From construction to automobiles and even cookware. You’ll find stainless steel everywhere. The material is a perfect choice for a variety of situations because of its apparent longevity, performance, and excellent strength.
However, stainless steel still undergoes corrosion very slowly. There are many inherent properties of the alloy that keep it protected from corrosion, but stainless steel passivation is one process that all types of stainless steel parts must go through.
This article will take a look at the passivation of stainless steel and discuss its process, advantages, and prevalent standards in the industry.
What’s Stainless Steel Passivation?
In the simplest terms, stainless steel passivation is the process of removing any molecules of iron from the surface. Steel is actually an alloy of iron, chromium, carbon, and various other elements. However, iron is the majority element and is susceptible to corrosion.
Chromium is a major part of stainless steel as it is mainly responsible for corrosion properties. The chromium content, along with the combination of other additives determines the durability and formability of the steel you use.
Steel becomes stainless, which means free from corrosion, because of its corrosion-resistant surface consisting of virtually inert elements like chromium and nickel. These elements form an extremely thin passivation layer on the surface which protects the iron inside from all kinds of contamination that can start the oxidation process.
However, chromium requires absolutely no surface contamination as it forms a very thin layer around the steel to prevent corrosion. The steel passivation process does that and enables the protection that causes steel to last longer and perform versatile conditions.
The passivation process is what activates the formation of the inert layer. Small iron molecules can stay on the surface and initiate the corrosion process. Passivation removes that iron from the surface and ensures a longer, more durable protective layer on the steel surface.
The History of Stainless Steel Passivation
Iron has been a part of our lives for thousands of years. From the start, it was apparent that Iron in its natural form is not suitable because of its oxidation properties that lead to corrosion. While alloys like stainless steel perform better, they still corrode if they don’t undergo processes like passivation.
Steel passivation started in the 1800’s when a chemist named Christian Friedrich Schönbein discovered that stainless steel showed little to no activity once it was dipped in a concentrated nitric acid solution. He compared the performance of the new treated steel with the regular alternative and took note of the enhanced corrosion protection of the new sample.
Since then, passivation through nitric acid became quite common throughout the world and became the industry standard. However, nitric acid had a lot of disadvantages because of its toxicity and environmental damage.
The use of nitric acid for steel passivation continued till the 1900s when a German brewing company identified citric acid as an alternative chemical for the process. By 1990, citric acid effectively replaced nitric acid in various areas because of its apparent non-toxic nature.
Today, industry standards are much more advanced and rely on both nitric and citric acid baths to safely conduct steel passivation.
Why Passivate Stainless Steel?
Iron has the right properties, is readily available, and is malleable enough to form various shapes without too many complications. These qualities make it the perfect metal for a variety of key applications. However, its corrosion-resistant surface properties leave a lot to be desired.
Stainless steel fares better than Iron (You can learn something from the comparison guide between cast iron and steel). However, it can still corrode as well. The effectiveness of its corrosion resistance depends on two main factors; the alloy’s composition and the passivation treatment.
Here are a few reasons why it’s so important to passivate stainless steel.
Iron’s Vulnerability to Corrosion
Iron constitutes a major part of any type of stainless steel. Additionally, there is no doubt that it has several advantages, but it’s also true that Iron corrodes fast, especially when it is exposed to water and oxygen. Air anywhere has both of these, which effectively means that corrosion is inevitable.
However, in the case of steel. Microparticles on the surface can start the corrosion process, and slowly but surely, it propagates to the whole part. Passivated stainless steel won’t have that issue because the process clears the surface and allows an inert protective layer to form on the surface for better performance.
Chromium Prevents Corrosion
Changing the composition isn’t always a viable solution because it directly affects the formability and other performance parameters of the steel. Therefore, stainless steel passivation becomes extremely important to maintain that durability.
Passivation of stainless steel facilitates the formation of a chromium layer on the surface. Chromium is inert and prevents corrosion even in the presence of water and other contaminants. Resultingly, the stainless-steel part lasts longer under the protection of chromium.
Contaminants in the Manufacturing Process
No matter how careful you’ve been, any manufacturing process will leave out contaminants that may induce corrosion. Even the machining tool can leave out some components on the surface that may reach under certain conditions and cause oxidation leading to corrosion. The passivation process starts with a thorough cleaning and minimizes the chances of any issues due to contaminants on the surface.
As discussed, stainless steel consists of many elements apart from iron, carbon, and chromium. During the material selection process, you need to be very careful and consider the effect of all components.
Consider the example of carbon content to understand it better. All steels have some carbon, and the quantity determines the physical properties. Generally, more carbon means higher tensile strength and hardness. However, it also means low ductility and more difficulty in the machining process. Chromium has a similar effect and is primarily responsible for the corrosion-related properties of steel.
Stainless steel passivation doesn’t change the composition and resultingly doesn’t reduce the machinability of the metal. Instead, it leverages the existing chromium in the alloy to maximize the protection against corrosion and enhance the overall durability.
An Overview of the Process for Passivating Stainless Steel
If you take a look at the process, it would look quite simple from the surface. However, you need to be aware that the process is extremely important and any mishap can create many issues for you down the line. Stainless steel is regularly used in numerous sensitive applications and any processing problem can quickly become too costly and resource-intensive to address properly.
Therefore, it’s important to ensure that the passivation process is flawless.
The Passivation Process
To put it simply, steel passivation is a 3-step process. It starts with ensuring that there are no contaminants, then you can facilitate the formation of the protection layer through different chemicals, and then finally test the results. Here’s a brief overview of the entire process.
Alkaline cleaning of stainless-steel parts
The first step is to ensure that the steel part is free from any contaminants on the surface. For that, an alkaline degreaser or surface cleaner is the best option as it is mild and effectively removes foreign objects like oil particles, chemicals, and other debris.
Passivation in citric acid or nitric acid bath
The next step is to remove all the traces of iron from the surface with an acid bath. For that, you have the option of using two stainless steel passivators. The first choice is citric acid and the second one is nitric acid which can also be used with sodium dichromate.
Citric acid passivation is a cost-effective and safe way to initiate the passivation process. It doesn’t emit any toxic gas and poses no threat to both the environment and the operators working on it. However, citric acid is not too popular because of its tendency to grow organic compounds like mold. Recent advances in technology have minimized this issue, but the problem still persists.
The other alternative, nitric acid passivation is a common alternative that’s traditionally popular in the industry. It delivers a more effective molecular distribution and gives you more durability. However, these qualities are at the expense of the processing time. Additionally, nitric acid has a hazardous classification and produces toxic fumes that are damaging to the environment.
To address the slow processing time, you have the option of combining nitric acid with sodium dichromate. However, remember that doing so would further enhance the toxicity and create further problems for you.
Regardless of the approach, you select. Immersion would result in stainless steel passivation which will create a protective layer around the surface and keep your steel part safe from contamination and corrosion for a very long time.
Testing passivated parts
Finally, once the passivation process is done, the final process is to simply conduct the appropriate tests to ensure that you did the right job. There are numerous standards that are prevalent across the industry and the right one depends on factors like your application, location, and the target industry.
Steps-by-Step Breakdown to Passivate Stainless Steel
Here’s a step-wise breakdown of the entire process to passivate stainless steel parts:
- Degrease the whole surface thoroughly to remove any chemical or physical contaminants
- Rinse the part with water to remove any leftover degreaser or cleaner you used
- Immerse the part in the passivating bath of either nitric or citric acid to initiate the process
- Once completed, rinse the part with water again to remove all traces of the acid
- Completely dry the part once cleaned
- Carry out the test specified by the standards you follow
Dos and Don’ts of Passivating Stainless Steel
Although the passivation of stainless steel requires a simple process, there are still a few best practices and things to avoid. Following them can help you get the best results with consistency. Here’s a brief look at the key dos and don’ts of the passivation process.
- Always ensure that the steel part is properly cleaned before starting
- Use dechlorinated water for rinsing whenever possible
- Regularly replace the acid baths to ensure proper passivation
- Minimize contamination from all sources by separating operations
- Use dedicated equipment and machinery for stainless steel part fabrication
- Implement a racking storage system that avoids metal-to-metal contact
Things to Avoid
- Never passivate parts with nitride elements. The treatment can cause the acid in the passivating tanks to start the oxidation process as well
- Avoid tools with iron content in the machine shop. Use carbide tools instead
- Never underestimate the role of heat treatment
- Ensure that nitric acid is in the right concentration to optimize the passivating process
Grades of Passivated Stainless Steel
Like all alloys, stainless steel also comes in different grades with varying mechanical properties and overall applications. Stainless steel has 3 main categories depending on the constituent alloying elements and the overall composition.
The 3 grades are:
1 – Austenitic Grade
The most common stainless-steel grade in various applications. This alloy composition comes with a chromium percentage in the range of 16% to 30% and some nickel, which means that it offers significant protection from corrosion for a long time. Moreover, austenitic grade stainless steel is non-magnetic and can’t undergo heat treatment which means that its mechanical strength is capped.
2 – Ferritic Grade
Ferritic alloys of stainless steel come with high chromium content as well. The range is around 10% to 30%, but the alloy also has a significant percentage of carbon which has an upper limit of 20%. Just like the previous grade, ferritic steel isn’t suitable for heat treatment as well. Moreover, other methods like cold rolling that work on austenitic grade don’t work on ferritic steel.
3 – Martensitic Grade
This is a versatile grade that is magnetic, heat treatable, and suitable for many tampering and aging processes. However, these qualities come at an expense of the overall effectiveness of corrosion resistance as martensitic stainless steel has a chromium content in the range of 12% to 17%.
Industry Standards for Stainless Steel Passivation
One of the best ways to ensure consistent quality and reliability is to follow prevalent standards in the industry. Apart from specific company standards, most global players either follow the ASTM or AMS standards for stainless steel passivation.
ASTM International is a standard body that operates on a global level and has more than 14,000 standards for material specification, processes, and tests. The following standards are more relevant to the testing and the passivation process of stainless steel.
- ASTM A96: For steel passivation with 5 combinations for both citric and nitric acid techniques.
- ASTM A380: Cleaning, descaling, degreasing, and finally passivating stainless steel parts and testing the quality of finish and even the steps to avoid specific issues.
- ASTM 380: Only descaling and passivating stainless steel parts.
- ASTM F86: Surface preparation of stainless-steel parts before passivation.
Aerospace Material Specifications (AMS) is another standard body that predominantly deals with the processes, materials, and standards for aerospace applications. However, since all aerospace applications use the best materials and processes, using AMS standards can help you maintain reliability.
The following two standards are prevalent for stainless steel:
- AMS 2700: Passivation standards for stainless steel that cover both citric and nitric acid methods.
- AMS-QQ-P-35: Standards are restricted to the nitric acid method and offer 4 different combinations.
Types of Equipment for Stainless Steel Passivation
The passivation process requires certain equipment that can help you optimize the 4-step process of cleaning, rinsing, passivating, and finally rinsing it again. You have the option of selecting the following equipment types for the process.
Small Benchtop Passivation Equipment
Manual passivation systems are ideal for areas with limited spaces. Generally, small parts and specimens are passivated in this equipment category.
Wet Bench Passivation Equipment
An integrated system that looks like an assembly line. You have ample space for all the passivation steps and safety features like in-tank flow, exhaust for nitric acid method, and much more. This equipment class is also manual.
Automated Passivation Systems
As their name suggests, automated passivation systems operate on the same principle but eliminate the need for human intervention throughout the process.
Agitated Immersion Passivation Systems
Semi-automated or automated equipment that uses pneumatics to agitate the part and provide an exceptional cleaning performance. More suitable for highly sensitive applications where you want the most durability and longevity.
RapidDirect: The Perfect Manufacturing Partner for Stainless Steel Passivation
When it comes to manufacturing services, outsourcing is always the better option for small and medium businesses. You get access to top-of-the-shelf technology and get to work with experienced teams who have solved similar issues for many industries on a global level.
For any manufacturing service ranging from part production to passivation of stainless steel. RapidDirect is easily among the top choices because of our commitment to quality and stringent standards that maintain consistent results.
It all starts with the right material and the right processes. We understand that and have an optimized system for stainless steel and all other metal parts. From stainless steel passivation to all the other surface finishing you’ll need to enhance your part’s durability and performance. Our team at RapidDirect has the skills, experience, and knowledge to handle all your requirements and deliver results in the shortest time.
Frequently Asked Questions on Stainless Steel Passivation
No. Pickling and passivation are two different processes. In welded parts, the pickling process removes all the debris, flux, and other contaminants from the surface of stainless steel parts to get them ready for passivation. Pickling can’t protect steel from corrosion, it only cleans the surface for the passivation layer to form and start its job.
There is no such thing as 100% corrosion-proof. However, stainless steel parts have an exceptionally long lifespan because of the passivation process. The protective chromium layer, albeit thin, can offer lasting protection that you can even extend by regularly maintaining the metal part and protecting it from oxidizing agents.
No, passivation is an essential process for stainless steel parts. Without the process, your part will be susceptible to attack from corrosion in a very short time span.