Stainless steel is well-known for being resistant to corrosion from things that make plain steel and iron (and other materials) rust away. However, many people don’t know why stainless steel is “stainless” and why it’s a preferred material for countless manufacturing applications such as Marlin Steel’s metal baskets, carts, trays, and racks.
Of course, it’s also important to know that stainless steel isn’t completely impervious to corrosion. Under certain circumstances, stainless steel alloys can become corroded, showing signs of rust or other problems. This often leads to the question: “What corrodes stainless steel?” In this blog, we’ll look at what makes stainless steel different from regular steel, and what factors can actually cause it to corrode.
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The term stainless steel is a reference to the alloy’s tendency to resist rust under normal conditions. Scientific American states that “the chemical elements in stainless steel react with oxygen from water and air to form a very thin, stable film … The presence of the stable film prevents additional corrosion by acting as a barrier that limits oxygen and water access to the underlying metal surface.
This layer of rust is so thin that the human eye typically cannot see it without aid, giving the surface appearance of the steel its characteristic “stainless” appearance. Now, let’s dive a little deeper!
One of the first things to know about stainless steel is that there are countless formulations of it on the market. What separates one of these stainless steel alloys from a plain steel alloy (aside from having different component metals in different ratios) is that protective oxide layer of film. As long as this oxide layer remains unhindered, the steel will retain a shiny surface appearance.
So, what causes stainless steel to form this oxide layer? The answer lies in the specific elements used in most forms of stainless steel: Iron, manganese, silicon, carbon, and chromium. Some forms of stainless steel also add nickel and/or molybdenum to further enhance the performance of the oxide layer. Of these elements, chromium has the highest impact on the rust resistance of stainless steel, meaning chromium-rich stainless steel alloys (such as most austenitic stainless steels) tend to have the highest overall resistance to corrosion.
Specific additives, such as molybdenum, can help to bolster a stainless steel alloy’s resistance to certain corrosive chemicals. For example, grade 316 stainless steel has molybdenum which grade 304 stainless steel lacks. Because of this, grade 316 stainless steel is more resistant to chlorides.
There are a number of reasons why a piece of stainless steel might start to develop rust. However, because there are hundreds of different alloys of stainless steel, what might cause one stainless steel alloy to corrode might not affect another one. Here’s a look at five factors that can cause stainless steel, including metal baskets and racks, to corrode.
Many types of stainless steel alloys will suffer extreme pitting corrosion when exposed to environments that are rich in chlorides (such as salt). For example, grade 304 stainless steel, when used in naval applications, may start to suffer pitting as a result of contact with seawater (which is rich in salt) or salt-enriched sea breezes.
To avoid pitting corrosion, it’s important to use a grade of stainless steel that is specifically resistant to chlorides—such as grade 316 stainless steel. Alternatively, a specialized coating can be applied to the steel to prevent direct contact with chlorides in the environment.
One basic mistake that some manufacturers may make when creating a custom steel wire or sheet metal form is that they may weld two dissimilar metals together—whether by accident or by design.
Why is this a problem? Because, when two metals with different properties are connected via a common electrolytic material (such as water or weld filler material), there may be a flow of electrical current from one material to the other. This will cause the less “noble” metal (meaning the metal that more readily accepts new electrons) to become an “anode” and start to corrode more quickly.
The speed of this corrosion will change depending on a few factors, such as the specific types of stainless steel being joined, what kind of welding filler was used, ambient temperature and humidity, and the total surface area of the metals that are in contact with one another.
The best preventative measure for bimetallic corrosion is to avoid joining two dissimilar metals permanently in the first place. A close second is to add a coating to the metals to seal them off with a coating to prevent the flow of electrons from the cathode to the anode.
It should also be noted that using a weld filler that is too dissimilar to the metals being joined can also result in galvanic corrosion at the weld site.
In some applications, particulate residue from a plain steel or iron workpiece may be transferred onto the surface of a stainless steel part or basket. These plain iron or steel particles can disrupt the protective oxide layer of a stainless steel workpiece—ruining its corrosion resistance so that it starts to rust.
The difference between this and the bimetallic corrosion problem listed above is that in this case, the contact between the dissimilar metals is purely accidental and typically without the manufacturer’s knowledge.
A common reason why plain steel or iron residue gets transplanted onto a stainless steel part or workpiece is that equipment used to process one type of material may be used for the other without being properly cleaned between batches.
For example, say a wire bending robot was used to bend plain iron wires for several hours, then immediately used to bend stainless steel wires. Some iron particles would likely be left behind on the bending robot’s manipulators, which could then be transferred to the stainless steel wires being bent.
To prevent the transplanting of plain steel or iron (or any other metals) to stainless steel workpieces, it’s important to thoroughly clean and prepare equipment when changing over to new material. Some equipment, such as steel brushes, should never be shared between different metal types.
Stainless steel alloys typically have a very high melting point (typically well in excess of 1,200˚F). However, while the metal doesn’t melt at high temperatures, it may experience other changes that affect its ability to resist corrosion.
For example, scaling is a common problem with stainless steel alloys when they’re exposed to extreme temperatures (such as those used in many heat treatment/annealing processes). When scales form on hot metal, the flaky leftover material can cause bimetallic corrosion since the scales have a different composition from the base metal.
Additionally, temperature extremes can cause exposed stainless steel alloys to lose their protective oxide layer for a time, increasing the risk of corrosion until the oxide layer can re-form.
To prevent corrosion from scaling or other issues caused by temperature extremes, it’s important to check the recommended operating temperatures for any given stainless steel to see if the temperatures used in your manufacturing processes exceed those limits. This is part of the reason why Marlin’s engineering team always asks clients about their process’s temperatures prior to designing any custom wire basket or sheet metal form.
There are many cases where a manufacturer can make a custom stainless steel wire basket or tray perfectly to specification, only for it to corrode because of some previously unaccounted-for environmental factor. The presence of salt and moisture in the air because of a factory’s coastal location is one example of an environmental factor that might be missed in a design document.
When selecting stainless steel to use for making any custom wire or sheet metal form, it’s important to consider as many environmental factors as possible. This helps to ensure that the stainless steel basket, tray, or part will resist corrosion for as long as possible, rather than rusting right away.
Stainless steel is often able to offer the ideal combination of strength, corrosion resistance, and temperature tolerance to accommodate a wide variety of manufacturing applications like:
These are just a few of the applications that a custom wire basket made from stainless steel can be used for. Of course, the specific type of stainless steel used will depend on the exact nature of the process. Some variations of stainless steel work better for certain applications than others.
For example, grade 316 stainless steel is often preferred for applications where contact with salt or other chlorides is common because it is resistant to pitting corrosion caused by salt. Meanwhile, some prefer hardened and stress relieved grade 430 stainless steel for its incredibly high tensile strength. Grade 304 stainless steel, on the other hand, is widely useful for a variety of applications, making it one of the most commonly used stainless steel grades in all sorts of industries.
Need help picking the right type of stainless steel for your custom wire basket? Marlin Steel’s engineers have years of experience in helping manufacturers build the best steel wire baskets for their manufacturing needs. We also have a wide variety of in-stock products ready to ship. Reach out to the Marlin team to discuss your manufacturing process and your stainless steel needs.
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