One of the most common misconceptions about stainless steel is that it is impervious to rust.
While most grades are offer exceptional resistant, they are not rust-proof.
They simply stain less… hence the name.
Image via Cougartron
Fortunately, minimizing the risk of rust is straightforward.
We’ll cover some of the best options in this guide.
And if it’s already too late and you’re noticing rust on your parts or surfaces, we’ll cover tips to help remove rust from stainless steel as well.
What Causes Stainless Steel to Rust?
To understand what causes rust to appear, we first need to look at what rust is at a deeper level.
As mentioned in a previous guide, stainless steel has a special chromium oxide layer that gives it corrosion resistance.
Rust is also an oxide layer—in this case ferrous oxide. However, unlike chromium oxide, rust doesn’t form an even layer. As this layer builds and later falls off, it exposes more of the raw stainless underneath.
At the least, this is an unsightly blemish.
At its worst, it can lead to structural instability and, ultimately, failure.
In food service environments or other areas with a need for a high level of cleanliness or hygiene, rust can also lead to contamination concerns.
This makes rust a serious problem for anyone using stainless steel.
The best defence against rust, as with any form of corrosion,
is a solid passivation layer.
The best defence against rust, as with any form of corrosion, is a solid passivation layer.
When this layer is damaged or depleted, the risk of rust rises.
Situations that might damage the passivation layer and encourage corrosion include:
- Use in chloride-rich environments
- Physical damage—such as blunt force dents or scrapes and gouges from sharp edges
- Exposure to high temperatures—such as welding or heat treatment
- Exposure to chemicals
- Free iron contamination through machining, cleaning, or storage
- Use in an oxygen-depleted environment
We cover these factors and more at greater depth in our stainless corrosion guide.
Preventing Rust on Stainless Steel
With an understanding of what causes rust, it is easier to develop plans to avoid it.
Specific steps and precautions will vary based on how and where you use your stainless equipment and parts.
However, as a general rule, the tips below should help you in a broad range of scenarios.
Disclaimer: The information in this guide is intended to serve as a general overview of stainless steel maintenance practices and options for resolving issues regarding rust. Always consult a professional, engineer, or cleaning product manufacturer before attempting maintenance yourself. Consulting MSDS data for any chemicals involved and disposal information will further minimize risk to persons, materials, and the environment.
Tip #1: Establish Regular Cleaning Routines for Your Stainless
Surface contaminants and debris make it harder for the passivation layer to heal when damage occurs.
A good wash with a soft cloth, warm water, and a mild detergent goes a long way to keeping that layer in optimal condition.
Be sure to check the chemicals used in any cleaners to avoid abrasives, high-strength acids or bases, and chlorides.
If you need more power than water and mild soap can provide, a range of cleaners formulated for stainless is available to ensure you won’t cause more damage than you fix while keeping your surfaces and parts clean.
Tip #2: Always Clean Weld Sites
When exposed to extreme temperatures—such as those involved in welding—stainless steel will change colors.
Any area exhibiting a color change also has a weakened or non-existent passivation layer.
Tip #3: Avoid Abrasive Scrubs and Sharp Tools
Any time you scrub the surface with steel wool or a metal scouring pad you risk leaving behind microscopic iron deposits.
The same holds true if you damage the surface using a tool containing iron.
If you store or ship your stainless steel near other metal objects, bumps and rubbing might also leave behind invisible particles.
This leads to free iron contamination.
While not always visible, this contamination reduces the ability of the passivation layer to heal. Over time, this can lead to rust formation and a reduced level of corrosion resistance.
Unlike standard cleaning, removing free iron from the surface of your stainless often requires specialized steps.
While claim cleaners claim to remove free iron contamination, be sure to check the chemistry involved to ensure results and avoid unintended interactions or safety risks to both materials and personnel before use.
Removing Rust from Stainless Steel
If you’ve found this guide too late to prevent rust from forming, don’t worry!
Unless the rust damage poses a significant structural threat or has compromised the integrity of the stainless steel parts or equipment, there’s a good chance you can remove the rust and return the stainless to something resembling its former condition.
For smaller areas or light rust, using a “soft” abrasive—such as baking soda will take care of any concerns.
Create a paste using baking soda and water and gently scrub the surface using a plastic scouring pad or dry cloth in the direction of the grain or finish until the rust disappears.
Once gone, give the surface an additional rinse using plenty of warm, clean water and allow it to dry. Within a day or two, the passivation layer should recover and provide excellent corrosion resistance once again.
For larger areas, you can apply the baking soda directly to the metal and then mist the surface with water until damp enough to scrub. Again, use a plastic scouring pad or dry cloth to avoid free iron contamination.
If rust is more severe, a cleaner containing oxalic acid might help.
Many manufacturers make industrial-specific blends to help with larger jobs. However, the chemical is safe enough for many consumer cleaners—such as Bar Keepers Friend.
Regardless of the cleaner you choose, be sure to consult MSDS documentation and follow all recommended safety precautions and guidelines to minimize risk.
While grinding rust away can work, it poses a risk of contaminating the underlying surface of the steel and spreading the free iron from the rust throughout the environment.
In most cases, it is recommended to try the above treatments first before resorting to abrasive measures.
For extreme cases of rust—or for removing rust from intricate parts or complex shapes—electrolysis is the best approach.
While the exact steps will vary based on the size of your steel parts and tools available, common components of electrolysis stations include:
- A non-conductive pool
- A well-ventilated area
- An electrolyte solution
- A current source
- The object you wish to clean
- A piece of “sacrificial” ferrous metal
Submerge the rusted part and scrap metal in the electrolyte pool, connect the negative terminal of your current source to the rusted item, and connect the positive terminal to the scrap metal.
When you apply current, the iron in the rust will transfer to the scrap metal, leaving behind a black coating on the formerly rusted piece. This can then be brushed away, and the surface passivated.
NOTE: None of these methods can replace the metal lost due to rust damage. In the event that you need the object to return to exact manufactured standards, replacement is the best option.
A Quick Review
- Stainless steel CAN rust
- Rust, like other forms of corrosion, results from contamination or damage to the passivation layer on stainless steel
- Routine, gentle cleaning is the best way to prevent rust
- Avoid chlorides, strong acids or bases, abrasives, and iron tools when cleaning or maintaining stainless steel
- “Soft” scrubs—such as baking soda—are great for spot and light cleaning
- Electrolysis provides a powerful means of removing widespread rust or cleaning complex parts
Rust removal can never recreate the stainless lost to rust damage
If you’re dealing with rust issues or planning a project where rust is a concern, Unified Alloys has more than 40 years of experience meeting the stainless steel needs of industries across North America. From the environment in which you plan to use your stainless steel to the exact grades and forms ordered, each aspect will impact your final product—and financials. Call us today to discuss options tailored to your exact needs.