Does stainless steel rust?

Have you wondered, does stainless steel rust or tarnish?

First stainless steel is a low-carbon steel that contains chromium, giving it an additional layer of protection and increased rust resistance.

Stainless steel is corrosion and heat-resistant, hygienic, robust, aesthetic, and 100% recyclable.

While these qualities make stainless steel more expensive, they also make it a superior material choice for manufacturing culinary products, surgical tools, and various construction hardware and fixings.

Any application where steel would corrode under the elements is better off being protected.

Why stainless steel doesn’t rust?

Stainless steel’s ability to effectively ward off rust comes from the trace amount of chromium in its composition, which’s no lower than 10%.

Chromium forms an extremely durable and translucent film on the steel surface called chromium oxide [1].

Chromium oxide is highly resistant to corrosion and weathering and can even self-heal from mechanical and chemical damage so long as oxygen is present.

Stainless steel is also alloyed with other elements, such as nickel and molybdenum, further adding to its durability.

What are the different stainless steel grades?

While stainless steel has many different grades, it’s usually divided into 5 main classifications based on its alloyed microstructure: 

• Austenitic. Classified as the 200 or 300 series, austenitic stainless steel is often composed of 17 – 30% chromium and about 8 – 25% nickel. This classification is non-heat-treatable but has high weldability, ductility, and heat resistance. Commonly used in making culinary and chemical equipment.
• Ferritic. Classified as the 400 series, this type of stainless steel has around 10-30% chromium and is stronger than carbon steel. This classification is magnetic but is also non-heat-treatable. Its low weldability, average ductility, and high heat resistance make it great for appliance trims and cooking utensils.
• Martensitic. Also classified as the 400 series, martensitic stainless steel is high-strength and can be hardened by heat treatment. It contains 11 – 17% chromium with less than a percent of carbon. It has low weldability, ductility, and heat resistance and is mainly used for fasteners and pump shafts.
• Duplex. Classified as the 2000 series, duplex alloys contain a mixture of austenite and ferrite with about 22% chromium and around 5% nickel. Duplex stainless steel offers added strength and better resistance to stress, corrosion, cracking, and pitting. Mainly used for manufacturing pipes, tubes, and tanks thanks to its high weldability. It has average ductility and low heat resistance.
• Precipitation hardening. Classified as having a dash 17 (17-) grade and are often referred to as semi-austenitic. Containing 12 – 30% chromium with about 4 – 7% nickel and can undergo various heat treatments for better strength-to-weight ratios. It has low weldability and heat resistance, but average ductility makes it ideal for making valves, gears, and other machinery parts. 

When does stainless steel rust?

Rust can still form on stainless steel when its protective chromium oxide layer gets damaged.

Improper cleaning and exposure to certain abrasives and extremely high temperatures can either tear through the chromium film or cause chemical reactions that would allow rust to form on the surface. 

What can cause steel to rust?

So, why does stainless steel rust? Does stainless steel rust in water? Does stainless steel rust in the shower? Does vinegar make stainless steel rust? Let’s find out.

When stainless steel rusts, it suffers the same way all other metals do – it reduces in strength, aesthetics, and structural integrity.

While stainless steel is practically corrosion-proof, improper care and treatment can still cause it to rust and degrade. Take a look at the following cases: 

• Water. Normally, stainless steel won’t rust and isn’t affected by water unless the chromium oxide film is damaged or it’s exposed to a chemical that reacts corrosively with water.
• Saltwater. Stainless steel can still rust in saltwater over long periods of time and without due maintenance.
• Vinegar. Long-term exposure or uncleaned vinegar on stainless steel can damage the chromium oxide layer, allowing rust to form at some point.

• Alcohol. If you ask does alcohol rust stainless steel, it’s the same as vinegar.
• Bleach. So, does bleach cause rust on stainless steel? Strong cleaners such as chlorine bleach can stain and damage stainless steel. Rinse clean with water and wipe dry to avoid this.
• In a pool/shower. Pools and showers treated with chlorine can form chloramine. If it contacts nitrogen-bearing compounds. This chemical becomes a very volatile metal-eater. 

How to remove rust from stainless steel?

Since most cleaning chemicals are more likely to damage the chromium oxide film on stainless steel, it’s actually better to use some of these common household items if you’re looking to de-rust it. 

Using a combination of equal amounts of lemon juice and baking soda or just baking soda and water, apply the solution on rusted spots and simply wash the gunk away with a damp sponge.

Repeat until all the rust has been removed. And if rust is deep you could just sand it and apply some primer.

How to prevent stainless steel placed outside from rusting?

Because of its appearance and unique properties, stainless steel is also used as a material for bike racks, handrails, and even benches.

Since these fixtures are commonly placed outside, they’re more exposed to the elements and are likelier to form rust.

To prevent this, be diligent and perform routine cleaning & maintenance using either homemade cleaning pastes or certain water-based coating, such as Stainless Shield, to add an extra layer of protection to the exposed chromium oxide film.

When should stainless steel be used?

If it weren’t for the cost, stainless steel can actually be used for almost any general purpose given its ability to resist corrosion even more than galvanized steel and even treated aluminum.

It’s certainly sturdy enough for construction, yet light enough to make long-lasting household items. To be more specific, below are some key examples of when best to use stainless steel.

1. As fasteners on tanalized timber

Also known as pressure-treated timber, is wood that has been chemically treated to be more durable and longer-lasting in wet environments.

The chemicals used for treatment make the wood more corrosive, to any steel fixings so it’s only fitting to use stainless steel so that the wood doesn’t eat away at the metal for any hardware that is structural.

2. Outside fixing hinges and latches

Anything outside that supports a function, like the hinges and latches on gates or garage doors, is better off being stainless.

Since stainless steel will better withstand prolonged exposure to the elements without maintenance or later replacement. 

3. Outside metals close to the sea

Pier rails, dock posts, and even bay-walk steel flooring should be stainless since these are closer to saltwater.

Saltwater has more dissolved ions and makes it easier for electrons to move, which is what quickens the build-up and progression of rust. 

4. On tools

Stainless steel tools will last longer and are easier to maintain, especially those that see common use along with various chemicals. For example, a stainless steel wrench for automotive engine repair is less likely to rust compared to a regular one.   

In cases you like to remove rust from your tools, here’s how.

When does steel or galvanized steel rust?

How does stainless steel rust?

To differentiate, galvanizing is when you coat a steel piece in zinc; galvanized steel is not an alloyed metal. In general, most metals rust when exposed to enough oxygen, creating iron oxide that begins eating away at a metal’s surface. 

Steel rusts when:

• An untreated surface comes into contact with water. 
• An untreated surface chemically reacts to increased moisture levels.

Galvanized steel rusts when:

• There’s damage or breach in the zinc coating.
• Doesn’t really rust, but instead gets a brown discoloration over time.
• Affected by exposure to elements such as saltwater, hydrogen sulfide, pollution, and moss/lichen growth. 
• When in contact with highly treated wood for long periods, this can be fixed with regular applications of grease.

Types of corrosion that lead to rusting

Not all corrosion results in rust and not all rust forms the same way. Below are patterns of how corrosion leads to rust formation. 

1. Uniform/General.

The most common type of corrosion is characterized by an even attack on the surface.

Considered the most benign as the extent of the corrosion is easily judged and the impact on material performance is easily evaluated. 

2. Galvanic.

This type of corrosion, also called bimetallic corrosion [2], occurs when one metal corrodes when in contact or close to a different metal through an electrolyte.

One metal stays relatively intact, while the other can experience severe corrosion.

E.G: A copper and a steel pipe are both in saltwater; one of them will experience more corrosion. 

3. Intergranular.

A form of local corrosion that occurs at the grain boundaries is caused either by impurities at the grain boundaries, enrichment of one of the alloying elements, or by the depletion of an alloying element at a grain boundary.

Crevice.

This is intense localized corrosion that forms within gaps of 2 joining surfaces.

The small microenvironment between even two completely corrosion-resistant materials can still experience corrosion and, while it will have little effect on the resistant pieces, it will eat away at all and any material medium between them. 

Pitting.

Pitting is a fast-spreading type of extremely localized corrosion that’s marked by the formation of very small holes.

Left untreated, it’s an extremely destructive form of corrosion and isn’t always easy to detect given that initial attack holes can be microscopic.

Stress corrosion cracking.

Usually identified by microscopic cracks within a material caused by both corrosions as well as tensile stress (E.G: heavy load-bearing such as the bolts of a shipping crane).

Common in the steel either near or submerged in saltwater. Dangerous attacks can go unnoticed and are mainly internal.

Comparing strength and lifespan of metals

Conclusion

Nobody who works with metal likes rust. While being understandably more expensive, investing in stainless steel saves you a lot in terms of micro-transactions in the future.

It doesn’t need an additional layer of zinc, or grease and unlike galvanized steel, exposure to oxygen actually regenerates the chromium oxide film. Besides, what else would you need? Stainless steel looks great and works to resist corrosion. 

So if there’s anything that can go wrong, you can always sand it bare and smooth.