What’s Stainless Steel Passivation?

From construction sites to automobiles and cookware, stainless steel is everywhere. Its popularity comes from its strength, durability, and reliable performance across various applications. However, even stainless steel isn’t completely immune to corrosion. Over time, it can slowly deteriorate unless properly treated. Stainless steel passivation is a crucial process that every stainless steel component should undergo to maximize its lifespan and maintain its integrity.

What’s Stainless Steel Passivation?

In simple terms, the passivation of stainless steel involves removing any free iron molecules from the surface. Since stainless steel is an alloy of iron, chromium, and other elements, iron, being the majority element, is prone to rust. The chromium content in stainless steel forms a chromium oxide layer on the surface, which acts as a passive oxide layer, shielding the steel from corrosion. However, this protective layer can only form properly if the surface is free from contaminants.

Passivating stainless steel ensures that all traces of iron and other potential contaminants are removed from the surface, allowing the chromium oxide layer to form consistently. This process helps the material resist corrosion and enhances its durability, making it suitable for long-term use in challenging environments.

The History of Stainless Steel Passivation

Iron has been used for thousands of years, but its natural tendency to corrode made it less ideal. Stainless steel, while more resistant, still requires passivation to prevent corrosion. The process began in the 1800s when chemist Christian Friedrich Schönbein found that dipping stainless steel in concentrated nitric acid significantly improved its corrosion resistance. Nitric acid became the standard, despite its environmental drawbacks.

In the 1900s, a German brewing company discovered citric acid as a safer, non-toxic alternative for passivation. By 1990, citric acid had largely replaced nitric acid in many applications. Today, both acids are used in modern passivation processes.

Why Is Stainless Steel Passivation Important?

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, they propagate 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, a passivated finish 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.

Improved Machinability

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.

How to Passivate Stainless Steel: A Step-by-Step Guide

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.   

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.

1. 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.

2. Passivation in citric acid or nitric acid bath

The next step is to remove all the traces of iron from the surface with a passivating acid bath. For that, you choose either of the two options:

1. Citric Acid
2. Nitric Acid with dichromate (or without)

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 the environment and the operators working on it. However, citric acid is not popular because it can cause mold growth. Although technological advances have minimized this issue, the problem persists.

Nitric acid passivation is another alternative, currently 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. Moreover, nitric acid is hazardous and produces toxic fumes.

To address the slow processing time, the best option is to combine it with sodium dichromate. However, keep in mind that doing so would further enhance the toxicity and create further problems.

Regardless of the approach, immersion in the acidic bath would initiate the passivation, re-forming the chromium oxide layer. Slowly, it surrounds the surface and keeps your steel part safe from contamination and corrosion for a long time.

3. 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. Numerous standards are prevalent across the industry and the right one depends on factors like your application, location, and the target industry.  

Some common testing (as per A967) for passivate parts include:

• High Humidity Test: Put stainless steel parts in a humidity cabinet for 24 hours to assess their resistance to high moisture levels.
• Salt Spray Test: Expose the parts to a salt spray chamber for about 2 hours to simulate corrosive conditions and evaluate their durability.
• Water Immersion Test: Submerge the parts in distilled water for at least 24 hours to check for signs of corrosion.
• Free Iron Test: An alternate method, when other tests like Water Immersion or High Humidity aren’t feasible. Use it for larger parts that don’t fit in a tank or cabinet.
• Copper Sulfate Test: Apply a copper sulfate passivating solution to the part’s surface for 6 minutes. If free iron is present, a copper-colored deposit will form. Avoid using it for stainless steel with less than 16% chromium or food processing parts.

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.

Best Practices

• 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 the expense of the overall effectiveness of corrosion resistance as martensitic stainless steel has a chromium content in the range of 12% to 17%.

The following table shows the popular alloys of these 3 grades and their respective applications:

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 Standards

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.  

AMS Standards

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:

British Standards

British Standards (BS) are developed by the British Standards Institution (BSI) for use in the UK and Commonwealth countries.

BS EN 2516 is one of the standards that specifically addresses the chemical passivation of corrosion-resistant steels, including all three grades and precipitation-hard enable grades. It also covers decontamination methods for nickel and cobalt-based alloys.

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 space. 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