Stainless steel is a universal material that forms the basis of countless industrial systems. Passivation of stainless steel is a fundamental process to maintain its quality and longevity.
In this article we will cover important questions about how to passivate stainless steel and what it is.
What is stainless steel passivation?
Passivation of stainless steel is a chemical process that makes it impermeable to moisture and oxygen, the main substances that cause rust. The passivation process achieves this by creating a very thin layer (a few atoms thick) of chromium oxide (2Cr2O3) on the exposed surface of the stainless steel.
This layer prevents atmospheric compounds from reacting with the iron content of the stainless steel, thus affecting its quality. The results are very similar to anodizing aluminum.
It serves a very important purpose in numerous industries such as food processing, cosmetics, pharmaceuticals, etc., where corrosion of equipment can cause safety hazards and large losses. Properly passivated stainless steel also improves productivity and reduces maintenance.
Why is passivated stainless steel necessary?
The thin layer of chromium (and nickel) oxide on stainless steel makes it resistant to corrosion. However, sometimes this layer does not form properly on its own or can be damaged, leaving the stainless steel vulnerable to corrosion. This is where the passivation process comes into play. It serves to form or restore the protective layer with the desired quality.
There are several reasons why stainless steel may need to be passivated. Some of them are the following:
– Low quality raw materials : Impurities from low quality raw materials may not be completely removed during steel production. This can lead to a reduction in chemical passivity.
– Machining/grinding : Most stainless steel parts undergo manufacturing processes such as machining and grinding. The tools used in these processes contain iron, which is embedded in the surface of the stainless steel during machining. Machining lubricants and coolants can also accumulate in surface holes, creating another form of contamination.
– Welding : Welding weakens the chromium oxide layer, especially in the heat-affected area.
– Assembly : Stainless steel products are mainly an assembly of several parts. During assembly, the individual components rub against each other and the resulting friction can damage the oxide layer, often to the point that it disappears completely in some places.
– Normal wear : The chromium oxide layer weakens over time as the part is exposed to various thermal, chemical and physical environments during operation.
All of these scenarios are very common in an industrial environment. Therefore, professional engineers apply a regular steel passivation schedule to extend the life and quality of industrial equipment and save money, time and effort in the long run.
3 steps for the stainless steel passivation process
We will now discuss the passivation process in detail. Passivation of stainless steel is a three-step process: cleaning, acid bath and quality testing.
Step 1: Cleaning
Cleaning the stainless steel surface is a crucial step for successful passivation. Accumulated surface contaminants can prevent passivating acids from reaching the surface, rendering all efforts in vain.
Common surface contaminants include dirt, grease, mineral and synthetic oils, hydrocarbons and other residues from industrial environments.
There are different methods for degreasing a stainless steel surface. Alkaline cleaning agents and high temperature baths (up to 65°C) dissolve and remove contamination very effectively.
Engineers often check the quality of cleaned surfaces using techniques such as the camphor test.
Step 2: Acid Bath
The second stage of stainless steel passivation is the acid bath. The cleaned stainless steel surface is immersed in a bath containing an acidic solution and some other additives such as accelerators and inhibitors.
This chemical reaction removes free iron from the metal surface. The main objective is to achieve a high chromium to iron ratio so that more chromium than iron is available for oxidation.
This stainless steel passivation step is controlled by three parameters: concentration, temperature and duration of the acid bath. Different combinations of these parameters produce different results. The experience and skill of engineers are crucial in adjusting these parameters for optimal results.
The most common acids for steel passivation are nitric acid and citric acid. Below we will briefly discuss both and compare them.
Nitric acid vs. citric acid: a technical comparison
Nitric acid was the original passivating agent for stainless steel. It acts on the metal, dissolving the iron, which is then washed by the circulating bath. Its main features are:
- The concentration of the nitric acid solution is between 10 and 50%.
- Acid bath temperatures can be up to 60°C.
- Additives such as sodium dichromate and phosphoric acid can further improve the passivation efficiency.
- Nitric acid makes passivated stainless steel more resistant to lightning strikes.
- Applies to austenitic and duplex steel grades only.
- Highly toxic and dangerous to the environment, requires careful handling and disposal.
Now let's move on to citric acid. It is a relatively new means of passivating stainless steel. It reacts chemically with the iron on the surface of the steel, but also removes chromium and nickel. However, the prepared surface is well prepared to naturally react with the surrounding air and form a thick oxide layer.
- An acid concentration of approximately 10% is used.
- Passives all types of stainless steel.
- Citric acid is safe to use and relatively more environmentally friendly.
- Approved for food processing.
- Citric acid passivation is faster and can be adjusted to produce results in less than 5 minutes.
Step 3: Quality Check
The final step in stainless steel passivation is quality testing. The passivated metal goes through some tests that it must pass.
At this stage, engineers use various testing methods. One strategy is to create an environment where rust occurs easily. This is called water immersion testing, where stainless steel is exposed to continuous cycles of wet and dry environments and is constantly monitored for signs of corrosion. Typically, poor passivation results in visible surface rust.
Chemical tests like the Ferroxyl test are also very useful. Releases iron by reacting with it and changing its appearance. Devices like passivity meters are also very popular for checking the reactivity of stainless steel.
It is now also common practice to perform quality controls during the acid bath stage. Monitoring the iron concentration and acidity of the circulating acid bath indicates the progress and effectiveness of the passivation process.
Common Industry Standards for Stainless Steel Passivation
Due to its importance, stainless steel passivation is highly standardized. ASTM and ASM industry standards are the most common and cover the passivation of useful grades of stainless steel such as 304 and 316 SS.
ASTM
The American Society for Testing and Materials (ASTM) has two comprehensive standards for stainless steel passivation.
ASTM A967/A967M covers various types of chemical passivation with nitric and citric acids, as well as electrochemical treatment. Contains detailed information on the passivation process, quality testing methods and criteria, as well as recommendations for the first cleaning and descaling steps.
ASTM A380/A380M provides detailed process guidelines and precautions for the passivation of stainless steel parts, assemblies, devices, and installed systems. This includes several process steps such as cleaning, descaling and passivation.
AMS
SAE International maintains the AMS 2700 specifications for passivation of stainless steel surfaces. It is more focused on professional practices in sectors such as aerospace and automotive.
Provides guidelines and procedures for dissolving iron and other less noble metallic elements from the surface of stainless steel to make it more resistant to corrosion.
Features of stainless steel passivation equipment
There is a wide range of machines for stainless steel passivation. We will highlight some of the main characteristics of the passivation devices available on the market.
Size : Passivation devices come in various sizes, from benchtop equipment to large industrial equipment capable of passivating large parts.
Passivation capacity : This is defined by factors such as the number of tanks, their volume and the total yield.
Functionality : Some passivation equipment comes as a complete package for degreasing, passivation, rinsing and drying.
Manual/Automatic : Users can choose between manual and automatic units. Automatic units are more precise, safer and have intuitive human-machine interfaces. PLC controlled systems give the machine an additional level of precision and reliability.
Best Practices for Passivated Stainless Steel Parts
As with any industrial process, certain professional practices improve the quality of steel passivation processes. Below are some key best practices for engineers working with passivated stainless steel.
1. Always passivate new and repaired/replaced parts
Whenever a new component is added or an old one is replaced, passivation is mandatory. A chemically reactive part can rust very quickly and damage other parts as well as the industrial process itself.
2. Monitor equipment regularly
Most industries use stainless steel parts whose passivity decreases over time through use or through compounds such as chlorides. The corrosion potential of such components should be checked regularly to determine if repassivation is necessary.
3. Separate machines and tools for SS
From an economic standpoint, it makes sense to process stainless steel in a separate environment where it is less exposed to iron and contaminants. In the long run, this can save huge passivation costs and downtime.
4. Quality control of industrial fluids
In an industry that works with corrosive liquids, their composition must be constantly checked. Fluids must be cleaned or replaced if they become too dangerous for stainless steel equipment.
5. Adapt the passivation process to the type of steel
Engineers need to understand that each type of stainless steel is different and not all are suitable for the same passivation cycle. Therefore, some intuition and experience is required to adapt the process to each variety and each piece and achieve optimal results.
University Degree
Passivation of stainless steel is an important process that helps stainless steel maintain its new appearance and prevents corrosion. Therefore, it is undoubtedly an effective method for increasing productivity and service quality.
Although the process is not complicated, the usual passivation can even be done at home with a simple kit. If you want production-grade passivated stainless steel parts, you need to hire a professional company.
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Common questions
Does 304 or 316 stainless steel need to be passivated?
For long service life, passivation is recommended for grades 304 and 316. Although 316 stainless steel has better corrosion resistance due to its higher molybdenum content, it does not provide adequate protection for most industrial environments.
How often should you passivate stainless steel?
The frequency of stainless steel passivation depends on its industrial application. Parts exposed to excessive dirt, friction, wear, or other extreme conditions should be passivated more frequently than parts in a mild industrial environment. For example, parts with a high chloride content should be passivated 2 to 3 times a year. In general, stainless steel parts should be passivated once a year for normal applications.
Can you passivate stainless steel twice?
Yes. The protective oxide layer on stainless steel weakens with time and use. In this case, the stainless steel must be passivated again to restore its corrosion resistance. Depending on the condition of the stainless steel parts, they can be passivated more than twice.
