ISO 7441:2015

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 (<1 Ω between plates).
• Evaluation methods
  • Mass loss (primary), visual examination, mechanical-property assessment where applicable.
  • Calculation of the bimetallic effect as a relative measure; site corrosivity classification (e.g., ISO 9223) is recommended for context.
• Exposure durations
  • Rectangular plates and washers: generally ≥ 1 year; wire-on-bolt shorter exposures may be acceptable under conditions.

Practical applications and users

ISO 7441:2015 is aimed at:

• Corrosion engineers and materials scientists evaluating galvanic risks in atmospheric environments.
• Testing and certification laboratories performing exposure tests for metals, alloys and coatings.
• Coatings manufacturers and specifiers assessing protective system performance when dissimilar metals are in contact.
• Asset owners (infrastructure, marine, construction, transportation) needing to manage bimetallic corrosion risk.

Use cases:

• Comparing corrosion rates of candidate material pairings.
• Verifying design choices where dissimilar metals are joined or in proximity.
• Supporting material selection, maintenance planning and failure investigations.

Related standards

• ISO 9223 (atmospheric corrosivity classification)
• ASTM G116 (wire-on-bolt practice)
• ISO 8565, ISO 8044, ISO 8407, ISO 6892-1
• ISO 1456, ISO 2081, ISO 7599, ISO 15510

Keywords: ISO 7441:2015, bimetallic corrosion, atmospheric exposure tests, rectangular plates, washers, wire-on-bolt, corrosion testing, bimetallic effect, mass loss evaluation.