Passivation method by stainless steel grade: 303, 304, 316, 400
Summary
Austenitic 300-series grades suit nitric or citric passivation; 303 and martensitic 400-series grades need a more oxidizing nitric bath, usually dichromate-inhibited, to avoid flash attack. Citric baths also serve them when the parts get alkaline steps before and after the acid (Carpenter Technology, 2003). A treatment named on the drawing or purchase order overrides this grade guidance.
ASTM A967/A967M-25, the active edition, last updated 24 January 2025, gives grade guidance in its nonmandatory Appendix X1 and Appendix X2 (ASTM, 2025).
SAE AMS2700G, revised 27 May 2025, is the current revision of the aerospace passivation specification (SAE International, 2025).
In the ASTM A967 edition summarized by Advanced Plating Technologies in 2019, Nitric 1 is 20 to 25 vol% nitric acid with 2.5 wt% sodium dichromate at 49 to 54 °C (120 to 130 °F) for at least 20 minutes (Advanced Plating Technologies, 2019).
US OSHA sets a permissible exposure limit for airborne chromium (VI) of 5 µg/m3 as an 8-hour time-weighted average (OSHA, 29 CFR 1910.1026).
This grade table covers chemical passivation of clean, machined or fabricated stainless steel parts in nitric or citric baths. It does not cover electropolishing, parts with heat tint or scale that need descaling first, nitrided or carburized stainless steel, or contracts that name their own method. The grade rows come from public summaries and maker documents, not the paid standard text.
Which ASTM A967 or AMS2700 method matches each stainless steel family?
The ASTM A967 method that matches each stainless steel family is Nitric 2 or Nitric 3 for standard austenitic grades, Nitric 1 for free-machining grades, and Nitric 1 or Nitric 4 for most martensitic and precipitation-hardening grades. That mapping comes from a 2019 summary of the grade table in ASTM A967 by the finisher Advanced Plating Technologies (APT, 2019). ASTM A967/A967M-25 states that Appendix X1 and Appendix X2 give "some general guidelines regarding the selection of passivation treatments appropriate to particular grades of stainless steel" (ASTM, 2025). NASA Johnson Space Center specification PRC-5002G applies AMS 2700 and lists allowed bath types per alloy (NASA, 2021). The table below decides which passivation bath to order for a stainless grade and what to change if parts etch.
Stainless family and grades | ASTM A967 nitric treatment | Other published options | If flash attack appears |
|---|---|---|---|
Austenitic: 304, 304L, 316, 316L, 321 | Nitric 2 or Nitric 3 (APT, 2019) | Type VI low-temperature nitric; Type VII, not recommended for new design (NASA, 2021); 10 wt% citric at 66 °C (150 °F), plain rinse (Carpenter Technology, 2003) | Use water below about 50 ppm chloride where possible, degrease first, replace the bath on schedule (Carpenter Technology, 2003) |
Free-machining austenitic: 303, 303Se | Nitric 1 only (APT, 2019) | Type II dichromate; Type VIII high-concentration nitric, not recommended for new design (NASA, 2021); 20 vol% nitric with 22 g/L sodium dichromate, or 10 wt% citric at 66 °C (150 °F), both with alkaline steps before and after (Carpenter Technology, 2003) | Move from plain nitric to dichromate or 50 vol% nitric (Carpenter Technology, 2003); add the alkaline neutralizing soak; rack parts apart (Carpenter Technology, 2003) |
Martensitic: 410, 420, 440C | Nitric 1; Nitric 4 also for 410 and 440C (APT, 2019) | Type II; Type VIII, not recommended for new design (NASA, 2021); citric at 49 to 54 °C (120 to 130 °F) with alkaline steps for 410 and 420 (Carpenter Technology, 2003) | Check hardening and temper first; improperly heat-treated parts can be attacked in the bath (Carpenter Technology, 2003) |
Free-machining 400 series: 416, 416Se, 430F | Nitric 1 (APT, 2019) | 416: Type II; Type VIII, not recommended for new design; 430F: Type II only (NASA, 2021); for 416 dichromate nitric is "preferred vs. citric" (Carpenter Technology, 2003) | As for 303: dichromate or 50 vol% nitric plus alkaline steps (Carpenter Technology, 2003) |
Ferritic: 409, 430 | 430: Nitric 3; 409: Nitric 1 or Nitric 4 (APT, 2019) | 430: Type VII only, not recommended for new design (NASA, 2021); 409Cb: dichromate nitric, "Use care: low Cr" (Carpenter Technology, 2003) | Check bath temperature, immersion time and contamination; citric was more prone to flash attack in lab tests (Carpenter Technology, 2003) |
Precipitation-hardening: 17-4PH, 15-5PH | Nitric 1 or Nitric 4 (APT, 2019) | Type II or Type VI; Type VIII, not recommended for new design; for asterisked alloys such as 17-4PH and 15-5PH, Type VI bath chloride below 1000 ppm, tested monthly or before processing (NASA, 2021) | For Type VI, test bath chloride (NASA, 2021); dichromate or 50 vol% nitric lowers flash attack risk (Carpenter Technology, 2003) |
Table sources: ASTM A967 grade table as summarized by APT (2019); NASA PRC-5002G Table 1 (2021); Carpenter Technology Figure 4 (2003). Limits: the APT summary predates ASTM A967/A967M-25, so confirm each row against Appendix X1 of the 2025 edition before you write it into a specification. NASA marks Type VII and Type VIII as not recommended for new design, so for 430 its table offers no recommended type (NASA, 2021). The key row is 303: none of the sources above lists plain low-temperature nitric for it.
Can you passivate 303 stainless steel without flash attack?
You can passivate 303 stainless steel without flash attack if the bath is more oxidizing than plain nitric and trapped acid is neutralized. Carpenter Technology explains that sulfides in sulfur-bearing free-machining grades are partially or totally removed in a typical nitric acid bath, leaving microscopic discontinuities where acid stays after rinsing (Carpenter Technology, 2003). Adding sodium dichromate or raising nitric acid to 50 vol% "both reduce the chance of undesirable flash attack" (Carpenter Technology, 2003).
Carpenter's alkaline-acid-alkaline procedure for 303 and other free-machining grades has three baths. First, a 30-minute soak in 5 wt% sodium hydroxide at 71 to 82 °C (160 to 180 °F), then a water rinse. Second, 30 minutes in 20 vol% nitric acid with 22 g/L sodium dichromate at 49 to 60 °C (120 to 140 °F), then a rinse. Third, another 30 minutes in the sodium hydroxide, a rinse and drying (Carpenter Technology, 2003). Each bath is hazardous: work from the SDS of the nitric acid, sodium hydroxide and sodium dichromate, and in the US treat the dichromate bath under OSHA 29 CFR 1910.1026 for chromium (VI) (OSHA, 29 CFR 1910.1026). In the EU, Commission Regulation (EU) No 348/2013 added sodium dichromate to REACH Annex XIV, the authorisation list, with a sunset date of 21 September 2017 (European Commission, 2013). Dispose of spent dichromate, nitric and caustic baths under the SDS disposal section and local hazardous waste rules; Carpenter notes the disposal problems of mineral acid and sodium dichromate solutions (Carpenter Technology, 2003).
Rack 303 parts individually, because free-machining grades need free flow of passivating and rinse solutions to carry away sulfide corrosion products (Carpenter Technology, 2003). More on what passivation does is in what stainless steel passivation is and does.
Do 304 and 316 stainless need a different passivation bath from 400-series grades?
Yes, 304 and 316 stainless steel need a different passivation bath from most 400-series grades, because the lower chromium and nickel of martensitic and ferritic grades makes them easier to etch. APT explains that sodium dichromate raises the oxidizing ability of nitric acid, making it better for precipitation-hardened, martensitic and ferritic grades, which have less nickel and chromium (APT, 2019). Plain 20 vol% nitric at 49 to 60 °C (120 to 140 °F) is Carpenter's practice for 300-series chromium-nickel grades (Carpenter Technology, 2003).
Worked example (illustrative order): a drawing calls for 316 housings and 416 shafts to be passivated per ASTM A967. Split the lot into two loads, because Carpenter advises against passivating more than one stainless steel at a time to prevent mixups and galvanic reactions (Carpenter Technology, 2003). Run the 316 housings in Nitric 2 or Nitric 3 (APT, 2019). Run the 416 shafts in Nitric 1 (APT, 2019) or in Carpenter's alkaline-acid-alkaline sequence (Carpenter Technology, 2003); the two dichromate baths differ in strength, temperature and time, so the order names one. Both baths are hazardous: keep each chemical's SDS accessible in the work area, as US OSHA 29 CFR 1910.1200 requires (OSHA, 29 CFR 1910.1200), and run the 416 dichromate bath under 29 CFR 1910.1026 and, in the EU, REACH Annex XIV, as in the 303 section (OSHA, 29 CFR 1910.1026). Test the loads differently as well: Carpenter warns that the copper sulfate test gives likely false positives on martensitic stainless steel and recommends a 24-hour humidity cabinet at 35 °C (95 °F) for free-machining grades (Carpenter Technology, 2003). The 316 load can take the humidity test or, except for food-processing parts, the copper sulfate test (Carpenter Technology, 2003).
Why do sources disagree on nitric or citric passivation for free-machining grades?
Sources disagree on nitric or citric passivation for free-machining grades because they balance flash attack risk against hexavalent chromium differently. The table below sets the published 303 options side by side, so a buyer can see which source a drawing or supplier is following.
Source | Treatment listed for 303 | Reason or condition given |
|---|---|---|
ASTM A967 grade table, as summarized by APT | Nitric 1, dichromate (APT, 2019) | Nitric options only in the summary |
NASA PRC-5002G | Type II dichromate; Type VIII high-concentration nitric, not recommended for new design (NASA, 2021) | Type VI is preferred where Table 1 permits, as it uses no hexavalent chromium, but 303 is not listed for Type VI |
Carpenter Technology | Dichromate nitric, or 10 wt% citric at 66 °C (150 °F), both with alkaline steps (Carpenter Technology, 2003) | Lab tests found citric procedures more prone to flash attack than nitric |
Gleco Plating | Citric for most 300 and 400 series grades; names 303 and 416 as needing a confirmed method (Gleco Plating, 2026) | For free-machining grades the finisher confirms the method so the bath does not attack the alloy |
The key row is Carpenter's: citric is allowed for 303, but only inside the alkaline-acid-alkaline sequence. The disagreement follows from chemistry and from policy. Sodium dichromate and higher nitric acid strength raise oxidizing power and lower flash attack risk on sulfur-bearing grades (Carpenter Technology, 2003), while NASA prefers its Type VI bath because it uses no hexavalent chromium (NASA, 2021). A purchase order for 303 parts should therefore name the treatment, any alkaline steps and the acceptance test, not only the standard. We found no standard or maker document that limits 303 to citric only. Gleco states that citric is increasingly preferred for safety and environmental reasons and leaves no sodium dichromate or hexavalent chromium in the waste stream (Gleco Plating, 2026).
Where the stainless grade passivation table does not apply
The stainless grade passivation table does not apply to surfaces that passivation cannot fix or must not touch. NASA PRC-5002G states that passivation "is not generally effective in removing oxide scale or heat tint" (NASA, 2021); those need descaling or pickling first under ASTM A380/A380M-25, which covers cleaning, descaling and passivating of stainless steel parts (ASTM, 2025). NASA PRC-5002G also says not to passivate nitrided or carbonitrided stainless steel, because severe corrosion or pitting of the hardened surfaces could occur, or carburized stainless steel, whose surface chromium carbides do not benefit from passivation (NASA, 2021).
Some uses fall outside ASTM A967 criteria. Carpenter notes that special procedures were needed for stainless steel beverage fittings and that ASTM A967 criteria may not apply to free-machining grades there (Carpenter Technology, 2003). ASTM A967/A967M-25 itself "makes no recommendations regarding the suitability of any grade, treatment, or acceptance criteria for any particular application" (ASTM, 2025). Aerospace parts follow the AMS2700 revision and type on the drawing, and NASA allows unlisted alloys only with approval from the cognizant materials engineer (NASA, 2021). When a grade, a coating history or a failed lot does not fit the table, ask your finisher and a corrosion specialist; Lubinpla can work such a case from your drawings, certificates and site history and return one cited document for your review.
Frequently asked questions
Should 440C be passivated before or after hardening?
After. Carpenter Technology states that high-carbon, high-chromium martensitic grades must be hardened to become corrosion resistant, and that passivation is frequently performed after a subsequent temper at a temperature that keeps the corrosion resistance (Carpenter Technology, 2003).
What does an old QQ-P-35 passivation callout map to?
QQ-P-35 has been superseded, and its modern equivalents are ASTM A967 and AMS 2700 (Gleco Plating, 2026). NASA PRC-5002G still uses the Roman numeral types from AMS-QQ-P-35, such as Type II and Type VI (NASA, 2021). Send the original drawing to the finisher rather than translating the type yourself.
Does the bath water matter for passivating 303 and 416?
Yes. Chloride contamination is the usual trigger for flash attack (Gleco Plating, 2026). Carpenter recommends water with less than about 50 ppm chlorides where possible (Carpenter Technology, 2003), and ASTM A967 caps final rinse water at 200 ppm total solids (Gleco Plating, 2026).