Guides / Specs

Metal Finishing Specs Explained: ASTM, MIL-SPEC and AMS Cross Reference

October 4, 2026

Drawings accumulate spec callouts over years, copied from one revision to the next, sometimes from a document that has since been superseded or cancelled. Buyers ask shops what a spec number actually means often enough that it is worth writing down plainly, without the marketing language that usually surrounds it.

This is not a substitute for reading the actual document. It is a starting point for knowing which family a spec belongs to and roughly what it controls, before you go read the standard itself.

The three spec families

Almost every plating or coating spec you will see on a drawing comes from one of three sources:

• ASTM International publishes commercial and industrial specs, identified with a letter and number such as B633 or A967. These are the most widely used baseline specs across industries.
• SAE International publishes AMS (Aerospace Material Specification) documents, identified as AMS followed by a number, such as AMS 2404. These are written for aerospace but used well beyond it.
• The U.S. Department of Defense, through the Defense Logistics Agency, publishes MIL-SPEC, MIL-DTL (detail specification) and MIL-PRF (performance specification) documents, such as MIL-PRF-8625 or MIL-DTL-5541.

A part can carry specs from more than one family at once. A hard anodized aluminum part might call out MIL-PRF-8625 for the anodize and a customer-specific note for color, both on the same drawing.

Zinc and zinc alloy plating

ASTM B633 covers electrodeposited zinc on iron or steel, in four thickness classes (called out as SC 1 through SC 4) and five supplementary finish types, from as-plated to painted. It is the default commercial spec for zinc plating.

ASTM B841 and AMS 2417 both cover zinc-nickel alloy plating. AMS 2417 is the aerospace and automotive version, commonly seen on fasteners and fittings that replaced cadmium plating on newer drawings. The two are close in intent; check class and thickness minimums rather than assuming interchangeability.

Electroless and electrolytic nickel

ASTM B733 and AMS 2404 both cover electroless nickel plating, specified by phosphorus content and often by a service condition number that sets minimum thickness for a given corrosion environment. AMS 2404 adds aerospace-specific process control on top of the ASTM baseline.

AMS-QQ-N-290, an older federal spec still referenced on some drawings, covers electrolytic nickel plating. Many drawings written against it now reference ASTM B689 instead, since the federal spec was cancelled; a shop familiar with legacy government drawings can usually confirm the current equivalent.

Anodizing and chemical conversion coatings on aluminum

MIL-PRF-8625 is the primary anodize spec, covering Type I (chromic acid, now uncommon due to hexavalent chromium restrictions), Type II (sulfuric acid, dyeable), and Type III (hard anodize). Class within each type sets whether the finish is dyed or left natural. How to word an anodizing callout is in how to cite an anodizing specification.

MIL-DTL-5541 covers chemical conversion coatings, commonly called chem film or Alodine, used where anodize’s insulating properties are unwanted, such as grounding surfaces. Type I uses hexavalent chromium and is being phased out on new designs; Type II is the trivalent, RoHS-compliant version most shops now run by default.

Passivation and stainless steel

ASTM A967 and AMS 2700 both cover passivation of stainless steel, listing accepted chemical treatments (nitric or citric acid based) and the test methods used to confirm free iron has been removed. AMS 2700 is the aerospace version and is more specific about test method selection; ASTM A967 is the common commercial reference.

Passivation does not add thickness or change dimensions, since it is a surface treatment rather than a deposited coating. It is often confused with electropolishing, which does remove a small amount of material; the two solve different problems and are sometimes specified together.

Hard chrome and PVD

AMS 2460 covers hard chromium plating, typically applied thick and ground to final dimension, used on wear surfaces such as hydraulic rods. There is no single dominant spec for PVD coatings such as TiN, CrN and DLC; most PVD work is called out against a customer’s own performance spec, naming coating type, thickness and hardness rather than a published document.

Powder coat and paint performance specs

Powder coat and liquid paint are usually called out against a customer-specific spec rather than a single dominant published standard, but architectural and exterior work commonly references the AAMA (American Architectural Manufacturers Association) family. AAMA 2603 covers a baseline organic coating, AAMA 2604 adds better weathering performance for high-performance applications, and AAMA 2605 is the top tier, typically a fluoropolymer coating specified for long-term outdoor exposure such as architectural aluminum.

The AAMA number sets performance requirements, such as color retention and chalk resistance after accelerated weathering, rather than dictating a specific paint chemistry, so two shops meeting the same AAMA class can still use different coating systems to get there. For non-architectural parts, most powder coat and e-coat callouts simply name the color, gloss and film thickness range without a published performance spec attached.

Reading a spec callout on a drawing

A complete callout usually has three parts: the spec number, a type or class letter, and a thickness or service condition. “ASTM B633, Fe/Zn 8, Type III” tells a shop the process (zinc electroplate), the minimum thickness class (8 microns) and the supplementary treatment (a clear or colored passivate, without a supplementary coating). A callout missing the type or class leaves those choices to the shop’s default, which varies between shops.

If your drawing carries an old or cancelled spec number, do not assume a shop can quietly substitute the modern equivalent. Ask, and get the substitution agreed in writing, since a customer receiving your parts may check the callout against their own approved spec list.

Full spec text is published by ASTM International for ASTM documents and by SAE International for AMS documents. A shop’s summary, including this one, is a starting point for a conversation, not a replacement for reading the document your drawing actually calls out.

See also: Anodizing specifications compared: MIL, AMS, ISO, ASTM and JIS.

How we help

We are a matching service, not a plating shop. Tell us the spec on your drawing, or describe what the finish has to do if you have not chosen one yet, and we pass your RFQ to up to three finishing shops that run that spec every week, not occasionally. For help writing the rest of the RFQ, see the coating RFQ checklist, or go straight to the RFQ form.

Frequently asked questions

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