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Briefing #1095: Corrosion & Bonding

Executive Summary

<p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> Corrosion management keeps underwater metals, engines, and electrical systems reliable. This executive summary explains marine corrosion modes, how bonding systems and anodes work, and a practical onboard program that pairs inspection with measurement and corrective action. It is written for passagemakers and cruising sailors who need clear, actionable methods that hold up offshore. </p> <h2 style="font-size:20pt;font-weight:600;line-height:1.2;margin:18px 0 10px;">Goals</h2> <ul style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px 24px;"> <li>Prevent rapid metal loss and leaks from galvanic or stray-current corrosion.</li> <li>Design, inspect, and maintain an effective bonding system.</li> <li>Select and manage sacrificial anodes for changing waters (fresh, brackish, salt).</li> <li>Differentiate symptoms, confirm root causes, and verify fixes with simple tests.</li> </ul> <h2 style="font-size:20pt;font-weight:600;line-height:1.2;margin:18px 0 10px;">Corrosion Modes That Matter</h2> <ul style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px 24px;"> <li><b>Galvanic corrosion:</b> Dissimilar metals electrically connected in an electrolyte (seawater). The less noble metal sacrifices to the more noble.</li> <li><b>Stray-current corrosion:</b> DC leakage into water (often from a damaged conductor or appliance) drives extremely fast metal loss—days, not months.</li> <li><b>Crevice corrosion:</b> Oxygen-depleted gaps (e.g., under deposits, gaskets) attack stainless steel despite its “stainless” reputation.</li> <li><b>Electrolysis (colloquial):</b> Often misused; focus instead on the specific galvanic or stray-current mechanism at work.</li> </ul> <p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> A correct diagnosis aligns <i>electrical continuity</i>, <i>water chemistry</i>, and <i>metallurgy</i> with observed damage patterns. Single anecdotes mislead—confirm with measurements. </p> <h2 style="font-size:20pt;font-weight:600;line-height:1.2;margin:18px 0 10px;">Bonding Systems: Purpose and Architecture</h2> <p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> A bonding system ties underwater metals (thru-hulls, strainers, struts, shafts, rudders) to a common potential via green conductors and a central bonding bus. Its aims are: </p> <ul style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px 24px;"> <li>Provide a low-impedance path for fault currents to trip protection devices.</li> <li>Maintain equal potential among fittings to reduce galvanic driving force.</li> <li>Reference metals to sacrificial anodes sized for the installation and waters.</li> </ul> <p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> Best practices: clean terminations; proper lugging; anti-rotation support; short runs to a copper bus; inspect for continuity and corrosion every haul-out and mid-season. </p> <h2 style="font-size:20pt;font-weight:600;line-height:1.2;margin:18px 0 10px;">Shore Power, Isolators, and Isolation Transformers</h2> <ul style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px 24px;"> <li><b>Galvanic isolators:</b> Inserted in the AC green safety ground to block low-voltage DC galvanic current while passing AC fault current. Choose fail-safe, UL marine–rated units; test per manufacturer guidance.</li> <li><b>Isolation transformers:</b> Galvanically isolate the entire shore system; heavy and costly but very effective for corrosion and safety.</li> <li><b>Dockside realities:</b> Multiple boats on one grid create potentials; bonding and anodes must assume imperfect neighbors.</li> </ul> <p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> When adding shore gear, re-test continuity and potentials; label changes in the maintenance log for trend analysis. </p> <h2 style="font-size:20pt;font-weight:600;line-height:1.2;margin:18px 0 10px;">Anodes: Selection, Sizing, and Service</h2> <ul style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px 24px;"> <li><b>Material:</b> Zinc (salt), aluminum (salt/brackish—modern standard), magnesium (fresh only; too active for salt).</li> <li><b>Coverage:</b> Focus on props, shafts, tabs, thrusters, struts, and bonded thru-hulls. Large fittings may benefit from dedicated anodes.</li> <li><b>Inspection:</b> Track consumption rate (% loss per month/season). Rapid loss points to stray current or undersized anodes.</li> <li><b>Installation:</b> Bright-metal contact; anti-rotation; isolate from paint; re-torque after first run and mid-season.</li> </ul> <p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> Log water type, marina, and season; these factors dominate anode life. Move between waters? Re-evaluate material and quantity. </p> <h2 style="font-size:20pt;font-weight:600;line-height:1.2;margin:18px 0 10px;">Diagnostics and Verification</h2> <ul style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px 24px;"> <li>Continuity checks along bonding conductors (ohms/milliohms). Repair any open/loose runs.</li> <li>Reference-electrode tests (Ag/AgCl) to check protection potential at key fittings.</li> <li>Leakage surveys for stray current: clamp meter on DC negatives; voltage from battery negative to seawater.</li> <li>Targeted haul-out inspections: dezincification (“pink” bronze), pitting on aluminum, crevice corrosion on stainless.</li> </ul> <p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> Confirm fixes with before/after measurements and photo logs. A good program turns myths into monitored facts. </p> <h2 style="font-size:20pt;font-weight:600;line-height:1.2;margin:18px 0 10px;">Operations, Safety, and Documentation</h2> <ul style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px 24px;"> <li>Record anode changes, shore gear additions, and any electrical work; correlate with anode wear and inspections.</li> <li>Use PPE and ventilation when working with cleaners and metals; isolate shore power before service.</li> <li>Educate crew about the dangers of in-water shock risk and why the green wire exists.</li> </ul> <p style="font-size:14pt;font-weight:300;line-height:1.2;margin:0 0 12px;"> With sound bonding, right anodes, and shore isolation where needed, you can stop corrosion from stealing reliability and enjoy a quieter, safer ship. </p>

Date Created

9/2/2025

Last Updated

9/26/2025

Topic ID

1095

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