CitriSurf Passivation Chemistry for Citric Acid Passivation

What is CitriSurf^®? Passivation Chemistry?

CitriSurf products are citric acid passivation solutions used for stainless steel passivation and corrosion prevention in metal finishing applications. CitriSurf chemistry removes free iron from the stainless steel surface and supports the formation of a protective passive oxide layer that improves resistance to rust and corrosion.

Best Technology offers CitriSurf passivation chemistry for manufacturers that need a reliable citric acid process for stainless steel parts. The product line includes formulations for immersion, spray, on-site work, and different stainless steel grades, allowing manufacturers to match the chemistry to their parts and passivation requirements.

What is Citric Passivation?

Citric passivation is a post-fabrication finishing process performed by immersing stainless steel parts in a citric acid bath. By removing free iron ions and forming a protective passive oxide layer on the surface, the stainless steel or other metal becomes highly resistant to rust.

Historically, manufacturers used nitric acid to passivate stainless steel surfaces. However, the workplace safety and environmental hazards associated with nitric acid prompted researchers to identify citric acid as an effective, safer alternative, like CitriSurf.

When CitriSurf passivation chemistry is used, citric passivation provides a safer and more controllable alternative to traditional nitric acid processes while still supporting effective stainless steel corrosion resistance. This makes CitriSurf useful for manufacturers that need a reliable passivation process for stainless steel parts without relying on more hazardous nitric acid chemistry.

For details on how citric passivation is performed, see our article “Citric Acid Passivation of Stainless Steel”.

Benefits of CitriSurf Citric Acid Passivation Chemistry

The key advantage of CitriSurf is that it uses citric acid passivation chemistry instead of nitric acid or hydrochloric acid. This gives manufacturers a safer option for stainless steel passivation while still supporting effective free-iron removal and corrosion resistance. CitriSurf is especially useful when facilities want to reduce chemical handling risks without giving up a controlled passivation process.

CitriSurf is available in various concentrations and can be used in both immersion and spray applications, providing versatility in its usage for different passivation requirements. CitriSurf’s ability to effectively passivate metals without the use of harmful chemicals makes CitriSurf a popular choice among industries seeking environmentally-friendly and effective passivation chemistry.

Another benefit of CitriSurf is that the treatment removes free iron from the surface without removing other metals from the alloy. This helps minimize dimensional change, which can be important for close-tolerance parts, high-precision machining, and stainless steel components that require consistent passivation results.

CitriSurf products are designed to support passivation requirements under standards such as ASTM A967 and AMS 2700 and can be used with a wide range of stainless steel grades. Properly passivated parts can be evaluated using salt spray, water immersion, copper sulfate, and high-humidity testing. This gives manufacturers an alternative to traditional nitric acid passivation while maintaining controlled corrosion-resistance performance.

Common Applications of CitriSurf

From aerospace to food and beverage processing, CitriSurf citric passivation chemistry is used to passivate stainless steel components. In the aerospace industry, it is used to prepare parts for flight by removing free iron and other contaminants that could cause corrosion. In the food and beverage industry, CitriSurf is used to ensure that stainless steel equipment used in production processes is free from impurities that could affect the quality and safety of the products.

Additionally, CitriSurf is also commonly used in the medical device and pharmaceutical industries to passivate stainless steel instruments and equipment, ensuring that they meet the strict requirements for cleanliness and safety. Its versatility and effectiveness make CitriSurf citric passivation chemistry a trusted choice for a wide range of applications.

Which CitriSurf Product is Right for You?

Best Technology offers the complete line of CitriSurf citric passivation chemistry products.

Selecting the right CitriSurf product depends on the stainless steel grade, application method, part size, required concentration, operating temperature, and whether the process will be run by immersion, spray, or on-site application. Best Technology can help match the CitriSurf formulation to the part material and passivation requirement.

The following CitriSurf products are primarily used for passivation in manufacturing and fabrication:

• CitriSurf 2050
• CitriSurf 2250
• CitriSurf 2450
• CitriSurf 3050
• CitriSurf 3250
• CitriSurf 77
• CitriSurf 2210

CitriSurf 2050 is the most cost-effective solution and is most useful for 300 series austenitic grades of stainless steel. CitriSurf 2050 has a pH of about 1.8 at normal working concentration. CitriSurf 3050 is a low-foaming version for spraying applications or tanks with submerged air blowers.

CitriSurf 2250 is the citric acid solution most commonly used by our customers. CitriSurf 2250 uses an increased pH to ensure the surface is maintained on the more sensitive 400 series ferritic and martensitic grades of stainless steel. CitriSurf 2250 has a pH of about 3.0 at normal working concentration. CitriSurf 3250 is a low-foaming version.

CitriSurf 2450 uses a higher pH for the most sensitive, extremely low chromium grades of stainless steel. CitriSurf 2450 has a pH of about 4.3 at normal working concentration.

CitriSurf 77 and CitriSurf 2210 are designed for on-site passivation and larger components that may be difficult to process in a tank. CitriSurf 77 is a liquid formulated for spray application, while CitriSurf 2210 is a thicker gel that clings to vertical surfaces and can be used for localized passivation after laser marking.

CitriSurf 77 is especially useful when spray application is preferred or when stainless steel parts are too large or difficult to move into an immersion tank. Its liquid formulation makes it a practical option for field work, installed equipment, and other on-site passivation applications.

For more information on CitriSurf compatibility with your grade of stainless steel, see our CitriSurf Passivation Material Compatibility Guide.

CitriSurf 77 vs. CitriSurf 77 Plus: What’s the Difference for Stainless Steel Passivation?

CitriSurf 77 and CitriSurf 77 Plus are both liquid products used for stainless steel passivation, but they serve different conditions. CitriSurf 77 Plus includes rust-removal capability, while standard CitriSurf 77 is intended for surfaces that do not require existing rust to be removed. Manufacturers passivating newly machined stainless steel parts may therefore use CitriSurf 77 or another suitable CitriSurf formulation.

CitriSurf Product Comparison

	CitriSurf 2050	CitriSurf 2250	CitriSurf 2450
Chemical composition	Citric acid, water, proprietary ingredients	Citric acid, water, proprietary ingredients	Citric acid, water, proprietary ingredients
Operating Temperature	Room temp or higher (120-160°F preferred)	Room temp or higher (120-160°F preferred)	Room temp or higher (120-160°F preferred)
Flash Point	None	None	None
Water solubility	Complete	Complete	Complete
Normal working concentration	7-13% by volume in water	9-18% by volume in water	10-20% by volume in water
pH at working concentration	approx. 1.8	approx 3.0	approx. 4.3
Data sheets	CitriSurf 2050 Data Sheet	CitriSurf 2250 Data Sheet	CitriSurf 2450 Data Sheet

For passivation with citric acid, the ratio of citric acid to water ranges from 7-20%, measured by volume. On the chart it is shown as Normal working concentration.

CitriSurf Passivation Material Compatibility Guide

Use this table as a reference for compatibility of your grade of stainless steel or other alloy with CitriSurf passivation solution.

Austenitic Stainless Steels (Non-Magnetic)
	CitriSurf 2050	CitriSurf 2250	CitriSurf 2450
S20100 (201)	✔	✔	✔
S20200 (202)	✔	✔	✔
S30100 (301)	✔	✔	✔
S30200 (302)	✔	✔	✔
S30400 (304)	✔	✔	✔
S30403 (304L)	✔	✔	✔
S30409 (304 H)	✔	✔	✔
S30430 (18-9LW)	✔	✔	✔
S30451 (304N)	✔	✔	✔
S30500 (305)	✔	✔	✔
S30800 (308)	✔	✔	✔
S30900 (309)	✔	✔	✔
S30940 (309Cb)	✔	✔	✔
S31000 (310)	✔	✔	✔
S31400 (314)	✔	✔	✔
S31600 (316)	✔	✔	✔
S31603 (316L)	✔	✔	✔
S31609 (316H)	✔	✔	✔
S31620 (316F)	✔	✔	✔
S32100 (321)	✔	✔	✔
S32109 (321H)	✔	✔	✔
S34700 (347)	✔	✔	✔
S34709 (347H)	✔	✔	✔
Free Machining Stainless Steels
	CitriSurf 2050	CitriSurf 2250	CitriSurf 2450
S30300 (303)	✔	✔	✔
S30323 (303Se)	✔	✔	✔
S30310	✔	✔	✔
S30330 (303Cu)	✔	✔	✔
S30345 (303 MA)	✔	✔	✔
S30360 (303 Pb)	✔	✔	✔
S34720	✔	✔	✔
S34723	✔	✔	✔
S43020 (430F)		✔	✔
S43023 (430FSe)		✔	✔
S44020 (440 F)		✔	✔
Martensitic Stainless Steels (Magnetic)
	CitriSurf 2050	CitriSurf 2250	CitriSurf 2450
S40300 (403)		✔	✔
S41000 (410)		✔	✔
S41400 (414)		✔	✔
S41600 (416)		✔	✔
S41623 (416Se)		✔	✔
S42000 (420)			✔
S42020 (420F)			✔
S43100 (431)		✔	✔
S44002 (440A)		✔	✔
S4403 (440B)		✔	✔
S44004 (440C)		✔	✔
Ferritic Stainless Steels (Magnetic)
	CitriSurf 2050	CitriSurf 2250	CitriSurf 2450
S40500 (405)		✔	✔
S40900 (409)		✔	✔
S42900 (429)		✔	✔
S43000 (430)		✔	✔
S43400 (436)		✔	✔
S44200 (442)		✔	✔
S44600 (446)		✔	✔
S44627		✔	✔
Precipitation Hardening Stainless Steels (Magnetic)
	CitriSurf 2050	CitriSurf 2250	CitriSurf 2450
S66286 (A286)		✔	✔
S13800 (13-8 Mo)		✔	✔
S15500 (15-5)		✔	✔
S15700 (15-7 Mo)		✔	✔
S17400 (17-4)		✔	✔
S17700 (17-7)		✔	✔
S35500 (AM 355)		✔	✔
S36200 (362)		✔	✔
Other Materials
	CitriSurf 2050	CitriSurf 2250	CitriSurf 2450
Titanium (Ti)	✔	✔	✔
Aluminum (Al)	✔	✔	✔
Inconel ( austenitic nickel-chromium alloy)	✔	✔	✔
Altemp 625, Haynes 625, Nickelvac 625 and Nicrofer 6020	✔	✔	✔
Inconel 600	✔	✔	✔
Inconel 617	✔	✔	✔
Inconel 625	✔	✔	✔
Inconel 718	✔	✔	✔
Inconel X-750	✔	✔	✔
* Source: Stellar Solutions

CitriSurf^® is a registered trademark of Stellar Solutions, Inc. McHenry IL USA

Passivation Chemistry FAQs

• What is the best method of passivation?

  Have you ever noticed that many day-to-day things seem to get divided into two opposing camps?  Things like asking what’s the best computer quickly become the PC versus Mac debate.  Phones? iPhone versus Android.  So, passivation?  Yup, nitric versus citric.

  There are real advantages to each nitric and citric passivation, but that is a topic for other FAQ’s because once you pick one you still have several choices to make.  Choices like: which standard do I use? which method do I choose?

  There are two primary standards that address passivation. The SAE Aerospace “Passivation of Corrosion Resistant Steels” (AMS2700) and the ASTM “Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts” (A 967).

  Each allows for citric or nitric and each allows for several options within each citric and nitric.  So what’s the difference? Each standard is copyrighted (and protected) and has to be purchased from the publishing agency, so getting into a lot of detail is not allowed. But here’s a short version of the differences in the various methods:

  Concentration of the specific acid – each method varies based on how concentrated the citric or nitric acid is – and in the case of nitric if the nitric acid is used alone or with sodium dichromate.

  Temperature of the bath – again, each method specifies a temperature range that must be maintained for the duration of the passivation bath.

  Time – the length of time a part must remain within the passivation solution is spelled out as well.

  What seems complicated becomes really simple. The standards spell out the concentration, the temperature and the time. Each method simply varies one or more of those three items.

• What is passivation?

  Passivation is a non-electrolytic process that enhances the corrosion resistance of stainless steel by removing free iron from its surface. Typically performed using nitric or citric acid, this treatment creates an inert, protective oxide layer. In metal finishing, “to passivate” means to perform this crucial process that prevents rust formation by eliminating reactive iron that could otherwise oxidize when exposed to the atmosphere.

  During machining and manufacturing processes, stainless steel parts may have imperfections from iron being embedded or smeared onto the surface from the machining tool steels. These free irons on the surface of the stainless steel need to be removed to prevent a corrosive reaction from occurring between the two different metals. To ensure optimal corrosion resistance, manufacturers must passivate the surface after machining, removing any embedded contaminants that could compromise the material.

  The passivation process chemically removes these free irons and forms a passive oxide “film” layer, which further improves corrosion resistance. When exposed to air, the stainless steel undergoing passivation will form a chemically inactive or inert surface. This is one advantage of using citric acid or nitric acid to passivate stainless steel, as both methods enhance durability. Regularly passivating stainless steel components ensures they maintain their protective properties, extending their lifespan in harsh environments.

  For industries that require the highest levels of corrosion resistance, it is essential to passivate stainless steel components as part of a routine maintenance and quality control process. Proper passivation not only removes surface contaminants but also helps maintain the structural integrity and longevity of critical parts, ensuring they perform reliably in demanding applications.

  For more detailed information on what passivation means, please see our “What is Passivation?” page.

• What’s the difference between nitric and citric passivation?

  Both nitric acid and citric acid are effective in passivating many grades of stainless steel, and both methods are described in the industry standards ASTM A967 and AMS 2700.

  Citric acid passivation offers the following key differences. Compared to nitric acid passivation, citric acid passivation is:

  • Newer
  • More environmentally friendly
  • Safer.

  In the past, some manufacturers avoided citric acid due to potential organic growth and molding issues. Today, modern formulations for biocides prevent organic growth in the solution. See our complete write up on the advancements of citric acid for passivation.

  These advancements in citric acid have allowed smaller manufacturers that lacked experience with chemical handling and processing to bring their passivation in-house. For a more detailed comparison, please see our “Nitric vs. Citric Acid Passivation” page.

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To learn more about surface finishing chemistries, visit our Surface Finishing Chemicals: Passivation, Electropolishing & Conversion Coatings page