A Practical Guide to Passivation for CNC Machined Parts 2025

Passivation is a key chemical treatment for metals to improve corrosion resistance and part longevity, like stainless steel, titanium, and tantalum—but how does it actually work, and how should you passivate CNC-machined parts?

These are questions frequently asked by manufacturers and precision machine shops. Our blog will give you a practical guide, answering these common questions.

Key Takeaway:

• Passivation chemically creates an ultra-thin, dense, and stable oxide film, transforming the metal from an “active” to a “passive” state. This process significantly enhances corrosion resistance without altering the part’s dimensions.
• Material selection and passivation solution must be precisely matched. Different metals (stainless steel, aluminum, titanium, copper) require specific chemical solutions and mechanisms. Using the wrong solution can fail to create the protective layer and may severely corrode the workpiece.
• Successful passivation starts with thoughtful part design. Key design-for-manufacturing practices—such as avoiding sharp internal corners, properly designing blind holes, and carefully considering fixturing locations—are crucial for ensuring uniform and complete passivation.
• Process standardization and verification are the cornerstones of quality assurance. Adhering to industry standards (ASTM, AMS, ISO) and performing rigorous tests (e.g., salt spray, water immersion) is essential to guarantee passivation quality meets application requirements.
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1. What is Passivation?

1.1. Core Definition

Passivation is a chemical surface treatment process primarily used for reactive metals and their alloys, such as stainless steel, aluminum, and titanium.

Its main purpose is to chemically create an extremely thin, dense, and stable protective oxide layer (typically an oxide) on the metal surface. This layer significantly inhibits further chemical reactions, thereby greatly enhancing the metal’s corrosion resistance.

In simple terms, passivation isn’t an added “coating.” Instead, it transforms the metal surface from an “active state” to a “passive state,” making it less reactive and less sensitive to the external environment (like air and moisture).

1.2. How Does Passivation Works

The principle of passivation involves electrochemical and chemical processes, which can be summarized in three key steps:

Step 1: Surface Cleaning & Activation

First, pickling solutions (like nitric or citric acid) thoroughly clean the metal surface, removing contaminants—especially free iron particles, cutting fluid residue, and oils. These contaminants are the main culprits for initiating pitting and rust.

Step 2: Oxide Layer Formation

In the presence of acid and oxidizers, the metal’s key corrosion-resistant elements (like chromium in stainless steel or aluminum in aluminum alloys) are preferentially oxidized. They dissolve into the solution and react with oxygen.

Step 3: Stable Protective Layer Creation

When this clean, chromium (or aluminum)-enriched metal surface is exposed to air or an oxygen-containing medium, it spontaneously forms an ultra-thin oxide film (like Cr₂O₃ on stainless steel, Al₂O₃ on aluminum). This film is:

Dense and Non-porous: Effectively blocks external corrosive agents (like chlorides, moisture, oxygen) from contacting the base metal.

Chemically Inert: Very stable and unlikely to participate in chemical reactions.

Self-Healing: If the film gets slightly scratched, the exposed chromium/aluminum can react with oxygen again in a suitable environment, reforming the oxide layer and “healing” the protection.

Passivation does not change the part’s physical dimensions, appearance, or mechanical properties. It simply maximizes and optimizes the material’s inherent, built-in corrosion resistance potential.

1.3. How Passivation Differs from Other Anti-Corrosion Treatments

This is an important distinction. For a clear comparison, see the table below:

Comparison of Surface Treatment Methods

Summary of Key Differences:

Passivation vs. Electroplating: Passivation “unlocks inner potential,” while electroplating “puts on an external coat.” The electroplated layer is an added layer of a different metal, providing sacrificial protection. The passivation film is a transformed version of the base metal itself.

Passivation vs. Anodizing: Both form oxide layers. However, anodizing is an electrochemical process creating a much thicker layer, mainly for aluminum, and it changes dimensions, appearance, and conductivity. Passivation is primarily chemical, creates an extremely thin film, and doesn’t alter these properties.

Passivation vs. Painting/Coating: Painting is a physical covering, creating a completely separate, usually thick, isolating layer. It changes appearance and texture and is non-conductive. Passivation is almost “invisible,” preserving the metal’s original look, feel, and conductivity.

2. Which Materials Can Be Passivated?

Passivation is not suitable for all metals. It primarily targets metals that can form stable oxide films and require enhanced corrosion resistance. Based on common materials for CNC machined parts, the applicability can be summarized as follows:

2.1. Stainless Steel

Stainless steel is the most common material for passivation. Its inherent corrosion resistance relies on its chromium content.

The passivation process of stainless steel uses chemicals (like nitric or citric acid) to remove surface free iron contaminants.

Simultaneously, it promotes chromium enrichment on the surface, which reacts with oxygen to form a thicker, more stable chromium oxide (Cr₂O₃) protective film.

The main applicable series are listed below.

Austenitic Stainless Steels: Such as 304, 304L, 316, 316L. These are the most frequently passivated types, widely used in medical, food, and chemical industries.

Martensitic Stainless Steels: Such as 420, 440C. These often require passivation to enhance corrosion resistance, but process parameters may need adjustment (e.g., adding inhibitors).

Precipitation-Hardening Stainless Steels: Such as 17-4PH. Passivation is performed after solution treatment and aging hardening to provide optimal corrosion resistance.

Note: Sulfur-containing “free-machining” stainless steels (like the 303 series) are not suitable for nitric acid passivation, as sulfide inclusions can be dissolved by the acid, leading to micro-pitting on the surface.

2.2. Aluminum and Aluminum Alloys

Aluminum is very reactive, but naturally forms an oxide film. Passivation can enhance this layer.

Aluminum and its alloys naturally have a thin aluminum oxide film (Al₂O₃) that provides some corrosion resistance. However, in acidic or chloride-rich environments, this film can become unstable. Passivation treatments (using chromate or chromium-free solutions) promote a chemical reaction where surface aluminum combines with oxygen to form a denser, more uniform, and corrosion-resistant aluminum oxide film, thereby enhancing surface protection.

Aluminum-Silicon Alloys: Such as 6061, 6063, commonly used in aerospace, structural components, and electronic enclosures.

Aluminum-Copper Alloys: Such as 2024, used in aircraft parts, but the passivation solution must be chosen carefully (low-corrosion formulas) to avoid damaging the copper phases.

Aluminum-Zinc-Magnesium Alloys: Such as 7075, which have high strength but are corrosion-sensitive, requiring strict control of passivation parameters.

Important Considerations:

Chromium-free passivation is environmentally friendly and suitable for most aluminum alloys.

Thorough cleaning to remove oils and oxide scale is essential before passivation; otherwise, the surface film will be uneven.

2.3. Titanium and Titanium Alloys

The naturally formed oxide film (TiO₂) on titanium already provides good corrosion resistance.

However, it can still be compromised in strong acid or chloride environments. Passivation treatment uses acid pickling (e.g., with dilute nitric or hydrofluoric acid solutions) to remove surface contaminants and promote the thickening and densification of the TiO₂ passive film, thereby enhancing corrosion resistance and durability.

Commercially Pure Titanium: Such as Ti Grade 1, Grade 2, used in chemical piping, medical instruments, etc.

Titanium Alloys: Such as Ti-6Al-4V (Grade 5), widely used in aerospace components, medical implants, and high-performance mechanical parts.

Important Considerations:

Temperature, time, and acid concentration during passivation must be strictly controlled to prevent over-etching of the surface.

Passivated titanium parts should be protected from contamination or mechanical scratches to maintain the integrity of the passive film.

2.4. Copper and Copper Alloys

Copper and its alloys readily react with air and moisture, forming oxides or basic copper carbonate, leading to tarnishing or patina. Passivation typically uses nitric acid or specialized copper passivation solutions.

These chemicals remove surface oxides and residual contaminants while forming a stable copper oxide film or metal complex layer, improving corrosion resistance and appearance.

Copper: Such as C1100, used in electronic devices and precision components.

Brass: Such as C36000, commonly used for mechanical parts and decorative items.

Bronze: Used for mechanical bearings, nuts, and wear-resistant parts.

Important Considerations:

The surface color of copper and its alloys may change slightly after passivation; the process should be chosen based on customer requirements.

For alloys containing tin or zinc, the passivation solution formula and time must be adjusted to prevent localized corrosion or an uneven film.

Chemical passivation for copper reduces oxidation and tarnishing, enhancing both corrosion resistance and aesthetic quality.

2.5. Other Special Alloys

Less common but possible: Nickel-based alloys, magnesium alloys (require specialized passivation solutions).

Passivation is most commonly used for stainless steel, followed by aluminum alloys, titanium alloys, and copper alloys. When selecting a material, consider the part’s operating environment, corrosion resistance requirements, and machining precision.

3. How to Choose a Passivation Solution?

Different materials truly require different types of passivation solutions during the process.

This is because each metal has unique chemical properties, leading to different reaction mechanisms with the passivating agent and resulting in protective oxide films with varying compositions and stability.

Passivation solutions work by triggering chemical reactions that form a dense oxide film on the metal surface. The specific activity of different metals in acidic, alkaline, or oxidizing environments dictates the required chemical medium.

For instance:

Stainless steel needs an oxidizing environment to promote the formation of a chromium-rich film.

Aluminum, conversely, requires a milder system to prevent excessive corrosion or etching.

Using the wrong passivation solution can have serious consequences. It may not only fail to create the protective layer but can also severely corrode or even ruin the workpiece.

Comparison of Passivation Solutions for Different Materials

4. Types of Passivation

Now that we understand which materials can be passivated, along with their mechanisms and suitable solutions, it becomes easier to grasp how passivation processes are classified.

4.1. Classification by Material

Passivation can be categorized based on the target material:

• Stainless Steel Passivation
• Aluminum & Aluminum Alloy Passivation
• Titanium & Titanium Alloy Passivation
• Copper & Copper Alloy Passivation

4.2. Classification by Core Chemistry & Mechanism

This classification primarily applies to the most common stainless steel passivation, as it directly relates to environmental, safety, and biocompatibility requirements.

• Nitric Acid Passivation

Uses a nitric acid (HNO₃) solution. Leverages the strong oxidizing power of nitric acid to dissolve free iron and rapidly form a stable chromium oxide (Cr₂O₃) protective film on the surface.

Best For: Highly efficient with a short process time. Ideal for most industrial-grade stainless steel parts where cost and efficiency are key drivers.

Limitations: Nitric acid is corrosive, poses operational hazards, and produces toxic nitrogen oxide fumes, resulting in higher environmental handling costs.

• Citric Acid Passivation

Uses a citric acid solution. This is an environmentally friendly and safe process that gently removes free iron ions from the surface through chelation.

Best For: The preferred choice for precision stainless steel components in medical, food & beverage, and pharmaceutical industries due to its non-toxic nature and high biocompatibility.

Key Advantage: Safer than nitric acid and complies with stringent environmental regulations.

• Nitric Acid with Dichromate Passivation

Adds dichromate salts to the nitric acid bath as an extra-strong oxidizing agent.

Best For: Specifically used for some harder-to-passivate stainless steels or those with lower chromium content (e.g., 400 series). The strong oxidizer helps form the passive film quickly and prevents “flash attack” (etching) of the base metal by the acid.

Limitations: Due to the use of highly toxic hexavalent chromium, this method is being progressively phased out or restricted under increasingly strict environmental regulations.

5. Benefits of Passivation

The benefits of passivation are listed below.

• Prevents Metal Oxidation and Corrosion

Even corrosion-resistant materials like stainless steel can develop localized rust if surface contaminants like iron particles or machining marks are present. Passivation removes free iron ions and creates a stable oxide layer, significantly reducing the risk of oxidation and corrosion.

• Enhances Part Surface Properties

Passivated surfaces are smoother and more uniform. This not only improves corrosion resistance but also facilitates subsequent processes like painting, plating, or assembly.

• Extends Part Service Life

Over the long term, passivation reduces the micro-cracks and wear initiated by corrosion, thereby increasing the part’s durability and overall reliability.

• Maintains Quality for High-Precision Parts

CNC-machined parts often have extremely tight dimensional tolerances and surface finish requirements. Since passivation does not alter the part’s dimensions, it is ideally suited for precision components, especially in critical applications like aerospace and medical devices, where part reliability is paramount.

6. Surface Preparation Requirements for CNC Parts Before Passivation

• Deburring and Edge Breaking

After CNC machining, parts may have burrs or sharp edges. Deburring and breaking edges prevent solution entrapment during passivation and ensure uniform treatment.

• Cleaning and Degreasing

Oils, cutting fluids, or dust on the surface can interfere with the passivation process. Thorough cleaning and degreasing are essential to achieve a perfectly clean surface.

• Impact of Surface Finish on Results

Smoother surfaces lead to a more uniform passive film. Optimal results are typically achieved with a surface roughness of Ra ≤ 1.6 μm.

7. The Passivation Process for Stainless Steel

Stainless steel is the most common and classic material for passivation treatment. Let’s use stainless steel passivation as an example to detail the process steps.

The core objective of stainless steel passivation is to remove the contaminated layer and rebuild a stable, dense Cr₂O₃ oxide film.

7.1. Stainless Steel Passivation Processes

Step 1: Alkaline Cleaning & Degreasing

This is a critical step in the entire process, yet one that is often overlooked.

Any oil, grease, coolant, marker ink, or fingerprints can create a barrier on the part surface, preventing the passivation solution from making uniform contact with the base metal. This leads to passivation failure or uneven results.

Purpose: To remove surface oils, grease, dust, and other organic contaminants.

Method: Immerse or spray the CNC-machined stainless steel parts with a hot alkaline cleaner (e.g., sodium hydroxide or sodium carbonate solution).

Typical Parameters: Temperature 60–80 °C, Time 10–20 minutes.

Step 2: Water Rinsing

Before entering the acid bath, parts must be thoroughly rinsed with clean, room-temperature water (ideally deionized or purified) to completely remove all residual alkaline cleaner.

Step 3: Acid Passivation / Activation

This is the core chemical treatment stage of the process. Parts are immersed in a specific acid solution – the passivation solution – which primarily achieves two things:

• Dissolves and removes embedded free iron particles and other metallic contaminants from the surface.
• Chemically promotes the reaction between chromium in the stainless steel surface and oxygen (from the solution or air) to form an extremely thin, dense, and stable chromium oxide protective film.

Common types of stainless steel passivation solutions include Nitric Acid-based, Citric Acid-based, and Nitric Acid with Dichromate. These were detailed in the previous section.

The following table provides a comparison. Specific choices depend on the stainless steel grade and requirements.

Comparison of Common Stainless Steel Passivation Solutions

Passivation Solutions for Common Stainless Steel Grades

Note:

1. Prefer citric acid (environmental trend):

For easily passivated alloys like the 300 series (especially 316), citric acid passivation is becoming the global preference due to its safety, environmental friendliness, and mandatory use in applications requiring high biocompatibility.

2. Challenges with hard-to-passivate alloys (400 series):

For martensitic and ferritic stainless steels (400 series), which contain higher carbon or lower chromium content, passivation is more difficult. If customers require high corrosion resistance, a more aggressive yet compliant modified nitric acid process must be used, accompanied by rigorous corrosion testing (e.g., salt spray tests) to validate effectiveness.

3. Special handling for 303 free-machining steel:

303 steel contains sulfide inclusions to improve machinability. These inclusions may dissolve during passivation, leaving microscopic pits that can reduce corrosion resistance. Therefore, 303 requires a specialized passivation process with low concentration and short duration, along with additional post-treatment measures.

Step 4: Thorough Final Rinsing and Drying

This step is critical to prevent secondary corrosion after passivation. It is essential to completely remove any residual acid solution adsorbed on the part surface.

Method: Use pure water, preferably deionized (DI) water, for multiple rinsing cycles.

Purpose of Deionized Water: Using deionized water prevents minerals and chloride ions present in tap water from leaving water spots or creating corrosion initiation sites on the part surface. This step is mandatory for high-demand components like medical devices and aerospace parts.

Drying: Dry the parts promptly and thoroughly using filtered, oil-free compressed air or by baking in an oven at a low temperature. Do not allow the parts to air dry naturally, as evaporating residual water can concentrate impurities on the surface, potentially damaging the newly formed passive film and leaving water stains.

Step 5: Verification and Testing (Quality Control)

After processing, standardized tests must be conducted to verify the effectiveness of the passivation.

Common methods include:

Water Immersion Test / Copper Sulfate Test: Immersing passivated parts in a specific concentration of copper sulfate solution to check for the presence of free iron, which would indicate incomplete passivation.

Potassium Ferricyanide Test: Applying a test solution to the part surface; if free iron is present, it will show blue spots.

Salt Spray Test: A longer-term accelerated corrosion test used to evaluate the long-term protective capability and corrosion resistance performance of the passive film.

7.2. Electropolishing + Passivation

For stainless steel, passivation is often combined with electropolishing to create a surface with superior cleanliness and maximum corrosion resistance.

Electropolishing as a Precursor: This electrochemical process preferentially dissolves microscopic peaks on the surface, significantly reducing surface roughness and creating a smooth, mirror-like finish. This smooth surface inherently reduces the areas for contaminant adhesion and corrosion initiation, and effectively eliminates micro-cracks, burrs, and strained layers left by CNC machining.

Chromium Enrichment: During electropolishing, iron is dissolved and removed more readily than chromium. This results in the formation of a thin, chromium-enriched layer on the part surface, which itself provides an excellent foundation for corrosion resistance.

Synergistic Effect: The freshly formed, active chromium-enriched layer from electropolishing is ideal for the subsequent passivation treatment. Passivation at this stage most effectively and rapidly grows a thicker, denser, and more chemically stable chromium oxide passive film on this prepared surface.

This powerful combination first uses electropolishing to create a “near-perfect” substrate and then uses passivation for “final strengthening and sealing.” Together, they produce an ideal surface with top-tier corrosion resistance, ease of cleaning, and excellent biocompatibility.

7.3. Passivation as a Pretreatment for Plating and Coating

When stainless steel requires subsequent electroplating (e.g., nickel or chromium plating) or organic coating (e.g., painting, powder coating), passivation is no longer the final step. Instead, it serves as a critical pretreatment process.

The alkaline cleaning and acid passivation steps thoroughly remove oils and free iron from the substrate, providing an impeccably clean interface.

Passivation brings the entire part surface to a uniform, stable chemical state, eliminating local variations in activity.

For Electroplating: A clean, uniform, passivated surface ensures that the electroplated layer can form a uniform, dense, atom-level bond (metallic bonding) with the substrate. This drastically improves the coating’s adhesion and uniformity, preventing blistering or peeling.

For Coating: The stable passive film prevents any potential underlying corrosion of the substrate beneath the coating (“undercutting” corrosion). If the substrate itself were to rust, the corrosion products could swell and lift the coating from within, leading to premature coating failure. Passivation provides an inert, stable “anchor” layer, enabling the organic coating to adhere durably and firmly.

The goal of passivation is not to provide its own corrosion resistance, but rather to “set the stage.” It creates a perfect, stable, and clean substrate to ensure the adhesion, uniformity, and long-term durability of the subsequent plating or coating layer. Without proper passivation pretreatment, even the best electroplated or painted coating may fail early due to poor adhesion.

8. Passivation Standards and Specifications

Adherence to established standards is crucial for ensuring the quality, consistency, and reliability of the passivation process. Here are some of the most critical specifications:

8.1. ASTM A967

ASTM A967 is Standard Specification for Chemical Passivation Treatments of Stainless Steel Parts

This is the most versatile and widely referenced commercial standard.

It offers significant flexibility, encompassing various chemical treatment methods (e.g., nitric acid, citric acid) and multiple test methods (e.g., water immersion, high humidity, salt spray).

It defines different passivation levels and allows manufacturers and customers to agree on specific test methods and acceptance criteria.

8.2. AMS 2700

AMS 2700 is for Passivation of Corrosion Resistant Steels.

This is the authoritative standard in the aerospace industry, known for its stringent requirements. It is published by SAE International.

AMS 2700 has explicit requirements for chemical purity, water quality (mandating deionized water), process validation, and supplier qualification.

It specifies detailed acid solution formulations and process parameters based on the stainless steel type and the required level of corrosion resistance.

Compliance with AMS 2700 typically represents the highest level of quality assurance.

8.3. ISO 16048

ISO 16048 is for Passivation of corrosion-resistant stainless-steel implants for surgery.

This standard is specifically designed for surgical implants, such as bone screws and plates.

It imposes extremely high requirements for biocompatibility and strictly limits surface residues to ensure the long-term safety of the implant within the human body.

8.3. ISO 10074

ISO 10074 is for Specification for passivation of austenitic stainless steel parts for hydraulic power systems.

This standard is specifically tailored for stainless steel components used in hydraulic systems.

Below is a chart to show how to select a passivation standard.

9. Best Practices for Passivation of CNC Parts

Considering the requirements of the passivation process during the design stage is crucial for ensuring the final part’s performance, avoiding production delays, and additional costs.

A well-designed part that is easy to passivate ensures uniform chemical action and facilitates thorough subsequent cleaning. Here are the core best practices to follow during the design phase.

The rationality of the design directly determines the success of the passivation outcome. Seemingly minor design details can cause significant issues during the passivation process.

9.1. Avoid Sharp Internal Corners and Dead Zones

Sharp internal corners (radius < 0.5 mm) create dead zones for solution flow. Passivation solution and rinse water cannot flow or exchange effectively here, leading to the accumulation of contaminants and chemical residues.

Best Practice:

Design sufficient radii (recommended radius ≥ 1 mm) on all internal corners. Radii guide the solution to flow smoothly, ensuring uniform treatment of the entire surface and facilitating complete rinsing.

You can check more information about what is fillet in this blog.

9.2. Properly Design Blind Holes, Deep Slots, and Deep Holes

Blind holes and deep slots are prone to trapping solution. If passivation solution or acidic rinse water remains inside, it can cause sustained, localized corrosion, damaging the part from within.

Best Practice:

• Prefer through-holes over blind holes whenever possible.
• If blind holes are necessary, design a drain/vent hole at the bottom to allow complete solution drainage.
• For deep holes, consider increasing the entrance diameter to allow easy access for rinsing tools.

9.3. Carefully Consider Fixturing and Clamping Locations

Clamping points during CNC machining and passivation can leave marks on the part surface. The microstructure of the metal in these areas might be altered, or the area could be contaminated by the fixture, preventing the passivation solution from acting effectively and creating potential corrosion initiation sites.

Best Practice:

• Communicate with the manufacturing engineer during the design phase to plan clamping strategies on non-critical surfaces.
• Clearly mark critical functional surfaces on the drawing and specify that clamping should be avoided in these areas.

9.4. Manage Sharp Edges and EDM Recast Layers

Sharp edges can show color differences or etching pits after passivation. EDM (Electrical Discharge Machining) surfaces often have a recast layer that is porous and can hold contaminants, compromising passivation.

Best Practice:

• Specify a slight break on all sharp edges.
• Control surface roughness (recommended Ra ≤ 0.8 µm) to facilitate the formation of a uniform passive film.
• If EDM is used, specify post-EDM machining or polishing to remove the recast layer before passivation.

9.5. Account for Contact with Dissimilar Metals

If the design includes embedded dissimilar metals, or if the part contacts dissimilar metal fixtures during passivation, it can lead to galvanic corrosion.

Best Practice:

• Ideally, remove dissimilar metal components before passivation.
• If removal is impossible, clearly inform the processing vendor; they may use specific fixtures or apply local masking.

9.6. Correctly Specify Passivation Requirements on Drawings

Clear communication of requirements is essential for achieving the desired result.

Best Practices:

• Clearly Identify Material: Always clearly and accurately specify the material grade. This is the most critical first step.
• Specify the Standard: In the “Technical Requirements” or “Surface Treatment” section of the drawing, clearly call out the standard to be followed. For example, PASSIVATE PER ASTM A967 or PASSIVATION: AMS 2700, TYPE 1.
• Specify the Method (if critical): For projects with specific requirements, you can specify the passivation method in the callout. This is especially important for projects with environmental or nitrate-free requirements. For example: PASSIVATE PER ASTM A967, CITRIC ACID METHOD
• Define Pre-Treatment State: If a specific surface finish is required before passivation, this should also be noted on the drawing. For example for a part requiring electropolishing: ELECTROPOLISH TO Ra < 0.4 µm PRIOR TO PASSIVATION.

10. Conclusion

Passivation is far more than a simple chemical dip; it is a critical bridge between machining and final application in precision manufacturing.

By understanding its scientific principles, mastering material characteristics, following design best practices, and strictly implementing standardized processes, manufacturers can fully unlock the inherent corrosion resistance of materials like stainless steel and aluminum alloys.

11. FAQ

Q1. My parts are made of stainless steel. Do they really need passivation? I thought stainless steel doesn’t rust.

While stainless steel is highly corrosion-resistant due to its chromium content, the machining process can compromise this inherent property.

During CNC machining, tiny particles of free iron from the cutting tools can be embedded into the part’s surface. These iron particles are highly susceptible to rust and can act as initiation sites for corrosion, even on an otherwise resistant part.

Passivation chemically removes this surface iron and enhances the natural chromium oxide layer, ensuring your stainless steel parts live up to their full “stainless” potential.

Q2: Will passivation change the size or tolerances of my precision CNC parts?

This is a critical concern for precision components, and the answer is reassuring: No. Passivation works at a microscopic (nanoscale) level.

The oxide layer it creates is so exceptionally thin that it does not measurably alter the part’s dimensions, geometry, or tightly held tolerances. This is one of the key advantages over processes like electroplating or anodizing, which add measurable thickness.

Q3: What’s the difference between Citric Acid Passivation and Nitric Acid Passivation, and which one is better?

The main differences are safety, environmental impact, and performance on certain alloys.

• Nitric Acid Passivation is a traditional, highly effective method. However, it involves handling a hazardous, corrosive acid and produces toxic fumes, requiring special safety and waste treatment measures.

• Citric Acid Passivation is a modern, eco-friendly alternative. It is non-toxic, safer for operators, and environmentally benign. For most stainless steels (like 304 and 316), it often provides superior performance by more effectively removing free iron through chelation.

For most new projects, especially in medical, food, and aerospace, Citric Acid Passivation is the recommended and often superior choice due to its safety and environmental benefits.

Q4. Which materials require passivation?

Mainly:

• Stainless steel (most common)

• Titanium and its alloys

• Aluminum (via conversion coating or anodizing)

• Copper (special anti-tarnish passivation)

Note: Carbon steel does not get passivated—it is usually plated or coated.

Q5: Will passivation remove discoloration, welding stains, or scale?

No. Passivation only works on clean, oxide-free surfaces.
For discoloration or weld oxides, pickling or mechanical polishing is needed first, then passivation.

Q6: How long does passivation protection last?

It is not a coating that wears off easily.

As long as the surface is not damaged or exposed to harsh chemicals, protection is long-term. However, if exposed to chlorine or strong acids, corrosion can still occur.

Q7: How much does passivation cost?

Generally low compared to coatings or plating.
Cost depends on:

• Part size

• Quantity

• Stainless steel grade

• Required testing or certification