Passivation of Stainless Steel is a metal finishing process to prevent rusting and other forms of corrosion. (Rust is one type of metal corrosion)
Like all ferrous metals, stainless steel contains iron. Similar to other types of steel, it’s made of iron and carbon.
But there is something specific about steel, rust:
When exposed to oxygen and water, the iron content in stainless steel reacts with water and oxygen to form rust (iron oxide). Apart from the appearance deteriorating, rust also weakens the metal, reducing the lifespan.
That’s where passivation comes in.
We will cover everything about the passivation of stainless steel.
- the history
- the benefits
- how to do it
The History of Passivation
Passivation dates back to the mid-1800s when a German-Swiss chemist by the name of Christian Friedrich Schonbein discovered the effect of passivation. He is touted as the chemist behind the discovery and the naming of the process.
He noted that if a piece of iron was dipped in concentrated nitric acid, then dipped in dilute nitric acid, the iron had no chemical reaction. Compared to the iron that was only dipped in the concentrated nitric acid alone. His attention was drawn to the latter iron with no reaction.
That Lack of reaction or Reactivity is what he Described as ‘passive’.
By the mid-1900s, the passivation of stainless steel using nitric acid had become widespread across Europe and North America.
There was a problem with Using this Technique for Passivation.
Experts had already raised concerns at the time about environmental and safety issues that came along with the use of nitric acid.
Adolf Coors, a famous researcher in the 1900s had in his research identified citric acid as a possible alternative to nitric acid. It therefore didn’t come as a surprise that by the end of the 1900S, citric acid had almost fully replaced nitric acid in the passivation of stainless steel. Citric acid had by that time, stood the test of time as a safer and more Eco-friendly alternative to nitric acid.
Much has changed since Christian Friedrich’s first discovery. Notably, the industry standards have improved.
Why It’s Important to Passivate Stainless Steel
- It is needed is to remove contaminants that are embedded on the surface of the metal.
- Passivation helps to mitigate corrosion by introducing a corrosion-resistant layer on top of the metal.
- Stainless steel that has been passivated is also able to withstand a higher degree of wear and tear.
- After welding, passivation is a great post-fabrication process to improve aesthetics.
- Passivated stainless-steel lasts longer and requires less maintenance, minimizing repair expenses and down times from damages.
- Compared to other cleaning methods, passivation is less aggressive than pickling for instance. The acids used in the passivation process are less reactive to stainless steel.
How Passivation of Stainless Steel Works
Stainless steel is an iron-based alloy, with iron and carbon as the base materials. Then there’s chromium and nickel.The passivation process is pegged on chromium.
Stainless steel contains between 11% and 30% chromium. For instance, 316 stainless steel has about 16% chromium, while 304 stainless steel has about 18% chromium.
Chromium is used to make the stainless steel tough and increase corrosion resistance.
Passivation is based on the fact that, when chromium is exposed to oxygen, a reaction ensues which then forms a thin film of chromium oxide.
The thin film covers stainless steel and ultimately protects iron, which is beneath the film, from rust. It is, therefore, safe to conclude that the main purpose of passivation is to optimize and augment the formation of the chromium oxide layer.
Once stainless steel is fully immersed in an acid bath, all the free irons on the surface dissolve. This doesn’t happen to chromium, which remains intact. The acid then chemically gets rid of the free irons.
What’s then left behind is a uniform surface with a high chromium proportion compared to the underlying metal.
Note that the chromic oxide layer doesn’t occur immediately after chromium is exposed to oxygen after acid immersion. The process takes place between 24 and 48 hours.
Types of Passivation
The types of steel passivation depend on the acid that is used during the process.
- Citric Acid Passivation
- Nitric Acid Passivation
- CitriSurf Passivation
- A-A-A (Alkaline-Acid-Alkaline) Passivation (Citric or Nitric)
Each process is guided by the standards laid down by the relevant organizations.
The common industry standard for Passivation of stainless steel include:
These standards define how passivation for different stainless steel should be done.
How Passivation of Stainless Steel is Done
As explained above, the passivation process of steel is guided by industry standards. Most widely used industry standard is the ASTM A 967. This standard provides several processes with a specific outline of the chemical properties of the acid and temperatures.
The only choice you have is deciding on the method to use among the provided methods depending on your specific needs.
Step 1: Is to clean and de-grease the metal to remove physical contaminants.
Step 2: Rinse the metal with water to remove any remaining contaminants and the cleaning agent.
Step 3: Prepare the passivating bath depending on the industry standard you are following.
Step 4: Submerge the metal in the passivating bath and let it sit for the amount of time specified by the industry standard you are following.
Step 5: After the specified time is over, remove the metal and neutralize it in a bath of sodium hydroxide. Then rinse it with water and dry the metal.
Step 6: Test the metal using any of the specified testing methods. The different testing methods include elevated temperature, humidity, and exposure to a rusting agent such as salt spray.
Passive Layer Maintenance of Stainless Steel
Iron is the largest component of stainless steel. Unfortunately, it takes just a little amount of damp air and oxygen to break down iron. This mostly happens through rusting which occurs when iron reacts with either oxygen or water.
As already noted, passivation prevents this reaction. It doesn’t end there though. Maintenance is still a vital issue after passivation.
Once stainless steel has been fully passivated, the passive layer may at some point get abraded. This happens through contraction or expansion that comes along with temperature changes. If enough oxygen is present to mix with chromium in the stainless steel alloy, the passive layer will quickly heal itself. This alone is why stainless steel will always have an edge over other metals.
Passivation of Stainless Steel Misconceptions
The most common misconception about passivation is that passivation is an electrolytic process. This isn’t the case because passivation is a chemical treatment. More importantly, passivation doesn’t depend on electrochemical reactions like anodizing and electro-polishing.
It has nothing to do with removing oxide scale from machined parts after welding or heat treating.
Then there’s also the misconception that passivation is the coating or rather a coat of paint. This isn’t correct because first off passivation doesn’t change the surface appearance or color of the metal. Passivation isn’t even necessary for items that need to be powder coated or painted.
Point To Note about the Passivation of Stainless Steel
It is correct to conclude that passivation is a form of controlled corrosion. Think about it this way –the acid bath corrodes and dissolves the free iron at the surface. The acid does this in a uniform and controlled manner.
But what happens when the process isn’t monitored and controlled properly? Runaway corrosion becomes imminent and inevitable. The aftermath is what passivation experts refer to as a ‘flash attack’. It occurs when the metal ends up with a dark and heavily etched surface, which is the exact same thing that passivation should prevent.
It is also important to ensure that the acid solution used is 100% free of contaminants. This will go a long way to preventing flash attacks.
The remedy here is a simple one – refill the acid bath with a fresh acid solution. In other words, replace the acid regularly so as to prevent the build-up of contaminants in the acid solution.
Alternatively, you can use high-grade water like DI water or RO water. Both water options have fewer chlorides compared to tap water. They can therefore resolve and prevent flash attack-related incidents.
Lastly, avoid mixing different grades of stainless steel. For instance, you shouldn’t mix 400 series and 300 series of stainless steel at the same time, in the acid bath.
Such a mixture always guarantees galvanic corrosion. In such cases, the less noble metal ends up corroding faster than it normally does in the absence of the other metal.
Frequently Asked Questions About Stainless Steel Passivation
How long does passivation of stainless steel last?
This is a great question but unfortunately there is no clear or easy answer. There are several factors which can affect the longevity of the passivation of stainless steel. If the thin protective passive layer has been damaged then this will obviously have a negative effect on the duration of this process. In contrast, if the object in question has been electropolished then this will greatly extend the passivation period.
Other factors could be;
- Environmental conditions i.e., temperature and other weather features
- Types of application of work
- In storage or not
Can you passivate stainless steel twice?
It is okay to rework passivation. However, there is no need to do it more than twice a year.
What happens if stainless steel is not passivated?
If stainless steel is not passivated, it’s more susceptible to corrosion such as rusting when exposed to different corrosion agents such as water, humidity, and salt among others.
Is 316 stainless steel passivated?
While 316 stainless steel has better corrosion resistance properties, it requires passivation since there is a type of steel that has a natural passivation layer.
Can 303 stainless steel be passivated?
Stainless steel can be passivated using an appropriate process. This is because 303 stainless steel contains either sulfur or selenium.
Can you passivate 416 stainless steel?
Like with all grades of stainless steel, it’s possible to passivate 416 stainless steel. The secret is using the correct solution.
How can you tell if stainless steel is passivated?
The most accurate way of knowing whether stainless steel is passivated is by testing it. Various testing methods include exposing the metal to water, spraying salt, or doing the copper sulfate test.
Can passivation be removed?
The only way to remove the passivation layer on stainless steel is by mechanically removing the layer.
Does passivation affect conductivity?
No, the passivation of stainless steel does not affect the conductivity. Stainless steel is itself a poor conductor of electricity.
Does passivation add thickness?
The passivation layer formed on stainless steel adds thickness. However, it’s so minute, somewhere between 1 and 3 nanometres thick. The best thing about this layer is that it self-heals.
Do you rinse after passivation?
Stainless steel should be rinsed with clean water after passivation.
Should you passivate Stainless steel after welding?
It’s key to passivate stainless steel after welding to make it corrosion resistant.
Does passivated stainless steel rust?
Since there is no iron that is in contact with humidity and oxygen, passivated stainless steel should not rust. The passive chromium oxide layer that is formed should not rust even when exposed to water and other corrosion agents.
What metals can be passivated?
Apart from stainless steel other metals that can be passivated include:
- Silicone And
- Ferrous Metals
Does passivation remove material?
Yes, the process removes free irons and contaminants such as grease and oils from the surface of stainless steel.
Does passivation improve surface finish?
No, passivation does not interfere with the finish. The best method for improving the finish is electropolishing.
Does passivation prevent galling?
No, the passivation of stainless steel does not prevent galling. The right process to prevent galling is electropolishing.
What is the difference between pickling and passivation?
While both passivation and pickling are ways to make stainless steel corrosion-resistant, there are some key differences. Passivation does not interfere with the metal, unlike pickling which removes some sections of the metal.
What is the difference between passivation and anodizing?
Passivation is to stainless while anodizing is to aluminum and aluminum alloys.
What does sodium dichromate do in passivation?
During the passivation process, sodium dichromate is used to mitigate the chances of a flash attack.
Wrapping up Passivation of Stainless Steel
There we have it, everything you should know about how to passivate stainless steel. As you have seen, passivation is a way to form a protective layer on the surface of stainless steel to reduce corrosion such as rusting. Different passivation industry standards provide a guide on how the process should be done. The most popular passivation of Stainless steel standards are the ASTM A967 and AMS 2700.