Galvanic corrosion is caused by having dissimilar metals in the gasket and flange, or two flanges, which become electrically connected in an electrolyte – often sea water. The electrode potential difference of the metals causes them to act as an anode and cathode, accelerating corrosion on the anode. Electrode potential and geometry ratio also have an effect, so using the correct materials is vital to prevent excessive corrosion. Flange face corrosion can lead to leaks and potential down time, and often require expensive and time-consuming repairs.
When dealing with a sealing solution, the flanges are either already installed or they have been specified. The challenge is to select a sealing solution that has the appropriate design and materials to minimize the potential for galvanic corrosion. Some standards, such as NORSOK, prohibit the use of graphite in sealing materials for offshore applications. Graphite is more noble than all metals, therefore any metal in contact with graphite becomes an anode, or a negatively charged surface, and starts to corrode. To make the challenge even greater, many of the applications prone to flange face corrosion must also be fire safe.
The challenges with graphite have created the need for a sealing material that will be fire safe, does not promote galvanic corrosion, and delivers a tight seal. Some operators have specified mica-based materials to meet this challenge. Mica is fire safe and relatively inert, but fails to deliver a tight seal. Vermiculite based products such as Corriculite and Thermiculite resist corrosion, are fire safe, and provide a tight seal.
If the flange face has been partially damaged by corrosion and cannot be removed for repair, Flexitallic has developed a special solution called the Flange Rescue Gasket. This gasket will prevent further corrosion and allow operation until proper maintenance can be performed. For applications with severe cycling, Flexitallic has also developed the Change-CR gasket, which combines the corrosion barrier with a dynamic seal.
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