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ISO 27831-2: Cleaning and Preparation of Metal Surfaces — Non-Ferrous Metals
Standard processes for cleaning non-ferrous metal surfaces prior to coating, including aluminium, copper, titanium, and magnesium alloys
Introduction to ISO 27831-2
ISO 27831-2:2008 specifies cleaning and preparation processes for non-ferrous metals and alloys, including aluminium, copper, nickel, titanium, magnesium, zinc, tin, tungsten, lead and their alloys, as well as coated items. This part complements ISO 27831-1 (ferrous metals) and is designed as a “stand-alone” document — Clauses 1 through 6 are reproduced from Part 1, and the cleaning processes follow the same designation system (Processes A through M from Part 1, extended with Processes N through Z for non-ferrous-specific treatments).
Non-ferrous metals present unique cleaning challenges: aluminium forms a tenacious oxide layer that requires specific etching treatments, titanium is susceptible to hydrogen absorption and stress corrosion cracking, and magnesium alloys are highly reactive and require special handling to avoid excessive chemical attack.
Metal-Specific Cleaning Requirements
The standard dedicates detailed clauses to each non-ferrous metal group, recognizing their distinct surface chemistry and cleaning requirements. Aluminium and its alloys receive the most extensive treatment (Clause 11), with specific processes for degreasing, corrosion product removal, oxide removal (etching), preparation for anodizing, adhesive bonding, electroplating, autocatalytic nickel plating, conversion coating, metal spraying, vitreous enamelling, and PVD supplementary treatments.
| Metal Group | Clause | Key Cleaning Considerations |
|---|---|---|
| Aluminium and alloys | 11 | Oxide layer removal (etching); avoid strong alkalis; separate abrasive media required |
| Copper and alloys | 12 | Dichromate/sulfuric acid cleaning; bright dipping; nitric acid pickling |
| Nickel and alloys | 13 | Electropolishing and chemical polishing options; passivation considerations |
| Titanium and alloys | 14 | Hydrogen embrittlement risk; specific acid pickling formulations (Process S) |
| Magnesium alloys | 15 | High reactivity; special chromate conversion pre-treatments; avoid aqueous acid contact |
| Zinc-based alloys | 16 | Die casting surface preparation; chromate conversion compatibility |
Titanium alloys require special attention: certain halogenated solvents (trichloroethylene, perchloroethylene) can cause stress corrosion cracking in titanium. The standard explicitly restricts their use, and Process S provides dedicated acid pickling formulations using nitric and hydrofluoric acid mixtures for titanium surface preparation.
Extended Cleaning Processes (N through Z) and Engineering Application
Part 2 extends the cleaning alphabet with 12 additional processes: Process N (acid pickling of aluminium alloys, 6 methods), Process O (oxide removal/etching of aluminium), Process P (corrosion product removal, etching, chemical polishing, electrobrightening, and electropolishing of aluminium), Process Q (cleaning copper and copper alloys), Process R (acid descaling of copper alloys), Process S (titanium pickling), Process T (nickel pickling and polishing), Process U (tin alloy preparation), Process V (gold coating contamination removal), Process W (aluminium preparation for electroplating), Process X (nickel preparation for electroplating), Process Y (titanium preparation for electroplating with 3 methods), and Process Z (zinc preparation for electroplating with 2 methods).
For engineering practice, the most critical consideration is matching the cleaning process to both the base metal and the intended subsequent coating. For example, aluminium intended for anodizing requires a different oxide removal treatment (Process O) than aluminium intended for electroplating (Process W). The standard provides explicit process selection guidance throughout, with cross-references to ensure the correct sequence is followed.
When processing mixed-material assemblies, the standard requires that the chosen treatment be suitable for each constituent material and combination. This is particularly important for assemblies combining ferrous and non-ferrous components — the cleaning process must not damage either material type.
Frequently Asked Questions
Q: Can ISO 27831-2 be used independently of Part 1?
A: Yes. The standard is designed as “stand-alone” — Clauses 1 through 6 (Scope, Normative references, Terms, Essential information, Requirements, Standard cleaning methods) are reproduced from Part 1, and the same process designation system is used throughout.
A: Yes. The standard is designed as “stand-alone” — Clauses 1 through 6 (Scope, Normative references, Terms, Essential information, Requirements, Standard cleaning methods) are reproduced from Part 1, and the same process designation system is used throughout.
Q: What is the recommended method for cleaning aluminium prior to anodizing?
A: Process O (oxide removal/etching) is specified for aluminium preparation prior to anodizing and adhesive bonding. The treatment involves degreasing followed by immersion in a sodium hydroxide-based etching solution, with specific formulations and conditions detailed in the standard.
A: Process O (oxide removal/etching) is specified for aluminium preparation prior to anodizing and adhesive bonding. The treatment involves degreasing followed by immersion in a sodium hydroxide-based etching solution, with specific formulations and conditions detailed in the standard.
Q: How should corrosion products be removed from aluminium surfaces?
A: Method P1 (immersion) or P2 (swabbing) are specified, using a phosphoric acid and sodium dichromate solution, or a nitric acid solution. The choice depends on the extent of corrosion and the component geometry.
A: Method P1 (immersion) or P2 (swabbing) are specified, using a phosphoric acid and sodium dichromate solution, or a nitric acid solution. The choice depends on the extent of corrosion and the component geometry.
Q: What are the special considerations for cleaning magnesium alloys?
A: Magnesium alloys are highly reactive and require chromate-based conversion pre-treatments before further coating. Aqueous acid solutions must be carefully controlled to avoid excessive attack, and the standard provides specific formulations for different magnesium alloy types and conditions (as-cast, machined, forged, sheet).
A: Magnesium alloys are highly reactive and require chromate-based conversion pre-treatments before further coating. Aqueous acid solutions must be carefully controlled to avoid excessive attack, and the standard provides specific formulations for different magnesium alloy types and conditions (as-cast, machined, forged, sheet).
