One of the most coveted benefits of stainless steel is its corrosion resistance, as other metals such as iron and steel easily rust under the right circumstances. Its high resistance to corrosion makes stainless steel a popular option for endless manufacturing applications.
However, most people don’t understand the factors due to which the corrosion of stainless steel occurs. We’ll explore the varying factors that result in stainless steel corrosion and rust.
What Is Stainless Steel?
Before we delve into factors that corrode stainless steel, it’s important to understand the material’s basics. It is a low-maintenance metal that has at least 10.5 percent chromium present. This chromium reacts with the surrounding oxygen to coat the surface in a thin exterior layer. This layer is so thin that it’s difficult for the human eye to see it without some sort of magnifying aid.
This stable exterior layer explains why stainless steel is resistant to corrosion, acting as a barrier between the underlying surface metal while limiting oxygen and water exposure.
How Is Stainless Steel Different?
One noteworthy mention about stainless steel is that it has countless formulations on the market, with each formulation having a unique makeup of stainless steel alloys. Stainless steel alloys contain this protective oxide layer that plain steel alloys don’t have. The shiny surface often associated with stainless steel will remain intact.
The cause behind this oxide layer is the specific elements used in the majority of stainless steel, including iron, manganese, silicon, carbon, and chromium. Some types of stainless steel may also include nickel and/or molybdenum to further improve how the oxide layer performs.
Chromium has the most significant impact on whether or not the stainless steel is rust-resistant, so stainless steel alloys that are rich in chromium are typically the most corrosion-resistant. However, it’s possible to add molybdenum to improve the stainless steel alloy’s resistance to certain corrosive chemicals.
Grade 316 steel is a great example of this, as it has molybdenum, something that grades 304 stainless steel does not. As a result, grade 316 steel is more resistant to chlorides.
Top 5 Factors Why Stainless Steel Rusts and Corrodes
Various scenarios may result in stainless steel beginning to rust. Because stainless steel is composed of hundreds of different alloys, the reasoning behind one stainless steel corroding versus another isn’t always clear. We’ve outlined five corrosion factors for stainless steel below.
#1: Strong Chlorides
The first corrosion factor for stainless steel is exposure to extremely strong chlorides (as salt alone will not cause damage). For example, if the stainless steel is exposed to an environment rich in incredibly strong chlorides, it can result in pitting corrosion. Grade 304 stainless steel is a great example of this, as it is often used in naval applications where salt water is involved.
To avoid pitting corrosion, the grade of stainless steel needs to be specifically resistant to chlorides, such as grade 316 stainless steel. It’s also possible to place a protective coating on top of steel to prevent it from coming into direct contact with chloride-heavy environments.
#2: Bimetallic/Galvanic Corrosion from Welding Dissimilar Stainless Steel Alloys
Some inexperienced or unknowledgeable manufacturers may weld two different metals together when creating a custom steel wire or sheet metal form. This may not seem like a huge deal, but these two metals have different properties and will react differently throughout the product’s lifecycle.
Even though the metal pieces are connected via a common electrolytic material such as water or weld filler, there could be a flow of electrical current between metals. This results in the metal that more readily accepts new electrons becoming an anode, meaning it will corrode faster.
Factors that can further speed up corrosion include the metal types, the welding filler, temperature, humidity, and more. The ideal solution for preventing this type of corrosion is to choose two similar metals to weld in the first place.
#3: Transplanting of Plain Iron or Steel
This is another example of what can cause erosion of stainless steel. In some cases, residue from plain steel or iron can transfer onto stainless steel, impacting the protective oxide layer and causing the piece to rust. This is an accidental particle transfer that often occurs when equipment is used across metal types without being properly cleaned beforehand.
To prevent this type of corrosion, the equipment must be properly cleaned, and certain pieces of equipment such as steel brushes should only be used to work on one metal type.
#4: Applying Temperature Extremes
Although stainless steel has a remarkably high melting point, it doesn’t mean that it is corrosion-resistant regardless of the temperature. When stainless steel alloys are exposed to temperature extremes, it may result in scale formation on the metal. These scales have a different composition than the base metal, resulting in bimetallic corrosion.
To prevent this issue from occurring, always check the recommended operating temperature when working with stainless steel. If the limits are exceeded, it may result in corrosion.
#5: Environmental Factors
Another example of what can cause corrosion is environmental factors. Many manufacturers may find themselves making custom wire baskets or other specialty stainless steel products only for them to corrode due to an unexpected environmental factor. Be mindful that 304 stainless steel products would need long-term exposure to intense environmental factors before they begin showing signs of wear.
Types of Corrosion of Stainless Steel
To better understand how stainless steel corrodes, it’s important to mention the different types of corrosion.
General or uniform corrosion is the most common type that impacts stainless steel and other metals. It occurs when there is an even layer of corrosion across the material’s surface.
Even though this type of corrosion covers the most surface area of the metal, it is seen as the most benign form of corrosion. It’s easy to see the impact it’s having on the material and can be tested.
Pitting corrosion on stainless steel is one of the most destructive forms of corrosion, and it can be difficult to identify. A cathodic point or local anodic creates a small pitted area of corrosion and begins to grow. These pits often travel quickly in a downward motion and can result in structural failure.
If there are different ion concentrations between two areas of a metal, it may result in crevice corrosion. This is a localized type of corrosion stemming from environments that have different ion concentrations between two parts of a metal.
For example, crevice corrosion occurs in bolts or washers since there isn’t much circulation that would allow the corrosive agents to enter. Levels of oxygen diminish in this case, resulting in a lack of re-passivation.
This type of corrosion results from impurities taking place at boundaries between the grains that form while an alloy is being solidified. Intergranular corrosion can gravely impact the metal’s mechanical properties while the rest of the metal piece may remain intact.
Stress Corrosion Cracking (SCC)
This stainless steel corrosion type results from metal exposure in a corrosive environment, typically when temperatures are high. When rapid temperature changes occur, it may result in stress corrosion cracking. SCC may also occur during the manufacturing process, such as cold forming, grinding, machining, etc.
Choosing the proper materials for each environment can prevent stress corrosion cracking.
In the case that two electrochemically dissimilar metals come into contact when immersed in a conductive or corrosive solution, it results in the production of an electron flow. The metal with the lowest corrosion resistance is typically impacted by corrosion more heavily.
How to Prevent Stainless Steel from Rusting and Corrosion
Preventing stainless steel corrosion starts at the design and production stage and is carried throughout the lifecycle of the metal. Routine maintenance can also help minimize corrosion.
The design of the product is the foundation for what makes stainless steel corrosion-resistant in the long run. Planning is a proactive measure in minimizing water penetration and reducing the potential for damage. For example, drainage holes should be used whenever possible, and air should be able to flow freely throughout the application.
The fabrication stage is another important aspect of stainless steel rust prevention. The stainless steel mustn’t come into contact with iron or regular steel as the carbon steel dust particles can increase the risk of rust formation. To this point, some stainless steel fabricators will not allow any carbon steel product in the shop for fear of cross-contamination or migrating.
Fabricators should always check their workstations, tools, storage units, steel turning rolls, and chains for contamination throughout the process. The cleaning and grinding tools should also remain separated.
Maintenance is another example of what makes stainless steel resistant to corrosion. Not only can maintenance limit existing rust from forming, but it can prevent additional rust from forming as well. Try to prevent scratches and to keep any products, such as racks, clean throughout their lifecycle.
When properly manufactured and cared for, stainless steel is a great choice for a variety of applications, such as metal racks and baskets. Our team at Schaumburg Specialties is extremely knowledgeable about stainless steel, and we’re happy to assist your business with all of your fabrication needs.
Call us today to learn more about how we can help.