Chemical
Surface
Treatment

Sulphuric acid anodising for aluminium CNC parts at moderate current density — producing a 5–25μm porous oxide layer that can be dyed in a wide colour range before sealing. The standard cosmetic surface treatment for aluminium consumer electronics, instrument panels, and architectural components. The porous oxide accepts dye before the final sealing step (hot DI water or nickel acetate seal closes the pores and locks the colour). Colour options: clear (natural silver), black, gold, champagne, red, blue, and custom RAL/Pantone matching. Hardness HV 200–300. Corrosion resistance improved vs bare aluminium. Dimensional change: each surface adds approximately 50% of total layer thickness outward — account for this in pre-anodise machining dimensions for tight fits. RoHS compliant (no hexavalent chromium).

High current density, low temperature sulphuric acid anodising producing a hard, dense oxide layer 25–50μm thick. Hardness HV 400–600 (comparable to hard chrome). Colour: dark grey to black in natural form due to layer thickness. Wear resistance excellent — standard for sliding aluminium components (pistons, valve bores, guide rails, aerospace structural parts). Corrosion resistance superior to Type II. Dimensional change is significant — typically 12–25μm added to each external surface — pre-anodise machining dimensions must be adjusted. For threaded holes: thread measured after hard anodise to confirm fit; for bore fits (H7), we calculate pre-anodise bore size at DFM. Compliance: MIL-A-8625 Type III.

Alkaline blackening (black oxide, bluing) for steel and stainless steel CNC parts. A chemical conversion coating producing a thin magnetite (Fe₃O₄) layer <1μm thick — dimensions are essentially unchanged (no dimensional allowance required). Black finish for aesthetic and light-absorption applications. Mild corrosion resistance (significantly improved with post-treatment oil or wax). Primary applications: optical and photographic equipment components (reduce reflectivity), tooling and gauges (identification and mild corrosion protection), and fasteners (cosmetic appearance). Steel black oxide: alkaline salt bath at 130–150°C. Stainless black oxide: different chemistry — lower temperature process. Must be followed by oil or wax treatment for meaningful corrosion resistance.

Nitric or citric acid treatment for stainless steel and titanium CNC parts — removing free iron, iron oxide particles, and machining contamination to restore and enhance the chromium oxide passive layer. Machining operations introduce iron contamination (from steel tooling and fixtures) onto stainless steel surfaces — these particles cause premature rust spots in humid environments. Passivation removes this contamination. Specifications: ASTM A967 (nitric or citric acid passivation of stainless steel); AMS 2700 (aerospace). Citric acid passivation preferred for medical and food-contact parts (RoHS, no nitric acid disposal issues). Testing: copper sulphate test or salt spray per ASTM A380 to verify effective passivation. Mandatory for stainless steel medical device parts, food-processing equipment, marine hardware.

Hydrochloric or sulphuric acid pickling for carbon steel CNC parts — removing mill scale, rust, heat tint, and heavy surface oxide contamination that undermines paint and coating adhesion. Mill scale (formed during hot rolling of steel bar stock) is an iron oxide layer up to 50μm thick that is cathodic to the steel substrate — if left under paint, it accelerates undercutting corrosion at any coating discontinuity. Acid pickling removes mill scale completely, exposing clean reactive steel that accepts paint, zinc phosphate, powder coating, and electroplating with maximum adhesion. Inhibited acid formulation prevents hydrogen embrittlement in high-strength steels (>1200 MPa UTS). Post-pickling rinse and immediate flash primer or conversion coating application required to prevent flash rust.

Nitric-hydrofluoric acid (HNO₃-HF) pickling for stainless steel and titanium CNC machined and welded parts — removing heat tint (the discoloured oxide zones around weld beads), heavy surface contamination, and embedded iron particles. Stainless steel weld heat tint is a chromium-depleted zone under the oxide discolouration — pickling removes both the tint oxide and the chromium-depleted layer, restoring the alloy's full corrosion resistance over the weld. Followed immediately by passivation (nitric or citric acid) to maximise the passive layer. Required for food-processing vessels, pharmaceutical equipment, and marine stainless steel assemblies where weld heat tint would compromise corrosion performance. Titanium pickling: removes alpha case (oxygen-enriched brittle surface layer formed during forging/heat treatment) and heat tint from titanium machined parts.