Harteloxieren

Harteloxieren, or hard anodizing, is a surface-treatment process for aluminum and its alloys that creates a thick, hard aluminum oxide layer (Al2O3) by electrochemical oxidation. The oxide layer is integral, ceramic-like, and grown in an electrochemical bath, typically sulfuric acid at low temperatures (near 0–5 °C). The process is carried under controlled current or pulsed current to achieve thick coatings with minimal shrinkage. Common layer thickness ranges from about 25 to 150 micrometres, with hardness often in the 350–600 HV range, and strong wear and corrosion resistance. The porous as-formed coating can be impregnated with lubricants or sealants; sealing by hot water or specific sealants closes pores and enhances barrier properties.

Common electrolytes include sulfuric acid (the most widely used) and, less commonly, oxalic or phosphoric acids, which influence pore structure and sealing behavior. The coating is attached to aluminum alloys, especially 2xxx and 6xxx series, sometimes 5xxx. The procedure usually involves cleaning, pre-oxidation, anodizing, rinsing, and sealing.

Applications span high-wear-resistance components such as piston rings, cylinder liners, gears, hydraulic components, cutting tools, and die-casting mold components, as well as aerospace and automotive parts. The process increases hardness and wear resistance while preserving dimensional accuracy. Limitations include brittleness of the oxide, potential cracking under impact, and the need for careful control of bath temperature, current density, and post-treatment sealing. Environmental and safety considerations arise from acidic electrolytes and waste management.