ULTRA LOW FREQUENCY CORROSION CONTROL SYSTEM DESIGNED FOR STERN TUBES

Semb-Eco, a wholly owned subsidiary of the Sembcorp Marine group in Singapore, has announced that its Elmag technology has been successfully installed in the seawater-cooled stern tube cavity of a fleet of tankers operating in the US. First installed in Sembcorp Marine’s Admiralty Shipyard in Singapore in August 2014, the Elmag system yielded positive results confirmed after more than five years in operation when the stern tube was inspected in Admiralty Shipyard.

In the stern tube void, there are several different types of metallic materials in the same area: mild steel for the housing, duplex stainless steel for the shaft, and steel / noble metal for the bearings. These dissimilar metals challenge even the best coatings, says Semb-Eco.

Elmag represents a technology breakthrough as the only stern tube corrosion control system that has the capability of actively generating magnetite. Typically, a protective magnetite layer can only be formed by using chemicals to treat steel surfaces submerged in a closed loop water system or under high temperature and pressure conditions. However, ElMag is able to provide protection to steel or metals in water at ambient temperature, using very low power and without the use of chemicals.

If the protective layer is damaged, the ElMag system will produce a new protective layer, unlike traditional coating which requires physical restoration. Additionally, the magnetite layer is not susceptible to interference corrosion caused by AC current.

This 24/7 corrosion protection makes ElMag unique, says a spokesperson for Semb-Eco. While chemical additives can be used to control corrosion, they pose a potential pollution risk. In contrast, the ElMag system is chemical free. “Cathodic protection (CP) and coatings are the traditional solutions for corrosion. However, CP has moderate effect in controlling dissimilar metal corrosion, pitting corrosion and hard-to-reach areas such as crevices. ElMag is able to overcome these issues and at the same time produces the active protective magnetite layer.

“Another common solution is to mount sacrificial anodes but their effectiveness is limited. Over time, the surfaces of these anodes tend to disintegrate leaving fragments in the water of the stern tube cavity, which may end up in the cervices, thereby possibly damaging the stern tube bearing.”

The system is designed to be installed or retrofitted to a wide range of propulsion trains.