Material Selection for Custom Parts: 304 vs 316 Stainless Steel, Aluminium Alloys and How to Specify

Last updated: 19 September 2026.

Every standard number, grade designation and figure below is quoted from an official standards-body or industry-association source listed at the end of this page. Material selection depends on your service environment and your drawing; confirm the final grade and the inspection document with your supplier before you order.

1. Start from the service environment, not the grade

Material selection goes wrong most often at the very first step, when a familiar grade is chosen before the environment has been described. The environment decides the family; only then does the grade follow.

Four questions belong on the table before any grade is named:

• What is the part exposed to? Atmospheric corrosion is classified by ISO 9223:2012, which sets up corrosivity categories for atmospheric environments by the first-year corrosion rate of standard specimens. ISO 9223 identifies the key factors as the temperature-humidity complex, pollution by sulfur dioxide and airborne salinity. Two coastal sites can therefore demand different material choices even when the drawings are identical.
• What temperature does it see? Temperature narrows the usable range long before mechanical load does. As an example, the fasteners standard ISO 3506-1 is built around an ambient test range of 10 °C to 35 °C and states that its grades are used without restriction from −20 °C to +150 °C, with use outside that band to be decided case by case.
• How is it made and joined? Machining, forming, welding and finishing all constrain the material. A grade that machines cleanly may not form or weld as readily, so the process route matters as much as the service conditions.
• What does the part have to prove? Product-contact parts, pressure parts and safety-relevant parts usually carry a documentation requirement that has to be decided at selection time, not after production.

A material decision that answers all four questions is a specification. One that names a grade alone is a preference.

2. Stainless steel: what actually separates 304 from 316

Type 304 and Type 316 are the two stainless grades most often specified on custom industrial parts, and they are close relatives rather than competing materials. Both are austenitic stainless steels. The Nickel Institute lists Type 304 (UNS S30400) and Type 316 (UNS S31600) as commonly used grades in the chemical and petrochemical industries, and describes Type 304 as deployed in cryogenic, caustic and oxidizing acid environments.

The difference that matters is molybdenum. The Nickel Institute states plainly that the addition of molybdenum in 316 provides superior resistance to chlorides and reducing acids relative to 304. That single sentence explains most real-world grade choices:

• Where the service is general and chlorides are not the governing threat, 304 is the ordinary choice.
• Where chlorides or reducing acids are present, 316 earns its extra alloy content. In pharmaceutical plants the same source notes that Type 304 is used for non-contact applications while Type 316 is used for product-contact applications, because its added molybdenum provides greater corrosion resistance.

There is a ceiling to that advantage. The Nickel Institute states that Type 316 and its derivatives may be used with chloride concentrations up to 500 mg/l, and that at higher chloride concentrations — particularly where combined with increased operating temperatures — a duplex stainless steel such as 2205 (UNS S32205), or a super austenitic or super duplex grade such as 2507 (UNS S32750), may be required for resistance to pitting corrosion and stress-corrosion cracking. Treat these figures as published industry guidance for a first screening pass, and confirm the final grade against your own service conditions.

The same source material, condensed into the decision itself:

Decision point	Type 304 (UNS S30400)	Type 316 (UNS S31600)
Alloying difference that matters	No molybdenum addition described in the source	Molybdenum added
Resistance to chlorides and reducing acids	Lower than 316	Superior to 304
Service the source associates with the grade	Cryogenic, caustic and oxidizing acid environments; non-contact applications in pharmaceutical plants	Chloride-bearing service up to 500 mg/l; product-contact applications in pharmaceutical plants
When the source says to step up	—	Where chlorides pass the pitting resistance of 316, or where high temperatures promote chloride stress-corrosion cracking: duplex 2205 (UNS S32205) or a super duplex grade such as 2507 (UNS S32750)

Use this as a screening aid, not a substitute for your own service conditions.

Whichever grade is chosen, the drawing has to point at a material standard as well as a grade name, because "304" on its own does not tell a mill what product form or what set of requirements applies:

• Bar and shapes: ASTM A276/A276M, Standard Specification for Stainless Steel Bars and Shapes, covers hot-finished or cold-finished bars — rounds, squares and hexagons — and hot-rolled or extruded shapes in the more commonly used types of stainless steels.
• Plate, sheet and strip: ASTM A240/A240M covers chromium, chromium-nickel and chromium-manganese-nickel stainless steel plate, sheet and strip for pressure vessels and for general applications.
• Fasteners: see Section 3.

Because the same stainless steel carries different designations in different systems, it is worth naming the grade by more than one. ISO 15510 lists the chemical compositions of stainless steels agreed by ISO/TC 17/SC 4, mainly on the basis of the specifications in existing ISO, ASTM, EN, JIS and GB (Chinese) standards. That cross-referencing is exactly why a drawing should state the grade designation and the standard it is to be supplied to, rather than a designation alone.

3. Stainless fasteners and the wider family

Fasteners are usually bought as standard parts rather than machined to a drawing, so they are governed by a different document — and it is the one place where the other stainless families enter the picture.

ISO 3506-1:2020 specifies the mechanical and physical properties of bolts, screws and studs, with coarse and fine pitch threads, made of corrosion-resistant stainless steels, and it specifies property classes in relation to austenitic, martensitic, ferritic and duplex (austenitic-ferritic) steel grades. Practical points follow from its scope:

• The standard is written around testing at an ambient temperature range of 10 °C to 35 °C. Its grades are used without restriction from −20 °C to +150 °C; outside that band it places the responsibility for the choice on the user in consultation with a fastener metallurgist.
• It covers ISO metric threads (per ISO 68-1), with diameter and pitch combinations per ISO 261 and ISO 262, coarse pitch threads from M1.6 to M39 and fine pitch threads from M8×1 to M39×3.
• It does not cover set screws and similar threaded fasteners not under tensile stress, which are handled by ISO 3506-3. Related parts of the family address higher-temperature selection (ISO 3506-5) and general technical information on suitable stainless steels (ISO 3506-6).

The engineering consequence is simple: a stainless fastener is specified by property class and standard, not by material grade alone, and the grade family (austenitic, ferritic, martensitic or duplex) is part of that choice.

4. Aluminium alloys for machined parts: 6061, 7075 and the series logic

"Aluminium" is a material family, not a specification. Machined aluminium parts are specified by alloy and temper, and the alloy designations come from a system that the Aluminum Association maintains through ANSI H35.1/H35.1M.

The Aluminum Association established the wrought alloy designation system through its Technical Committee on Product Standards, adopted in the United States in 1954 and officially adopted internationally in 1970; the association notes that the list began with 75 unique chemical compositions and that there are now more than 530 registered active compositions. Two series dominate custom machined and extruded work:

• 6xxx series. The Aluminum Association describes this series as containing silicon and magnesium in order to form magnesium silicide within the alloy, and as heat-treatable, highly formable, weldable, of moderately high strength and with excellent corrosion resistance. Extrusion products from the 6xxx series are its first choice for architectural and structural applications, and alloy 6061 is the most widely used alloy in this series. For a general-purpose machined bracket, housing or fitting, 6061 is the ordinary starting point.
• 7xxx series. Here zinc is the primary alloying agent, and when magnesium is added in a smaller amount the result is a heat-treatable, very high strength alloy. The Aluminum Association names 7050 and 7075 as the most commonly known alloys in the series, widely used in the aircraft industry. Reach for 7xxx when strength-to-weight governs — and expect less forgiving machining and joining than 6061.

As with stainless steel, the alloy name is only half a specification. Extruded aluminium bar, rod, wire, profiles and tubes are specified under ASTM B221, whose alloy and temper designations follow ANSI H35.1/H35.1M and whose Unified Numbering System equivalents are formed by prefixing the alloy with A9 (for example, A91100 for aluminium 1100), in accordance with Practice E527; a metric companion, B221M, exists separately. A drawing should therefore read, in effect: alloy, temper, product form and product standard.

5. How to write the material on the drawing

A material requirement is only real when it is unambiguous on the drawing. Five elements do most of the work.

1. Grade designation and material standard, together. "304" and "A276/A276M" describe different things; a mill needs both. The same applies to aluminium (alloy plus B221) and to fasteners (property class plus ISO 3506-1).

2. Condition or temper. Stainless bar under ASTM A276/A276M is furnished in defined conditions — annealed, hardened and tempered, strain hardened, or cold worked — and aluminium is supplied in a temper. The condition changes the properties and cannot be left to the shop's default.

3. General tolerances. ISO 2768-1:1989 is intended to simplify drawing indications and specifies general tolerances for linear and angular dimensions in four tolerance classes; it applies to workpieces produced by metal removal or formed from sheet metal. State the standard and the class in the title block. Note that this edition is expected to be replaced by a revised ISO 2768, so confirm which edition a supplier is working to.

4. Geometrical tolerances, where geometry carries function. ISO 1101:2017 defines the symbol language for geometrical specification of workpieces and the rules for interpretation, and provides the foundation for geometrical specification. Apply geometric controls only where flatness, position, profile or run-out genuinely matter — each control adds inspection work.

5. The inspection document. Say what the material has to come with (see Section 6), or you will receive whatever the supplier issues by default.

If the part is a machined component, CNC machining covers the process side, and sheet metal design covers formed parts. For a worked comparison of which standard applies to which product form, ASTM, CE and UL standards is the companion page.

6. Material certificates and traceability: what to ask for

A material certificate is the evidence that the material delivered is the material specified. The trap is that "a certificate" can mean several different documents, with different amounts of testing behind them.

Two standards govern the family:

• EN 10204 is titled Metallic products — Types of inspection documents. It specifies the different types of inspection documents supplied to the purchaser, in accordance with the requirements of the order, for the delivery of all metallic products — plates, sheets, bars, forgings, castings — whatever their method of production. It defines four document types, numbered 2.1, 2.2, 3.1 and 3.2, and groups them by the basis of testing: types 2.1 and 2.2 rest on non-specific inspection, while types 3.1 and 3.2 rest on specific inspection. The standard also references EN 10168, Steel products — Inspection documents — List of information and description.
• ISO 10474:2013 defines the different types of inspection documents supplied to the purchaser, in accordance with the requirements of the order, for the delivery of steel products, and is to be used together with the relevant general technical delivery conditions (for steel and steel products, ISO 404; for steel castings, ISO 4990).

The practical difference a buyer cares about is the one EN 10204 makes explicit in its structure: a document resting on non-specific inspection is not tied to the testing of your particular batch in the same way as one resting on specific inspection. That is the substance of the "2.2 vs 3.1" question — so the drawing or purchase order should name the document type required, rather than asking for "a material certificate" in general terms.

A request list that produces usable documents rather than reassurance:

1. The document type by number — for example, an inspection certificate 3.1 — named on the order.
2. The material standard and grade actually supplied, so the certificate can be read against the drawing.
3. The certificate covering the batch that was used, not a generic mill certificate for the grade.
4. A stated issuer and date, since the value of the document depends on who signs it and when.
5. Documents for sub-components too — fasteners, inserts and bought-in items each carry their own material, governed by their own standard.

7. Corrosion and outdoor hardware

Outdoor hardware is where material selection is most often settled by assumption rather than by environment, because the failure mode is slow and the environment is invisible on a drawing.

ISO 9223:2012 provides the shared vocabulary. It establishes a classification system for the corrosivity of atmospheric environments, defining corrosivity categories by the first-year corrosion rate of standard specimens, and identifies the key factors as the temperature-humidity complex, sulfur dioxide pollution and airborne salinity. A sheltered inland site and an exposed coastal site are different corrosivity categories, and the material that is adequate for one may be marginal for the other.

For stainless outdoor hardware, the same molybdenum logic from Section 2 applies: 304 for general and inland service, 316 where chlorides and reducing acids are present, and duplex or super austenitic grades where the Nickel Institute's published chloride guidance for 316 would be exceeded. Because the corrosivity category and the chloride exposure are both environment-dependent, the honest specification practice is to state the environment on the drawing note, select the grade against it, and confirm the adequacy of the choice in the material standard you cite.

Finish and material are a system, not alternatives. Where steel or aluminium is selected for outdoor service, the coating carries part of the corrosion duty, and metal surface finishing covers how those finishes are specified and inspected.

8. Common material-specification mistakes

Most disputes that look like quality problems are actually specification problems. The frequent ones:

1. A grade with no standard. "316" does not say whether the material is bar, plate or a fastener, nor which requirements apply. Name grade and standard together.
2. Assuming "stainless" means corrosion-proof. Grade family and molybdenum content decide chloride performance; the environment decides what is needed. Stainless is a family with a wide spread.
3. Leaving condition or temper to the shop. The same grade in different conditions is a different material on the bench.
4. A general-tolerance default. If ISO 2768-1 is not cited with a class, tolerances are whatever the supplier habitually uses.
5. Asking for "a certificate". Without a document type, the paper you receive may not relate to your batch.
6. Selecting before the environment is defined. This is the expensive version of mistake 2, because it forces a redesign or a re-order rather than a document correction.
7. Assuming a designation travels alone. Because ISO 15510 cross-references ISO, ASTM, EN, JIS and GB specifications, the same material can be named differently at the mill and on your drawing. State both the designation and the standard.

9. When to ask the supplier

Material selection is not a contest between the buyer and the supplier; it is a decision that needs both sides' information. Three cases should always be raised with the supplier rather than resolved at the desk:

• When the service environment is unusual — high chloride, elevated temperature, product contact or pressure service. The supplier's metallurgist can tell you which of the available grades is normally used for the condition, and ISO 15510 gives the composition reference to check the answer against.
• When the grade is at a standard boundary — for example, stainless fastener grades whose usable range under ISO 3506-1 ends at +150 °C, or aluminium in a high-strength temper. Boundary cases are where assumptions fail.
• When documentation is the point — if the order has to be supported by a specific inspection document, confirm which types (per EN 10204 or ISO 10474) the supplier can actually issue before the order is placed, not after.

A short, specific question — the environment, the load, the operating temperature, and the document type required — will get a better answer than a general request for "the best material". When the drawings are ready, the request-a-quote route is where this information is best handed over; and if the material must be verified at the factory as well, how to find and verify a manufacturer in China covers the sourcing side. The Manufacturing hub collects the rest of this series.

All sources retrieved and HTTP-tested on 2026-09-19. Every standard number, grade designation and figure on this page has a row below.

Frequently asked questions

Sources

All sources retrieved 19 September 2026. This page is an independent reading of the sources listed; the official pages themselves are the specification.

Fact used on this page	Source
S1 — ISO 3506-1:2020, Fasteners — Mechanical properties of corrosion-resistant stainless steel fasteners — Part 1: Bolts, screws and studs with specified grades and property classes: specifies the mechanical and physical properties of bolts, screws and studs, with coarse pitch thread and fine pitch thread, made of corrosion-resistant stainless steels, when tested at the ambient temperature range of 10 °C to 35 °C; specifies property classes in relation to austenitic, martensitic, ferritic and duplex (austenitic-ferritic) steel grades; NOTE 1 — fasteners are used without restriction in applications ranging from ?20 °C to +150 °C, however they are also used outside this range down to ?196 °C and up to +300 °C; NOTE 2 — ISO 3506-5 assists in the selection of grades intended for use at temperatures up to +800 °C; applies to bolts, screws and studs with ISO metric thread in accordance with ISO 68-1, with diameter/pitch combinations in accordance with ISO 261 and ISO 262, with coarse pitch thread M1,6 to M39 and fine pitch thread M8×1 to M39×3, with thread tolerances in accordance with ISO 965-1 and ISO 965-2; does not apply to set screws and similar threaded fasteners not under tensile stress (see ISO 3506-3); ISO 3506-6 provides general rules and additional technical information on suitable stainless steels (page: Published, Edition 3, 2020-04, last reviewed and confirmed 2025)	ISO https://www.iso.org/standard/70045.html
S2 — ISO 2768-1:1989, General tolerances — Part 1: Tolerances for linear and angular dimensions without individual tolerance indications: "This part is intended to simplify drawing indications and specifies general tolerances in four tolerance classes. It applies to the dimensions of workpieces that are produced by metal removal or are formed from sheet metal. It contains three tables and an informative annex…"; expected to be replaced by ISO 2768 within the coming months (page: Published, Edition 1, 1989-11, last reviewed and confirmed 2022, stage 90.92 to be revised)	ISO https://www.iso.org/standard/7748.html
S3 — ISO 2768 (under publication), Geometrical product specifications (GPS) — Dimensional tolerancing — Tolerance limits for general specification of linear and angular sizes: will replace ISO 2768-1:1989; same abstract text — general tolerances in four tolerance classes; applies to dimensions of workpieces produced by metal removal or formed from sheet metal; three tables and an informative annex (page: Edition 2, stage 60.00 under publication)	ISO https://www.iso.org/standard/85741.html
S4 — ISO 1101:2017, Geometrical product specifications (GPS) — Geometrical tolerancing — Tolerances of form, orientation, location and run-out: "ISO 1101:2017 defines the symbol language for geometrical specification of workpieces and the rules for its interpretation. It provides the foundation for geometrical specification." (page: Published, Edition 4, 2017-02, last reviewed and confirmed 2022)	ISO https://www.iso.org/standard/66777.html
S5 — ISO 10474:2013, Steel and steel products — Inspection documents: "ISO 10474:2013 defines the different types of inspection documents supplied to the purchaser, in accordance with the requirements of the order, for the delivery of steel products." To be used with ISO 404 (steel and steel products) and ISO 4990 (steel castings) (page: Published, Edition 2, 2013-07, last reviewed and confirmed 2023)	ISO https://www.iso.org/standard/53736.html
S6 — ISO 9223:2012, Corrosion of metals and alloys — Corrosivity of atmospheres — Classification, determination and estimation: "ISO 9223:2012 establishes a classification system for the corrosivity of atmospheric environments. It defines corrosivity categories for the atmospheric environments by the first-year corrosion rate of standard specimens…" "ISO 9223:2012 specifies the key factors in the atmospheric corrosion of metals and alloys. These are the temperature-humidity complex, pollution by sulfur dioxide and airborne salinity." (page: Published, Edition 2, 2012-02, last reviewed and confirmed 2022)	ISO https://www.iso.org/standard/53499.html
S7 — ISO 15510:2014, Stainless steels — Chemical composition: "ISO 15510:2014 lists the chemical compositions of stainless steels agreed by ISO/TC 17/SC 4, mainly on the basis of a composition of the specifications in existing ISO, ASTM, EN, JIS, and GB (Chinese) standards. They apply to all wrought product forms including ingots and semi-finished material." (page: Published, Edition 2, 2014-05, last reviewed and confirmed 2025)	ISO https://www.iso.org/standard/61187.html
S8 — ASTM A276/A276M, Standard Specification for Stainless Steel Bars and Shapes: "This specification covers hot-finished or cold-finished bars except bars for reforging… It includes rounds, squares, and hexagons, and hot-rolled or extruded shapes, such as angles, tees, and channels in the more commonly used types of stainless steels." Conditions — "Condition A in which the bars are annealed, Condition H in which the bars are hardened and tempered at a relative temperature, Condition T in which the bars are hardened and tempered at a relatively high temperature, Condition S in which the bars are strain hardened or relatively light cold worked, and Condition B in which the bars are relatively severe cold worked"; bar for reforging per A314; free-machining grades per A582/A582M (cited page is A276/A276M-17)	ASTM International https://www.astm.org/a0276_a0276m-17.html
S9 — ASTM B221-21, Standard Specification for Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes: "This specification covers aluminum and aluminum-alloy extruded bars, rods, wire, profiles, and tubes in the aluminum alloys (Note 1) and tempers shown in Table 2." "Alloy and temper designations are in accordance with ANSI H35.1/H35.1M. The equivalent Unified Numbering System alloy designations are those of Table 1 preceded by A9; for example, A91100 for Aluminum 1100 in accordance with Practice E527." "A complete metric companion to Specification B221 has been developed—Specification B221M…" (cited page is B221-21; also references B211/B211M, B210/B210M, B429/B429M, B241/B241M, B234)	ASTM International https://www.astm.org/b0221-21.html
S10 — ASTM A240/A240M, Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications: "This specification covers chromium, chromium-nickel, and chromium-manganese-nickel stainless steel plate, sheet, and strip for pressure vessels and for general applications including architectural, building, construction, and aesthetic applications." (cited page is A240/A240M-20)	ASTM International https://store.astm.org/a0240_a0240m-20.html
S11 — The Nickel Institute, "Nickel in process engineering" (industry-association official page): "Type 304 (UNS S30400) and Type 316 (UNS S31600) are commonly-used grades. Type 304 is deployed in cryogenic, caustic and oxidizing acid environments. The addition of molybdenum in 316 provides superior resistance to chlorides and reducing acids relative to 304. When chlorides pass the resistance to pitting corrosion for 316 or extreme temperatures promote chloride stress-corrosion cracking, duplex stainless steel such as 2205 (UNS S32205)… or super duplexes such as Type 2507 (UNS S32750)… can provide useful resistance."; pharmaceutical section — "Typically Type 304 (UNS S30400) is used in 'non-contact applications', while Type 316 (UNS S31600) is used in 'product contact applications', as its added molybdenum provides greater corrosion resistance."; "Type 316 and its derivatives may be used with chloride concentrations up to 500 mg/l."	The Nickel Institute https://nickelinstitute.org/en/nickel-applications/nickel-in-process-engineering
S12 — The Aluminum Association, "Industry Standards" (industry-association official page): "70 years ago, the Aluminum Association established the wrought alloy designation system through its Technical Committee on Product Standards (TCPS), which was adopted in the US in 1954… the system was approved as American National Standard H35.1… officially adopted by the International Signatories of the Declaration of Accord in 1970"; "When the current system was originally developed in 1954, the list included 75 unique chemical compositions. Today, there are more than 530 registered active compositions…"; 6xxx — "Alloys in this series contain silicon and magnesium in order to form magnesium silicide within the alloy. Extrusion products from the 6xxx series are the first choice for architectural and structural applications. Alloy 6061 is the most widely used alloy in this series…"; 7xxx — "Zinc is the primary alloying agent for this series, and when magnesium is added in a smaller amount, the result is a heat-treatable, very high strength alloy… The most commonly known alloys are 7050 and 7075, which are widely used in the aircraft industry."; 1xxx — "comprised of aluminum 99 percent or higher purity"; also describes 2xxx (copper), 3xxx (manganese), 4xxx (silicon), 5xxx (magnesium) series	The Aluminum Association https://www.aluminum.org/standards
S13 — NS-EN 10204:2004, Metallic products — Types of inspection documents (Standards Norway, national standards body): standard title "Metallic products - Types of inspection documents"; abstract 1.1 — "This European Standard specifies the different types of inspection documents supplied to the purchaser, in accordance with the requirements of the order, for the delivery of all metallic products e.g. plates, sheets, bars, forgings, castings, whatever their method of production. 1.2 This standard may also apply to non-metallic products. 1.3 This standard is used in conjunction with the product specifications…"; NOTE 1 references prEN 10168 for steel (page: published 10 January 2005, edition 2004-10, ICS 01.110, 77.140.01, 77.150.01)	Standards Norway (SN) https://online.standard.no/en/ns-en-10204-2004
S14 — DIN EN 10204:2005-01, Metallic products — Types of inspection documents; German version EN 10204:2004 (DIN, national standards body): standard content structure (table of contents) enumerates the four document types and groupings — "Prüfbescheinigungen auf der Grundlage nichtspezifischer Prüfung" (inspection documents based on non-specific inspection) containing "Werksbescheinigung '2.1'" and "Werkszeugnis '2.2'", and "Prüfbescheinigungen auf der Grundlage spezifischer Prüfung" (inspection documents based on specific inspection) containing "Abnahmeprüfzeugnis '3.1'" and "Abnahmeprüfzeugnis '3.2'"; references EN 10168:2004-06 "Steel products - Inspection documents - List of information and description" (page: publication date 2005-01, 11 pages, ICS 01.110, 77.140.01, 77.150.01)	DIN (Deutsches Institut für Normung) via DIN Media https://www.dinmedia.de/en/standard/din-en-10204/59682782