Stainless Steel Passivation: All You Should Know
Stainless steel is used in almost all types of industries due to the excellent properties it possesses. However, stainless steel deteriorates after some time and needs appropriate treatment.
If you are looking forward to learning what is stainless steel passivation and the methods to achieve it, read this guide.
What is Stainless Steel Passivation?
Stainless steel passivation is a process in which the free iron molecules from a stainless steel surface are removed.
Stainless steel is an alloy made using chromium, iron, and other elements, and iron has a tendency to rust. The chromium layers on the stainless steel help protect it from corrosion.
Passivation of stainless steel makes sure that all the contaminants are removed from the surface of stainless steel so that chromium can form an oxide layer.
In case contaminants are present, this layer won't prove to be protective enough. The process ensures to resist corrosion and improves the durability of stainless steel.
Why is Passivation Essential for Stainless Steel?
Iron is known for various properties, but it doesn't offer corrosion resistance. Hence, stainless steel is created by mixing the elements in which chromium ensures corrosion resistance.
However, passivation still proves to be extremely important for a lot of reasons, and these are as follows:
Protection Against Corrosion
Iron is the major element in stainless steel and offers advantages but it does corrode when exposed to oxygen and water. In this case, passivation allows it to clean the surface and adds a layer of protection on stainless steel to ensure its performance.
Removal of Contaminants
Contaminants are always left when it comes to the manufacturing process, no matter how careful you are. The machining tool even has the tendency to leave behind contamination, and that leads to oxidation, which causes corrosion to take place.
The process of passivation takes place with a very thorough cleaning process, and hence, the contaminants are removed from the surface.
Enhanced Machinability
The application of stainless steel passivation doesn't change the composition of the stainless steel, and it doesn't even impact the machinability of the stainless steel. In fact, it offers protection against corrosion and makes it more machinable.
3 Steps for Stainless Steel Passivation Process
The process of stainless steel passivation has three crucial steps, which are as follows:
Cleaning the Surface
The first step requires cleaning stainless steel parts from all types of contaminants. An alkaline degreaser is used for the process of cleaning it, as it can remove contaminants like debris, dust, chemicals or oil.
Passivation Process
The second stage is the main stage where citric acid or nitric acid is used for bathing the surface of the stainless steel. It helps to remove iron traces from the surface.
Citric acid is affordable and also environmentally friendly but has a tendency for mould growth. Nitric acid, however is more effective but also is expensive and produces fumes that are toxic.
Testing of Passivated Components
As soon as the passivation of the components is carried out, some tests are done to ensure that the process has been completed perfectly. These tests are as follows:
- A high humidity test is carried out where the passivated component is left in humidity for a day, and its resistance against moisture is tested.
- A salt spray test is carried out by keeping the passivated parts in a salt spray chamber for a span of two hours so that the durability and corrosion can be evaluated.
- The passivated components are also subjected to distilled water for a day so that the signs of corrosion can be checked.
- Copper sulfate is also applied to the passivated components to test if it creates a deposit due to the presence of free iron or not.
Methods and Techniques in Stainless Steel Passivation
The process of stainless steel passivation has the following methods and techniques:
Citric Acid Passivation
Citric acid passivation offers environmental friendliness and creates a very thin layer over the component of stainless steel. It removes contaminants and also free iron from the surface. It is a highly affordable and effective method of passivation due to less environmental damage.
However, one downside of this process is that moulds may grow on it, which is why it is not very popular for the passivation process.
Nitric Acid Passivation
Nitric acid passivation is a process in which stainless steel is submerged in a nitric acid solution. It helps remove free iron and enhances corrosion resistance. It is preferred by manufacturers due to the effectiveness it offers.
However, it is a hazardous method due to the nature of nitric acid. Moreover, it is not even suitable for the environment.
Many manufacturers also add sodium dichromate, which helps to add more effectiveness to the process and ensures high corrosion resistance. Despite this, the hazardous nature of the process makes it less common.
Comparing Both Techniques
The process of citric acid passivation is safe and mild compared to nitric acid passivation and also ensures better stability. However, it is less compatible with all types of stainless steel alloys and also is a slow process compared to the nitric acid method.
On the other hand, nitric acid passivation is fast, offers compatibility with different stainless steel alloys, and also is more effective. However, due to the safety concerns and its hazardous nature, it is a risky process.
Advantages of Stainless Steel Passivation
The process of stainless steel passivation is a beneficial one, and some of its benefits are as follows:
Improved Corrosion Resistance
Stainless steel gains excellent corrosion resistance due to the passivation process. In cases where the chromium layer is damaged, the stainless steel may be oxidized, but the process of passivation adds a new layer, which makes it highly corrosion-resistant.
Improved Lifespan
The passivation process can increase the lifespan of stainless steel components. Chemical treatment of the components increases their strength and also ensures enhanced wear resistance.
Hence, applications that need a long-lasting stainless steel component may find it suitable.
Enhanced Aesthetic Finish
Passivation enhances the surface finish of the component and makes it highly corrosion-resistant. If the surface has fingerprint spots or other marks, the passivation leads to a cleaner look, making it appealing.
Factors Affecting Passivation Efficacy
There are three most important factors that impact the efficacy of passivation and must be considered; these are as follows:
Impact of Environment
The role of temperature impacts the passivation’s effect. If the temperature is high in the surroundings, the passivation is more effective.
Also, the cleaning agents used affect passivation. If the chloride level is high in the cleaning solution, it may cause some corrosion in the stainless steel.
Composition of Material
Material composition is another consideration, as it impacts the efficacy of the passivation process. Some alloying elements may be different, like nickel or molybdenum, which impacts the firmness of the layer of oxide, and the efficacy of the passivation can be impacted.
Secondary Treatments after Passivation
In order to achieve a successful layer of passivation on the stainless steel, it is important to manage the time the component is left in the acid bath. It should be extracted in due time and dried up appropriately; otherwise, water spots or contaminants may appear on the surface.
Challenges and Concerns in Stainless Steel Passivation
Stainless steel passivation comes with some challenges, which are as follows:
- It is very important to clean the stainless steel appropriately to prevent embedded iron. In case the iron residue is left on the surface, the entire process may go in vain and lead to a corrosion appearance.
- The entire component should be treated with uniformity. In case of inconsistency in the process, corrosion may appear on the component.
Common Industry Standards for Passivation of Stainless Steel
ASTM
ASTM is an international body that has laid down standards for the passivation of stainless steel, and the ones that are relevant to the process are as follows:
ASTM A967: It provides a clear definition of the passivation treatments for the stainless steel components, which includes testing methods, cleaning, descaling, etc.
ASTM A380: This standard offers suitable recommendations for passivating stainless steel parts using different methods like water-jetting and acid pickling.
ASTM F86: It specifies the significance of surface preparation to avoid corrosion.
AMS
AMS stands for Aerospace Material Specifications, which also deals with the materials and standards relevant to aerospace. The standards relevant to stainless steel are as follows:
AMS 2700: It defines the temperature, immersion time, and concentration for the passivation process.
AMS-QQ-P-35: This standard also covers the cleaning, passivation, and descaling of components that are corrosion resistant, like in aerospace and medical.
AMS-STD-753: This standard provides for testing methods to assess the passivation of the stainless steel components.
Best Practices for Passivated Stainless Steel Parts
The process of passivation requires some best practices so that it can be carried out in an effective way, and some of these are as follows:
- Ensure that before starting the process of passivation, the component is clean.
- Make sure to use dechlorinated water whenever you need to rinse the components.
- Try to keep operations separated to minimize contamination.
- The equipment you are using for stainless steel passivation should be only for this process and must not be used for other processes.
- Use a proper racking storage system so that metal-to-metal contact can be avoided.
Conclusion
Stainless steel passivation enhances the durability and corrosion resistance in stainless steel components, and since these components are so common in applications, the process is highly significant.
If you are also looking for custom passivated stainless steel components, you can get in touch with us at DEK.
FAQs
How can I passivate stainless steel safely and economically?
Citric acid passivation is the most safe and economical passivation method. Make sure to clean the stainless steel part when you start the process.
How much material does passivation remove?
It removes a negligible amount of material from the surface.
How long does passivation last on stainless steel?
The passivation lasting depends on the application use and the environment it is subjected to, but the components can be subjected to re-passivation when needed.
Is passion the same as plating?
No, plating deposits a layer of one metal on another, whereas passivation only creates a layer of oxide.
