Why Passivation is Necessary and How Can It Benefit Your Company
Simply put, the passivation process assists in removing corrosion contaminants such as free iron from the surface of stainless steel. At first glance, the procedure may appear to be purely cosmetic, but there are internal benefits as well.
Whether you're aiming to precision clean valves, filters, or any other metallic component, passivation is needed to keep processing smooth and long-lasting. What exactly is passivation, and how can it benefit as a precision cleaning method? Let's find out.
The Science Behind Passivation
Without all the chemistry jargon and complexities, at its core, Passivation is the process of treating or coating a metal in order to reduce the chemical reactivity of its surface. In stainless steel, passivation means removing the free iron from the surface of the metal using an acid solution to prevent rust.
Free irons and/or iron compounds are stripped off the surface through an acid solution. If free irons are not removed, a rust build-up can occur due to repeated exposure to oxygen and rainfall.
When the surface iron is removed, the other components of the alloy (primarily chromium, often nickel as well) are left behind as a surface layer over the underlying steel.
Upon exposure to air, these elements react with oxygen to form an oxide layer that protects the rest of the steel from corrosion.
An acidic solution is a chemical agent that is able to passivate -- with nitric acid being the most commonly used. It serves to remove free irons without jeopardizing internal components or structures.
The structure of stainless steel must be considered as well. Chromium makes up at least 10% of stainless steel and has anti-erosion properties. When Chromium is combined with iron at a 1.5 to 1 ratio or greater, a thin yet effective corrosion resistance film is naturally formed on the surface. The more compatible the ratio, the stronger the film becomes. This film may also be referred to as a "passive" layer. But, this film can be damaged in two common ways:
- Too much free iron will remove or weaken the protective film.
- Welding, fabrication, and machining leave behind metallic oxides and debris that disrupt the ratio.
When precision cleaning is done through passivation, contaminants are removed from the surface of the stainless steel, thus reestablishing the optimal ratio between the stable chromium and the reactive iron. When the ratio is optimal, the “passive” layer will be stronger and thicker.
Why is Passivation Necessary
If passivation were just a cosmetic feature, then it would be completely fine to ignore. But, rusting machinery is a different story and needs to be actively managed. To emphasize how much of a problem rust actually is, it costs U.S. companies over $300 billion per year.
That’s exactly why passivation is necessary. Rather than letting rust create dents and damages, performance issues, and eventually lead to complete machine failure, snip the problem before it gets out of hand.
If you wait too long, you may have to replace entire sets of equipment that are multiple fold more expensive than precision cleaning.
Passivation processes are controlled by industry standards, the most prevalent among them being ASTM A380, ASTM A967 and AMS 2700. These industry standards list several passivation processes that can be used, with the choice of specific method left to the customer and vendor. The "method" is either a nitric acid-based passivating bath, or a citric acid-based bath, these acids remove surface iron and rust, while sparing the chromium.
Benefits of Passivation
Now that we know what passivation is and why it’s necessary, let’s examine the benefits that it can provide.
Enforced “Passive” Protective Layer
As mentioned before, passivation helps to maintain the natural outer film that protects stainless steel from rusting. In its natural state, the film is effective but very thin. But with passivation, we manipulate the ratio between Chromium and Iron to be the most compatible and create a thicker "passive" layer. This helps to protect your components.
When working with precision cleaning experts, they’ll utilize tests to determine how much free iron there is and how close they are from that optimal ratio. These include:
- High-humidity test
- Copper Sulfate
- Water wetting and drying
- Ferroxyl test
This one is a pretty obvious benefit. When passivation is completed, the yellowing and bumps of rusting metal will return back to a newer look.
Maintains Strong Stainless Metal Structure
Rust weakens the very structure of stainless steel. The typical strong iron is replaced by flaky powder; that’s why rusted metal is easier to dent and damage valuable components. Even more, rusting accelerates with time. It’ll begin to spread and start concentrating rust on certain parts of the surface.
Keeps Electricity Conducting Properties
Rust is an insulator of electricity, meaning that electricity flow will decrease when there's rust present. Companies choose to use stainless steel (or similar metals) because electricity does a great job at transporting electrical flow from one spot to another.
This is a problem for equipment such as transducers which use electric signals to communicate measurements. If transducer cleaning is not maintained regularly and begins to rust, it may interfere with accurate measuring.
For high-powered machinery, this efficiency and speed are often necessary for things to even function optimally. When you precision clean and utilize the passivation process, there's no need to worry about your machinery's conducting capabilities.
Allows for Extended Maintenance Intervals
With regularly scheduled passivation precision cleanings, total system maintenance will not be required as often as before. With unrusted stainless steel, your equipment will last longer and require less monitoring.
Because rust itself is made up of iron and oxygen, it poses no biological threat to humans. But, rust can jeopardize overall safety. Like you see with bridges and roads that collapse, rust can compromise structure and security. If you want to avoid malfunction or machine failure-related injuries, passivation can help.
Passivation is Versatile
Many methods of passivation can be employed for a variety of machine parts.
- Submersion - Metals are submerged in a tank of acidic solution. This is typical for smaller equipment and parts that can be dismantled easily. Submersion offers quick and uniform passivation. Submersion is the most common approach and works well with gauge and transducer cleaning.
- Circulation - An acidic liquid can be circulated through values and filters to de-rust your machinery's internal components. Even if these parts are not visible to us, it’s still important to passivate the inside of your systems as well. If you check valves afterward, they'll look brand new no matter how many years they've been at work.
- Spray Application - This method is for any parts that cannot fit into the tanks. The method involves manually spraying acidic solution in order to remove free iron and rust.
How do you verify passivation solution strengths?
Titration is the slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization, which is often indicated by a color change.
The solution called the titrant must satisfy the necessary requirements to be a primary or secondary standard. In a broad sense, titration is a technique to determine the concentration of an unknown solution.
Discoloration after passivation
Heating stainless steel to temperatures in excess 800°F can cause metallurgical changes and reduce corrosion resistance in a manner that is not corrected by surface cleaning or chemical passivation. Typically, stainless steels contain less than 30% chromium and more than 50% iron. While stainless steels are resistant to rusting at room temperatures, they're prone to discoloration by oxidation at elevated temperatures due to the presence of chromium and other alloying elements such as titanium and molybdenum.
Proper handling of parts that have been passivated
Once a part is passivated, proper handling procedures must be followed to prevent the surface from being damaged and allowing iron to be introduced back onto the surface.
Many people would think that the stainless-steel parts of a machine should be able to withstand normal handling procedures. During normal handling, a passivation layer can be removed in two ways. Parts can come into contact with corrosive agents that chemically damage the chromium oxide or parts are not handled properly and mechanically remove the oxide layer.
After the surface is protected, it's important not to expose it to any chemicals that can damage the oxide or strip away a section of the native surface. This means protecting parts from exposure to acids during passivation, as well as exposure to organic acids, such as vinegars and lemons which may be present in the workplace.
Storage of the passivated parts should be done in such a manner to not allow damage to occur. This can be accomplished by using a container that will not allow contact from foreign objects.
Precision Fabricating & Cleaning is Here to Help
Are the benefits of passivation appealing to you and your company? PFC is here to help! With knowledgeable and experienced staff, class 7 cleanroom and class 5 workstations, and time-sensitive turnarounds, we're here to say. Passivation is much more than a cosmetic procedure. There are many real-world benefits to passivation-based precision cleaning, and we would even consider it necessary. Act sooner than later, and it could save you thousands of dollars.