Most industries depend upon the efficient and reliable operation of large-scale material handling systems that manufacture, transport and store different chemicals. These systems require huge amounts of forethought, capital, and regular maintenance to function satisfactorily.
As a result, engineering designers do everything in their power to increase longevity while reducing the need for maintenance as far as possible for such systems.
In this article, we take a look at the process of passivation. Though often overlooked, passivation plays a key role in many large (and small) scale systems to ensure their durability and performance, especially when it comes to stainless steel.
What Is Passivation?
Passivation is a post-fabrication process that makes a material passive or inert to chemical reactions that can change its composition and ultimately lead to failure. In the industry, the passivation process is typically carried out to make a metal surface more resistant to corrosion or oxidation by building a protective film over it.
This thin film, also known as a passivation layer or passivation film, covers the material’s surface but does not make any changes to the base metal. The passive film acts as a barrier to reduce the chemical reactivity of the material making it more resistant to corrosion and contamination. While surface passivation can be done for many ferrous materials, it is mostly associated with stainless steels.
Stainless steel has self-passivating properties which allow it to form a chromium oxide layer. This layer imparts corrosion-resistant properties. However, when the steel surface undergoes any fabrication processes, the metal loses the protective passive film and with it, the corrosion resistance property. The loss of this layer exposes the free iron from underneath and can initiate corrosion. When left unchecked, it can lead to ultimate failure.
Through the passivation process, we can reestablish the protective chromium oxide layer and reduce the concentration of free iron at the surface. The chrome-to-iron ratio must be greater than 1. A ratio of 1.5:1 provides optimum protection from corrosion attacks.
Stainless Steel PassivationPassivation vs Pickling
Pickling and passivation are often misunderstood to mean the same. However, the two are separate processes with different natures and working principles.
Fabrication processes such as welding often result in a heat-affected zone. This heat-affected zone introduces contaminants as well as destroys the high chromium content layer in the part. This reduces the chromium content at the surface.
Pickling is used to remove oxide scale from a material’s surface to diminish the heat-affected zone and the low chromium content layer. The part is immersed in an acid tank for a set duration. The process also removes any embedded iron particles and contaminated carbon steel. However, if the contamination is high, the part will require cleaning with an alkaline solution before the pickling bath.
Passivation, on the other hand, is a more complex process, and in many instances, consists of pickling as the first step. The passivation process uses a different set of acidic solutions to not only clean the surface but also prompt the development of a passive oxide layer.
Thus, pickling is used for cleaning metal parts and is often a pretreatment process whereas passivation not only cleans the surface but forms a chemically non-reactive layer. While pickled parts obtain a dull, matte grey look, passivated products do not undergo any change in appearance.
When Is Passivating Used?
Passivation is a rather quick and automatable surface treatment process. As a result, it has many use cases. Some of the situations where passivating proves to be a feasible and effective solution are as follows:
- Before putting stainless steel parts into use
- After mechanical machining operations
- After welding
- When new components are joined to existing components
- Passivation must be done after contamination
- As a form of preventive maintenance
Before putting stainless steel parts into use
The passivation of stainless steel and other metals is generally carried out as the last process before putting the parts into use. This allows the material to enter the service environment with an intact passive layer.
Many operations have the ability to disrupt the passive layer and expose the material to corrosion. This is the reason why passivation is carried out after all the manufacturing processes are completed and the part is ready to enter service.
After mechanical machining operations
Mechanical operations can remove the passive layer from the surface. Cutting, grinding and mechanical polishing are some examples.
While the passive layer is capable of self-restoration, the differences in layer thickness mean that it could result in corrosion initiation in the part. This is why experts recommend passivating a part that has undergone mechanical operation whether it be during manufacturing or service.
If possible, the tools used for machining should be suited for stainless steel to prevent contamination of the surface.
Embedded iron as a contaminant poses a greater risk than others. A stainless steel surface with embedded iron particles will pass the ferroxyl test but as soon as it is put into service or sterilised by steam, it will develop copious amounts of rust. The only two solutions, in that case, would be to conduct multiple passivation procedures or grind and refinish the affected area.
After welding
We already discussed in an earlier section how welding can destroy the chromium-rich layer responsible for protection as well as embed the part with different contaminants. This is why it is recommended to passivate welded stainless steel parts. A suitable passivation process will restore the passive layer and make it impervious to oxidation.
When new components are joined to existing components
Joining new tubing with old tubing could also be an optimum moment to passivate the entire system. When new tubing is connected to old tubing, it generally undergoes pre-service passivation. However, welding may be used to connect the two tubes which can initiate corrosion as discussed in the above section.
The pre-service passivation of the new tubing may also not be enough to protect the tubing system if the internal surfaces of the existing system already have rouging. Thus, in these situations, it is important to weigh your options carefully to choose the most suitable solution. For example, it is possible to either passivate the entire system or swab passivate the old system upon welding or even use ferruled connections.
Choosing to derouge and passivate the entire system is the safest choice as the passivation contractor is at the site with the chemicals, and these chemicals are going to be put into a portion of the system anyway so the entire system can also be passivated.
After contamination
Experts recommend passivation when the system is exposed to contaminants such as chlorides and iron. We have discussed iron contamination above. Chlorine contamination can be equally damaging as it has the ability to penetrate the chromium oxide layer and attack the base metal.
Exposure to chlorine is known to destroy entire systems as it initiates rouging and will cause metal failure if left unchecked. This is especially true for austenitic stainless steel systems.
Another disadvantage of chlorine is that it dissolves the chromium oxide layer forming ferric oxide and hypochlorous acid. The hypochlorous acid releases free iron and chlorides, both of which exacerbate the situation leading to out-of-control corrosion in the system.
As a form of preventive maintenance
Passivation may also be carried out at regular intervals as preventive maintenance. This type of maintenance prevents breakdowns by addressing issues while they’re still in the early stages. However, the challenges associated with the process such as the passivation cost, system downtime and lost labour productivity may prevent it from being scheduled effectively.
In such cases, production lines can also shift to a condition monitoring plan that checks for signs that a system requires passivation. The rouge formation quantity, location, and uniformity should be assessed to determine when to schedule passivation to minimise future losses. Systems that are highly prone to rouging are passivated once every year or once every two years.
Passivation process
The passivation process generally consists of four main steps. These steps are:
- Cleaning
- Passivation chemical application
- Rinsing
- Oxidation to form the passive layer
Cleaning
For passivation to have maximum effect, the surface must be as clean as possible. Surface contamination can lead to the formation of electric potential on the surface which will result in uneven passivation.
The first step removes all contaminants such as dirt, dust, rust, grease and surface oil right down to the surface grain boundaries. Once removed, we have a pure stainless steel layer that passivates much more effectively.
Passivation chemical application
Passivation is performed in nitric acid or citric acid baths. The process removes free iron from the surface.
Nitric acid is more affordable but due to environmental concerns, the industry is moving towards citric acid. Citric acid is also safer than nitric acid. We add sodium dichromate when using a nitric acid bath to encourage oxygen formation and build the passivation layer. This compound is a toxic hexavalent chromium compound that requires very careful handling.
However, nitric acid can handle flash attacks better than citric acid passivation. The passive layer also forms faster and is more effective.
An alternative is to use ultrasonic machines with citric acid baths. This combination prompts oxygen formation at the surface and, therefore, a quick formation of the protective oxide layer.
Stainless steel parts are passivated by holding them in the bath for about 20 to 30 minutes at a temperature ranging between room temperature and 65 °C (149 °F).
Rinsing
After the chemical bath immersion, the parts are rinsed in a suitable solvent to remove all traces of the acid solution. This step also removes any residual free iron compounds.
Oxidation to form the passive layer
The metal parts may be exposed to certain chemicals that prompt metal oxide film formation after rinsing. These include chemicals such as potassium ferricyanide, copper sulphate and salt spray. The protective outer layer formed in this manner is more robust and reliable.
Conclusion
Corrosion is one of the most common issues with metals in service, especially if they interact with water or water-based process fluids. Corrosion, in these systems, can not only damage the system but also contaminate the process fluid (rouging). Through passivation, we can ensure their protection and optimum functioning by improving their corrosion resistance.
But passivation can be a difficult process to master. There are many factors that influence this chemical treatment. This is why we must always entrust it to metal finishing specialists and competent fabricators for a consistent and reliable finish.