ISO 28340:2013 – Combined Coatings on Aluminium — Electrophoretic Organic Coatings and Anodic Oxidation

Introduction to Combined Coating Systems on Aluminium

ISO 28340:2013 establishes the general specifications for combined coating systems on aluminium that integrate anodic oxidation with electrophoretic organic coatings. This hybrid approach delivers superior corrosion protection and aesthetic durability compared to either coating applied alone. The standard addresses a critical need in architectural construction, automotive trim, and consumer electronics where aluminium components require both decorative finish and long-term environmental resistance. The synergy between the two processes is scientifically grounded: anodic oxidation produces a porous oxide layer with controlled pore diameter (typically 20-80 nm depending on electrolyte composition and current density), which serves as an ideal mechanical interlock for the subsequently applied electrophoretic organic coating. This interlocking mechanism at the nanoscale produces adhesion strengths that far exceed simple surface bonding, typically achieving cross-cut adhesion ratings of classification 0 (no detachment) per ISO 2409 even after 1000 hours of salt spray exposure.

Technical Requirements and Performance Metrics

The standard specifies minimum anodic oxide mass per unit area of 10 g/m² for exterior applications and 5 g/m² for interior use. The electrophoretic organic coating must achieve a minimum thickness of 12 μm measured in accordance with ISO 2360. Combined coating systems must pass a 500-hour neutral salt spray test per ISO 9227 with no blistering exceeding 2 mm from the scribe line. Additional performance requirements include a minimum impact resistance of 2.5 J (ASTM D2794), cross-cut adhesion classification of 0 to 1, and a 3000-hour accelerated weathering test (ISO 4892-2) with maximum 50% gloss retention for exterior grades. The standard introduces rating number charts in normative annexes B and C that provide standardized photographic references for evaluating corrosion and alkali resistance on a 0 to 5 scale, eliminating subjective interpretation across different laboratories and inspectors.

Engineering Design Insights for Combined Coating Applications

Substrate alloy selection is critical for achieving consistent coating quality. Alloys from the 6xxx series (Al-Mg-Si) consistently produce the most uniform anodic oxide layers with pore diameters in the 15-25 nm range that are ideal for electrophoretic deposition. High-copper 2xxx series alloys should be avoided due to non-uniform oxide formation characterized by coarse porous structures and localized dissolution. For 5xxx series alloys (Al-Mg), the magnesium content should not exceed 3.5% to prevent MgO segregation at the oxide-metal interface. Electrophoretic bath parameters — pH maintained at 5.8-6.2, solid content 10-15%, and bath temperature 28-32°C — directly influence coating smoothness and edge coverage. The applied voltage profile during deposition should follow a controlled ramp: initial 5-second hold at 30 V for wetting, followed by a linear ramp to 100 V over 30 seconds, then a 3-minute constant voltage hold for film consolidation. This profile minimizes edge defects and ensures uniform coating thickness on complex geometries with deep recesses and sharp corners.

Fabrication sequence matters significantly: mechanical forming operations such as bending, stamping, and roll-forming should be completed before anodizing, as post-coating deformation causes microcracking at the organic-inorganic interface due to the modulus mismatch between the brittle oxide (E ≈ 80 GPa) and the compliant organic coating (E ≈ 2 GPa). When post-coating machining is unavoidable, edges must be sealed with compatible two-part polyurethane touch-up paints that meet the full performance requirements of ISO 28340, including salt spray and weathering resistance.

Quality Assurance and Testing Protocol

ISO 28340 mandates comprehensive testing including adhesion assessment by cross-cut test (ISO 2409), impact resistance (ASTM D2794), and chemical resistance evaluation against specified reagents including 10% HCl, 10% NaOH, and methyl ethyl ketone. Production facilities must implement statistical process control for anodic oxide thickness with Cpk values exceeding 1.33 and conduct daily calibration checks on coating thickness gauges using certified reference standards. The normative annexes provide photographic rating charts for corrosion and alkali resistance evaluation, with specific reference images for rating levels 0 (no attack) through 5 (severe attack). These visual standards ensure consistent evaluation across different inspectors and laboratories, eliminating the subjective variability that historically plagued coating performance assessment. The standard also requires that production lots be sampled according to ISO 2859-1 with an AQL of 1.0% for appearance defects and 0.65% for performance-related failures.

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