Stainless steel is Corrosion resistant by design, but it can rust in certain conditions—though not as quickly or badly as ordinary steel. Stainless steel corrodes when subjected to harsh chemicals, salt, grease, moisture, or heat for an extended period.
The amount of chromium present largely determines the corrosion resistance of stainless steel. If there is insufficient chromium concentration close to the surface, the top layer of stainless steel is removed, and no new chromium oxide layer can form. The upshot is that the material is highly susceptible to numerous types of Corrosion.
Introduction:
While it may appear impossible to rust stainless steel, the reality is that stainless steel like 430 or 440a Stainless steel can rust and corrode just like any other material. This curious occurrence has puzzled me and my fellows, who have experienced their fair share of rusty fabrications made from stainless steel, such as gates and handrails.
From barber-pole type rust patterns on tubular handrails to rusty seawater exposed gates and even rust on a stainless steel bicycle rack, there are hundreds of questions about why these seemingly impervious materials become rusty so quickly.
The answers lie in the examination of the entire fabrication process undertaken during its production – determining what went wrong during these processes requires the skills of only the best forensic detectives.
Why is rusting of normal steel different from Stainless Steel?
To understand what causes stainless steel to rust, first grasp the science that keeps it from rusting in the first place.
Iron and carbon are the main components of steel, while stainless steel also has iron, carbon, and up to 30% chromium. Chromium is the essential component that prevents rusting in stainless steel, though it can also contain other elements like nickel and manganese.
The well-known red rust color is caused by iron oxide (Fe2O3), typically generated when oxygen comes into Contact with the surface of regular steel. The ferric oxide cannot build up in a continuous layer on steel because the oxide molecule’s volume is greater than the iron atoms beneath it. Fresh steel is exposed when it eventually spills off, starting a vicious rusting cycle.
Due to chromium’s high affinity for oxygen, stainless steel develops chromium oxide on its surface when in Contact with oxygen. The fragile layer of chromium oxide protects the stainless steel from additional oxidation and doesn’t spall off. Even if the rust-resistant coating is scraped off and removed from stainless steel, a fresh layer will guard the remaining stainless steel underneath.
The chromium oxide layer will continue to shield the stainless steel and keep it from rusting if enough chromium is present.
Is stainless steel more prone to rust than carbon steel?
Absolutely No! The reason for corrosion resistance in stainless steel is resistant to continuous degradation, which is impossible in normal steel. In normal steel, corrosion continues as long as there is free iron present to react with oxygen. As previously stated, the oxide layer generated on the surface of regular steel spalls off, whereas it stays in the case of stainless steel.
Stainless Steel Basics
What makes stainless steel “stainless?”
Stainless Steel is called Stainless because of its unique rust resistance properties and smooth finish even after being used for a long-time in-service condition.
As discussed, resistance to rusting comes from the chromium oxide layer on the surface formed due to the strong affinity between oxygen and chromium present in stainless steel.
What distinguishes stainless steel from regular steel?
Stainless steel differs from plain steel majorly in terms of corrosion resistance. When rusting of plain steel when started, it can propagate to whole steel, while rusting of stainless steel is very localized and protects from further degradation. It’s explained in more detail in the above sections.
Scientific Reasons Behind Rusting of Stainless Steel Appliances
Although this isn’t accurate, stainless steel is considered rust-resistant. There are situations where stainless steel starts rusting. We are covering some of the factors that may lead to rusting stainless steel.
Factor # 1: Pitting-type corrosion caused by deposition of airborne sea salts
Chlorides are an incredibly versatile, yet dangerous element in our environment. They have a wide variety of applications, given their ability to act as an electrolyte and carry electrical current. Unfortunately, their corrosive nature means they can cause significant damage to stainless steels by staining and rusting them. In fact, chlorides are so powerful that they can even erode Teflon, which is generally considered a highly resistant material.
Research has shown that stainless steels exposed to a marine atmosphere will undergo surface tarnishing as a result of pitting corrosion. This is caused by aggressive airborne chlorides in the environment which react with the metal’s surface.
Despite considerable research into pitting corrosion of stainless steel under bulk solutions containing chloride, no studies have yet been done on the reaction mechanism when these metals are subjected to a marine atmosphere.
Pitting Resistance Equivalent Number and Stainless Steel:
The ability for metal to withstand pitting and crevice corrosion is determined by the alloying elements that are present. For example, austenitic and duplex grades are improved with an increase in chromium, molybdenum and nitrogen content.
Ferritic grade strengthens its resistance to pitting and crevice corrosion primarily through increased chromium and molybdenum levels. The correlation between wall thickness and chromium additions is complex so suppliers of stainless steel have made attempts to describe stainless steel resistance in terms of a “pitting resistance equivalent number” (PREN).
This numerical index takes into account composition parameters such as chromium, molybdenum, nitrogen, and sometimes tungsten. So when looking to improve your metal’s resistance to pitting or crevice corrosion you should always consider the beneficial effects that these alloying elements can provide.
Prevention Tips:
AISI 304 is widely used in fabrications due to its economic value, but when greater corrosion resistance is needed, the addition of molybdenum can provide an enhanced level of protection. AISI 316 and AISI 317 are two options here as both contain 2-3 and 3-4 percent molybdenum respectively and show higher resistance to halogen salt exposure.
Although stainless steel is naturally able to form passive films in presence of oxygen, additional treatment like chemical passivation may be required for use in specific scenarios. However, during immersion in halogen salt solutions, Copper-Nickel alloys such as Monel prove to be better equipped to handle environmental abuse than stainless steels.
Ultimately, engineering compromises are frequently dictated by cost, meaning the most appropriate alloy choice must be made on a case-by-case basis.
For more information, you can start reading stuff on Chloride Stress Corrosion Cracking in Austenitic Stainless Steel.
Another article you should read is on, “Pitting corrosion mechanism of Type 304 stainless steel under a droplet of chloride solutions”.
Factor # 2: Intergranular Corrosion / Sensitization of Stainless Steel during Welding and Steel Heat Treatment
Intergranular corrosion is a common issue experienced in metals undergoing welding or other high-temperature exposures. This occurs due to the depletion of chromium from the alloy matrix near the grain boundaries during such manufacturing processes.
In these instances, temperatures become too hot to keep chromium dissolved within the matrix, yet remain too low for chromium to be rediffused into it. As a result, chromium carbides and nitrides precipitate forming grain-boundary sites prone to corrosion.
Although this process is sometimes unavoidable, manufacturers must be aware of its presence and take active steps to reduce its impact on their products and operations.
304 Stainless Steel Welding:
Sensitization, different from annealing, is an important consideration when discussing stainless steel. For example, heating a type 304 stainless steel containing 0±039% carbides for 10 hours at 700°C can lower the chromium levels from 19% to less than 13%, which affects its corrosion resistance.
Prevention Tips:
This can be remedied by adjusting the carbon content, adding titanium or niobium-tantalum stabilizers, using post-weld heat treatments appropriately, or opting for duplex stainless steels that don’t typically exhibit sensitization at austenite-ferrite grain boundaries since the chromium diffuses and carbide growth is faster in the ferrite phase.
Factor # 3: Stress-Corrosion Cracking because of some loading and Corrosive Environment
Stress corrosion cracking (SCStress corrosion cracking (SCC) is a type of corrosion that typically occurs in stainless steel when exposed to a specific corrosive environment. This form of corrosion is caused by the combination of tensile stress and a specific corrosive element, such as elevated temperature, dissolved oxygen in water, marine water or halogen solutions.
Prevention Tips:
If the corrosive environment is due to temperature, one possible remedy is sensitization. Other remedies include the use of corrosion inhibitors, shot peening to introduce compressive stress, stress-relief annealing or similar treatments. Additionally, increasing the amount of nickel in austenitic steel can improve its passivity and reduce the risk of SCC.
These remedies can reduce the risk of SCC and help ensure the protection of stainless steel from corrosion. It is important to take appropriate measures to prevent Stress Corrosion Cracking so that stainless steel will remain in its best condition for a long time.
Factor # 4: Bimetallic Corrosion or Surface Contamination / Improper Handling of Stainless Steel Appliances Leads to Rusting or Tarnishing
Bimetallic rusting of stainless steel is a form of corrosion that occurs when stainless steel appliances come into contact with iron or steel particles. This sets off a reaction in which an electrochemical cell is activated and causes the formation of rust.
The process begins when iron particles come into contact with the stainless steel surface, significantly increasing the surface area. This then causes the chromium oxide layer on the stainless steel surface to become depleted, reducing its ability to protect itself from rusting.
It is important that all stainless steel surfaces are properly protected and maintained in order to reduce the risk of bimetallic rusting.
Because of the propensity of iron or steel particles to rust in the presence of oxygen, it is essential to ensure that fabrication of stainless steel be completed in an area separate from grinding and welding of ferrous alloys.
This is due to the fact that the air in manufacturing plants carries metallic dust particles that could easily settle on a satin finished handrail and cause oxidation. These static electric charges also attract dust from other sources in the environment.
As an example, Kane Behling, supervisor of the polishing department at R & B Wagner lives in a suburb close to manufacturing plants. He commented on how easy it is for airborn particles to travel when he mentioned “(that) he can’t even walk across wet grass on his lawn without collecting rust on his socks”.
More examples related to Contamination of Stainless can be found in this article.
How long does it take for stainless steel to rust?
When Stainless steel is exposed to oxygen, the chromium in stainless steel forms a passive film that protects the underlying metal from further oxidation. This protection can last for decades; however, depending on the environment, its durability may vary.
For example, in a damp environment with low levels of oxygen and high salinity, the protective layer can wear away more quickly. If this happens, stainless steel will start to corrode and rust at an accelerated rate.
Generally speaking, it can take anywhere from several months to several years for unprotected stainless steel to show signs of rusting. To ensure its longevity, it is important to regularly inspect and maintain stainless steel surfaces. Proper care can help extend the life of your stainless steel and minimize any corrosion or rusting that may occur over time.
How to Prevent Stainless Steel from Rusting?
- To prevent stainless steel from rusting, it is important to keep the surface of the material clean and free of contaminants.
- Regularly wipe down your stainless steel surfaces with a damp cloth or soft sponge to remove any dirt or debris that may be present.
- Additionally, regular cleaning and polishing with a mild detergent and stainless steel cleaner can help to prevent rusting.
- It is also important to use only water-based cleaning solutions when cleaning your stainless steel surfaces, as the chemicals in other types of cleaners may corrode the surface if used too frequently.
- Finally, it is important to keep any stainless steel that is exposed to moisture covered in a rust-resistant paint or coating, as this will help to protect the material from rusting.
By following these simple tips, you can ensure that your stainless steel surfaces remain durable and corrosion-free for many years.
You can read more about prevention tips in another article, “Stainless Steel Prevention“.
Does Stainless Steel rust easily?
No, stainless steel does not rust easily. This is because it contains a minimum of 10.5% chromium content by mass that forms an invisible and adherent oxide film which prevents the further oxidation of stainless steel surfaces. This oxide layer protects the metal surface from atmospheric corrosion, making it very resistant to rusting compared to other metals or alloys.
Still, rusting is something that can not be prevented even if steel is stainless in the presence of Corrosive Environment. For Example, using chloride containing detergents for cleaning of stainless steel pans can lead to corrosion, as chloride ions can penetrate the passive film on stainless steel surface and cause pitting corrosion. Hence, it is important to use suitable cleaning agents for maintaining the rust resistance of stainless steel.
Overall, stainless steel does not rust easily due to its composition but it is important to take proper care when using it in order to prevent corrosion.
Read more about Cleaning of Stainless steel, if looking for exact recipes to keep steel clean.
Can you repair Stainless steel rust easily?
Stainless steel is an alloy composed of iron and at least 10.5% chromium, which creates a corrosion resistant surface. While stainless steel can resist rusting, it is not completely immune to corrosion. Factors such as humidity, salt spray, and dirt can cause stainless steel to corrode over time. If not treated properly, rust can form on stainless steel surfaces.
Fortunately, there are ways to repair and restore rusty stainless steel. The first step is to remove the rust using either a stiff brush or chemical cleaners such as white vinegar or citric acid. Afterward, you should apply a topical sealant or protective coating to keep the surface from corroding further. Finally, you should take steps to maintain the surface and keep it clean by wiping down frequently with a cloth dampened in warm water.
With these steps, you can repair stainless steel rust and protect its integrity for years to come. Read more about repairing rusted stainless steel appliances by checking out this article on Stainless steel prevention tips.
Does vinegar remove rust?
Yes, vinegar can be used as a rust remover. Vinegar contains acetic acid which reacts with the iron oxide (rust) and breaks it down, leaving a less concentrated version of the original iron molecule. To use vinegar to remove rust, simply soak the rusty object in white vinegar for a few hours and then scrub off the rust with a wire brush. For more stubborn rust, you may need to soak the object in vinegar overnight.
Why is my brand new stainless steel sink rusting?
Rust can occur on stainless steel sinks due to a variety of factors.
The most common cause is an electrolytic reaction between the sink and other metals. This is usually caused by water containing high levels of chlorine, galvanized pipes or fixtures, steel wool pads used for cleaning, and other metal objects placed in or near the sink.
In some cases, rust can from various cookware made of cast iron or carbon steel like woks, skillets and pans.
Additionally, acids found in certain cleaning products can cause a reaction on the sink as well. Finally, exposure to direct sunlight or even regular heat from common kitchen use may also lead to rusting of stainless steel sinks.
What causes brown spots on Stainless Steel?
Brown spots on stainless steel are usually caused by mineral deposits. These deposits, often called ‘scaling’, can be caused by impurities in the water used to clean or rinse the metal surface. If not removed promptly, these minerals can react with oxygen and create a brownish-colored stain on the steel.
In order to avoid this issue, it is important to use clean water when cleaning and rinsing stainless steel surfaces. Additionally, using a mild detergent will also help remove any built-up dirt or deposits that may be present on the surface of the metal.
Does Stainless Steel rust in all types of water?
No, it does not. Water has its own composition and each composition has its own salt concentration, and free oxygen concentration. The salt content, the temperature and the free oxygen all play a role in how much corrosion can occur. Salt water is more corrosive than fresh water and warm water is more corrosive than cold water.
In general, stainless steels are considered to be very resistant to corrosion in most types of waters, but with certain exceptions such as hot chlorine or seawater. It is important to understand the composition of the water that is in contact with stainless steels in order to determine whether corrosion will take place and if it will, how much corrosion will occur.
