Stainless steel is a durable and widely used material. While it is highly resistant to corrosion, it is prone to rust over time if not cared for. In this blog post, we will explore how corrosion occurs in different types of stainless steel and how you can prevent rust formation in your products.
Stainless Steel and Rust – An Overview.
Rusting occurs in iron-containing alloys when iron oxide forms on the surface of the metal. It is a reddish-brown coating that can make the steel weaker and less resistant to further corrosion.
This process, also known as corrosion, causes the metal to deteriorate and weaken over time.
The Causes of Rusting in Stainless Steel.
You’ll find your stainless steel rusting if you expose it to moisture and oxygen. When water and air come into contact with it, they cause a chemical reaction that initiates the rusting process. This can also be caused by exposure to high temperatures and abrasive chemicals.
The Role of Oxygen in Rusting.
Oxygen plays a critical role in the rusting process of stainless steel. It reacts with iron atoms on the surface of the metal, causing them to lose electrons and become oxidized. This process creates the reddish-brown coating we see.
The more oxygen that comes into contact with the metal, the faster the rusting process will occur.
How Stainless Steel Resists Rusting.
Chromium in stainless steel protects it from rust. It does so by creating an inert oxide layer on the material’s surface.
This chromium oxide layer acts as a protective barrier that prevents oxygen and water vapors from coming into further contact with the underlying steel surface. You can add elements, such as nickel and molybdenum, to further enhance the alloy’s stability against chemicals and environmental agents.
Types of Corrosion in Stainless Steel.
This material can experience several types of rusting if you do not protect it well. Let’s discuss them:
1. Surface Rusting
Surface rusting is the most common type of rusting in stainless steel. You’ll see it occur when the metal’s surface is exposed to moisture for an extended period. This creates a reddish-brown discoloration on the surface of the metal.
Surface rusting does not typically penetrate deep into the metal.
2. Pitting Corrosion.
This process occurs when small pits or holes form on the surface of the metal. It is caused by exposure to chloride ions, such as those found in saltwater or specific cleaning agents.
Pitting corrosion can weaken the metal and make it more susceptible to further corrosion. It’s the reason why many stainless steels are powder coated to protect them from coming in contact with moisture in the air.
3. Crevice Corrosion
As the name suggests, crevice corrosion commonly affects tight spaces or crevices. This type of corrosion is common in welded or bolted joints, where two pieces of metal are joined.
Crevice corrosion can cause deep pits in the metal and weaken the joint.
4. Stress Corrosion Cracking
This process is caused by a combination of tensile stress and corrosion that lead to the formation of cracks in the metal. It can be difficult to detect and can lead to catastrophic failure of the metal.
You’ll commonly find it in environments with high chloride ions or metals under constant stress.
5. Intergranular Corrosion
Intergranular corrosion can happen along the grain boundaries of the metal. It is typically caused by exposure to high temperatures or welding without proper post-weld cleaning.
Intergranular corrosion can cause the metal to become weak and brittle, leading to failure over time.
Factors Affecting Corrosion in Stainless Steel
Several factors can influence the rate and severity of rusting in stainless steel:
Temperature plays a critical role in the rusting process: high temperatures can accelerate the corrosion process, causing the metal to deteriorate faster. Similarly, it follows that low temperatures can slow down the corrosion process.
This sensitivity is why stainless steel is more prone to corrosion in industrial environments.
2. Chloride Concentration
Exposure to chloride ions, such as those found in saltwater or specific cleaning agents, can cause pitting corrosion and amplify the rusting process. The higher the concentration of chloride ions, the more significant the risk of corrosion.
3. The pH Levels
The rusting process can be affected by the pH levels of the environment. Stainless steel is most corrosion-resistant in a neutral or slightly alkaline environment. Acidic environments, such as those in industrial or manufacturing settings, can cause your metal to corrode faster.
4. Surface Finish
Rough surfaces can trap moisture and dirt, allowing corrosion to occur. A smoother surface finish, such as a mirror finish, can be more corrosion-resistant than a rougher surface finish.
Preventing Corrosion in Stainless Steel
You can take several preventive measures to reduce the risk of corrosion in stainless steel, four of which are below.
1. Proper Material Selection
You must select the correct type of stainless steel for your application, as different grades show different chemical stability levels. For example, austenitic stainless steel is more corrosion-resistant than martensitic stainless steel.
Consider the environment and conditions your stainless steel will be exposed to and select the right grade accordingly.
2. Proper Surface Finish
A smooth surface finish is more corrosion-resistant than a rough surface finish. Consider a mirror finish or electropolishing to create a smoother surface finish. You can also opt for coatings that will protect your metal.
3. Proper Cleaning and Maintenance
You must regularly clean your stainless steel objects to prevent them from rusting. Dirt and contaminants can trap moisture on the material’s surface, accelerating the corrosion process.
It’s best to use mild soap and water to clean the metal’s surface. Dry thoroughly with a soft cloth to remove any remaining moisture on the surface. Ensure you avoid using harsh cleaning agents or abrasive tools, as they can scratch the metal’s surface and damage the chromium oxide layer on it.
4. Proper Use and Handling
Make sure you’re not exposing the stainless steel to environments with high moisture levels, chloride ions, or extreme temperatures. Moisture buildup can be avoided by storing the metal in a dry and well-ventilated area.
Understanding the causes and types of rusting in stainless steel, as well as the preventive measures, is essential to maintain the longevity and performance of your metal products.
You can prevent costly replacements and ensure the continued reliability of your metal products by providing them with proper maintenance and controlling the factors that influence corrosion.
Always select the proper grade of stainless steel for your application. Follow the guidelines mentioned above to ensure the continued durability and corrosion resistance of your stainless steel.
1. Can you savage rusted stainless steel?
Even if your stainless steel has started to corrode, it can still be salvaged. However, the particular salvaging method will depend on the extent of damage from corrosion and the type of corrosion.
If the corrosion is limited to the surface of the metal, then simple methods like grinding and sanding can be used to remove the rust from the metal.
However, advanced methods like electrochemical polishing and passivation must be used in case of severe corrosion.
2. How do I determine the extent of rusting in stainless steel?
There are several methods to detect the extent of rusting in stainless steel; these include.
- Visual inspection: Rust in stainless steel often occurs as a discoloration that can be easily seen on light-colored stainless steel. Always inspect your material for discoloration or staining on the surface of the metal.
- Chemical inspection: You can also use chemical testing to detect rust in stainless steel. Chemical tests can identify the presence of specific chemicals, such as chloride ions, that can contribute to rusting. These can also help you diagnose the exact cause of corrosion.
- Electrochemical testing: Electrochemical testing is an advanced method for detecting rusting in stainless steel. This method involves measuring the electrochemical potential of the metal and comparing it to a known standard.
Electrochemical testing can help determine the severity of corrosion and identify any underlying issues.