Overview
ISO 7441:2015 - "Corrosion of metals and alloys - Determination of bimetallic corrosion in atmospheric exposure corrosion tests" - specifies and compares standardized methods to determine bimetallic (galvanic) corrosion of metals and coated metals under natural atmospheric exposure. The standard describes how to prepare specimens, expose them, evaluate corrosion (amount and type), and calculate the bimetallic effect - the relative increase in corrosion caused by electrical contact between dissimilar metals in an atmosphere.
Key features:
- Compares three test configurations: rectangular plates, washers, and wire-on-bolt.
- Covers specimen types (test, reference, standard, control), preparation, exposure and evaluation procedures.
- Intended to quantify bimetallic effects that typically act within about 0.5 cm of the contact point in atmospheric conditions.
Key technical topics and requirements
- Test methods compared
- Rectangular plates - allows evaluation of mechanical properties in addition to mass loss; recommended for tensile testing specimens (thickness 1–6 mm).
- Washers - useful when the anodic material is not known in advance.
- Wire-on-bolt - shorter exposure time (typically >90 days acceptable) but consult ASTM G116 for full method details.
- Specimen preparation
- Types: test, reference, standard, control.
- Surface condition: free of visible defects; surface roughness better than 2.5 µm (0.4–0.5 µm recommended).
- Coated metals must follow appropriate coating standards (e.g., ISO 1456, ISO 2081, ISO 7599).
- Assembly and electrical checks
- Use insulating washers/sleeves (ceramics, polyethylene/polypropylene) to isolate bolts.
- Bolts/washers typically stainless steel grade 1.4301/304 or higher.
- Typical tightening torque ≈ 1.0 ± 0.1 N·m; verify insulation (>10 kΩ between bolt and washers) and contact resistance (