passivation parts

What is Passivation and How Does It Work

Passivation is a vital process in many industries to protect metals against corrosion. This extensive corrosion protection method involves forming a protective oxide layer on the surface of the metal to shield it from damaging chemical reactions. In essence, passivation renders the metal passive or inert. Hence, the resulting passive layer can be a barrier between the reactive metal and its surrounding environment. It protects the underlying metal from oxidative degradation and blocks further corrosion. 

This article will cover everything you need to know about passivation. We’ll explore what it is, how it works, common passivation techniques, pros and cons, its applications across various industries, etc. Let’s dive in!

What is Passivation?

In physical chemistry and engineering, passivation refers to coating a material to render it “passive”, meaning that it is less susceptible to being affected or corroded by its surrounding environment. Passivation entails the formation of an outer shield material layer, which may be applied as a micro coating, generated through a chemical reaction involving the base material, or permitted to develop by spontaneous oxidation in the air. 

From the perspective of technique, passivation is a process that involves applying a thin layer of a protective substance, like metal oxide, to provide a barrier against corrosion. The thin layer, also called a passivation layer or passivation film, provides surface coverage to the material without altering the base metal. The passive layer lowers the material’s chemical reactivity, enhancing its resistance to corrosion and contamination. 

Passivation is predominantly applied to stainless steel. Nitric acid or citric acid is employed in the passivation process of stainless steel to remove free iron on its surface. This results in an inert, shielding oxide layer on the stainless steel, enhancing its resistance to rust due to the absence of iron reacting with the atmosphere. Note that the chrome-to-iron ratio in stainless steel should be higher than one during the removal of free iron. An optimal level of protection against corrosion attacks may be achieved with a ratio of 1.5:1.

passivation operation

What Materials can be Passivated?

Several materials can undergo passivation, which is a process that increases corrosion resistance:

Stainless Steel

Stainless steels possess corrosion resistance but are not entirely immune to rust formation. In contrast to nitric acid, a passivating acid frequently employed to passivate stainless steel, citric acid is increasingly favored due to its reduced toxicity, biodegradability, and handling danger, making disposal easier. 

aluminum passivation

Aluminum

Aside from plating, painting, and other barrier coatings, passivation of aluminum alloys is typically accomplished via chromate conversion coating or anodizing. In addition to aluminum, chromate conversion is frequently used to passivate tin, zinc, cadmium, copper, silver, and magnesium alloys.

Ferrous Materials

Ferrous materials, such as steel, can be safeguarded to some extent by inducing oxidation (commonly known as “rust”), transforming the oxidation into a metalophosphate using phosphoric acid, and adding further protection by surface coating. 

Titanium 

By anodizing titanium, a thicker passivation layer can be generated. Like numerous other metals, this layer induces thin-film interference that imparts a colored appearance to the metal surface. The thickness of the passivation layer directly influences the color generated.

Nickel

The utilization of nickel to process elemental fluorine is possible due to the development of a passivation layer composed of nickel fluoride. This fact is suitable for applications involving water and sewage treatment.

Silicon

Surface passivation in microelectronics and photovoltaic solar cells is typically accomplished via 1000 °C thermal oxidation to produce a silicon dioxide coating. 

Perovskite

Passivation is the simplest and most extensively researched technique for enhancing perovskite solar cells. In addition to increasing photoelectric conversion efficiency in perovskite cells, passivation enhances device stability. 

Carbon

In carbon quantum dot (CQD) technology, CQDs are tiny carbon nanoparticles (less than 10 nm in size) with some surface passivation.

How Does Passivation Work?

How Does Passivation Work? Generally speaking, the passivation process is divided into four main steps. Here, we take stainless steel passivation as an example. 

Step 1: Component Cleaning

The passivation process starts by thoroughly cleaning the stainless steel component, eliminating any surface oils, chemicals, or debris from the machining process. Skipping this particular step will result in foreign substances on the metal’s surface, hindering passivation’s effectiveness.

Step 2: Acid Bath Immersion

Following cleaning, the component is submerged in an acid bath to eliminate any free iron particles from its surface. In this stage of the process, three methods are commonly employed.

Nitric Acid Bath

The traditional passivation method is nitric acid, which produces the most efficient molecular redistribution of the molecular structure of the metal’s surface. However, nitric acid has several downsides because it is a hazardous chemical. In addition to necessitating specialized handling, this substance comes with hazardous emissions, poses environmental risks, and may demand an extended duration for processing.

Nitric Acid with Sodium Dichromate Bath

Adding sodium dichromate to nitric acid enhances or accelerates the passivation process in certain alloys. It is a less popular alternative because sodium dichromate increases the hazards of nitric acid bathing.

Citric Acid Bath

Citric acid is a safer alternative to nitric acid passivation. Citric acid demands no particular handling, emits no hazardous gasses, and is an environmentally friendly solution. Citric acid passivation failed to acquire popularity since its compounds posed a risk of organic development and molds. Still, new advancements have overcome these issues, making it a cost-effective and environmentally benign option.

Stainless steel components undergo passivation by immersing them in a bath for approximately 20 to 30 minutes at a temperature ranging between room temperature and 65 °C (149 °F).

Step 3: Component Rinsing

Following the immersion in the chemical bath, the components are rinsed with an appropriate solvent to eliminate any residual acid solution. Furthermore, any residual free iron compounds are eliminated during this process.

Step 4: Oxidation to form the passive layer

After rinsing, the metal components may be exposed to specific chemicals that induce the formation of a metal oxide film. Chemicals such as potassium ferricyanide, copper sulfate, and salt spray are among these. This manner produces a more durable and reliable protective outer layer.

Common Passivation Techniques

Some often used passivation techniques include:

Immersion Method:  The immersion method involves submerging metal components in a passivation solution, often consisting of nitric or citric acid, for a certain duration and at an appropriate temperature to eliminate contaminants.

Galvanizing: Galvanizing is a process where steel or iron pieces get immersed in molten zinc to prevent corrosion. Additionally, the zinc can serve as a sacrificial anode if needed.

Bluing: Bluing is a type of conversion coating on steel. It creates a thin, blue surface layer, which helps to minimize glare. However, it needs to be regularly maintained using oil.

When to Use the Passivation Process?

Passivation is a quick and easy surface treatment method. It consequently has numerous use cases. The following are some examples of situations where passivation is a viable and effective solution:

After Mechanical Machining Operations

The passive layer on the surface can be removed after machining operations. Examples include cutting, grinding, and mechanical polishing. Although the passive layer can self-restore, the changes in layer thickness may result in corrosion starting in the part. 

Before Putting Stainless Steel Parts into Use

Passivation of stainless steel and other metals is often performed as the final procedure before putting the products into service. This enables the material to move into the service environment with its passive layer intact. 

After the Welding

Welding frequently yields a heat-affected zone. This heat-affected zone adds contaminants and damages the part’s high chromium content layer. This decreases the surface chromium content. An appropriate passivation process can restore the passive layer and render it oxidation-resistant.

After Contamination

Passivation is advised when the system is exposed to contaminants, including chlorides and iron. Chlorine contamination can cause significant damage since it may penetrate the chromium oxide layer and corrode the base metal.

When New Components are Joined with Old Components

When new tubing is joined to existing tubing, it is usually pre-service passivated. Welding may be used to join the two tubes, which might cause corrosion. In this case, it is advisable to passivate the whole system, passivate the old one while welding, or even employ ferruled connections.

As a Preventive Maintenance

Passivation can also be done regularly as preventative maintenance. This maintenance form helps avoid breakdowns by treating problems while they are still in their early stages. However, the process’s obstacles, such as the passivation cost, system downtime, and lost labor productivity, may prohibit it from being properly planned.

passivation of stainless steel

The Advantages and Disadvantages of Passivation

Passivation is a practical, preventive practice that can increase the lifespan of parts and systems. Here are some potential advantages and disadvantages of passivation:

Advantages:

Prevents Corrosion. Passivation forms a protective passive layer on a metal’s surface that acts as a barrier to prevent corrosion reactions. This helps improve the metal’s durability and longevity.

Improves Wear Resistance. The passive layer improves the surface hardness of the metal, making it more resistant to wearing down from friction and abrasion. This can extend the service life of metal parts.

Saves Money and Reduces Downtime. Passivized parts reduce the risk of contamination-related production losses and unscheduled system shutdowns.

Lowers the Risk of Product Contamination. The process eliminates chemically reactive iron and other contaminants.

Allows for Extended Maintenance. Regular passivation lowers the need for system shutdowns and maintenance, resulting in longer operational durations.

Requires no Power Source. Passivation is a spontaneous chemical reaction and does not require an external power source to maintain the passive layer, like some active anti-corrosion measures (e.g., cathodic protection). This makes it simple and cost-effective.

Disadvantages: 

Be Limited to Certain Metals/Alloys. Only metals that spontaneously form passive oxide films, like stainless steel, aluminum, and titanium, can be passivated. Other metals may require different anti-corrosion treatments.

Be Removed by Abrasion. The passive layer offers protection but can be worn off from friction or abrasive action over time. The metal would need to be re-passivated for continued protection.

Less Effective in Some Environments. While offering good corrosion resistance, passivation may be less effective in highly acidic or alkaline environments that can dissolve the passive film more readily.

Requires Surface Preparation. Metals need proper cleaning and surface preparation before passivation to enable the formation of a consistent, protective, passive layer. This may marginally extend the time to complete the fabrication process.

Temporary Protection. Passivation slows down corrosion but does not completely stop it in most cases. Periodic reapplication may be needed for continued protection.

No Smooth the Metal: Passivation does not smooth the metal. Thus, if this is necessary for the final product, it must be addressed before treatment.

Environmental Impact: Passivation processes frequently involve using chemicals that may damage the environment if not properly managed.

Specialized Equipment: The process necessitates using specialized equipment, which might be expensive and difficult available. The hazardous chemicals attached to the equipment also provide possible safety risks. 

part of passivation

Factors Affecting the Effectiveness of Passivation

Here are some key factors that can affect the effectiveness of passivation:

Surface Cleanliness – The metal surface must be free of oil, grease, dirt, oxides, etc., for the passive film to properly form. Any contaminants will prevent the film from forming correctly. Proper cleaning and surface preparation are essential.

Passivating Agent – The choice of passivating agent, such as nitric acid or citric acid, and its concentration might impact the passivation process. Thus, try your best to choose a suitable passivating agent.

Alloy Composition – The alloying elements in the metal influence the formation and protectiveness of the passive film. Some alloying additions, like chromium, promote passivation, while others may hinder it. The composition must be optimized for passivation.

Environmental Conditions – Factors like temperature, pH, and the presence of corrosive ions impact passivation. Each alloy has optimal conditions for passive film formation. Temperature extremes or pH outside ideal ranges can prevent passivation.

Surface Defects – Scratches, pores, cracks, or imperfections in the surface act as sites for corrosion. Passivation is less effective on surfaces with defects. Care must be taken to avoid damaging the surface.

Film Rupture – Damage to the passive film from abrasion, wear, erosion, etc., will break down the protective layer and expose the bare metal. The film must remain intact for passivation to work.

Film Thickness – A thicker passive film provides greater protection. Factors like passivation time, temperature, and method influence film thickness. Optimization is needed to grow an adequate barrier.

Maintenance and Repassivation – The passive film may deteriorate over time. Proper maintenance procedures and re-passivation treatments may be needed to restore the protective layer.

Practical Applications of Passivation

A controlled passive layer enhances corrosion resistance, electrical characteristics, optical properties, biocompatibility, and aesthetics. The choice of coating depends on the substrate material and application. Here are some practical applications of passivation:

Common Metals – A passive coating like chromium or nickel to a metal surface can protect it from corrosion. This is commonly done on stainless steel, aluminum, zinc, and other materials to make them more durable, especially in harsh environments.

passived medical parts

Surgical Instruments – Surgical instruments require corrosion resistance and a chemical composition that is both extremely inert and safe for use during surgery, both of which are properties that are beyond the capabilities of many conventional coatings.

Ball Bearings – A high accuracy is necessary for ball bearings. Even a trace amount of rust or numerous conventional coatings would alter the tolerances to the point where functionality would be compromised. 

Fasteners – Under normal conditions, parts fastened together by passivating treatment remain impervious to corrosion. Conversely, when exposed to moisture, a non-passivated stainless fastener may quickly corrode an aluminum or mild steel part.

Semiconductor Devices – A passivation layer of silicon dioxide or silicon nitride helps protect semiconductor devices like integrated circuits from moisture, contamination, and mechanical damage. This helps improve their reliability and lifetime.

Solar Cells – Anti-reflective coatings used on solar cells act as passive films to increase light absorption and reduce reflection. This improves the solar cell efficiency.

Medical Implants – Passive films of oxides help prevent corrosion and ion leaching of metallic medical implants like stents, orthopedic prosthetics, etc. This increases biocompatibility. 

Food Packaging – Tin cans have a passive tin oxide layer to prevent the iron substrate from corroding when in contact with acidic foods. This helps increase shelf life.

Sensors – Sensitive electronic sensors may have a passivation coating to prevent corrosion or catalytic reactions at the sensor surface that could interfere with measurements.

Jewelry – Clear coatings on jewelry create a passive barrier against tarnishing and discoloration of silver, copper, and other materials.

passivation applications

Passivation vs. Anodizing: What is the Difference?

Passivation is a chemical treatment applied to metals like stainless steel or titanium to increase corrosion resistance. It forms a thin, transparent oxide layer on the surface that prevents further corrosion. Passivation is typically done by exposing the metal to an acid solution. The acid oxidizes the surface and makes it passive or less reactive. 

Anodizing is an electrolytic process used on metals like aluminum. The metal part is immersed in an acid solution, and an electric current is passed through it. This grows a thick oxide layer on the surface. Anodizing can create colored finishes and provides excellent corrosion and wear resistance. The anodic oxide layer is much thicker than the passivation layer.

Summary

Passivation refers to a post-fabrication process that renders a material passive or inert to chemical reactions that may alter its composition and eventually result in failure. In the industry, the passivation process is often used to increase the resistance of a metal surface to corrosion or oxidation through the formation of a protective film.

The applications of passivation processes can be found in the aerospace and medical industries to protect essential components and extend the life of surgical tools. Passivation provides other benefits beyond corrosion prevention, such as increased durability, biocompatibility, and general performance.

Reference

Passivation (chemistry) – From Wikipedia

FAQs

Passivation forms a very thin protective layer on the metal surface, whereas pickling removes a very thin layer from the metal surface. Both techniques include the application of acids, and pickling can be performed on previously corroded parts before passivating them. 

Although passivation is not necessarily required for all stainless steel applications, it is typically advised for 316 stainless steel, particularly when the steel may be exposed to corrosion.

Yes, the self-healing capability of the passive layer enables minor damage to be repaired. Exposed areas will re-oxidize quickly when in contact with oxygen. However, severe damage may require re-passivation.

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