Galvanic Corrosion Prevention: Mixing Dissimilar Metals in Fasteners

Galvanic corrosion is an electrochemical process that occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte (water, salt spray, humidity). The less noble (more anodic) metal corrodes at an accelerated rate, while the more noble (cathodic) metal is protected. Fasteners are particularly vulnerable because they often have a smaller surface area than the parts they join — when a fastener is the anode in the couple, corrosion can be 10-100× faster than uniform corrosion. The galvanic series ranks metals by their electrode potential in seawater. The anodic index is a simplified, normalised version (measured in volts relative to a calomel electrode in seawater) used by fastener engineers. Metals with higher anodic index values are more noble (cathodic), and metals with lower values are more anodic. **Anodic index of common fastener materials (vs. calomel electrode, seawater):** - Zinc: -1.10 V (most anodic, sacrifices itself) - Magnesium: -1.73 V - Carbon steel (A36/SAE 1020): -0.76 V - Galvanized steel: -1.10 V (zinc) - Stainless steel 304 (passive): -0.05 V - Stainless steel 316 (passive): -0.05 V - Stainless steel 410 (passive): -0.15 V - Aluminum 2024-T3: -0.75 V - Aluminum 6061-T6: -0.75 V - Copper: -0.20 V - Brass (C36000): -0.30 V - Titanium grade 2: -0.05 V - Graphite: +0.30 V (most cathodic) **Why this matters in fasteners:** A stainless steel 316 bolt in aluminum 6061-T6 sheet creates a corrosion couple where the aluminum is the anode. The galvanic potential difference is 0.70 V — well above the 0.25 V threshold typically associated with significant galvanic corrosion. If the joint is exposed to rainwater or salt spray, the aluminum will pit and eventually perforate, even though the stainless steel bolt remains intact. The reverse scenario (aluminum bolt in steel sheet) would be far less damaging because the small aluminum bolt has limited anode area, and the corrosion current is bounded. **Rule of thumb:** Keep galvanic potential difference below 0.25 V for non-critical applications and below 0.15 V for critical or wet service. Use the anodic index to verify, and use isolation methods (next section) when the potential difference cannot be avoided. TradeGo's engineering team can review your fastener material pairings against the anodic index and recommend the safest combination for your service environment. For typical construction fastener applications, our stainless steel bolts range includes A2-70 (304) and A4-70 (316) for marine-grade service, while our galvanized bolts offer a cost-effective sacrificial coating for non-marine applications. Use our fastener material selection guide (below) to verify compatibility for your specific environment.

The most reliable way to prevent galvanic corrosion is to use the same metal for fastener and base material, or to choose metals that are very close on the anodic index. When this is not possible (for example, when stainless steel is required for corrosion resistance but the structure is aluminum), the design must incorporate isolation, coatings, or both. **Recommended fastener-base metal pairings (no isolation needed for non-marine service):** - **Stainless steel 316 ↔ Stainless steel 316:** Ideal. No potential difference. - **Stainless steel 304 ↔ Stainless steel 304:** Good. Identical metal. - **Galvanized carbon steel ↔ Carbon steel (structural):** Good. Zinc coating is sacrificial and protects the underlying steel. - **Aluminum 5052 ↔ Aluminum 6061:** Good. Same alloy family. - **Brass ↔ Copper:** Good. Both copper-rich. **Pairings requiring isolation or coating:** - **Stainless steel 316 ↔ Aluminum 6061-T6:** Use EPDM isolation washer, nylon sleeve, or zinc-rich primer. - **Stainless steel ↔ Galvanized steel:** Generally safe because the zinc is already a sacrificial coating, but if the coating is damaged, the underlying carbon steel may corrode. Use isolation when the galvanized coating is known to be scratched. - **Copper ↔ Steel:** Avoid. 0.56 V potential difference. Use brass or bronze transition fittings. - **Aluminum ↔ Concrete:** Generally safe if the concrete is dry; problematic if the concrete is wet and contains chlorides. Use epoxy-coated or PVC-isolated anchors. - **Stainless steel ↔ Concrete with chlorides:** Marine concrete often contains chloride accelerators. Use epoxy-coated stainless or hot-dip galvanized with isolation. **Common real-world examples:** 1. **Roofing:** Self-drilling screws (carbon steel, zinc-plated or zinc-flake coated) in galvanized or painted steel roofing. Compatible. 2. **Curtain wall:** Stainless steel 316 anchors in aluminum curtain wall mullions. NOT compatible. Use stainless steel with EPDM isolation gaskets, or use aluminum-compatible coated fasteners. 3. **Solar PV mounting:** Stainless steel 304 module clamps in anodized aluminum rails. Marginal — requires EPDM or rubber isolation, and the design must keep the joint dry. 4. **Coastal fencing:** Stainless steel 316 fasteners in powder-coated aluminum posts. Compatible only with isolation; otherwise the powder coat must be the dielectric. 5. **Marine deck hardware:** Stainless steel 316 bolts in fiberglass or aluminium marine decks. Compatible with isolation; the standard solution is stainless steel with butyl rubber or EPDM sealing washers. TradeGo's stainless steel fastener range covers A2-70 (304), A4-70 (316), and A4-80 (316 high-tensile) grades, and we provide compatibility guidance for the most common base metal combinations used in construction, marine, and energy applications. TradeGo also supplies a complete nylon washer and EPDM washer range for isolation applications, with standard and custom thicknesses from 0.5 mm to 5 mm. We also provide flat washers in stainless steel 304, 316 and galvanised carbon steel for conventional applications.

When dissimilar metals must be joined and the anodic potential difference exceeds 0.25 V, isolation methods break the electrical circuit and prevent galvanic current flow. The most common isolation methods are non-conductive washers, bushings, sleeves, gaskets, and sealants. **Non-conductive washers and bushings:** - **EPDM (Ethylene Propylene Diene Monomer):** Most common isolation material. Operating temperature -50°C to +150°C, excellent UV resistance, good chemical resistance. Available as flat washers, sealing washers (with EPDM bonded to a metal washer), and bushings. Used in curtain wall, solar PV, and roofing applications. - **Neoprene (polychloroprene):** Older technology, still common in marine applications. Operating temperature -40°C to +110°C. Good oil and ozone resistance. - **Nylon (PA66, PA6):** High mechanical strength, but limited UV resistance (degrades in direct sunlight unless carbon-black filled). Operating temperature -40°C to +120°C. Used as bushings and sleeves. - **PTFE (Teflon):** Excellent chemical resistance and temperature range (-200°C to +260°C), but expensive and lower mechanical strength. - **HDPE / UHMW-PE:** Lower cost, used in less demanding applications. - **Fibreglass (GFK / GRP / G10 / FR4):** High strength, excellent temperature resistance, used in electrical and chemical applications. **Isolation sleeves:** When a fastener passes through a hole in a dissimilar metal, a non-conductive sleeve around the shank prevents sidewall contact. Sleeves are typically nylon, PTFE, or HDPE. Common in electrical equipment, aluminium curtain wall, and marine applications. **Isolation gaskets:** Flat gaskets placed under the head of a fastener and under the nut prevent both head-to-base and nut-to-base contact. EPDM and neoprene are common. For high-load applications, EPDM-bonded-to-metal washers (sealing washers) provide both electrical isolation and water sealing. **Sealants and caulks:** Silicone and polyurethane sealants applied at the fastener interface provide both electrical isolation and water exclusion. Best used in combination with mechanical isolation, as sealants can degrade over time. **Thread sealants:** PTFE tape or anaerobic thread sealants (Loctite 567, ThreeBond 1104) prevent water ingress into the threaded interface. They are not primary isolation, but they reduce the electrolyte availability. **Isolation design checklist:** 1. **Coverage:** Isolate BOTH the head-to-base and the nut-to-base interfaces — partial isolation is ineffective. 2. **Compression:** Isolation washers must be thick enough to prevent metal-to-metal contact under preload, typically 1-3 mm for EPDM, 0.5-1 mm for hard plastic. 3. **Temperature:** Match the isolation material to the service temperature range. 4. **UV exposure:** Use carbon-black-filled materials for outdoor applications. 5. **Compression set:** EPDM and neoprene can take a compression set over time; specify low-compression-set grades for long-life applications. 6. **Chemical compatibility:** Check compatibility with cleaning chemicals, fuel, hydraulic fluid, etc. TradeGo's technical service can recommend the appropriate isolation washer and bushing dimensions for your application, with stock of EPDM, neoprene, and nylon isolation components in our Shanghai warehouse.

Coatings and finishes provide another defence against galvanic corrosion, either by acting as a barrier between the metals, by serving as a sacrificial anode (like zinc), or by matching the electrochemical potential of the coupled metals. **ASTM F1941 — Electrodeposited Coatings on Fasteners:** ASTM F1941 is the most-cited standard for electrodeposited coatings on mechanical fasteners, covering both inch and metric screw threads. It specifies requirements for: - Coating thickness (Class 1: 0.0002 inch / 5 µm; Class 3: 0.0005 inch / 12.7 µm; Class 5: 0.0010 inch / 25.4 µm) - Supplementary conversion coatings (chromates, e.g., yellow chromate over zinc for additional corrosion resistance) - Salt spray corrosion resistance per ASTM B117 (Class 1: 24 hours; Class 3: 48 hours; Class 5: 96 hours before red rust) - Precautions against hydrogen embrittlement for high-strength fasteners (baking at 200-230°C for 4-24 hours after plating) - Dimensional accommodation (oversized tapped nuts for coated threads) Common electrodeposited coatings per F1941 include: - **Zinc, clear (chromate-free):** Environmentally preferred, modest corrosion resistance - **Zinc, yellow chromate:** Traditional, good corrosion resistance (RoHS-restricted in some markets) - **Zinc, blue-bright:** Decorative, lower corrosion resistance - **Zinc-nickel (12-16% Ni):** Premium coating, 500-1000+ hours salt spray, used in automotive and military - **Zinc-cobalt, zinc-iron, zinc-tin:** Specialty variants - **Tin, tin-zinc, tin-lead:** Specialty, often for electrical/electronic applications - **Cadmium:** Excellent corrosion resistance, but restricted under RoHS for most commercial applications **ISO 14713 — Zinc Coatings for Steel:** ISO 14713 is the European counterpart to F1941 for zinc coatings on steel. It covers hot-dip galvanizing (ISO 1461), zinc flake (ISO 10683), zinc-rich primers (ISO 12944-5), and electroplated zinc (ISO 2081). ISO 14713 provides guidance on coating selection for different service environments per ISO 9223 corrosivity categories (C1-C5). **Engineered multi-layer coatings:** For demanding applications, multi-layer coating systems provide superior galvanic corrosion protection: 1. **Zinc-rich primer + topcoat (duplex):** Epoxy zinc-rich primer (50-75 µm) plus polyurethane topcoat (40-60 µm). Used in offshore, C5-M environments. Salt spray life > 2,500 hours. 2. **Zinc flake + topcoat:** Zinc flake base (10-15 µm) plus PTFE or Xylan topcoat (5-15 µm). Salt spray life > 1,500 hours. 3. **Geomet / Dacromet + topcoat:** Similar to zinc flake + topcoat; favoured by automotive OEM. 4. **Electroplated zinc + passivation + topcoat:** Class 3 zinc (12.7 µm) + trivalent chromium passivation (0.5 µm) + organic topcoat (5-10 µm). Salt spray life 240-500 hours. **Coating integrity:** Coatings are vulnerable to damage during installation. Cutting threads, head-marking, and friction at the bearing surface can all damage the protective coating and expose the base metal. To preserve coating integrity: - Use thread-forming screws rather than thread-cutting screws (no chip removal) - Specify head-marking before coating, not after - Use washers under the head and nut to distribute the bearing load - Apply a touch-up coating (zinc-rich paint, e.g., CRC Zinc-It) to any installation damage **Coating selection by environment:** | Environment | ISO 9223 class | Recommended coating | |---|---|---| | Interior, dry | C1 | None or zinc Class 1 | | Interior, humid | C2 | Zinc Class 3, zinc-rich primer | | Exterior, rural | C3 | HDG to ISO 1461, zinc Class 5 | | Exterior, industrial | C4 | HDG + paint, zinc flake | | Coastal, offshore | C5-I / C5-M | Zinc flake + topcoat, duplex systems | TradeGo supplies fasteners with the full range of ASTM F1941 and ISO 14713 coatings, plus engineered multi-layer systems. All coated fasteners come with a coating certificate specifying the standard, class, and corrosion resistance rating.

Below are the most common questions TradeGo receives about preventing galvanic corrosion in fastener assemblies. For application-specific engineering support, contact our materials engineering team.