A customer specified ASTM A967 Nitric 1 for a batch of carbon steel parts. After processing, an operator handled the parts without gloves. Clear fingerprint-shaped rust marks later appeared on the surface. In the same project, 303 stainless steel parts received the same process callout and did not develop visible rust. This carbon steel passivation failure was not primarily a handling problem. ASTM A967 was written for stainless steel, so the finish specification did not match the base material.
ASTM A967 Was Written for Stainless Steel
The full title of ASTM A967/A967M is Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts. Its scope is unambiguous: it covers stainless steel.
ASTM A967 passivation is not paint, plating, or another deposited coating. The chemical treatment removes free iron and other surface contamination left by machining and fabrication. It then helps a clean stainless steel surface form or restore its chromium-rich passive film.
Carbon steel does not contain enough chromium to form the same protective film. Acid cleaning may remove rust, scale, or contamination, but it does not give carbon steel the corrosion behavior of stainless steel. Calling the operation a passivation process does not change that metallurgical limitation.
If ASTM A967 appears on a carbon steel drawing, the supplier should stop and request clarification before production. The parts may look clean immediately after treatment, but the specified process will not provide the corrosion protection the buyer may expect.
Fingerprints Exposed the Missing Protection
Human perspiration leaves moisture, chlorides, and other salts on a metal surface. These deposits can absorb moisture from the air and create a small local electrolyte. On bare or inadequately protected carbon steel, this fingerprint corrosion develops along the areas touched by the operator.
Clean gloves, dry handling, and prompt packaging would have reduced the contamination. They would not, however, have corrected the finish specification. In this project, bare-hand contact triggered the visible staining; insufficient protection of the carbon steel made it possible.
This distinction matters when investigating a complaint. Blaming only the operator may prevent another fingerprint, but the next source of moisture—from humid storage, condensation, or shipping—can expose the same underlying problem.
Why the 303 Stainless Steel Parts Behaved Differently
The 303 stainless steel parts contained enough chromium to form a thin, stable passive film. ASTM A967 Nitric 1 therefore had a valid material basis on those parts: it removed surface iron contamination and supported the alloy’s natural corrosion resistance.
That explains why the 303 parts in this project did not show the same fingerprint-shaped rust under similar handling conditions. The difference came from the base metals, not from the word “passivation” on the process documents.
Type 303 is a free-machining stainless steel. Its added sulfur improves machinability but also gives it lower corrosion resistance than 304 stainless steel. It is corrosion resistant, not corrosion proof. Finished 303 parts should still be handled with clean gloves and packed in a clean, dry environment. A useful technical overview of this material difference is available from Advanced Plating Technologies.
The Customer Changed the Carbon Steel Parts to Manganese Phosphate
The customer later had the carbon steel parts reworked in Israel using a phosphate coating specified as MIL-DTL-16232 Type M, Class 3. After treatment, the parts had a black appearance.
Under this specification, Type M identifies a manganese phosphate base. The process forms a crystalline conversion coating on ferrous metal. A manganese phosphate coating is commonly selected to improve break-in behavior, reduce friction, support wear performance, and retain oil or another post-treatment.
The black appearance was not unusual. MIL-DTL-16232 allows phosphate coatings to range from gray to black, although color alone does not establish corrosion performance.
Class 3 is the more important detail: it means no supplementary treatment. In the current MIL-DTL-16232H with Amendment 1, the basic salt-spray requirement for Type M, Class 3 without a supplementary coating is only 1.5 hours. Type M, Class 2 includes a specified preservative oil and requires at least 48 hours.
The phosphate process was appropriate for the carbon steel substrate, but Class 3 should not be treated as a long-term corrosion system. If parts must survive ocean freight, humid storage, repeated handling, or a defined salt-spray exposure, the drawing should specify the required supplementary protection and acceptance test.
What Buyers Should Specify Instead of Carbon Steel Passivation
Manganese phosphate is one option for carbon steel, not the only one. The right finish depends on what the part must do after manufacturing.
| Primary requirement | Finish to evaluate |
|---|---|
| Break-in performance, oil retention, or a dark gray-to-black appearance | Manganese phosphate with the appropriate supplementary treatment |
| A base for paint or powder coating | Iron or zinc phosphate followed by the specified coating system |
| General industrial corrosion protection with a metallic appearance | Zinc plating with the specified conversion coating and/or sealer |
| Higher wear and corrosion resistance with controlled buildup | Electroless nickel, subject to dimensional and functional requirements |
| Temporary protection during in-plant handling or storage | Rust-preventive oil or VCI packaging with humidity control |
The finish should be selected after considering corrosion exposure, appearance, wear, dimensional tolerance, electrical requirements, and shipping conditions. A treatment name by itself cannot define all of those outcomes.
Information to Put on the Drawing or RFQ
Avoid callouts such as “passivate” or “black finish” without additional requirements. Give the supplier enough information to select and verify the process:
- Base material and exact grade
- Governing specification and revision
- Finish type and class
- Required color or acceptable color range
- Whether an oil film or sealer is permitted
- Salt-spray, humidity, or application-specific corrosion criteria
- Storage and shipping conditions
- Masked surfaces, critical fits, and dimensional limits after coating
- Clean-handling and packaging requirements
These details allow the finishing supplier to quote the correct process and give both parties an objective basis for acceptance.
The Purchasing Lesson From This Project
The lesson from this carbon steel passivation case is straightforward: ASTM A967 belongs on stainless steel parts, not as a protective finish for bare carbon steel. Gloves might have prevented the visible fingerprint pattern, but they would not have corrected the specification.
For carbon steel, choose the complete finishing system around the service environment and the part’s function. If MIL-DTL-16232 Type M, Class 3 is specified, recognize that it excludes a supplementary treatment. A black surface is an appearance result—not proof of long-term rust protection.
Before releasing a purchase order, send the material grade, finish standard, class, corrosion target, and shipping conditions with the drawing. A short finish review before production costs far less than stripping, reprocessing, and shipping parts for local rework.