Your drawing says cadmium, Type II, Class 2 — or it still says QQ-P-416. Here is the type and class structure, the thickness table, the embrittlement bake, and an honest answer on substitution.
QQ-P-416 was the federal specification for electrodeposited cadmium plating. Revision F, 1 October 1991, ran through Amendment 3 and was canceled by Notice 1 on 28 March 2002, directing future acquisition to SAE-AMS-QQ-P-416, “Plating, Cadmium (Electrodeposited)”. That document is now at Revision G, 1 September 2022.
The classification did not change in the handover, which is why both numbers still circulate. If your drawing cites the federal number, the callout is not invalid — it points at a canceled document. A shop will process to AMS-QQ-P-416 and say so on the certification. If you are revising the drawing anyway, change the number. If not, add a note — “QQ-P-416 canceled; process per AMS-QQ-P-416” — so the source inspector is not left to interpret it.
All three types get the same electrodeposited cadmium. What differs is what happens afterwards, and that is driven by what goes on top.
Bare cadmium. Conductive, and the least corrosion-resistant of the three because nothing suppresses the white corrosion product. Specify it where a conversion film would interfere — electrical contact and bonding surfaces.
A chromate conversion film applied to retard white corrosion product. This is what most drawings mean by “cad plate”; colors run bronze, brown, olive drab, yellow and forest green. The film degrades with heat — not for continuous service at 149 °F or above, or intermittent exposure at 300 °F or above.
A tightly adherent phosphate film whose stated purpose is to act as a coating base. This is the type for parts that will subsequently be completely coated. Not a standalone corrosion finish; it is the tooth under paint.
The mistake to avoid is a heavy Type II chromate primed over: chromate slows corrosion of an exposed surface, it is not there to bond an organic film. Painted parts get Type III — the design guide covers the same logic on aluminum.
Class does not change the process, the color or the post-treatment. It sets a minimum deposit thickness on significant surfaces — the areas a 0.75 inch diameter sphere can contact — and it is a minimum, not an average.
| Class | Minimum thickness | Approx. metric | Normally specified for |
|---|---|---|---|
| Class 1 | 0.0005 in | 13 µm | Severe service. Marine and salt exposure, long outdoor life, parts that will not be recoated in service. |
| Class 2 | 0.0003 in | 8 µm | The general-purpose class. Moderate exposure, most airframe and ground-support hardware, brackets and fittings. |
| Class 3 | 0.0002 in | 5 µm | Mild exposure and, more often, dimensional constraint — threaded fasteners and close-fit hardware. |
On threads, class is a fit problem before it is a corrosion problem. Cadmium builds on both flanks, so effective pitch diameter grows by roughly four times the deposit thickness, and a Class 1 deposit on a 3A external thread will not gauge unless the thread was cut undersize for it. If the drawing carries no plating allowance, call Class 3 or call the allowance.
Cadmium on high-strength steel is the textbook embrittlement case. Atomic hydrogen generated at the cathode diffuses into the steel, and the cadmium deposit traps it there. Parts pass inspection, go into service, and fail under sustained load days or weeks later with no plastic deformation and no warning.
1. There is a hard upper limit. Unless otherwise specified in the ordering information, AMS-QQ-P-416 states that parts with an ultimate tensile strength greater than 200 ksi (or HRC 43) shall not be plated to this specification. Above that, it does not claim its bake makes the part safe.
2. Below that limit, hardened steel gets baked. The trigger threshold and the required hours come from Table 1 or Table 1A of the revision your drawing invokes. Read that table — the threshold has moved between revisions and is not a number to take from a website, including this one.
3. The bake starts within four hours of plating. Parts must be baked within 4 hours of the plating operation being completed. It is the requirement most often missed, because it is a scheduling requirement rather than a process parameter. Hydrogen keeps diffusing while parts wait for oven space, so a bake that starts next shift is a nonconformance even when temperature and hours are right.
4. The bake comes before the chromate. For Type II and Type III parts requiring baking, the bake precedes the supplementary coating — baking a chromate film dehydrates it and destroys most of its corrosion performance. It cuts both ways: the bake passivates the cadmium, so surfaces passivated by baking must be reactivated before they will accept the Type II chromate.
Typical relief conditions are 375 °F ± 25 °F for a minimum of 23 hours, reduced to 275 °F ± 25 °F where the part cannot tolerate the higher temperature — carburised work at HRC 55 and above. The specification separately requires stress relief before plating for hardened steel machined, ground, cold-formed or cold-straightened after heat treatment. Neither operation substitutes for the other.
Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments. Notched specimens are plated with production work and held under sustained load. It qualifies the process, not the part.
Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement. A guide, not a specification: useful where a drawing gives no bake conditions, but imposing nothing itself.
Hydrogen Embrittlement Relief (Baking) of Steel Parts, currently Revision E. Where a drawing invokes it, it governs the bake instead of the plating specification’s own table.
Cadmium survives on legacy and defense programs because four of its properties happen to be the four an airframe fastener needs, and no replacement carries all four.
Cadmium sits between zinc and aluminum in the galvanic series, close enough that a steel fastener in aluminum structure drives very little galvanic current at the joint. Comparative fastener testing ranks it best coupled to aluminum, and has found zinc-nickel turning strongly cathodic to aluminum after extended cycling as its surface enriched in nickel.
Soft and low-friction as deposited, so threaded fasteners run smoothly and, more to the point, predictably — the torque-tension relationship is consistent enough to trust published torque tables. Alternatives need a matched lubricant to reach comparable clamp load.
Sacrificial protection with a corrosion product that stays loose and non-bulking. In marine and coastal service it is still the benchmark others are measured against.
Class 3 cadmium is 0.0002 in minimum. It plates thin, and its corrosion product does not swell and lock threads or bind close-tolerance bores. Replacements specified for equivalent salt-spray life run heavier and harder — a dimensional problem on hardware designed around a cad allowance.
The limits are equally specific. Cadmium melts around 600 °F, putting its useful service ceiling near 450 °F. Contact with titanium at temperature is a recognized embrittlement risk many programs prohibit outright, and it is not a space-flight material.
Two mature substitutes. Both are good coatings. Neither is a drop-in, and on most legacy programs the obstacle is not chemistry.
| Cadmium (AMS-QQ-P-416) | Zinc-nickel (AMS2417 and equivalents) | IVD aluminum (MIL-DTL-83488) | |
|---|---|---|---|
| Applied by | Aqueous electroplating | Aqueous electroplating | Vacuum deposition — ion vapor deposition, sputtering, or electrodeposition from an organic electrolyte |
| Hydrogen embrittlement | Significant. Relief bake on hardened steel; 200 ksi / HRC 43 upper limit | Conventional baths embrittle high-strength steel. Low-hydrogen-embrittlement formulations exist and are qualified for aircraft use | No aqueous hydrogen charging, so usable on very high-strength steel where cadmium is barred |
| Against aluminum structure | Best in class | Can turn cathodic to aluminum with age and cycling — evaluate the couple, do not assume it | Aluminum on aluminum; the couple is essentially neutral |
| Threaded hardware | Predictable torque-tension as plated | Usually needs a matched supplementary lubricant to reach equivalent clamp load | Line-of-sight process; as-deposited film is porous and normally glass-bead peened |
| Temperature | About 450 °F ceiling; Type II chromate limited well below that | Higher ceiling than cadmium | Highest of the three |
| Practical cost | Baseline | Comparable to plating generally | Substantially higher; capital-intensive vacuum equipment |
The part nobody says out loud: on a legacy airframe or a fielded weapon system, substitution is a paperwork problem, not a chemistry problem. Changing the drawing means an engineering change, re-qualification of the joint, updated torque values and usually a customer or government approval — work larger and slower than the plating itself. Cadmium lines still run because thousands of released drawings still call for it and nobody has funded the change.
Designing new: specify zinc-nickel plating and be done — the head-to-head is in zinc-nickel vs cadmium plating. Sustaining an existing program: plate cadmium to the drawing and run substitution as its own project. Where low friction is the real requirement rather than corrosion protection, dry film lubricant sometimes solves the problem the cadmium was there for.
Exposure control. Cadmium has its own OSHA substance-specific standard, 29 CFR 1910.1027: a permissible exposure limit of 5 µg/m³ as an 8-hour TWA and an action level of 2.5 µg/m³, with monitoring, medical surveillance, regulated areas and recordkeeping behind it. None of that scales down for a single line.
Waste. Cadmium-bearing rinse water and treatment sludge are handled as hazardous waste with segregated treatment and manifested disposal. Cadmium cannot share a waste stream with the rest of the shop.
Regulation abroad. Under REACH Annex XVII entry 23, cadmium plating of metallic articles is prohibited across most industrial sectors in the EU, with derogations preserving the aeronautical, aerospace, mining, offshore and nuclear sectors. Cadmium was not banned in aerospace; it was banned nearly everywhere else.
The result: a defense program with a cadmium print has a short list of sources, and it has been getting shorter for twenty years.
Specification, type, class. Then, if the part is hardened steel, say what it is and what you want done about the hydrogen.
CADMIUM PLATE PER AMS-QQ-P-416, TYPE II, CLASS 2.
Chromate finish, 0.0003 in minimum. The default for a bracket or fitting that lives exposed. Add the color if it matters; otherwise you get the shop’s standard.
CADMIUM PLATE PER AMS-QQ-P-416, TYPE III, CLASS 2, PRIOR TO PRIMER AND TOPCOAT PER (paint spec).
Phosphate post-treatment as the coating base. Naming the paint system on the same note stops anyone chromating a part about to be primed.
CADMIUM PLATE PER AMS-QQ-P-416, TYPE II, CLASS 3. MATERIAL HEAT TREATED 180–200 KSI UTS. HYDROGEN EMBRITTLEMENT RELIEF BAKE 375 °F ±25 °F, 23 HOURS MINIMUM, COMMENCING WITHIN 4 HOURS OF PLATING AND PRIOR TO CHROMATE. PROCESS QUALIFIED PER ASTM F519. THREADS MACHINED TO ACCOMMODATE PLATING.
Long, and worth every word: strength, bake, window, sequence and thread allowance all stated, so nothing rests on the shop inferring your intent.
Worth adding: require the certification to report actual bake start time. It is the only way the four-hour window is auditable after the fact.
Gleco Plating has been finishing metal in Texas since 1979. Cadmium is not a process we run at every site — it runs at the McAllen plant, 3800 W Ursula Ave, McAllen, TX, 956-800-4069. Send cadmium work straight there and you save it a leg.
We process Type I, II and III to AMS-QQ-P-416 in Class 1, 2 and 3, with embrittlement relief baking on hardened steel and the bake sequenced ahead of the supplementary chromate. If your drawing still cites the canceled federal number, tell us and we will note the AMS reference on the certification rather than leave it ambiguous at incoming inspection.
We hold AS9100D and ISO 9001:2015 — the AS9100D certificate covers three sites — and we are ITAR Registered and DFARS compliant, which is why defense and aerospace work makes up so much of the cadmium line. Controlled technical data is handled under our ITAR registration; the rest is on certifications and approvals. Process background sits on the cadmium plating page, and the Metroplex logistics under cadmium plating in Dallas, TX. Before a drawing is released, the design guide is worth a read — plating allowance on threads is cheaper to add there than to argue at first article.
We do not quote a coating we cannot certify to the drawing. If your part is above the specification’s 200 ksi limit, or will contact titanium at temperature, say so on the RFQ and we will tell you what we think before you buy plating.
The drawing is usable, but the document it points at is canceled. QQ-P-416F was canceled by Notice 1 on 28 March 2002, directing future acquisition to SAE-AMS-QQ-P-416. The type and class structure carried over unchanged, so Type II Class 2 means the same thing either way, and a shop will process and certify to AMS-QQ-P-416. Adding a drawing note tying the old number to the new one removes an argument at receiving inspection.
If they are hardened steel, almost certainly. Table 1 or Table 1A sets the strength or hardness at which relief baking becomes mandatory and the hours required. Read the table in the revision your drawing invokes rather than a remembered number — the threshold has moved between revisions. Above 200 ksi UTS or HRC 43, the part shall not be plated to the specification at all unless the ordering information says otherwise. Austenitic stainless, aluminum and several nickel and cobalt alloys are exempted as not susceptible.
Because hydrogen keeps diffusing into the steel while the part sits. The specification requires baking to begin within 4 hours of plating being completed. It is a scheduling requirement, which is why it is most often missed — temperature and hours get audited, start time frequently does not. If it matters, ask for actual bake start time on the certification.
Baking a chromate conversion film dehydrates it and destroys most of its corrosion performance, so a chromate applied first is wasted. AMS-QQ-P-416 states directly that for Type II and Type III parts requiring baking, the bake precedes the supplementary coating. One knock-on effect: the bake passivates the cadmium surface, which must then be reactivated before it will take the chromate properly.
Type II is a supplementary chromate treatment that slows white corrosion product on an exposed surface. Type III is a supplementary phosphate treatment serving as a coating base, and the specification points at it for parts that will subsequently be completely coated. Type II if the cadmium is the final finish, Type III if paint goes on top. Type II also carries a service temperature limit Type III does not — not above 149 degrees F continuous or 300 degrees F intermittent.
Technically, often yes — zinc-nickel is a mature aerospace finish and low-hydrogen-embrittlement formulations are qualified for aircraft use. Contractually, usually not without work: changing the drawing is an engineering change, meaning re-qualification of the joint, revised torque values and customer or government approval. There are real differences to evaluate rather than assume — zinc-nickel can turn cathodic to aluminum structure over time, and needs a matched lubricant to reach cadmium’s clamp load on threads.
Up to the specification’s limit, yes, with the relief bake sequenced correctly. Above an ultimate tensile strength of 200 ksi or HRC 43, AMS-QQ-P-416 says parts shall not be plated to it unless the ordering information specifies otherwise. That is not a shop capability question — it is the specification declining to vouch for the outcome. Above that strength, IVD aluminum per MIL-DTL-83488 is the usual route: a vacuum process with no aqueous hydrogen charging.
McAllen — 3800 W Ursula Ave, McAllen, TX, 956-800-4069. Zinc-nickel and the rest of the zinc family run at Rowlett. If a job has both cadmium and Rowlett-only processes on the router, tell us at quote and we will sequence the transfer rather than discover it mid-job.
Cadmium type, class, material strength and any bake requirement — we will read the callout and tell you what we would process, before you commit to it.
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
