Materials Performance

AUG 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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34 AUGUST 2018 W W W.MATERIALSPERFORMANCE.COM C L A S S I C c los e to t hat of t it a n iu m (UNS R56400). The fastener alloys are l i sted i n order of t hei r ten si le st reng t hs. Those mater ials w it h poor marine corrosion resistance (and thus difficult to use success fully) are identified in the tables. Table 4 lists the coatings that are frequently applied to steel fas teners to improve t heir corrosion resis t a nc e. Bot h me t a l l ic a nd non metallic coatings are used to pro tect steel fasteners during ex tended storage and construction periods. None of the coatings adds signifi cantly to the service life of the fas teners after they have been placed in service below the water line. The coatings are helpful in atmospheric exposures, but they are generally overcoated with the coating system used to protect the structure. Guidelines for Marine Fastener Selection Below the Waterline Tables 5 and 6 offer guidelines for using alloy fasteners below the waterline. Several of the most com mon fasteners of the family of al loys discussed above are in cluded as examples in this table. Some of t he common materials to be fas tened or bolted are other metals, such as steel, copper, etc. (Table 5) and various nonmetallics (Table 6). Three basic ratings for the fastened or bolted system are G = generally satisfactory; Y = may be satisfac tory, but more de tailed information is needed to fully assess the system; and R = the combination of materi als should be avoided due to known galvanic problems, crevice corro sion susceptibility, or poor corro sion resistance of the materials. Below waterline fasteners, such as bolts in the keel and bilge areas, are an integral part of the safety con siderat ion s of a l l boat s a nd ships at sea. For steel hulls, only a lu m i nu m is u n sat isfac tor y be cause it is anodic to steel. Thus, this fastener combination receives an "R" rating (Table 5). Most of t he other fasteners can be safely used in steel base plate because they are all cathodic to steel, and except for tita nium, are resistant to damage from hydrogen released at cathodically protected surfaces. Type 316 stain less steel (UNS S31608) and alloy 400 (Monel † , UNS N04000) are pro tected by steel even in structures that are not cathodically protected by anodes or impressed c urrent systems. The nonmetallic base plate mate rials present another chal lenge to fastener materials. Under the water M A T E R I A L S S E L E C T I O N & D E S I G N

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