Progress in Pressure Resistant Packaging and Corrosion Protection Technology of High Voltage Power Supply for Deep Sea Exploration
Deep sea exploration has opened new frontiers in oceanography, marine biology, and resource discovery. The extreme environment of the deep ocean, with high hydrostatic pressure and corrosive seawater, poses unique challenges for electronic equipment. High voltage power supplies for deep sea applications require specialized packaging that can withstand the pressure and prevent corrosion while maintaining electrical performance.
The ocean depth determines the hydrostatic pressure, which increases by approximately one atmosphere for every ten meters of depth. At six thousand meters, the pressure is about six hundred atmospheres or sixty megapascals. Equipment at this depth must withstand this pressure on all external surfaces. Air filled enclosures experience the full pressure differential, while oil filled enclosures can equalize the pressure.
Pressure resistant packaging approaches include pressure tolerant and pressure compensated designs. Pressure tolerant designs use rigid enclosures that withstand the external pressure without significant deformation. The enclosure walls must be thick enough to resist buckling under the pressure. Pressure compensated designs fill the enclosure with a fluid, typically oil, that transmits the external pressure to the internal components. The components must be designed to operate under pressure.
Pressure tolerant enclosures are typically spherical or cylindrical with domed ends, shapes that efficiently resist external pressure. The material is usually aluminum, titanium, or stainless steel, selected for strength, corrosion resistance, and weight. The wall thickness depends on the material strength, the enclosure dimensions, and the design pressure with safety factor.
penetrators allow electrical connections to pass through the enclosure wall while maintaining the pressure seal. The penetrator must seal against both the enclosure wall and the cable. Epoxy potting can seal the cable conductors within a metal body that bolts to the enclosure. Glass to metal seals provide hermetic connections for permanent installations.
Corrosion protection is essential for long term reliability in seawater. Seawater is a corrosive electrolyte that attacks most metals. The corrosion rate depends on the metal, the oxygen content, the temperature, and the flow conditions. Protection strategies include material selection, coatings, and cathodic protection.
Material selection for seawater service favors corrosion resistant alloys. Titanium has excellent corrosion resistance in seawater but is expensive. Stainless steels, particularly the austenitic grades, have good resistance but may suffer pitting or crevice corrosion in stagnant conditions. Copper nickel alloys have good resistance and antifouling properties. Plastics and composites are immune to corrosion but have strength limitations.
Coatings provide a barrier between the metal and the seawater. Organic coatings such as epoxy paint systems protect the underlying metal if the coating is continuous and well adhered. Metallic coatings such as zinc galvanizing provide sacrificial protection. Coating selection must consider the operating environment and any compatibility requirements with cathodic protection.
Cathodic protection uses sacrificial anodes or impressed current to protect the structure. Sacrificial anodes of zinc or aluminum corrode preferentially, protecting the structure. Impressed current systems use an external power source to drive protective current to the structure. The protection system must be designed for the geometry and the environment.
Internal components require protection from the internal environment. In air filled enclosures, the internal atmosphere must be dry and non corrosive. Desiccants absorb moisture that enters through seals or permeates plastic materials. Inert gas fill excludes oxygen that could cause corrosion. In oil filled enclosures, the oil must be compatible with all materials and remain stable under pressure and temperature.
Testing verifies the pressure and corrosion resistance. Pressure testing in hyperbaric chambers subjects the enclosure to the design pressure and beyond. Long term pressure testing identifies any creep or fatigue effects. Corrosion testing in seawater tanks or field exposure measures the corrosion rate and identifies susceptible areas. The testing program must cover the expected service life of the equipment.
