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.

Issue link:

Contents of this Issue


Page 37 of 92

35 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018 Practical Guide To Using Marine Fasteners sequently, there is much greater use of coated carbon steel fasteners. However, the thin coatings do not provide long term corrosion protec t ion , e sp e c i a l ly whe n ex p o s e d closer to the seawa ter. Continual maintenance of the fastener system app ea r s to b e mor e acc ept able above the water line, since it is more accessible and may not be as critical of a service condition. In exposures above the water line, the galvanic effect is limited to the immediate area of contact and is not spread over the larger, wetted line, a fastener used in wood, con crete, plastic, or rubber will not re c e ive a ny b e n e f it o f c a t h o d ic protection. The nonmetallic mate rial usually produces a severe crev ice condition. Therefore, the alloy fastener's performance de pends on the alloy's inherent sea water corro sion resistance. The crevice attack that occurs on fas teners in nonme tallic materials is more damaging to type 316 stain less steel and other common stain less steels t han to alloy 400. Alloy 400 has long been used success f u l ly i n wood a nd other nonmetallics. The new 6% Mo stainless steels 1 , Ni Cr Mo alloys, MP35N, and titanium are more re sistant than alloy 400 and are in creasi ngly bei ng con sidered for these difficult applications. Some small boat manufactur ers have reported good perfor mance for type 316 stainless steel bolting i n f iber glass rei n forced plast ic (FRP) hulls. The bolts were used below t he waterl i ne. They were packed with water repellent lubri cant and recessed in the FRP. The use of t he g raph ite f ree, water repellent lubricants that fill the bolt hole is good practice for stainless steel and aluminum, bot h above and below the waterline. Table 6 presents guidelines for fastener selection in nonmetallic based materials below the water line. The "R" rating for most fas teners exposed in graphite warns against t he use of any graphite containing lubricant, pack ing, or gasket in contact with these mate rials. The carbon (graphite) gal vanic couple wit h most metals in sea water greatly accelerates corro sion of the less noble metal. This gal va n ic combi nat ion may even be harmful to more noble metals, such a s a l loy 40 0, Ni Cr Mo a l loys, MP35N, and titanium. Consider able caution is advised when using any fastener a l loy i n g raph ite com posites until more thorough eval uations have been undertaken. Aluminum and coated steel are not suggested for use in con crete below the waterline. The im proved rating for type 316 stainless steel in concrete is based on the fact that the alkalinity of the con crete has been found to protect the stainless steel partially embedded in concrete for several inches be yond the concrete surface. Above the Waterline Fasteners are generally not sub jected to as critical conditions above the waterline with respect to vessel, platform, or structure safety. Con

Articles in this issue

Archives of this issue

view archives of Materials Performance - AUG 2018