create favorable conditions for rust formation. Improper use of gloves or handling tools during transportation can leave more or less degrees of contamination on the metal surface, making corrosion more likely to occur in these areas. Furthermore, packaging with untreated materials containing acids or moisture may create a humid, corrosive gas environment within the sealed packaging space.
Therefore, metal rusting is a systemic issue that places higher demands on a factory’s “inter-process rust prevention” and “finished product rust prevention”. If you have concerns in this aspect, Pingheng Machinery is your ideal choice. Following acid pickling of metal castings, Pingheng Machinery can conduct salt spray testing according to your casting requirements to ensure our products meet your rust prevention standards.
Types of Rust
As mentioned above, rust primarily refers to various iron oxide hydrates produced by the chemical reaction between iron, oxygen, and water. Understanding the types of rust helps us correctly identify different forms of corrosion, analyze their causes, and select appropriate rust removal methods. Based on their appearance and form, rust can be primarily categorized into the following types.
Red Rust
Red rust is the most common type of rust, which is primarily composed of Fe₂O₃•H₂O. It exhibits a characteristic reddish-brown appearance. Red rust has a soft, powdery texture and forms a uniform rust. This rust develops when moisture comes into contact with iron in an oxygen-rich environment. Due to its porous structure, the underlying metal continues to corrode until it is completely consumed.
Yellow Rust
The primary component of yellow rust is FeO(OH)₂O, which is also called goethite in mineralogy. The color of this type of rust ranges from pale yellow to orange-yellow to brown. It typically forms only in highly humid and poorly ventilated environments (such as underwater), and exhibits a loose rust layer which is similar to red rust.
Brown Rust
Brown rust mainly consists of Fe₂O₃, which is formed by atmospheric oxygen and water. It is the final stage of typical rust development, appearing as an uneven, hard, brown crust on metal surfaces. Brown rust is drier than the aforementioned rust forms because its water molecules have separated and evaporated, leaving behind a more solid iron oxide that typically manifests as spots. However, it still can not prevent the further rusting of the underlying metal.
Black Rust
Black rust, which is also called black iron, primarily consists of Fe₃O₄ and is not commonly found on metal products. It typically forms in high-temperature, low-oxygen environments, manifesting as a dense black film on the metal surface. Unlike the rust mentioned above, this stable oxide film prevents further oxidation of the underlying metal.
It is worth noting that different types of rust may appear simultaneously, and the cause of each may vary. For instance, brown rust may result from contaminants left behind during the manufacturing process.
Effects of Rust
There is a reason that metal rusting has long been a persistent problem for us all, from procurement professionals to individual buyers. The financial, labor, and material losses it causes warrant our serious attention. Let’s now examine the multifaceted impacts of metal rust in detail.
Weakened Material Performance
Rust may transform still, hard metals into powdery or loose layered structures, compromising their structural integrity and weakening the material. Localized rust can create holes in metal sheets, accelerating material deterioration. Rust reduces both the strength and hardness of metal, while generating internal stresses that make it more prone to deformation and cracking.
Rusted metal may cause functional failure in certain equipment and tools. For instance, rust acts as an insulator, potentially causing electrical circuits to malfunction. Moving mechanical parts may seize up due to rust.
Reduced Lifespan
The reduction of the service life of metal is the inevitable outcome of metal rust, while the rusting will intensify the ageing and failure of the material, thus curtailing its longevity.
Increased Costs
The increase in the cost is primarily reflected in the following aspects:
Maintenance costs: In order to prevent further detriment, regular maintenance is indispensable. Including, but not limited to, frequent rust removal and touch-up painting
Repair costs: Promptly replace or repair rusted components that are difficult to maintain daily.
Downtime costs: Production stoppages caused by equipment maintenance or repairs also result in financial losses.
Waste of Resources
If materials rust prematurely due to improper maintenance or usage, leading to early scrapping will increase the waste of resources. It runs counter to the globally advocated principles of sustainable development.
Decreased Aesthetics
When metal with decorative requirements rusts, its reddish-brown color and porous, loose appearance inevitably diminish its aesthetic value, making it look unpleasing.
Safety Hazard
Metal rust reduces the load-bearing capacity of materials, which may cause certain structural components to collapse or detach, posing potential safety hazards.
How to Prevent Metal Rust?
As discussed above, we have seen the impact of metal rust, which can cause significant additional losses to your industry. However, investing a small amount in routine maintenance measures can save you substantial money, time, and effort. Therefore, we provide the following rust prevention methods for your reference, hoping they will be of assistance.
Use an Alloy/Rust-Resistant Metal
Selecting more corrosion-resistant metals is the most fundamental approach to preventing rust. Typically, people opt for more rust-resistant alloys to prevent metal rust. While all metals are susceptible to corrosion, their resistance varies significantly. Thus, alloys generally offer superior corrosion resistance compared to pure metals. Metals such as stainless steel, aluminum alloys, titanium alloys, and copper alloys spontaneously form a dense oxide layer on their surfaces. This layer acts as a barrier against atmospheric moisture and oxygen, fundamentally preventing rust formation.
Apply Oil
In the routine maintenance of metal products, particularly for short-term storage and daily upkeep of tools and mechanical components, oiling is a common practice. Lubricants effectively repel water to delay metal corrosion. However, oils may also cause damage to certain parts of the environment, so they must be used judiciously.
Coatings/Paints
Applying paint or coatings to metal surfaces is one of the most common rust prevention methods, forming a physical barrier that isolates the metal from water and oxygen.
You can select different paintings based on the metal’s usage environment. For instance, in humid conditions, choose paint with water-resistant properties.
Common rust-preventive coatings include liquid paints such as epoxy primers and polyurethane topcoats, as well as powder coatings like acrylic, vinyl, and epoxy resins.
Galvanization
Hot-dip galvanizing is a common and effective industrial method for preventing metal rust. By applying a dense, heavy layer of zinc to the metal surface, it effectively blocks water and oxygen, delaying rust formation. Since zinc is more chemically reactive than iron, it reacts with air first, providing effective protection for the underlying metal.
In addition to galvanizing, electroplating with nickel and chromium is also widely used.
Regular Maintenance
In addition to physical rust prevention methods, routine maintenance is also essential. This includes but is not limited to daily inspections, cleaning, repainting, identifying issues, and addressing them promptly.
Store Properly
Proper storage is equally important. Metals must be kept away from humid environments, with the core principle being to maintain a dry, clean, and well-ventilated storage area. The environment during material transportation must also remain dry, and desiccants can be used in all these settings.
Blueing
Bluing is commonly used to protect firearm barrels by forming a dense layer of iron(III) oxide on the metal surface, typically blue or black. While it is aesthetically pleasing and provides rust prevention, regular oiling is required to maintain optimal protective performance.
VCI Packaging
VCI packaging is a highly efficient technology, acting like an “invisible protective shield” for metal products. The primary composition of VCI (Vapor Corrosion Inhibitors) depends on which metal will be protected. However, they all can gradually transition from solid or liquid to vapor form at specific temperatures, filling the entire packaging space. This process creates an invisible, imperceptible, and dense protective film on the metal surface—even within crevices and intricate threads. This film effectively blocks water and oxygen from contacting the metal surface, preventing metal rust from the very fundamentals.
Conclusion
After reading this article, you should now have a clear understanding of the dangers of metal rust and the necessity of rust prevention. If you are seeking a suitable supplier for your metal casting projects, Pingheng Machinery will be your ideal choice.
At Pingheng Machinery, you needn’t worry about casting quality. We possess comprehensive quality inspection equipment, including salt spray testing equipment, to evaluate castings’ rust resistance. Our metal casting could withstand 360 hours of salt spray testing without red rust according to our customers’ requirements. Thus, rust will not upset you in Pingheng machinery.
Start your casting project with Pingheng without worrying about metal rust issues.
FAQ
How to Clean Metal Rust?
Generally, various rust removal methods can be chosen based on the type of rust. The most common method is manual rust removal, which involves using a steel wool pad or sandpaper. However, it is only suitable for localized rust, which is easily removable from metal surfaces. For larger rusted areas, power tools like angle grinders are a good choice.
Sandblasting and high-pressure water jetting are generally used to remove rust in the industrial field. The former is suitable for a thick, rusted crust, while the latter is used for a loose, rusted layer.
Additionally, for components with complex shapes and intricate structures, chemical methods are more efficient. Acidic rust removers, such as oxalic acid and phosphoric acid, are suitable for various types of rust but may cause metal corrosion. For thin rust, a conversion rust remover is a good choice. As the name suggests, it converts rust into a protective layer that can be painted over directly. In contrast, chelating rust removers (such as EDTA) are better suited for treating surface rust and impurities on metal surfaces. They are relatively mild and cause less damage to the substrate. However, after applying any kind of chemical rust remover, all metal surfaces must be thoroughly rinsed with clean water or alkaline water and dried promptly; otherwise, it will accelerate the corrosion of metal.
It is important to note that any method of rust removal can not stop the rust of metal. Therefore, protective treatment is indispensable after the metal rust removal. But the best answer is to keep regular maintenance during everyday use. Prevent the deterioration of metal rust from the very beginning.
