Although stainless steel is widely known for resisting corrosion, it isn’t completely impervious to corrosion. Under the right (or is that wrong) circumstances, stainless steel alloys can become corroded. Many a manufacturer has ordered a stainless steel part or basket, only to find that their “stainless” steel has started to show signs of rust and/or other problems within a few days or weeks of being put to use.
This often leads to the question: “What corrodes stainless steel?” Why is it that a corrosion-resistant metal would fail—and sometimes fail drastically—when put to use?
Here is a quick explanation of some of the things that can corrode stainless steel:
1: Strong Chlorides Can Cause Pitting Corrosion in Stainless Steel
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.
2: Bimetallic/Galvanic Corrosion from Welding Dissimilar Stainless Steel Alloys
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.
3: Transplanting of Plain Iron or Steel onto Stainless Steels
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 were 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.
4: Applying Temperature Extremes to Stainless Steel
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 a given stainless steel and check 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’ temperatures prior to designing any custom wire basket or sheet metal form.
5: Unaccounted-for Environmental Factors
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 a 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.
Need help choosing the right grade of stainless steel for your next custom wire basket or tray? Reach out to the team at Marlin Steel to get started!