Added coating. Grown oxide. Different physics.

Yes. Aluminum plating possible but requires special preparation. Zincate process: dips aluminum in zincate solution creating zinc layer, then plating adheres to zinc. Common for: aluminum with chrome plate (appearance), aluminum with copper plate (electrical). Direct plating of aluminum difficult due to natural oxide layer forming instantly. Specialty process — not common outside specific applications.

No, not conventional anodizing. Anodizing requires substrate to form stable protective oxide — aluminum does, iron does not (iron oxide is Fe2O3 = rust, not protective). For steel surface treatment: bluing (iron oxide formation, different process), phosphate conversion, black oxide. None equivalent to aluminum anodizing. For protective decorative coating on steel: electroplating or powder coat.

Possible and sometimes done. Examples: (1) Aluminum part anodized, then specific area masked and plated (for electrical contact spots). (2) Hard-chrome plated onto anodized aluminum (rare specialty process). (3) Anodize aluminum, then paint (paint adheres to anodized surface well). Most common: choose one primary surface treatment per application. Mixing adds cost and complexity.

Hard chrome plate: 60+ HRC, 25-50 µm thick, excellent wear resistance. Type III anodize: 300-500 HV (~50 HRC equivalent), 25-75 µm thick, good wear resistance. For very severe wear: hard chrome on steel substrate. For aluminum wear applications: Type III anodize. Hard chrome is tougher per µm but anodize is sufficient for most aluminum applications.

Hexavalent chromium (Cr⁶⁺) plating: toxic, regulated. Phasing out globally — moving to trivalent chromium (Cr³⁺) which is safer but different cosmetic appearance. Zinc plating: relatively benign. Electroless nickel plating: uses hypophosphite, relatively controlled. Anodizing: sulfuric acid process, controlled discharge. Both processes require environmental controls and wastewater treatment. For RoHS compliance, specify trivalent chromium or alternative.

For same aluminum part: Type II anodize $5-15, Type III anodize $10-25, chrome plate (after zincate prep) $20-50. Plating more expensive on aluminum due to preparation. For steel part, plating is primary option — anodize not applicable. Cost driven by: part size, surface area, coating thickness, rack setup, masking requirements. Volume production reduces per-part cost significantly.