Stainless steel is known for its beautiful appearance and excellent durability. It’s easy to clean, can last a long time, and sees regular use in everything from kitchen flatware to industrial machinery.
But that doesn’t mean it’s invincible.
In this guide, we’re going to look at what gives stainless steel its 'stainless' qualities, what can cause stainless steel to rust, and some best practices to help get the most out of your stainless steel.
Before we can explain why stainless rusts, we have to talk about how rust forms.
This process—also known as oxidation or corrosion—is a chemical reaction that occurs when iron, water, and oxygen interact.
This reaction uses elements of iron to create hydrous iron(III) oxides and iron(III) oxide-hydroxide—leading to the brownish-red substance we know as rust.
These are the basic elements required to initiate corrosion.
However, there are also plenty of ways in which this reaction might accelerate or become more aggressive.
The addition of salt, heat, or an electric current, for example, will typically speed up how quickly rust forms.
So what does this have to do with stainless steel?
All stainless steel alloys contain iron in varying ratios depending on the exact alloy in question.
However, unlike other forms of steel or iron-containing alloys, stainless steel has a very important film across its surface known as a passive layer.
Stainless steel alloys also contain chromium which, like iron, oxidizes in oxygen and water.
However, unlike iron, the resulting film is highly resistant to further oxidation and protects the underlying metal instead of slowly breaking it down.
Similar films are why titanium and aluminum offer exceptional rust resistance as well.
This layer can be as thin as a few atomic layers, but that is all that’s needed to keep your stainless looking virtually brand new after years of elemental exposure or use.
With an understanding of what causes rust formation, it’s pretty clear that the biggest threats to your stainless steel are things that threaten the passive layer on its surface.
Without this layer, the iron in the stainless steel is exposed and open to oxidation and corrosion attacks.
Initiation of rust can happen shockingly fast, resulting in rapid metal loss.
Let’s look at 5 common ways that you might compromise the passive layer on stainless steel and increase the risk of rust formation.
Also known as galling corrosion, this is one of the biggest threats to stainless steel work surfaces, equipment, and other areas which might see regular interaction.
This makes it a common culprit for rust in residential, food service, and medical settings. It can also occur during fabrication as metals are cut, sanded, or otherwise worked.
When you bang other items into stainless steel—particularly sharp ones—or run hard items across the surface, you risk penetrating the passive layer.
If this happens frequently enough this could lead to rusting where the layer is compromised.
This is also common when using steel wool or other abrasives when cleaning stainless steel.
It is critical to ensure there is no iron contamination on anything used to scour the surface of stainless steel parts, equipment, or structures.
Failure to do so could cause tiny areas of weakness where iron is embedded in the passive layer, leading to layer failure and the initiation of rust attacks.
It can also happen when nuts, bolts, or other fasteners are forced together improperly, damaging the passive layer and allowing for corrosion to set into the areas impacted.
The passive layer resists water well enough, it doesn’t fare as well against some chemicals and organics.
This is typically referred to as pitting corrosion. Specifics will vary based on the exact grade of stainless steel you’re using.
For example, while stainless steel offers respectable acid resistance, there are certain grades that fare better when used with phosphoric acid, sulfuric acid, or nitric acid.
Bases, on the other hand, nearly all spell trouble for stainless steel at high concentrations, typically leading to cracking or etching corrosion.
When used with other metals, it’s important to consider the risk of galvanic corrosion.
This happens when more than one metal is exposed to an electrolyte in the presence of an electrical current.
The least noble of the metals will corrode—often very rapidly.
Fortunately, most of this information is readily available using charts and comparisons.
Consult our guide on galvanic corrosion to look deeper into the science behind this type of corrosion and tips to help prevent it.
Access to ample amounts of oxygen is essential to maintaining a healthy passive layer.
If you’re using stainless steel components and there are tiny gaps in your system design—particularly when using liquids—you could run the risk of crevice corrosion.
This may not occur right away. However, concentrations of chemicals can increase quickly in tiny spaces—or new compounds can form—leading to corrosion that spreads surprisingly fast with little or no warning.
When possible, always ensure that submerged parts or wet processes see a steady movement of any potential electrolytes or other chemicals and allow for plenty of air circulation in dry areas.
Welding is a stressful process for any metal.
For stainless steel, exposure to temperatures between 550C and 850C can lead to an issue known as weld decay.
When steel reaches these temps, the chromium used to generate the passive layer leeches out.
This makes it harder for an effective passive layer to form.
Always remain mindful of temperatures when welding stainless steel and, when possible, use low carbon alloys or alloys with additives designed to provide easier, safer welding performance.
As shown in the risk factors above, human interaction is often the root cause that leads to the formation of rust and other forms of stainless steel corrosion.
This is particularly relevant when it comes to attaching stainless steel parts and components to each other.
If using nuts, bolts, or other fasteners, ensure that threading is smooth and use lubricants to allow for easy tightening with minimal torque.
This will allow you to notice any potential threading or galling issues while they’re minor enough to address.
When welding stainless steel, consider the use of low carbon steels and be mindful of temperatures.
Stainless steel alloys which include titanium or niobium also help to reduce the risk of weld decay.
There are also post-weld pickling treatments available to help improve the corrosion resistance of welded areas.
Even in the most precise systems, there will often be areas of high and low concentration as you move throughout the piping process.
If you’re using holding tanks, have small areas of restricted flow in your design, or designing systems for maritime use, this is even more important.
Always consult with qualified engineers to conduct risk assessments and design with tolerances above what you plan to encounter during typical operations.
This provides a failsafe against catastrophic failure while also working to ensure that you’ll see a longer service life out of all parts and processing components involved.
When properly maintained, stainless steel is known for its easy-to-clean surface and outstanding durability. In most cases, warm water and a mild detergent are enough to handle most issues.
For cleaning which requires physical contact, be sure to use soft cloths or iron-free scouring pads.
While stainless will stand up well to most harsh chemicals, be sure to check any recommendations for cleaners that are highly basic.
When appearance is critical—such as in architectural or food service use—be sure to spot test any cleaners to avoid discoloration or other visual issues.
For other considerations when maintaining your stainless steel, consult our cleaning and maintenance guide.
Passivation treatments use chemical exposure to promote rapid healing or growth of your stainless steel’s passive layer.
However, they must be applied in a specific manner and typically involve the use of some fairly sensitive chemicals.
If performed improperly, passivation treatments might actually damage your stainless steel instead. So be sure to research options thoroughly.
You can read more about them in our guide to passivation methods.
Pickling treatments use acids to help counteract the effects of welding or high temperatures on the passive layer.
They’re ideal for removing mill scalings, removing oxides from stainless steel, reducing impurities, or removing stains.
They range from pastes to liquids to provide flexible application options.
Some types of pickling treatments even use proprietary blends to provide benefits for specific stainless steel grades or use cases.
You can read more about pickling treatments and their uses in our guide.
Rust and corrosion can spread very quickly.
If you discover rust forming, it’s important to act quickly to assess the damage and decide the appropriate way to either repair or replace the impacted components.
Should you decide to remove the rust, surface cleaners such as "soft" abrasives or options containing oxalic acid provide a safer alternative to simply grinding or blasting the rust from the surface.
If you must use an abrasive option, take care to avoid contaminating the underlying and surrounding steel.
You should also avoid using methods that might throw free iron into the surrounding environment.
Environmental contamination could lead to further corrosion attacks wherever free iron settles.
Also, be sure to analyze the impact of rust removal on structural integrity.
Rust removable does not replace any of the metal lost to the oxidation process.
As such, these assessments are critical to ensuring continued safe operation after removal.
Stainless steel isn't immune to rust
Stainless steel’s passive layer helps to prevent the formation of rust
Anything which contaminates or damages the passive layer on your stainless steel could lead to rust formation
Common threats to the passive layer include abrasion, chemical exposure, heat exposure, oxygen depletion, and galvanic reactions.
Passivation and pickling treatments can help to support passive layer health after exposure to chemicals or heat.
Proper stainless steel maintenance—including keeping the surface clean with warm water and a mild detergent—can greatly reduce corrosion risks.
If you notice rust formation, respond quickly to determine whether cleaning or replacement is the best option.
Even if you manage to remove the rust on your stainless steel, you cannot replace the metal lost in the oxidation reaction. Always consider structural integrity and component strength after cleaning.
As a leading provider of stainless steel components and products for more than four decades, Unified Alloys is a trusted supplier for industries big and small throughout Western Canada and beyond. Our comprehensive selection of stainless steel pipe, valves, flanges, and other components allow us to serve as your primary source for all things stainless. Call us today to speak with an expert sales analyst about your needs!
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