What each specification asks for, which chemistry suits which alloy, and how to write the callout so it reads only one way.
Passivation deposits nothing. It is a subtractive chemical clean that removes free iron and other exogenous contamination so the chromium oxide film can reform across the whole face of the part.
Stainless passivates itself in air. The problem is what machining, grinding and handling leave behind: iron from tooling, embedded abrasive, smeared metal that is locally iron-rich. An acid bath dissolves that iron preferentially and leaves the chromium, so the surface is chromium-enriched and the oxide reforms uniformly.
Two consequences matter on a drawing. Passivation cannot make an alloy more corrosion resistant than the alloy is, and it has no thickness — a thickness, color or class on the print means plating or a conversion coating. Our design guide covers where those belong.
Both remove free iron and both are recognized by the same specifications. They fail differently, and that should drive the choice.
| Factor | Nitric | Citric |
|---|---|---|
| How it works | Oxidising acid. Dissolves iron and drives the oxide film. | Chelating acid. Complexes iron and holds it in solution. |
| Free-machining and high-sulfur grades (303, 416, 430F) | Workable, but usually requires a sodium dichromate inhibited bath to avoid attacking sulphide inclusions. | Generally gentler on these grades and widely used for them. |
| 400 series martensitic and low-chromium ferritic | Requires an inhibited or higher-concentration bath. Selection is grade-specific. | Used successfully, but the bath must be controlled; verify by test, not by assumption. |
| 17-4 PH and other precipitation-hardening grades | Typically run in a higher-concentration bath. Condition of the part matters. | Common choice. Confirm the heat-treat condition before selecting either. |
| Flash attack risk | Real on free-machining and high-carbon grades if the bath is uninhibited or contaminated. | Lower, not zero. Contamination, over-concentration and poor rinsing still cause it. |
| Hexavalent chromium | Several AMS 2700 Method 1 types contain sodium dichromate, which is a hexavalent chromium source. | None. |
| Waste and environment | NOx fume control, nitrate-bearing rinse water, and hex-chrome waste streams on dichromate types. | Biodegradable acid, no NOx, simpler treatment. Baths need biological control. |
| Cycle time | Typically 20 to 30 minutes at temperature. | As short as a few minutes in a hot bath, longer as bath temperature drops. |
| Cost drivers | Chemistry is cheap; fume handling and waste treatment are not. | Chemistry costs more per gallon; disposal and permitting cost less. |
Flash attack is a rapid, uncontrolled etch leaving a dark or frosted surface with reduced corrosion resistance. It comes from the wrong bath for the alloy, contamination, excessive temperature, or parts that were not clean going in — a process control failure, not a property of one acid.
Process detail sits on the citric acid passivation and nitric acid passivation pages, under our stainless steel passivation overview.
Method 1 is nitric acid. Method 2 is citric acid. That is the whole of it, and mixing the two up is the most common misreading we see on incoming prints.
Method 1 carries numbered types: combinations of nitric concentration, sodium dichromate content (present in some types, absent in others), bath temperature and minimum time, plus one electrochemical variant. The alloy drives the choice — chromium and carbon content, whether the grade is free-machining, and whether it is austenitic, martensitic, ferritic or precipitation-hardening. Lower-chromium, higher-carbon and free-machining grades need an inhibited or more aggressive bath to clean without etching. Plain austenitic grades do not.
Method 2 has no type numbers — one citric method with a concentration range, temperature and time bands, and permitted wetting agents and inhibitors. “AMS 2700 Method 2 Type 3” is not a valid callout.
Method 1 type numbering carried forward from the canceled federal specification QQ-P-35, so a QQ-P-35 Type number and the same-numbered AMS 2700 Method 1 type generally describe the same bath. Treat that as a translation aid, not an authority — read the parameters in the revision you are held to.
A program that prohibits hexavalent chromium cannot use a dichromate-inhibited type, and the print should say so.
ASTM A967 covers the same ground for commercial and medical work, using treatment designations rather than methods and types: Nitric 1 through Nitric 5, Citric 1 through Citric 5. The higher-numbered designations are the escape hatch — alternative formulations, accelerants or inhibitors, acceptable if the part passes the test.
| A967 designation | Character of the bath | Closest AMS 2700 Method 1 type |
|---|---|---|
| Nitric 1 | Moderate nitric, sodium dichromate inhibited, warm | Type 2 |
| Nitric 2 | Wide-range nitric, no dichromate, ambient, long dwell | Type 6 |
| Nitric 3 | Moderate nitric, no dichromate, warm | Type 7 |
| Nitric 4 | High-concentration nitric, no dichromate, warm | Type 8 |
| Nitric 5 | Other nitric formulations that meet the test requirements | No direct equivalent |
| Citric 1 / 2 / 3 | Same citric concentration range at descending temperature bands with correspondingly longer dwell | Method 2 (no types) |
| Citric 4 / 5 | Other citric formulations, including pH-controlled baths | Method 2 (no types) |
A967 is written around outcome: unless the purchaser specifies a treatment, the processor selects one from those listed and the part passes a named test. AMS 2700 is written around process: a defined bath type, run inside defined limits, under a controlled quality system. Aerospace flowdowns lean on AMS 2700 for that reason — see certifications and approvals and aerospace metal finishing. A print citing both is not in conflict; say which governs if they disagree.
ASTM A380 sits upstream of both — the practice for cleaning, descaling and passivation, covering degreasing, heat-tint removal, pickling, rinse water quality and free-iron detection. Passivation cannot succeed on a part arriving with scale or oil. A380 is how to prepare; A967 or AMS 2700 is what to achieve.
Passivation leaves no visible evidence of itself, so every specification leans on a test. A967 names its practices by letter.
| Test | What it detects | Where it fits |
|---|---|---|
| Water immersion (A967 Practice A) | Rust or staining after alternating immersion and drying cycles | General-purpose, non-destructive, slow |
| High humidity (A967 Practice B) | Staining in a controlled humidity cabinet | Common aerospace acceptance test; also referenced by AMS 2700 |
| Salt spray (A967 Practice C, per ASTM B117) | Corrosion under neutral salt fog for a stated short duration | Screens gross contamination; not a coating-life test |
| Copper sulfate (A967 Practice D) | Free iron, shown as copper plating out on the surface | Fast and cheap. Not for martensitic 400 series or ferritic grades below 16% chromium — those give false failures |
| Potassium ferricyanide–nitric acid (A967 Practice E) | Free iron, shown as a blue reaction. Very sensitive | Best on equipment, welds and vessel interiors. Reagent residue must be removed; often disallowed on medical and food-contact surfaces |
| Free iron (A967 Practice F) | Free iron by the referenced detection method | Used where a direct contamination check is preferred over a corrosion test |
Match the test to the alloy and end use, and name it on the print. Copper sulfate on a 410 or 416 part will fail a correctly passivated lot. Medical work needs the test named — see medical device passivation and plating.
| What you are looking at | What it actually is | Correct process |
|---|---|---|
| Weld heat tint, forging scale, heat-treat oxide | A chromium-depleted oxide layer, not free iron | Pickling or mechanical removal first, then passivate |
| Roughness, cleanability or a bright reflective requirement | A topography requirement, not a chemistry one | Electropolishing. Passivation changes chemistry, not surface profile |
| Embedded abrasive or blast media | Mechanical contamination locked into the surface | Handle it at the blasting and surface preparation stage |
| Corrosion or wear life the base metal cannot reach | An alloy limit, not a cleanliness problem | Electroless nickel for barrier protection, or dry film lubricant against galling |
A complete callout names the specification and revision, the method or designation, the type where one exists, and the test.
PASSIVATE PER ASTM A967, CITRIC 2. TEST PER PRACTICE B (HIGH HUMIDITY).
PASSIVATE PER AMS 2700, METHOD 2 (CITRIC). NO HEXAVALENT CHROMIUM. TEST PER HIGH HUMIDITY.
PASSIVATE PER AMS 2700, METHOD 1, TYPE 2. DO NOT USE COPPER SULPHATE TEST. TEST PER WATER IMMERSION.
MATERIAL 17-4 PH, CONDITION H1025. PASSIVATE PER ASTM A967, CITRIC 1. TEST PER PRACTICE A.
Three habits prevent almost every passivation escape: state the alloy and its condition, name the test, and say whether hexavalent chromium is permitted.
| Alloy family / application | Default method | Why |
|---|---|---|
| 300 series austenitic (304, 316), general | Either. Citric where waste and cycle time matter | Both work reliably. No inhibitor needed on plain nitric |
| Free-machining austenitic (303) | Citric preferred; inhibited nitric if nitric is mandated | Sulphide inclusions are attacked by uninhibited nitric |
| Free-machining martensitic (416, 430F) | Citric, or nitric with dichromate inhibitor | Same inclusion problem, plus lower chromium margin |
| Martensitic 400 series (410, 420, 440) | Inhibited nitric or controlled citric; test carefully | Low chromium and high carbon. Avoid copper sulfate testing |
| Ferritic 400 series (430, 409) | Method by grade; confirm chromium content first | Test method choice hinges on the 16% chromium line |
| Precipitation hardening (17-4 PH, 15-5 PH) | Citric, or a higher-concentration nitric bath | Heat-treat condition changes surface behavior. State the condition |
| Medical, implant-adjacent, food contact | Citric, with the test named on the print | No hex chrome, no nitrate residue, cleaner validation story |
| Aerospace with an AMS 2700 flowdown | Whatever the flowdown says. Ask before substituting | Method and type are usually customer-fixed, not shop-selectable |
| Welded assemblies and fabrications | Descale or pickle first, then passivate | Heat tint is not free iron. ASTM A380 governs the prep |
Gleco has been finishing metal in Texas since 1979. Passivation runs in-house to AMS 2700 and ASTM A967 alongside plating, anodizing and coating, so a mixed print does not become a multi-vendor routing problem.
Citric passivation runs at our Rowlett plant in the Dallas–Fort Worth area. Nitric passivation is also processed in-house. Where a print calls out something we would have to interpret — a bare AMS 2700, an A967 designation that conflicts with the alloy, or a test that will fail your grade regardless — we raise it at quote, not after the lot is done.
On the RFQ, send three things: grade and heat-treat condition, the acceptance test if you have one, and any hexavalent chromium restriction. If no test is named we propose one and confirm it before production.
Related: passivation in Dallas, TX, the Rowlett metal finishing plant, under one roof routing, and plating quality.
QQ-P-35 was canceled in 1998, replaced by ASTM A967 and AMS-QQ-P-35; AMS-QQ-P-35 was later withdrawn in favor of AMS 2700. We still process to those prints, because a QQ-P-35 type number generally corresponds to the same-numbered AMS 2700 Method 1 type. Replace the callout at the next drawing revision.
Not measurably. It removes free iron and a very thin contaminated layer, well below what a normal tolerance band cares about. Significant material loss means flash attack or over-pickling. Pickling does remove measurable metal.
Usually flash attack: the bath etched instead of cleaning. Causes are an uninhibited nitric bath on a free-machining or high-sulfur grade, contamination, excessive temperature, or parts that were not clean going in. Poor rinse water leaves a film that wipes off; flash attack does not, and those parts should be rejected.
Yes. Machining cuts through the passive layer, exposes fresh metal and embeds iron from tooling and coolant. Mill-supplied passivation describes the bar stock, not your part. It belongs after the last operation that touches the surface.
Selection falls to the processor when the drawing is silent. Low risk on plain austenitic grades; not on free-machining, martensitic or precipitation-hardening grades, so we ask before quoting. Adding method and type removes the round trip.
You can and you should. If the print names a test, we run it; if not, we select one suited to the alloy — no copper sulfate on martensitic 400 series or ferritic grades below 16% chromium, because those give false failures on good parts. Where a test is prohibited for your end use, such as potassium ferricyanide on some medical surfaces, say so on the drawing.
Alloy, condition, method and test. If any of the four is missing or in conflict, you hear about it at quote, not at first article.
Precision metal finishing since 1979. Plating, anodizing, coatings and passivation from three Texas facilities.
Our paint, powder & Cerakote division: Gleco Paint & Powder Coating
Gleco Rowlett
2220 Grisham Drive
Rowlett, TX 75088
972-475-4300
Gleco McAllen
3800 W. Ursula Ave
McAllen, TX 78503
956-800-4069
