Guide
What does a QQ-P-416 cadmium plating callout mean, and what can replace it?
Cadmium plating is an electrodeposited cadmium coating on steel and other metals, usually specified on US drawings as QQ-P-416 or its successor SAE AMS-QQ-P-416, that protects the base metal sacrificially, stays lubricious in threads and sits close to aluminum in galvanic potential. A callout such as QQ-P-416, Type II, Class 2 names three things: the cadmium layer, the supplementary treatment over it (the Type) and the minimum thickness (the Class). The usual replacements are zinc-nickel plating to SAE AMS2417, ion vapor deposited (IVD) aluminum to MIL-DTL-83488 and tin-zinc plating to SAE AMS2434, and none of them is a drop-in swap without a review of torque, fit and approvals.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design engineers and buyers who have inherited a drawing with a cadmium note and need to either quote it as written or revise it. It decodes the callout, explains why cadmium was chosen in the first place, compares the three common replacements on the properties that made cadmium useful, and gives a step-by-step path for revising the drawing. It is a specification guide: plating itself is done by an outside finisher, and only the public scopes and summaries of the standards are quoted here.
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What do the Type and Class in a QQ-P-416 callout mean?
The Type tells the plater what goes on top of the cadmium: Type I is as-plated, Type II adds a chromate conversion coating and Type III adds a phosphate coating. The Class sets the minimum plated thickness, with Class 1 the thickest and Class 3 the thinnest.
Read a note like CADMIUM PLATE PER QQ-P-416, TYPE II, CLASS 2 as three separate instructions. The base process is electrodeposited cadmium. Type II means the plated part then goes through a chromate dip, which gives the familiar gold or iridescent yellow color and slows the white corrosion product that bare cadmium forms; Type I parts stay silvery gray, and Type III parts get a phosphate film that is meant as a base for paint. The Class is a minimum thickness on significant surfaces, and the table below lists the three values as QQ-P-416F defined them and AMS-QQ-P-416 carries them.
The document number itself also carries information. Federal specification QQ-P-416F was canceled by a 2002 notice that directs new buys to SAE AMS-QQ-P-416 (QQ-P-416F Notice 1), and SAE has revised its version several times since; the current revision is AMS-QQ-P-416G (SAE, 2022). A drawing that still cites plain QQ-P-416 is old, and a finisher will normally quote it to the AMS revision unless the purchase order says otherwise. Requirements such as the bake and test lots can differ between revisions, so copy the full designation from the drawing into the RFQ instead of paraphrasing it.
| Designation | What it means | Typical reason it was chosen |
|---|---|---|
| Type I | As plated, no supplementary treatment | Solderability, electrical contact, or a part that is painted or oiled later |
| Type II | Supplementary chromate conversion coating (gold or iridescent color) | Best stand-alone corrosion resistance; most common on fasteners and fittings |
| Type III | Supplementary phosphate treatment | Paint base |
| Class 1 | 0.0005 in (13 µm) minimum | Severe exposure, no tight fits |
| Class 2 | 0.0003 in (8 µm) minimum | General use; the usual default |
| Class 3 | 0.0002 in (5 µm) minimum | Threads and close fits where buildup must stay small |
Why did engineers specify cadmium plating in the first place?
Cadmium brought four useful properties together, and none of the replacements delivers all four at once: sacrificial corrosion protection in salt environments, natural lubricity that gives consistent torque-tension in threads, close galvanic compatibility with aluminum, and easy solderability. Knowing which of the four your part relies on is what decides the replacement.
On a steel fastener clamped into an aluminum airframe or housing, zinc sits well to the active side of aluminum in the galvanic series while cadmium sits close to it, so a cadmium-plated fastener drives less galvanic current through the joint. In threads, cadmium is soft and slippery, which is why torque values on many legacy assembly drawings were set for cadmium-plated hardware. On electrical connectors and terminals, cadmium solders readily and keeps a low contact resistance. Marine and coastal hardware used it because it holds up well in salt-laden air.
The cost of those properties is toxicity. Cadmium is a carcinogen, and US workplace exposure to it is controlled by a dedicated OSHA standard that sets a low airborne limit and requires monitoring and medical surveillance (29 CFR 1910.1027). Those controls make cadmium lines costly to run, and far fewer platers offer cadmium than zinc or zinc-nickel, which matters to a buyer as much as any regulation: a callout nobody nearby will plate is a supply risk.
Is cadmium plating banned, and which rules push a drawing off it?
Cadmium plating is not banned outright in the US, but three sets of rules squeeze it: the DFARS limit on hexavalent chromium (which hits the Type II chromate), the EU RoHS and REACH restrictions on cadmium itself, and OSHA exposure rules that make cadmium costly to plate. Which of them applies depends on who buys the part and where it ships.
For US defense work, DFARS clause 252.223-7008 prohibits deliverables containing more than 0.1 percent hexavalent chromium by weight in any homogeneous material unless the contracting officer specifies otherwise (DFARS 252.223-7008). The classic Type II cadmium finish uses a hexavalent chromate, so a new defense contract can block the callout even where cadmium itself is still allowed. The clause also bars deliverables that would need hexavalent chromium removed or reapplied during later sustainment, and it does not apply to hexavalent chromium produced as a by-product of manufacturing; exceptions have to come through the contracting officer.
For products sold into the EU, two regimes apply. The RoHS Directive restricts cadmium in electrical and electronic equipment to 0.01 percent by weight in a homogeneous material, which in practice rules out cadmium-plated parts inside covered equipment unless a listed exemption applies (Directive 2011/65/EU, Annex II). REACH Annex XVII entry 23 prohibits cadmium plating of metal parts used in a long list of equipment sectors, with exemptions for aeronautical, aerospace, mining, offshore and nuclear applications that require high safety standards, for safety devices in road and agricultural vehicles, rolling stock and vessels, and for electrical contacts (ECHA, Annex XVII entry 23). Read the entry against your product, because each exemption carries conditions beyond the sector name.
Which coatings can replace cadmium plating, and how do they compare?
Zinc-nickel to SAE AMS2417 is the most common general replacement, IVD aluminum to MIL-DTL-83488 is the closest match for steel parts against aluminum, and tin-zinc to SAE AMS2434 is the closest match for lubricity and solderability. Each one wins on some of cadmium’s properties and loses on others.
Zinc-nickel is an electroplated alloy that corrodes slowly and evenly, keeps much of its protection after heat exposure, and is offered by many platers; it is harder and less slippery than cadmium, so threaded joints need a lubricant or a fresh torque study. IVD aluminum is applied in a vacuum chamber rather than an aqueous bath, so the process itself does not charge the steel with hydrogen, and an Army Research Laboratory fastener study judged it comparable to cadmium on torque values and corrosion in its test program (ARL-TR-1952). Its limits are geometry and supply: the coating is line-of-sight, so blind holes, deep bores and internal threads coat poorly, and fewer shops run the equipment. Tin-zinc is an electroplated alloy whose SAE scope (revision D) lists corrosion protection where cadmium cannot be used, fastener lubricity and solderable parts among its typical uses (SAE AMS2434D; the current issue is AMS2434E, 2022).
| Property | Cadmium (AMS-QQ-P-416) | Zinc-nickel (AMS2417) | IVD aluminum (MIL-DTL-83488) | Tin-zinc (AMS2434) |
|---|---|---|---|---|
| Process | Electroplated | Electroplated alloy | Vacuum deposited | Electroplated alloy |
| Corrosion protection on steel | Sacrificial; strong in marine air | Sacrificial; strong, also after heat exposure | Sacrificial; strong with a supplementary treatment | Sacrificial; good |
| Lubricity and torque-tension | Reference: low, consistent friction | Higher friction; lubricant or new torque values needed | Judged comparable to cadmium in one Army fastener study | Good; fastener lubricity is a listed use |
| Galvanic fit with aluminum | Very good | Good | Best: same metal | Good |
| Hydrogen from the process | Yes; bake high-strength steel | Yes; low-embrittlement baths exist | None from the coating process | Yes; bake high-strength steel |
| Blind holes and internal threads | Thin but present | Thin but present | Poor: line-of-sight | Thin but present |
| Solderability | Good | Poor | Poor | Good |
| Thickness classes | Class 1, 2, 3: 13, 8, 5 µm minimum | Per the AMS2417 revision cited; check its options | Class 1, 2, 3: 25, 13, 8 µm minimum | Per the AMS2434 revision cited; check its options |
| Approval effort on a legacy drawing | None; already approved | Moderate; widely accepted substitute | Moderate; long defense history on steel | Higher; fewer platers and approvals |
Does a cadmium plating callout need a hydrogen embrittlement bake?
Yes, when the steel is hardened above the threshold the specification sets: the plating standard calls for stress relief before plating and a post-plate bake for high-strength steel. The replacement coatings that are electroplated carry the same requirement, so changing the coating does not remove the bake.
Cadmium is deposited from an aqueous bath, and acid cleaning plus the plating reaction put atomic hydrogen into the steel. In hardened parts that hydrogen can cause delayed cracking under load, and the cadmium layer slows its escape, which is why the bake has to follow plating promptly. The hardness trigger, bake temperature and duration are set in the AMS-QQ-P-416 revision you cite, so put the steel grade and hardness on the drawing and let the specification supply the numbers. For process verification, ASTM F519 defines mechanical embrittlement tests for plating processes (ASTM F519-23).
When the part moves to zinc-nickel or tin-zinc, check that the new note also calls for embrittlement relief, and ask whether the plater runs a low-hydrogen bath for high-strength steel. Moving to IVD aluminum removes plating hydrogen but not the hydrogen picked up in any acid cleaning before it, so keep the steel’s hardness on the drawing either way. The guide to hydrogen embrittlement baking covers how to write the bake and sequence on a purchase order.
How do you revise a legacy drawing that calls out cadmium plating?
Identify which cadmium property the part depends on, check which rules apply to its market, pick the replacement that keeps that property, rewrite the finish note completely, and requalify torque, corrosion and embrittlement before release. Do not simply cross out the cadmium spec and write a new one in its place.
- Find the function. Ask what the cadmium is doing: aluminum contact, controlled torque in a threaded joint, solderability, marine corrosion, or nothing in particular. Many cadmium notes were inherited from older drawings and protect nothing a plain zinc finish could not.
- Check the drivers. Note whether the part goes into a DFARS contract, EU electrical equipment, or a REACH-covered sector, because that decides whether cadmium, the hexavalent chromate, or both must go.
- Choose by function. Use the comparison table: IVD aluminum for steel against aluminum where the geometry is open, zinc-nickel for general corrosion protection with threads, tin-zinc where lubricity or solderability matter most.
- Rewrite the full note. State the new specification and revision, the thickness or class, a trivalent or other hexavalent-free supplementary treatment, the embrittlement relief, masking, and whether dimensions apply before or after plating.
- Requalify. Run torque-tension tests on the real joint, corrosion testing such as neutral salt spray to ASTM B117, and embrittlement testing where the steel is hardened. Update assembly torque values if the friction changed.
- Record the change. Issue the drawing revision with a note on interchangeability, and decide whether existing cadmium-plated spares may still be used.
A sample note for a hardened steel part moving to zinc-nickel might read: ZINC-NICKEL PLATE PER SAE AMS2417 [TYPE AND THICKNESS AS QUALIFIED], HEXAVALENT-CHROMIUM-FREE PASSIVATE. EMBRITTLEMENT RELIEF PER SPECIFICATION. DIMENSIONS APPLY AFTER PLATING. REPLACES CADMIUM PER AMS-QQ-P-416; NOT INTERCHANGEABLE IN TORQUE WITHOUT LUBRICANT PER NOTE 7. Fill the bracketed designation from the revision you qualify to, because type and thickness designations should be checked against that exact revision.
What does a machine shop need on the drawing when the plating is outsourced?
The shop needs to know whether dimensions apply before or after plating, which surfaces and threads are masked, and the exact finish specification and revision. Those three items decide how threads and fits are machined before the parts go to the plater.
Plating adds metal to every exposed surface, and on a thread it adds it on each flank, so the pitch diameter grows by a multiple of the coating thickness. If the drawing says dimensions apply after plating, the machinist cuts threads and bores undersize to leave room; if it says before plating, the plater has to stay inside whatever allowance the thread class leaves. The guide to threads after plating explains how to set that allowance.
When we review a drawing for CNC turning or CNC milling that carries a cadmium note, we flag the callout, ask whether a replacement has been approved, and confirm the before-or-after-plating rule for every thread and fit. Plating is done by an outside finisher, so we quote the machining and identify the finish requirement for you to approve rather than choosing a substitute on our own. Send the drawing, the finish note as it stands and any approved alternate with your request for a quote.
Frequently asked
Is cadmium plating still used?
Yes, mainly on legacy aerospace and defense hardware, electrical connectors and some marine fittings where the drawing has not been revised or no qualified substitute exists. New designs rarely call it out, and far fewer platers offer it than offer zinc or zinc-nickel.
What is the difference between cadmium plating and zinc plating?
Both protect steel sacrificially, but cadmium is more lubricious, closer to aluminum in galvanic potential and better in marine air, while zinc is cheaper, far less toxic and widely available. Plain zinc is not a general cadmium substitute on threaded or aluminum-contact parts; zinc-nickel is the usual zinc-based choice. See the zinc-nickel vs zinc plating guide.
What color is cadmium plating?
As plated (Type I) it is silvery gray. The Type II chromate gives the gold or iridescent yellow look most people recognize, and some chromates produce other colors. Color alone does not identify cadmium, because yellow zinc looks similar.
Can a drawing allow either cadmium or a replacement?
Yes, if the design authority has qualified both. Write the alternate as a complete second note with its own specification, thickness and treatment, and state whether parts with different finishes may be mixed in one assembly.
Does Type II cadmium without hexavalent chromate exist?
Some platers offer trivalent or other hexavalent-free supplementary treatments over cadmium. Whether one is acceptable in place of the Type II chromate depends on the specification revision you cite and on the design authority, so confirm both before relying on one to meet the DFARS clause.
Sources
- SAE AMS-QQ-P-416G, Plating, Cadmium (Electrodeposited), SAE International, revised September 2022
- QQ-P-416F Notice 1, cancellation of federal specification QQ-P-416F (1 October 1991) in favor of SAE AMS-QQ-P-416, notice dated 28 March 2002
- SAE AMS2417K, Plating, Zinc-Nickel Alloy, SAE International, revised May 2020
- SAE AMS2434E, Plating, Tin-Zinc Alloy, SAE International, revised January 2022 (scope wording quoted from revision D, 2011)
- ARL-TR-1952, The Performance of Three Alternative Coatings to Electroplated Cadmium for Corrosion Protection in Fastener Applications, US Army Research Laboratory, May 1999
- DFARS 252.223-7008, Prohibition of Hexavalent Chromium, Department of Defense, clause dated January 2023
- Directive 2011/65/EU (RoHS), Annex II, European Parliament and Council, 2011, as amended
- REACH Annex XVII, entry 23: cadmium and its compounds, European Chemicals Agency, consolidated text of Regulation (EC) No 1907/2006
- 29 CFR 1910.1027, Cadmium, US Occupational Safety and Health Administration, final rule 1992
- ASTM F519-23, Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments, ASTM International, 2023
- ASTM B117-26a, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM International, 2026