Stainless steel, just as its name suggests, is a steel that does not rust. However, in our everyday usage, especially in industries, we can also observe that steel still rusts. What’s the deal with this? If you are also interested in how it is going, just hang on to this article, and you will know the answer.
What is Stainless Steel?
Before we dig into why stainless steel rusts, let us have a basic understanding of stainless steel. Stainless steel is an iron-based alloy. Apart from iron, stainless steel contains a key element – chromium, as well as other elements such as carbon, manganese, phosphorus, sulfur, and silicon. The composition and proportion of different kinds of stainless steel vary; together, they form the diverse family of stainless steels.
Stainless steel exhibits a silvery-white appearance, but through chemical methods such as PVD coating and chemical coloring, it can be coated with a variety of colored finishes, greatly enhancing its aesthetic capabilities.
Stainless steel is primarily categorized into five major types: the most commonly used austenitic stainless steel, low-cost ferritic stainless steel, high-hardness martensitic stainless steel, austenitic-ferritic duplex stainless steel that combines the advantages of both, and high-strength precipitation-hardening stainless steel.
Stainless steel possesses strong corrosion resistance. However, in terms of mechanical properties, its hardness is inferior to carbon steel. Correspondingly, stainless steel exhibits superior toughness and ductility. Furthermore, the magnetic properties of stainless steel vary by type: austenitic grades are typically non-magnetic, while martensitic and ferritic grades are magnetic.
What Makes Stainless Steel “Stainless”?
The most prominent property of stainless steel is its corrosion resistance, which is largely attributed to its key composition – chromium. Generally speaking, when the content of chromium is higher than 0.5%, it can form an extremely thin and dense oxide layer to resist the continuous corrosion of the underlying metal after the stainless steel is exposed to air. This layer of chromium oxide passivation film not only exhibits excellent antioxidant properties, but also has self-repairing ability. When this passivation film is destroyed, as long as there is oxygen around, it will repair spontaneously, thus achieving the outstanding corrosion resistance of stainless steel. And the higher the content of chromium, the less prone the metal is to rust.
Why does Stainless Steel Still Rust?
Now we come to the most crucial issue of this article: Why does stainless steel still rust? The rust-resistant properties of stainless steel primarily depend on its surface passivation film. Any damage to this film or disruption to its self-repair mechanism can lead to rusting. Below are several specific causes of stainless steel corrosion.
Abrasive
Scratches and abrasions are the most direct means of damaging the passivation film on stainless steel surfaces. Processes such as cutting and grinding during metal fabrication can destroy this oxide layer. Prolonged impact or friction from hard objects may lead to wear corrosion in stainless steel.
Chemical Substance
Although the passivation film formed by chromium can resist most corrosive media, its resistance to strong chlorides remains insufficient. Chloride ions can locally penetrate the passivation film, causing stainless steel to undergo pitting corrosion. Simultaneously, strong oxidizing acids or reducing acids—such as high-concentration phosphoric acid and nitric acid—can uniformly dissolve the passivation film on the stainless steel surface, leading to general corrosion of the stainless steel.
Other Metals
When stainless steel comes into contact with other metals may cause electrochemical corrosion. Two metals immersed in an electrolyte solution may undergo a chemical reaction, causing the less-positive one to act as an anode and be consumed, thereby accelerating its corrosion.
Tiny Spaces
If there are tiny spaces designed in your components, the concentration of their chemical substances will rapidly rise and cause an intense reaction, which accelerates the corrosion of stainless steel.
Improper Welding
The weld temperature of stainless steel could achieve 450-850°C. In this temperature range, the inner chromium of stainless steel will react with carbon to form chromium carbide, which leads to significant chromium depletion, preventing the formation of a passivation film and causing internal material failure.
Oil Contamination
Oil itself can not cause the corrosion of metal; however, oil could absorb moisture and dust as well as other impurities, thus forming a corrosive environment on the surface of stainless steel. And oil can isolate oxygen to prevent the self-repairing of the passivation film.
Other Factors
Apart from the factors mentioned above, long-term exposure to damp, poorly ventilated environments or areas with significant temperature fluctuations can create ideal conditions for corrosion, leading to the deterioration of stainless steel.
Types of Stainless Steel Corrosion
After understanding the factors that may cause corrosion in stainless steel, we will now introduce the different types of corrosion. Each type of corrosion arises from distinct causes and requires different solutions.
General Corrosion/Uniform Corrosion
General corrosion, or uniform corrosion, refers to corrosion that uniformly covers the stainless steel surface. The likelihood of general corrosion occurring is relatively low, as it only happens when the passivation film on the stainless steel surface is completely dissolved in strong acids or strong alkalis. The harm caused by general corrosion is typically minor because it is predictable. In practical engineering applications, it can be effectively prevented by incorporating a corrosion allowance (increasing material thickness).
Localized Corrosion
Compared to general corrosion, localized corrosion typically occurs in specific areas, making it difficult to detect and more challenging to handle.
Galvanic Corrosion (or Bimetallic Corrosion)
As the name suggests, when two metals come into contact in a solution, a chemical reaction occurs. This causes the more chemically active metal with a more negative electrode potential (the anode) to corrode rapidly.
Intergranular Corrosion
This typically occurs within the temperature range of 450°C to 850°C. Prolonged exposure at these temperatures causes carbon within the stainless steel to react with chromium, forming hard but useless chromium carbides. Chromium will rapidly be depleted, creating “chromium-depleted zones” incapable of forming a passivation film. Consequently, when corrosion occurs, the unprotected areas are immediately compromised, and the damage remains undetectable from the stainless steel’s surface.
Pitting Corrosion
Pitting corrosion is highly prevalent in chloride-containing environments. Chloride ions excel at causing localized corrosion that creates small, deep holes in metal surfaces. Once the passivation layer is locally compromised, it accelerates the corrosion of the underlying metal.
Crevice Corrosion
Crevice corrosion occurs within narrow gaps in metals. It represents a specific form of pitting corrosion. It can develop between metal surfaces or between metal and non-metal interfaces.
Stress Corrosion Cracking (SCC)
Stress corrosion cracking, as the name suggests, is a form of metal fracture caused by the combined effects of stress and a corrosive environment. It is often referred to as the “cancer” of engineering because its onset is entirely unpredictable. Under sustained stress and specific corrosive conditions, stainless steel may suddenly fracture without warning.
How to Prevent Rust on Stainless Steel
Even though stainless steel possesses outstanding corrosion resistance, routine rust prevention should not be neglected to ensure its long-term service life and it can deliver greater economic benefits.
Design
Choosing the Proper Stainless Steel Grade
Rust prevention should be taken into consideration from the initial design, including selecting appropriate steel grades based on the usage environment. For example, 304 stainless steel is suitable for food-related applications, while 316 stainless steel should be chosen for demanding working conditions.
Optimizing Structural Design
In structural design, gaps and dead corners should be avoided as much as possible to prevent creating ideal environments for corrosion. Additionally, direct contact between stainless steel and metals with different potentials should be avoided; insulating spacers can be used for isolation.
Reserve for Corrosion Allowance
If foreseeable corrosion is expected during the use of stainless steel, the plate thickness should be appropriately increased to account for this condition.
Manufacturing
During the manufacturing process, ensure adherence to proper welding and fastening specifications to prevent intergranular corrosion in welded areas. In the meantime, maintain cleanliness throughout manufacturing to avoid residual contaminants.
Maintenance
Regular maintenance during use is crucial for preventing corrosion in stainless steel. For more information on metal rust prevention, click “All You Want to Know About Metal Rust”.
Conclusion
To be honest, for stainless steel casting, its machined surfaces will never rust. However, due to high-temperature casting (≥1000°C) during production, the raw surface develops an oxide layer that may exhibit red rust after several years of use. At Pingheng Machinery, though, you needn’t worry about metal rust issues. Pingheng Machinery has specialized in stainless steel investment casting for 26 years, boasting extensive casting expertise. Before shipping, we would perform solution treatment, acid pickling, and passivation on castings to meet customer requirements. In the meantime, we are equipped with a spray salt testing machine. Through testing, our castings can withstand 360 hours of salt spray testing without red rust on the surface, eliminating your concerns about stainless steel corrosion.
Contact us now to start your next casting project.
FAQ
Which type of stainless steel is most resistant to rust?
Among all the stainless steels, austenitic stainless steel exhibits the strongest resistance to corrosion. Among austenitic stainless steels, 304 and 316 stainless steel exhibit the most outstanding corrosion resistance.
