Marine and Offshore Magnetics: Salt Fog, Condensation and Vibration

Salt Air Is the Real Specification

Marine and offshore installations expose magnetic components to salt-laden air, condensation, and vibration from engines and hull motion. Standard industrial components fail there in predictable ways: varnish surfaces craze, terminal creepage paths become conductive after salt deposits, and laminated cores rust at the joints. The fix is not a single coating but a construction that assumes moisture will reach the part.

This note covers the construction choices that survive marine environments and the tests that demonstrate it.

Construction Choices That Matter

Marine exposure and the corresponding construction response
Exposure	Failure mode	Construction response
Salt fog	Surface leakage, corrosion at joints and terminals	VPI or full encapsulation; sealed terminals; salt-spray tested finish
Condensation cycling	Water ingress into winding voids, IR collapse	Potted or VPI construction, hydrophobic varnish, breather-free design
Vibration	Winding movement, enamel abrasion, terminal fatigue	Impregnated winding, clamped core, supported leads
Humidity plus heat	Accelerated insulation aging	Class H insulation with margin, lower hotspot

Encapsulation vs VPI

VPI fills the winding with varnish and leaves the outer surface coated. Full encapsulation (potting in a shell) adds a continuous plastic barrier that is far more effective against salt fog, at the cost of thermal resistance and repairability. For deck-mounted equipment, encapsulation with a defined thermal path to the mounting plate is usually the right answer. For cabinet-mounted parts in a protected enclosure, VPI plus a conformal coating on terminals is often sufficient.

Either way, the weak point is the termination. A perfectly encapsulated body with an unprotected solder joint will fail within months. Specify sealed terminals or a potted terminal block, and require the terminal area to be included in the salt-spray test.

Tests That Demonstrate Fitness

Salt spray per IEC 60068-2-52 (cyclic) rather than a simple continuous fog, because cycling exposes crevice corrosion.
Damp heat steady state per IEC 60068-2-78, followed immediately by an insulation resistance measurement — the IR after humidity is the meaningful number.
Thermal cycling with IR and hipot at the end, to catch cracking in the encapsulation.
Vibration per the applicable marine or classification rule, with hipot after the test.

Ingress Protection Is Not Enough

An IP65 enclosure protects the assembly, not the component inside it. Condensation forms inside sealed enclosures as temperature cycles, so a component rated for a protected environment still sees moisture. The correct specification is the environment inside the enclosure, which is why marine projects should state “condensing humidity, salt fog exposure on the deck, 95 % RH cycles” rather than only an IP rating.

What to Request

Environmental description: salt fog, condensing humidity, temperature range, vibration level.
Construction: VPI, encapsulation, terminal sealing, and the materials used.
Test reports: salt spray, damp heat with post-test IR, thermal cycling, vibration.
Insulation class and hotspot at the enclosure ambient, with margin for aging.
Service access: whether the part can be rewound or must be replaced as a unit.