Anodising

Oxide Layer Formation and Current Control

Anodising is an electrolytic passivation process that grows a controlled aluminium oxide layer on the workpiece surface. Unlike electroplating, the workpiece is the anode. The oxide thickness, hardness, and porosity are directly governed by current density, voltage, and process time. Type II sulphuric acid anodising operates at 1.5–3 A/dm² and 15–21V, producing 5–25 µm oxide layers suitable for architectural and decorative finishes. Type III hard anodising demands higher current densities of 3–6 A/dm² at 25–80V with refrigerated baths near 0°C, producing dense, wear-resistant coatings of 25–150 µm. The rectifier must deliver stable, regulated DC with precise current-density control across these ranges to ensure uniform oxide growth over complex part geometries.

Temperature and Voltage Interaction

Bath temperature is critical in anodising. Sulphuric acid dissolves the oxide layer at a rate that increases with temperature, so the balance between oxide formation (driven by current) and dissolution (driven by temperature) determines final coating properties. Hard anodising requires near-freezing electrolyte temperatures (0–5°C) to minimise dissolution and achieve dense, hard coatings. The rectifier must accommodate the higher voltages needed to push current through thick, resistive oxide layers as they grow — requiring a voltage range of 0–28V or higher with seamless crossover from constant-current to constant-voltage regulation.

Colour Sequencing with pe8705

Electrolytic colour anodising uses alternating current or modified DC waveforms to deposit metallic pigments (tin, cobalt, nickel) into the porous oxide structure. The pe8705 colour sequencing controller automates multi-step colour processes by programming current amplitude, polarity, and timing for each colour step. This eliminates operator error in batch-to-batch colour matching and enables repeatable production of bronze, black, champagne, and custom architectural finishes. The pe8705 integrates directly with PE 3000 series rectifiers via digital bus, managing up to 16 sequential process steps with programmable ramp rates, hold times, and transition profiles.