Galvanic Corrosion in Marine Fasteners: Why the Bolt Dies First

On board a vessel, the fastener is often the smallest metal part in contact with a much larger structural member. That is exactly why galvanic corrosion kills bolts and studs so fast. This article explains the three conditions that create a galvanic couple, why the anode-cathode area ratio decides the failure rate, and how to prevent dissimilar metal corrosion on your hull and deck hardware.

What Is Galvanic Corrosion?

Galvanic corrosion is an electrochemical process, not a simple chemical reaction. It occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte such as salt water or sea spray. Three conditions must be present at the same time: dissimilar metals, an electrical connection between them, and an electrolyte. Remove any single element and the galvanic cell cannot operate.

In the couple, the less noble (more electrochemically active) metal becomes the anode and corrodes at an accelerated rate, while the more noble metal becomes the cathode and is protected. Seawater is an exceptionally effective electrolyte because of its high ionic conductivity, so galvanic attack on a vessel proceeds far faster than in freshwater.

Galvanic Series: Use the Seawater Table, Not Standard Electrode Potentials

To predict which metal corrodes, engineers use the galvanic series — a ranking of metals by their measured potential in a given environment (typically flowing seawater), referenced to a standard half-cell such as the saturated calomel electrode or Ag/AgCl. A common mistake is to use the standard electrode potential (vs. SHE) table instead. Stainless steel in its passive state measures far more noble in seawater than its standard electrode potential would suggest, so judging by the SHE table can give the wrong answer.

Practical rankings for seawater (from active/anodic to noble/cathodic) include: magnesium < zinc < aluminum alloys < carbon and alloy steels < cast iron < stainless steels (active) < copper and copper alloys < stainless steels (passive) < nickel alloys < titanium < graphite. The wider the separation, the stronger the driving force for galvanic attack on the more active member.

The Area Ratio Is the Deciding Factor

The potential difference alone is not enough to predict how fast a fastener fails. The anode-to-cathode surface area ratio is the most critical practical factor, because the total galvanic current generated depends on the cathode area, and that current is concentrated on the anode surface.

A small anode connected to a large cathode experiences high current density and rapid localised penetration. This is why a carbon steel fastener in a large stainless steel or bronze fitting is the worst-case configuration on board: the small fasteners are sacrificed and can fail in months to a few years. Conversely, a stainless steel fastener (small cathode) on a large galvanized steel structure spreads the current across a large anode, so the corrosion is slow and the assembly is generally acceptable in atmospheric service. In short: make the more noble metal the smaller component wherever mixing is unavoidable.

Potential Difference Thresholds

There is no single universal threshold, but common engineering guidance is to keep the potential difference below about 0.25 V for non-critical applications and below about 0.15 V for critical or continuously wet service; a difference within about 50 mV is generally considered safe. Values above the threshold require isolation or other mitigation. Because measured potentials vary with environment, always confirm the couple against a recognised seawater galvanic series rather than relying on a single figure.

Stainless Steel Is Not Immune

Stainless steel owes its corrosion resistance to a passive oxide layer. In oxygen-depleted crevices or stagnant areas, this layer can break down, leading to pitting and crevice corrosion on the stainless part itself. This is the reason submerged stainless steel is sometimes bonded into a cathodic protection system with sacrificial anodes to help stabilise the passive film — counter-intuitive but established practice. Do not assume stainless fasteners are automatically safe underwater.

How to Prevent Galvanic Corrosion in Marine Fasteners

Standards Reference

The following documents provide the technical basis for predicting and testing galvanic corrosion:

• ASTM G71-81(2024) – Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes
• ASTM G82 – Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance
• MIL-STD-889 – Dissimilar Metals (galvanic series table used in industry)
• ISO 13174 – Cathodic protection of harbour installations (sacrificial anode application)

FAQ

Why does a carbon steel bolt fail faster when joined to stainless steel?

The carbon steel bolt is the anode (small area) and the stainless steel member is the cathode (large area). The galvanic current from the large cathode is concentrated on the small bolt, causing high local current density and rapid corrosion.

Is stainless steel always safe below the waterline?

Not necessarily. In oxygen-depleted crevices it can suffer pitting and crevice corrosion, and it can be galvanically detrimental when coupled with certain metals. Assess the full system rather than assuming stainless is universally safe.

Can I mix stainless fasteners with galvanized steel structures?

In atmospheric (non-immersed) service it is generally acceptable, because the fastener is a small cathode and the galvanized member is a large anode that spreads the current. In continuously wet or immersed coastal service, isolation or a different material selection is recommended.

How should I separate dissimilar metals in a bolted joint?

Use non-conductive gaskets, insulating washers and sleeves, or PTFE tape on the threads, and coat the more noble (cathodic) member so its effective cathode area is reduced.