All You Need To Know About Citric Acid For Passivation

What is Citric Acid Passivation?

Citric acid passivation refers to the process of using citric acid to prevent corrosion in stainless steel (SS) and other alloys. Through the process of removing free iron ions and forming a protective layer of passive oxide on the surface, stainless steel or any other type of metal can become extremely rust resistant. After the stainless steel parts have been fabricated, a finishing process called citric acid passivation is performed on them by submerging them in a bath of citric acid. Please refer to our article titled “What is Passivation?” for further information on the process of passivation. Nitric acid was traditionally used in the manufacturing process to passivate stainless steel. Nitric acid, despite being an extremely efficient passivation agent, presents significant risks to both the workplace and the environment and must be used in strict accordance with applicable regulations.

Due to the potential risks associated with nitric acid, businesses sought out citric acid passivation as an alternative. Earlier attempts at citric acid passivation, on the other hand, were plagued by problems involving organic growth and mould. Since then, a great deal of progress has been made. Citric acid has become much more resistant to the growth of organic matter as a result of recent developments in biocides. The majority of grades of stainless steel are now passivated using citric acid, which is the method that is considered to be the most environmentally friendly option.

Understanding Passivation

The manufacturing process often leaves behind contaminants that can interfere with the formation of an oxide layer on the surface of metals. This layer acts as a barrier against corrosion for metals like stainless steel by forming on the surface of the metal. In spite of the fact that stainless steel is, by its very nature, inert, the impurities that are left behind combine with the metal and cause a reaction that is corrosive. Manufacturers frequently utilise chemical passivation to enhance performance, durability, and corrosion resistance in sectors that demand a high standard of cleanliness, such as the medical, aerospace, laboratory, and food production. The following are some examples of these kinds of industries:

Why Citric Acid Passivation

In recent years, the practise of citric acid passivation has gained increasing acceptance in a variety of industries, including the medical and aerospace sectors. It is an effective alternative to nitric acid passivation with fewer concerns regarding its handling, and it is considered environmentally friendly as it is on the GRAS which stand for “Generally Recognized as Safe” list for the FDA. As a result, it is ideal for use in the medical manufacturing industry as well as the food manufacturing industry. In addition, citric solutions are superior to traditional nitric acid methods like ASTM A967, Nitric 2, in terms of their ability to effectively passivate a wider variety of stainless steel alloys.

Why does stainless steel require passivation?

The process of removing metallic or “free” iron from the surface of stainless steel materials is referred to as “passivation,” and it involves a series of procedures. On a surface made of stainless steel, the presence of free iron ions can result in corrosion or rust spots. The integrity of stainless steel can be easily compromised by contaminants, which leaves the material vulnerable to corrosion. Here are some common examples:

• Microscopically sized contaminants are released from the cutting surfaces of bits and blades on tools used for machining or repairing components.
• Damage to the surface can be introduced through handling or maintenance.
• Dirt or shop dust settles on the surface
• To facilitate machining, excessive amounts of sulphides are used, which contaminates stainless steel alloys.

This type of contamination can be harmful to stainless steel because it can disrupt the naturally occurring thin layer of chromium oxide that coats the material and protects it from the damaging effects of atmospheric oxygen and corrosion. The thickness of a human hair is approximately one hundred thousand times greater than the thickness of this delicate protective layer. During the manufacturing process, the stainless steel is exposed to air, which causes a layer of chromium oxide to form almost instantly. Therefore, any material or contamination that becomes ingrained in or spreads across the surface will probably cause the layer to become compromised, which will result in corrosion being introduced directly onto the metal. The inherent resistance to corrosion that was present in the stainless steel component prior to fabrication can be restored using a post-fabrication process called passivation. It is not a treatment for removing scales, nor is it similar to applying a coat of paint. It is a method of cleaning that preserves the chromium oxide layer that was there originally.

Benefits of citric acid passivation

Passivation with citric acid, as opposed to nitric acid, offers a number of advantages, the most important of which is that citric acid is less hazardous to humans and the environment. When citric acid is handled according to the FDA’s guidelines for good manufacturing practise, it is not considered to be a risk to human health and is therefore included on the GRAS list, which stands for “generally recognised as safe.” Citric acid, which is used in a wide range of foods and beverages, contains the same kind of naturally occurring acid that is present in oranges and other citrus fruits. It is degradable naturally and safe to use. Businesses that employ citric acid passivation are able to circumvent many of the regulatory concerns that are posed by the government since citric acid can often be disposed of in a sewer system with relatively minimal waste treatment needed (subject to individual municipal requirements).

In order for citric acid solutions to be effective, they must first remove the free iron from the surface of the metal and then form a complex that is water-soluble with the iron ions. This stops the iron from precipitating again, which stops it from having the harmful effect that nitric acid is known to have. The removal of iron contributes to the formation of a passive oxide layer on the surface that is more resistant to rusting. 

Citric acid passivation is used by manufacturers in industries that require high performance. This is particularly true for the aerospace and medical device sectors, where passivation of parts is essential for performance and durability, including cytotoxicity and bio-burden requirements. Citric acid passivation is a process that manufacturers in these sectors rely on. For many companies who are bringing their passivation needs in-house rather than outsourcing them to a metal plating facility, citric acid is the go-to solution. Citric acid is available in practically all supermarkets.

Principal advantages of citric acid passivation

• Environmentally safe chemistry – simple in both its use and its disposal
• Outstanding performance with all types of stainless steel
• Chemicals that pose no risk in the workplace – no toxic or corrosive fumes
• complies with all relevant industry standards – tests of high humidity, salt spray, immersion, and copper sulphate are passed successfully.
• Removal of free iron from the surface that is both improved and accelerated.
• tests of high humidity, salt spray, immersion, and copper sulphate are passed successfully.

Only iron is removed, leaving the alloy intact with its other metals. 

Citric vs. Nitric Passivation Comparison

Before research showed that citric acid provided a safer and more effective process, the only way to passivate stainless steel was to use nitric acid. Nitric acid was the only option for a long time. In contrast to nitric acid, citric acid is organic, does not pose any health risks, and is perfectly fine to consume. The process of citric passivation does not produce any toxic fumes and has the potential to be disposed of at a much lower overall cost than nitric waste does.

Numerous studies have shown that phosphoric and nitric passivation are not as effective as citric passivation, and in some cases, citric passivation is even more effective. The ratio of chromium oxide to iron oxide that can be discovered on passivated surfaces is evaluated through the use of electron spectroscopy, which is a common method of evaluation. The surface has a greater resistance to corrosion when there is a higher proportion of chromium to iron. Comparing the results of the two passivation processes, the citric process produced an average ratio of 2.36 chrome to iron content, whereas the nitric process produced an average ratio of 2.1 chrome to iron content. The citric process has been shown to be more effective at maximising the protective chromium oxide layer than the other process, despite the fact that both processes will eliminate excess free iron and provide superior protection.

On 316L coupons, it was discovered that citric passivation resulted in greater Fe/Cr oxide ratios than nitric passivation and produced an oxide layer that was almost 50% thicker than nitric’s. This discovery was made in another study that was funded by NASA. Nitric acid can be used to passivate most grades of stainless steel; Nevertheless, nitric acid cannot be used to passivate all grades of stainless steel. There are also some variations in chemistry that must be tightly controlled in order to meet industry specifications. According to the grade of stainless steel and the frequency of testing that is specified, there are eight different compositions of the nitric solutions. Citric acid solutions, on the other hand, can passivate the same variety of stainless steel grades with just one solution. Changes in temperature are only specified when necessary for the purpose of controlling the total amount of time spent immersed.

Citric acid is used as part of the passivation process for stainless steel.

Industry standards

In the end, manufacturers are the ones who are responsible for performing passivation in accordance with the acceptance criteria set forth by their customers. ASTM A967 and AMS 2700 are the two industry standards for citric acid passivation, and both of these standards cover the majority of acceptance criteria.

ASTM A967

Chemical treatments for the passivation of stainless steel parts are the subject of the ASTM A967 standard. It establishes standards for nitric acid immersion treatments in addition to citric acid immersion treatments. The production of citric acid can be broken down into five distinct categories. Citric 1-3 stipulate solution concentrations of 4–10% citric acid (based on the weight of the composition), with treatment times being shortened while the temperature was increased. Citric acid 4 and 5 are flexible in terms of other parameters and even the addition of additives. CitriSurf, manufactured by Stellar Solutions, is classified as a Citric 4 product; however, the recommended procedures keep the same concentration as well as the other parameters that were defined for Citric 1-3. As long as the surface treatment that is produced satisfies acceptance test criteria, the ASTM A967 standard allows for any combination of immersion time, temperature, and concentration of citric acid to be used.

AMS 2700

The passivation of corrosion-resistant steels is the subject of the AMS 2700 standard. The aerospace industry makes use of this particular standard. In a manner similar to that of ASTM A967, it established standards for both the nitric acid immersion treatment (Method 1) and the citric acid immersion treatment (Method 2). The citric acid passivation standard known as Method 2 calls for solution strengths ranging from 4% to 10% citric acid (based on the composition’s weight), along with more expedient treatment times and higher temperatures.

Citric acid passivation process steps

The following is a list of the typical steps involved in the passivation process for stainless steel:

1. The use of alkaline cleaning solutions on the materials in order to get rid of any and all contaminants, oils, and other foreign material etc – Utilizes detergent cleaners on a regular basis such as Micro90, Simple Green, and other brands.
2. Water rinse – In high precision industries, deionized (DI) water or reverse osmosis (RO) water is typically used as the source of water.
3. Immersion in citric acid (CitriSurf) in order to completely dissolve any free irons and sulphides and hasten the formation of a passive film or oxide layer.
4. Water rinse – most frequently with DI Water in industries requiring high precision
5. Second water rinse – common practise in high precision industries involving DI Water
6. Dry parts
7. Salt spray, high humidity chamber exposure, or copper sulphate testing are some of the methods that sample parts can be tested with using specification standards.

The precise steps of the passivation process are determined by the chromium content of the alloy, the characteristics of the machinability, and any other surface treatments that have been applied to the stainless steel, titanium, or other alloy.

Testing results of citric acid passivation

In most cases, testing of the components after passivation is performed on a per-lot basis. Industry standards such as ASTM A967 make it possible to conduct many different kinds of tests, including the following:

• Water immersion test
• High humidity test
• Salt spray test
• Copper sulfate test
• Free iron test

The copper sulphate test is particularly helpful because, in comparison to other tests, it can be completed in a shorter amount of time. In the copper sulphate test, a solution of copper sulphate and sulfuric acid is applied to the surface of a sample part that represents the lot that is being evaluated. At least six minutes should pass with the surface maintaining its wetness with the solution. After the solution has been removed, the component is analysed to look for copper deposits. Any sign of copper plating on the component indicates that the test was not successful. The copper sulphate test, on the other hand, is not suitable for everyone. It is not recommended for areas that have been laser-marked and cannot be applied to any surface that is used in the processing of food. It is not recommended to use this test on martensitic stainless steels from the 400 series or ferritic stainless steels from the 400 series.

What to look out for when passivating with citric acid

Don’t get cleaning and passivation confused with one another. It is possible to quickly jump to the conclusion that immersing the parts in citric acid will not only passivate them but also clean them. That is not the case. The components have to go through a cleaning process BEFORE being submerged in the citric acid solution. Otherwise, any debris from the shop, such as grease that was left over from the fabrication process, could potentially interact with the citric acid and cause gas bubbles to form on the surface, which would prevent passivation from occurring.

If this is the case, you might want to think about using a degreaser or switching detergents to make absolutely certain that the part is free of any and all contaminants. Grinding or pickling may be required in some instances in order to remove thermal oxides.

According to section 7.2.4 of ASTM A380, it is essential to conduct a water-break test after cleaning and rinsing the component and prior to introducing it to the citric acid solution. This step comes before placing the component in the solution. The water-break test is performed with the intention of locating any oily residue or hydrophobic contaminants, such as fingerprints or grease. Make sure there are no impurities in the solution of citric acid. The contamination of the citric acid solution can be remedied by simply replacing the existing solution in the citric acid bath with a new citric acid solution. If the issue continues to arise, you may want to consider making the citric acid solution with a higher quality of water, such as RO water or DI water, which is less likely to contain contaminants than regular tap water.

Utilizing racks is another recommendation for best practice, as this will prevent individual metal parts from coming into contact with one another. This makes it easier for the solution to flow freely, which helps remove corrosive contaminants and prevent pockets of acid from forming.

Take precautions to avoid galvanic corrosion. Galvanic corrosion, also known as bimetallic corrosion, can be avoided by avoiding the mixing of two different types of stainless steel in the same citric acid passivation bath. For example, the 300 series and the 400 series of stainless steel. This is particularly crucial to keep in mind when working and dealing with a sizable amount of mixed stainless steel grades in a same bath because the bigger volume raises the risk of galvanic corrosion. Because of this, the less noble metal corrodes at a faster rate than it otherwise would have if the dissimilar metals in the solution hadn’t been in contact with one another.

References

Citric Acid Passivation Explained. (2022, February 17). Stainless Steel Electropolishing Company – New England Electropolishing. https://neelectropolishing.com/citric-acid-passivation/

Best Technology. (2022a, June 28). Citric Acid Passivation for Stainless Steel | CitriSurf 2250 | CitriSurf 2050. https://www.besttechnologyinc.com/passivation-systems/citrisurf-citric-acid-passivation-solution/

Armoloy® of Illinois | Citric Passivation | Environmentally Friendly Corrosion Protection for Stainless Steel. (2022, October 24). Armoloy of Illinois. https://armoloy-il.com/citric-passivation/

Citric Acid Passivation of Stainless Steel. (2022, July 6). Crest Ultrasonics. https://crest-ultrasonics.com/citric-acid-passivation-of-stainless-steel/