Aluminium Anodising Knowledge App

Chromic-acid anodising typically operates at higher voltage and lower current density than sulfuric anodising. The film is relatively thin and has a branched or less regularly porous morphology.

Sulfuric acid attacks the outer oxide while the electric field grows new oxide at the metal/oxide interface. The balance between formation and dissolution creates ordered pores above a thin barrier layer.

Type III generally uses sulfuric-acid-based chemistry at low temperature, strong agitation and high current density. Cooling suppresses chemical dissolution so that a much thicker coating can grow.

• Increasing voltage is normally required as the electrically resistive film thickens.
• Local current concentration at edges and sharp features can cause burning.
• High-silicon and high-copper alloys generate more heat and may limit achievable thickness.
• Pulse, ramped-current or modified electrolytes may be used where permitted by the qualified process.

• Water-break-free surface after cleaning.
• Controlled cleaner concentration, temperature, time and contamination.
• Etching changes dimensions, surface roughness and appearance.
• Nitric-acid-based or proprietary deoxidisers remove copper, silicon and other smut.
• Mixed assemblies, inserts and trapped solution require special review.

• Contact must carry the required current without overheating.
• Contact marks must be located in permitted areas.
• Racks must resist the electrolyte and remain mechanically secure.
• Air pockets prevent coating; gas pockets and poor orientation cause defects.
• Contact pressure must account for oxide formation around the contact point.

• Control coating thickness and pore condition before dyeing.
• Control dye identity, concentration, pH, temperature, time and contamination.
• Use colour standards and defined lighting where colour matching is required.
• Rinse thoroughly before sealing to prevent bleed, staining and seal contamination.
• UV exposure, heat and chemicals can fade some dyes.

• Control seal chemistry, pH, temperature, time, water quality and loading.
• Under-sealing leaves excessive porosity and poor corrosion resistance.
• Over-sealing may produce bloom, smut, loss of colour or dimensional change.
• Hardcoat should remain unsealed where wear is primary unless sealing is explicitly required.

Weld metal and heat-affected zones may anodise to a different colour and thickness. Copper-bearing filler, silicon-rich filler and brazing residues can produce severe colour differences or local attack. Assemblies that trap acid require drainage and rinsing provisions.

Anodising does not normally create the classic steel hydrogen-embrittlement mechanism, but surface attack, residual stress, thick brittle oxide and sealing temperature can affect fatigue performance. Highly stressed parts, sharp radii and high-strength alloys need design-authority review.

• Compatible cleaning, etching (if required), deoxidising, anodising, dye, seal and rinse tanks.
• Rectifier with controlled current, voltage, ramp and recording capability.
• Cathodes correctly sized and positioned for uniform current distribution.
• Refrigeration and heat exchangers, especially for Type III.
• Air agitation or solution circulation designed to avoid local heating.
• Racks, clamps, masking plugs, tapes and stop-off materials.
• Hoists, timers, drainage stations and clean drying equipment.

• Acid titration and dissolved-aluminium analysis equipment.
• Calibrated temperature probes, pH meters and conductivity meters.
• Coating-thickness instruments and metallographic sectioning facilities where required.
• Analytical balance for coating-weight tests.
• Seal-quality, dye-bleed and spot-test equipment.
• Salt-spray cabinet, abrasion tester and dielectric test equipment.
• Calibrated current and voltage measurement independent of display indicators where required.

• Accurate surface-area calculation and current-density control.
• Controlled current ramp to avoid thermal shock and edge burning.
• High-capacity refrigeration and continuous temperature monitoring.
• Strong, uniform agitation around recessed and massive sections.
• Thickness checks at representative low- and high-current-density areas.
• Defined sealing status and allowance for final grinding or honing if permitted.

• Scheduled analysis and statistical review of trends.
• Control dissolved aluminium by decant, regeneration, ion exchange or bath replacement as applicable.
• Inspect cathodes, busbars, contacts, cooling coils and agitation.
• Prevent chloride, copper, iron, cleaner and dye cross-contamination.
• Process representative test panels after major adjustment or maintenance.

• Complete coverage except at permitted contact and masked areas.
• No burns, arcing, powder, pitting, corrosion, stains, scratches or handling damage.
• Uniformity appropriate to alloy and part geometry.
• Class 2 colour compared against the specified standard under controlled lighting.
• Contact marks located only where permitted.

Thickness may be measured by eddy-current instruments, microscopic cross-section or other approved methods. Curvature, roughness, substrate conductivity, alloy, edge effects and calibration standards influence results.

Neutral salt-spray testing is commonly used for process-control or qualification. Test panels, alloy, coating type, class, seal, edge preparation, exposure and acceptance criteria must match the governing requirement.

Seal quality may be evaluated by acid-dissolution, dye-stain, admittance, mass-loss or other specified methods. A visually acceptable sealed coating can still have excessive residual porosity.

Type III performance is often verified by abrasion resistance rather than indentation hardness alone. Alloy, thickness, seal, test wheel, load, cycles and conditioning significantly affect results.

Anodic oxide is electrically insulating. Breakdown voltage depends on thickness, porosity, defects, sealing, humidity and test geometry. Do not rely on anodising for safety-critical insulation without a defined and validated requirement.

• MIL-PRF-8625 and required revision.
• Type I, II or III and Class 1 or 2.
• Required coating thickness or permitted range.
• Areas to anodise, mask, plug or leave conductive.
• Final dimensional limits and whether dimensions apply before or after coating.
• Seal type or explicit “unsealed” requirement, particularly for Type III.
• Dye colour, colour standard, gloss and colour-matching requirements.
• Contact-mark locations and touch-up permissions.
• Post-anodise grinding, lapping, honing, impregnation or lubricant.
• Special corrosion, abrasion, dielectric, adhesion or fatigue requirements.

• Part number, drawing and revision.
• Alloy, temper, casting or forging condition.
• Specification, revision, type, class, thickness and sealing.
• Approved processor and customer-specific process specification.
• Required certification, test reports, retained panels and traceability.
• Sampling plan and lot definition.
• Packaging requirements to prevent scratching, fretting and moisture staining.
• Notification or approval before chemistry, source, seal or process changes.