What testing methods verify arc-anodized coating quality?
Comprehensive Testing Methods for Arc-Anodized Coatings
Verifying the quality of an arc-anodized coating requires a multi-faceted approach that assesses its physical dimensions, mechanical properties, and corrosion resistance. These tests are crucial for ensuring the coating meets the stringent requirements of industrial standards, such as MIL-A-8625 Type III, and customer-specific specifications.
Coating Thickness and Uniformity Measurement
The most fundamental verification is measuring the coating thickness, a primary determinant of performance. This is typically done using:
Eddy Current Method (ASTM B244): A quick, non-destructive method suitable for quality control on the production line. It provides immediate feedback on whether the coating meets the specified minimum, often 50 µm (2 mils) for hard coat applications.
Cross-Sectional Microscopy (ASTM B487): This is the most accurate method. A sample is sectioned, mounted, and polished, and the coating thickness is measured directly under a microscope. This also reveals the coating's microstructure, including its density, adhesion to the substrate, and the presence of any cracks or voids.
Mechanical and Wear Resistance Testing
The exceptional hardness and durability of the arc-anodized layer are verified through several key tests:
Microhardness (ASTM E384 / ASTM B647): A Knoop or Vickers indenter is used with a specific load (e.g., 500 gf) to measure the surface hardness. A quality arc-anodized coating on a compatible alloy like A360 should consistently exhibit values exceeding 400 HK, often reaching 500-600 HK or higher.
Abrasion Resistance (ASTM G65): The Dry Sand/Rubber Wheel test quantifies the coating's volumetric wear loss. A low wear volume confirms the coating's ability to withstand severe abrasive environments.
Adhesion (ASTM D3359): The cross-cut tape test is performed to ensure the ceramic coating is metallurgically bonded to the substrate and will not spall or delaminate under stress.
Corrosion Protection and Seal Quality Verification
Since corrosion resistance is a key function, several tests validate the coating's integrity:
Salt Spray Testing (ASTM B117): The industry-standard accelerated corrosion test. A quality arc-anodized coating is typically specified to withstand 500 to 1000+ hours of exposure without the appearance of pitting or base corrosion, depending on the application's severity.
Seal Quality Tests: The effectiveness of the post-anodizing seal is critical for corrosion resistance. This is verified by:
Acid Dissolution Test (ASTM B680): Measures the impedance of the sealed coating; a high impedance value indicates a well-sealed, less porous surface.
Dye Stain Test (ASTM B136): A drop of acidified dye is applied; if the seal is poor, the dye is absorbed, leaving a permanent stain.
Die-Casting Specific Considerations
When applying Arc Anodizing to die castings, additional scrutiny is required due to the inherent material characteristics. Cross-sectional analysis is particularly important to inspect the coating formation over the aluminum-silicon eutectic structure. The coating must be continuous and well-adhered even in areas with high silicon concentration to prevent localized failure. This is a key part of the final Post Process validation before components are approved for Mass Production or shipped to clients in demanding sectors like power tools or automotive.
