Surface Treatments for Machined Parts: Standards, REACH

A surface treatment on a machined part is a coating or chemical step after machining that adds corrosion protection, wear resistance or a defined appearance to the finished geometry: anodizing for aluminium, zinc plating for iron and steel, passivation for stainless steel. Each has its own current standard (ISO 7599, ISO 10074, ISO 2081, ISO 19598, ASTM A967), and wherever hexavalent chromium is involved, REACH and RoHS set limits on the process and on the finished part.

Which surface treatments are used on machined parts?

The treatment follows the base material and the job the surface has to do. Table 1 lists the common combinations with the standard that applies in October 2026. Choosing the base metal comes first, see aluminium alloys for CNC machining and stainless steel for machined parts.

Table 1. Common surface treatments for machined parts and the current standard, status 2026-10-30

Source: ISO catalogue data for the ISO standards, DIN Media for EN 12487, AENOR catalogue for ASTM A967. ISO 10074 and ISO 4527 are in systematic review in 2026.

A corrosion requirement can be written as a salt spray duration. ISO 9227:2022 defines the neutral salt spray test (NSS) for metals, metallic coatings, conversion coatings and anodic oxide coatings, so the duration to be reached should be agreed with the plater and named on the drawing.

What does anodizing do to aluminium parts and their dimensions?

ISO 7599:2018 is a method for specifying decorative and protective anodic oxidation coatings on aluminium. It covers coating properties, test methods, minimum performance requirements, suitable aluminium grades and pretreatment. It does not cover barrier-layer coatings, chromic or phosphoric acid anodizing, coatings that only prepare the surface for paint or plating, or hard coatings. ISO confirmed the standard in 2023.

Hard anodizing has its own standard. ISO 10074:2021 specifies hard anodic oxidation coatings and their test methods, and its Annex A lists the information the customer must give the anodizer. It excludes plasma electrolytic oxidation, micro-arc oxidation and spark anodizing.

Anodizing changes dimensions. The Aluminum Anodizers Council guide states that the coating penetrates the base metal as much as it builds up on the surface, 50 % each per surface. A coating thickness t therefore adds t/2 to every surface and t to an outside diameter, and removes t from a bore diameter.

Worked example: a hypothetical 20 µm coating on a 25 mm bore adds 10 µm per wall, so the bore ends up 20 µm smaller. An H7 bore at 25 mm has a tolerance band of 21 µm (IT7, ISO 286-1), so the coating uses nearly all of it, and any variation in coating thickness takes the bore out of tolerance. Either state on the drawing that the dimension applies after coating and agree the pre-coating size with the plater, or tolerance the coated bore wider than H7. See ISO 286 fits and IT grades for the band widths.

Which standard applies to zinc plating, and what changed in 2025?

ISO 2081:2018 was withdrawn when ISO published ISO 2081:2025 on 11 November 2025. The new title is "Electroplated coatings on iron and steel using zinc treated with solutions containing chromium (VI)", and its scope names supplementary treatments using hexavalent chromium compounds. It still covers the information the purchaser gives the electroplater and heat treatment before and after plating, and it notes that coating thickness on threaded components may be limited by dimensional requirements, including class or fit.

Zinc plating with a Cr(VI)-free passivate falls under ISO 19598:2016 instead. That standard covers electroplated zinc and the alloys zinc/nickel and zinc/iron on iron and steel, and it specifies the designations, the minimum corrosion resistance in defined tests and the minimum thicknesses. ISO last confirmed it in 2022. A drawing that says "zinc plated to ISO 2081, trivalent chromium passivate" cites a standard that no longer covers the passivate; ISO 19598 does.

Two points for the drawing: state the steel grade and its heat-treated condition, because ISO 2081 sets heat-treatment requirements before and after plating, and check the coating thickness on threads against the thread fit.

A withdrawn standard can still sit on old drawings. ISO 10546 for chromate conversion coatings on aluminium is withdrawn, and the European standard EN 12487:2007 replaced it. ISO 7599:2010 was replaced by the 2018 edition, and ISO 2081:2018 by the 2025 edition.

What does passivation do for stainless steel?

Passivation is a chemical treatment of stainless steel parts that mainly removes free iron from the surface. ASTM A967/A967M-25 covers nitric acid immersion, citric acid immersion and electrochemical treatment, plus alternative qualitative tests with acceptance criteria to confirm that the passivation worked. It makes no recommendation on which grade or treatment suits a given application, so the drawing has to name the treatment and the test.

ISO 16048:2003 is the ISO passivation standard, but it covers corrosion-resistant stainless steel fasteners only. ISO confirmed it in 2024. For turned and milled parts, name ASTM A967 or the customer specification.

The chemistry matters for compliance. In the 2013 edition of ASTM A967, one nitric acid treatment, Nitric 1, contains sodium dichromate. Sodium dichromate is a hexavalent chromium compound and is also listed in REACH Annex XIV. The citric acid treatments do not use it.

How do REACH and RoHS affect surface treatments?

Three EU instruments touch these processes through hexavalent chromium.

Table 2. EU rules that limit hexavalent chromium in surface treatments

Source: Regulation (EU) No 348/2013, Annex; Directive 2011/65/EU, Annex II; Directive 2000/53/EC, Annex II.

REACH Article 56(1) bars placing an Annex XIV substance on the market or using it unless an authorisation covers the use. Article 56(2) lets a downstream user rely on an authorisation held higher in the supply chain if it follows the conditions, and under Article 66 that user notifies ECHA within three months of first supply. For a buyer this means the obligation sits with whoever uses the substance, which for a plating bath is the plater, and the question to ask is which authorisation covers that line. Whether a given passivate or conversion bath uses an Annex XIV substance depends on its chemistry.

RoHS applies to electrical and electronic equipment, including cables and spare parts, so a machined housing for a sensor or a power unit inherits the limit from the end product. The maximum values apply per homogeneous material, defined as a material that cannot be separated into different materials by mechanical actions such as slicing, grinding or abrasion. A coating that can be removed by abrasion is therefore judged on its own, not diluted by the mass of the part. Cadmium has a limit of 0.01 %, lead and mercury 0.1 %.

For vehicles, the ELV Directive 2000/53/EC sets the same 0.1 % for hexavalent chromium. It is being replaced by an ELV Regulation, which the Commission lists as in force from 13 August 2026 and applying from 1 September 2028, with the Directive continuing until then. Check its substance rules against the customer's specification.

ECHA announced a restriction proposal for hexavalent chromium substances in April 2025 that would move them from the authorisation list into a restriction. As of this article it is a proposal, not law. Ask the plater whether a Cr(VI)-free alternative is qualified for your part, so the choice is made before any restriction takes effect.

What should the drawing say about the surface treatment?

A drawing that says only "anodize" or "zinc plate" leaves the standard, thickness, chromium chemistry and dimension reference open.

Check these entries before a drawing goes out for quotation:

• The standard with edition year, such as ISO 7599:2018, ISO 10074:2021, ISO 19598:2016 or ASTM A967/A967M-25.
• Whether dimensions apply before or after coating, and which surfaces stay uncoated or masked.
• Minimum thickness or the class in the standard, and the salt spray duration to ISO 9227 if corrosion is a requirement.
• Hexavalent chromium allowed or excluded, with the RoHS or ELV reason when it applies.
• The base material grade and, for zinc plating, the heat-treated condition.
• Surface roughness before treatment. Coatings change the final roughness, see Ra vs Rz surface roughness.

The technical drawing checklist lists the other title block entries a supplier needs.

Frequently asked questions

What is the difference between anodizing and zinc plating?

Anodizing converts the surface of aluminium into an oxide layer and is specified in ISO 7599:2018 (decorative and protective) or ISO 10074:2021 (hard). Zinc plating deposits a zinc layer on iron or steel and is specified in ISO 2081:2025 with hexavalent chromium passivation or in ISO 19598:2016 with a Cr(VI)-free treatment.

Does anodizing change the dimensions of a part?

Yes. The oxide layer grows into the base metal and out of the surface in roughly equal parts, so a coating of 20 µm adds about 10 µm to each surface. On a bore that is 20 µm smaller in diameter, which nearly fills the 21 µm tolerance band of an H7 fit at 25 mm.

Is hexavalent chromium banned in the EU?

Not outright. Chromium trioxide has been on REACH Annex XIV since the sunset date of 21 September 2017, so it may only be used under an authorisation. In electrical and electronic equipment, RoHS limits hexavalent chromium to 0.1 % by weight in any homogeneous material.

Which standard covers passivation of stainless steel parts?

ASTM A967/A967M-25 covers nitric acid, citric acid and electrochemical passivation of stainless steel parts. ISO 16048:2003 is the ISO standard, but it covers corrosion-resistant stainless steel fasteners only.

Does ISO 2081 still cover zinc plating with a trivalent chromium passivate?

No. ISO 2081:2025, which replaced the 2018 edition on 11 November 2025, is titled for zinc treated with solutions containing chromium (VI). Zinc and zinc-alloy coatings with a Cr(VI)-free treatment are specified in ISO 19598:2016.

Sources

1. ISO 7599:2018, decorative and protective anodic oxidation coatings (ISO catalogue)
2. ISO 10074:2021, hard anodic oxidation coatings (ISO catalogue)
3. ISO 2081:2025, electroplated coatings on iron and steel using zinc treated with Cr(VI) solutions (Genorma, ISO catalogue data)
4. ISO 19598:2016, zinc and zinc alloys with supplementary Cr(VI)-free treatment (Genorma, ISO catalogue data)
5. ISO 11408:1999, black oxide coating on iron and steel (Genorma, ISO catalogue data)
6. ISO 4527:2003, autocatalytic nickel-phosphorus coatings (Genorma, ISO catalogue data)
7. ISO 9227:2022, salt spray tests (Genorma, ISO catalogue data)
8. ISO 16048:2003, passivation of stainless-steel fasteners (Genorma, ISO catalogue data)
9. SS-ISO 10546, withdrawn, replaced by SS-EN 12487 (SIS)
10. DIN EN 12487:2007-07, chromate conversion coatings on aluminium (DIN Media)
11. ASTM A967/A967M-25, chemical passivation treatments for stainless steel parts (AENOR catalogue)
12. ASTM A967/A967M-13 treatments, Nitric 1 with sodium dichromate (Advanced Plating Technologies summary)
13. Anodizing Reference Guide, Aluminum Anodizers Council (50 % penetration, 50 % buildup)
14. Regulation (EU) No 348/2013, Annex XIV entries, chromium trioxide entry 16 (legislation.gov.uk)
15. Regulation (EC) No 1907/2006 (REACH), Article 56, use of Annex XIV substances (legislation.gov.uk)
16. Regulation (EC) No 1907/2006 (REACH), Article 66, notification by downstream users (legislation.gov.uk)
17. Directive 2011/65/EU (RoHS), Annex II, maximum concentration values (legislation.gov.uk)
18. Directive 2011/65/EU (RoHS), Article 3, definitions (legislation.gov.uk)
19. RoHS Directive, restricted substances (European Commission)
20. Directive 2000/53/EC (ELV), Annex II, tolerated concentrations (legislation.gov.uk)
21. End-of-life vehicles, replacement of the ELV Directive (European Commission)
22. ECHA chromium (VI) restriction proposal, announced 29 April 2025 (Tetra Tech summary)
23. ISO 286-1:2010 standard tolerance grades, IT7 for 18–30 mm (table reproduced on Wikipedia)

Questions on a drawing? sales@ertas.engineering

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