What is Stainless Steel Passivation?
The ability of a metal to resist corrosion is essential for its use in a wide range of applications, particularly those in which contamination can have a destructive effect either during the manufacturing process or when the product is being used by consumers. Surface contamination from grease and oil, debris, and chemicals are typically present on parts and components after machining stainless steel, despite the fact that raw stainless steel has a high level of corrosion resistance to begin with. In order to improve the resistance of metal components to corrosion, a process called passivation removes these contaminants from the surface of the component.
History of Passivation Process
Christian Friedrich Schonbein, a German chemist, made the discovery of the effect of passivation in the middle of the 1800s. When he compared the iron that had been treated with concentrated nitric acid to iron that had not been treated with concentrated nitric acid, he found that the iron that had been treated with concentrated nitric acid had very little or no chemical reactivity. His term for the condition in which there is no chemical reaction is the “passive” condition. Environmental and safety concerns regarding nitric acid became more apparent as the practise of passivating stainless steel with nitric acid became more widespread in the 1900s. Following extensive research in Germany, the Adolf Coors brewing company came to the conclusion that citric acid is an effective substitute. In the 1990s, many manufacturers started switching from nitric acid to citric acid because citric acid was seen as a safer and more environmentally friendly alternative.
What is Stainless Steel Passivation?
After the components made of stainless steel have been manufactured, they go through a process called passivation, which is a non-electrolytic finishing process. This process improves the components’ resistance to rust and corrosion. Depending on the particular alloy, stainless steel is made up of several different non-ferrous metals in addition to iron and chromium as its primary constituents. The presence of iron is essential for rust to form; consequently, removing iron molecules from the surface of the metal will result in an increased concentration of chromium molecules, which are naturally inactive. Due to the fact that chromium molecules make up the majority of the surface, this surface is thick, non-relative, and passive, making it difficult for rust to form.
There are currently three different kinds of surface passivation that are recognised as meeting industry standards. The chemical that is used for passivation determines the type of passivated material. The following are the three different types of passivation:
- Nitric acid
- Nitric acid with sodium dichromate
- Citric acid.
The requirements of the customer are typically taken into consideration when deciding which chemical to use for the passivation process. Every type of passivation approach comes with its own set of benefits and drawbacks.
Why passivate stainless steel?
After stainless steel parts and components have been newly machined, a process called passivation is considered to be the best post-fabrication practise. Among the many advantages are:
- Chemical film barrier against rust
- increased longevity of the product
- Decontamination of the product’s surface
- less maintenance is required.
How does passivation work?
The basic components of stainless steel are iron, nickel, and chromium. Stainless steel is an iron-based alloy. Chromium is the primary element responsible for giving stainless steel its ability to resist rust and corrosion. When chromium is exposed to oxygen (air), it forms a thin film of chromium oxide that covers the surface of stainless steel and prevents the iron beneath it from rusting. This film is called a chromium oxide film. Passivation is performed with the intention of enhancing and perfecting the process of forming a chromium oxide layer. The free iron on the surface of the stainless steel can be removed by immersing it in an acid bath, but this does not affect the chromium content. The free iron is removed from the surface through a chemical process by the acid, which results in the formation of a uniform surface that contains a greater percentage of chromium than the material beneath it.
After being submerged in acid, the stainless steel is then exposed to oxygen in the air, which causes it to form a chromic oxide layer over the course of the next 24 to 48 hours. Because there is a greater concentration of chromium near the surface, it is possible for a layer of chromium oxide to form that is both more robust and more robust. By removing free iron from the surface, it is possible to eliminate potential starting points for corrosion. The formation of the resulting passive layer results in a surface that is resistant to rust because it is chemically inert.
In general, stainless steel alloys start off with good levels of corrosion resistance; however, the level of this resistance can change depending on the molecular composition of the alloy. In the process of passivation, which aims to further improve resistance, there are three steps involved:
STEP 1. COMPONENT CLEANING
The first step in the passivation process is to completely clean the component made of stainless steel. This involves removing any surface oils, chemicals, or debris that was left over from the machining step. If this step of the process is skipped, the effectiveness of the passivation will be reduced because there will be foreign objects on the surface of the metal.
STEP 2. ACID BATH IMMERSION
After the component has been cleaned, it is then submerged in an acid bath in order to remove any free iron particles that may be clinging to its surface. In this stage of the process, there are typically three different approaches that are used.
NITRIC ACID BATH
The conventional method of passivation involves the use of nitric acid, which results in the most efficient molecular redistribution of the surface’s molecular structure. Nitric acid, on the other hand, is considered to be a hazardous material, which brings with it a number of drawbacks. It is not only something that needs to be handled in a specific way, but it also gives off toxic gases, is harmful to the environment, and may require more time to process.
NITRIC ACID WITH SODIUM DICHROMATE BATH
When combined with certain alloys, the addition of sodium dichromate to nitric acid results in a passivation process that is either more effective or completed more quickly. The use of sodium dichromate heightens the risks associated with bathing in nitric acid; consequently, few people choose this alternative.
CITRIC ACID BATH
Citric acid passivation is a much more secure option than nitric acid passivation. Citric acid is an option that is friendly to the environment because it does not require any special handling, it does not release any toxic gases, and it is not a hazardous substance. Citric acid passivation had a difficult time gaining popularity because the compounds it used posed a risk of organic growth and moulds. However, innovations in recent years have eliminated these risks, turning it into an alternative that is both cost-effective and friendly to the environment. This dipping or bathing process produces a chemical reaction on the surfaces of the component, adding a thin layer of oxide film with little to no presence of iron molecule, restoring the metal’s corrosion resistance to its state as raw material. This chemical reaction can take place regardless of the approach that is applied.
STEP 3. TESTING AND CERTIFICATION
The results of testing performed after the passivation process provide verification that the passivation process was successful and certification that the product complies with ASTM, Mil-Spec, and various other industry standards.
When is passivation of stainless steel required?
After stainless steel has been ground, welded, cut, and subjected to any number of other machining processes, the material is subjected to a post-fabrication process known as passivation. Because stainless steel has a natural resistance to corrosion in the absence of any corrosive elements, passivating the material is not always required. However, when conditions are normal and realistic, any of the following can prevent the formation of the oxide film that protects against corrosion:
- foreign substances in a manufacturing setting (shop dirt, grinding swarf)
- stainless steel with sulphides added for better machinability
- Parts made of stainless steel may have iron shavings from cutting tools embedded in their surface.
In order to restore a surface that is uniformly resistant to corrosion, such contaminants need to be removed all the way down to the surface grain boundaries. These problems are resolved through a process called passivation.
What passivation is NOT
- Not electrolytic. In contrast to electrolysis, passivation is a chemical process. In contrast to electropolishing or anodizing, passivation does not rely on electrochemical reactions.
- Not for scale removal. After heat treating or welding, oxide scale on machined parts cannot be removed by passivating.
- Not a coat of paint. Stainless steel’s colour or surface appearance are not altered by passivation. Items that will be painted or powder coated don’t need to be passivated.
How can I passivate stainless steel safely and economically?
Nitric acid and citric acid are both viable options for performing the passivation process on stainless steel. On the other hand, the utilisation of citric acid is recommended for a variety of reasons. Citric acid passivation has been shown to be more effective than nitric acid passivation in a number of different tests. This is because citric acid passivation only removes iron, whereas nitric acid passivation removes iron and leaves all of the chromium, nickel, and other “good” components intact. Nitric acid, on the other hand, in addition to removing some iron, will also remove some chromium and nickel. Passivation with citric acid is not only much safer but also much better for the environment. Nitric acid is extremely hazardous to the health of both people and animals, as well as the environment. The use of citric acid passivation should be considered the obvious option for anyone who places a high value on the wellbeing of their workforce.
The Passivation of Stainless Steel: What You Should and Should Not Do
The following are some things that you should do and things that you should avoid doing in order to successfully passivate:
- BEFORE you passivate, you should clean the surface thoroughly, removing any oxide or heat tint particles.
- DO stay away from chlorides, as too much of them can result in a dangerous flash attack. When possible, only use high-quality water with a chloride content of under 50 parts per million (ppm). In most cases, water from the tap is sufficient, and in some circumstances, concentrations of chlorides of up to several hundred parts per million can even be tolerated.
- REPLACE BATHS ON A REGULAR SCHEDULE IF AT ALL POSSIBLE To prevent a loss in passivation potential that could lead to a flash attack and ruined parts, you should replace baths on a regular schedule. Baths should be kept at the appropriate temperature because temperature that is allowed to get out of control could allow for a localized attack.
- In order to reduce the likelihood of introducing contamination during high production runs, you should DO remember to stick to very specific replacement schedules for your solutions. To determine whether or not the bath is beneficial, a control sample should be utilized. In the event that the specimen is contaminated, it is time to replace the bath.
- DO designate particular machines as the sole fabricators of stainless steels; when cutting stainless steels, always use the same preferred coolant, regardless of what other metals you might be working with.
- DO treat parts individually in order to prevent metal from coming into contact with other metals. This is especially important when working with free-machining stainless steels, as these steels require a free flow of passivating and rinse solutions in order to diffuse away corrosion products resulting from sulphides and avoid pockets of acid.
- When working with stainless steel parts that have been carburized or nitrided, passivating the material is NOT recommended. When parts are handled in this manner, their natural resistance to corrosion may be reduced to the point where they are susceptible to being attacked in the passivating tank.
- Tooling that contains iron SHOULD NOT BE USED in an environment in the shop (the floor, the equipment, the coolant, etc.) that is not exceptionally clean. If carbide or ceramic tools are used instead of steel ones, then steel grit can be avoided.
- REFRAIN FROM FORGETTING that an attack may take place in a passivating bath if the parts have not been properly heat treated. To make martensitic grades with high levels of carbon and chromium resistant to corrosion, the material must be hardened. It is common practise to perform a passivation procedure after a subsequent tempering at a temperature that preserves the material’s corrosion resistance.
- Passivate only one piece of stainless steel at a time; DO NOT passivate multiple pieces at once. The application of this discipline can help avoid costly mixups as well as galvanic reactions.
Grades of Passivated Stainless Steel
Stainless steel, like all alloys, is available in a variety of grades, each of which has its own unique set of mechanical properties as well as overall applications. There are three primary classifications of stainless steel, and each one is determined by the constituent alloying elements and the overall composition.
1 – Austenitic Grade
The grade of stainless steel that is used the most frequently in a variety of contexts. The fact that this alloy composition contains some nickel in addition to a chromium percentage that falls anywhere between 16% and 30% gives it the ability to provide significant protection against corrosion for an extended period of time. In addition, austenitic grade stainless steel is not magnetic, and it cannot be heat treated, which means that its maximum mechanical strength cannot be increased.
2 – Ferritic Grade
High chromium concentration is another characteristic of ferritic stainless steel alloys. The range is roughly 10% to 30%, but the alloy also contains a sizeable amount of carbon, which is never allowed to over 20%. This sets the upper limit for the percentage of carbon. The heat treatment process cannot be used on ferritic steel, just like it cannot be used on the preceding grade. In addition, techniques such as cold rolling, which are effective for austenitic grades of steel, are ineffective for ferritic steel.
3 – Martensitic Grade
This is a versatile grade that can be heat treated, has magnetic properties, and can be utilised in a variety of tampering and ageing procedures. However, these characteristics come at the expense of the martensitic stainless steel’s overall effectiveness as a corrosion resistant material because the martensitic stainless steel’s chromium content ranges from 12% to 17%.
Industry Standards for Stainless Steel Passivation
One of the best methods to guarantee consistent quality and dependability in one’s goods or services is to adhere to the industry standards. When it comes to passivation of stainless steel, the majority of global players, apart from any specific company standards, adhere to either the ASTM or AMS standards.
ASTM International is a standard-setting organisation that is active on a global scale and maintains a collection of more than 14,000 standards pertaining to the specification of materials, processes, and tests. The testing and passivation process of stainless steel is more closely aligned with the following standards.
- ASTM A96: Using five combinations of nitric and citric acid techniques for steel passivation.
- ASTM A380: Cleaning, descaling, degreasing, and ultimately passivating stainless steel parts while evaluating the finish’s quality and even the precautions to take in order to avoid particular problems.
- ASTM 380: only passivating and descaling stainless steel components
- ASTM F86: parts made of stainless steel that have been prepped for passivation.
Aerospace Material Specifications (AMS), which stands for “Aerospace Material Specifications,” is yet another standard-setting organisation that focuses primarily on the processes, materials, and standards for aerospace applications. Nevertheless, utilising AMS standards can assist you in maintaining reliability. This is because all aerospace applications make use of the best materials and techniques.
The stainless steel industry generally adheres to the two standards listed below:
- AMS 2700: Standards for stainless steel passivation that apply to both nitric and citric acid techniques.
- AMS-QQ-P-35: Only the nitric acid method is allowed, and there are 4 different combinations available.
Types of Equipment for Stainless Steel Passivation
In order to optimise the four-step process of cleaning, rinsing, passivating, and finally rinsing it again, the passivation process necessitates specific equipment. The following equipment types are available to you for the process.
- Small Benchtop Passivation Equipment
Systems for manual passivation work best in places with constrained space. Small components and samples are typically passivated in this equipment category.
- Wet Bench Passivation Equipment
a comprehensive system that resembles an assembly line. You have enough room for all the passivation processes and security features, including in-tank flow, nitric acid method exhaust, and many others. This category of equipment is also manual.
- Automated Passivation Systems
Automated passivation systems, as their name implies, work on the same principle but don’t require any help from people at any point in the procedure.
- Agitated Immersion Passivation Systems
Pneumatics are used by automated or semi-automated equipment to agitate the part and deliver excellent cleaning results. More ideal for extremely delicate applications where you want the greatest longevity and durability.
Benefits and Applications
Passivation offers a variety of advantages, such as:
- After machining, the elimination of any remaining contaminants.
- increased resistance to corrosion
- decrease in the danger of contamination during production processes
- an improved component’s performance
- lengthened time between maintenance periods
The advantages of passivation are widespread in a variety of industries and applications that rely on the non-corrosive qualities of stainless steel, including:
- Heavy equipment (ball bearings, fasteners)
- Aerospace and aircraft components
- Medical (orthopedic implants, surgical instruments)
- Pharmaceutical (pumps, fittings, and components used in manufacturing)
- Military (firearms, military equipment)
- Energy (power distribution and transmission)
Frequently Asked Questions on Passivation of Stainless Steel
1 – Is pickling and passivation the same thing? No. Passivation and pickling are two distinct techniques. To prepare stainless steel parts for passivation, the pickling procedure cleans the surface of welded parts of all debris, flux, and other contaminants. Steel cannot be protected from corrosion by pickling; pickling merely prepares the surface for the formation and functioning of the passivation layer.
2 – Is stainless steel made corrosion-proof via passivation? A product cannot be completely corrosion-proof. The passivation process, however, gives stainless steel components an exceptionally long lifespan. Even though it is thin, the protective chromium layer can provide long-lasting protection, which you can even increase by giving the metal part regular maintenance and shielding it from oxidising substances.
3 – Is it necessary to passivate stainless steel? No, passivation is a necessary step for parts made of stainless steel. Without the process, corrosion attack on your part will be possible in a very short amount of time.
What is Stainless Steel Passivation? (2022, September 27). FZE Manufacturing Solutions LLC. https://fzemanufacturing.com/blog/what-is-stainless-steel-passivation/
Stellar Solutions. (2022, June 29). What is Passivation? How to Passivate Stainless Steel, and Why. | Stellar Solutions – Stainless Steel Passivation. Stellar Solutions – Stainless Steel Passivation | Citric Acid Passivation. http://citrisurf.com/what-is-passivation/
Stainless Steel Passivation: For Enhanced Durability and Versatility. (2022, September 29). Rapiddirect. https://www.rapiddirect.com/blog/what-is-stainless-steel-passivation/
What is Stainless Steel Passivation? (2022b, September 27). FZE Manufacturing Solutions LLC. https://fzemanufacturing.com/blog/what-is-stainless-steel-passivation/