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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32 AUGUST 2018 W W W.MATERIALSPERFORMANCE.COM C L A S S I C F astener materials can be sub jected to five primary condi tions or zones in the marine envi r o n m e n t : m a r i n e at m o s p h e r e, splash, tidal, full immersion, and mud zones. Each of these environ ments provides different degrees of corrosiveness dependi ng on t he material. In addition to these cor rosion zones, other seawater con ditions greatly affect fastener cor rosion resistance: seawater veloc ity, pollutants (industrial airborne and dissolved gases from decaying or ga n ism s), temperat u re, a nd hy d rogen evolut ion f rom elec t ro chemical reactions in the marine environment. The first fastener failure was ap parently documented in 1761. Iron nails corroded from a copper hull sheathing with partial loss of the sheathing from the H.M.S. Alarm. This classic example of gal van ic corrosion in seawater was one of This article is published as originally appeared in MP, Vol. 29, No. 4 (1990). Practical Guide To Using Marine Fasteners Ralph W. (Bud) Ross Jr. Consultant, Huntington, WV 25775 Arthur H. Tuthill Tuthill Associates Inc., P.O. Box 204, Blacksburg, VA 24060 M A T E R I A L S S E L E C T I O N & D E S I G N the first forms of marine corrosion identified. Today, galvanic corrosion is one of the most important forms of cor rosion to consider when de signing a fastener system, because by its na ture, a fastener system is normally made up of two or more different metals in a metallic couple. One or more of the metals acts as an anode (corrodes), and the other metal acts as a cat hode (pro tec ted aga i n st corrosion). Stress corrosion cracking of improperly heat-treated, stainless-steel valve body bolts has occurred during h ot summe r month s in some plants along the Gulf Coast. Galvanic effects are of primary importance in selecting fasteners. The fastener should be cathodic to (more noble t han) t he base plate material. It should never be t he anode (less noble) material. The use of stainless steel fasteners in alumi num structures is common and fol lows the galvanic guide lines just mentioned. However, in aluminum struc tures exposed in the marine atmo sphere, galvanic corrosion of the aluminum causes enlarging of the fastener hole allowing the uncor roded stai n less steel fastener to drop out. The use of copper alloy fasteners in aluminum results in se vere pitting and corrosion of the aluminum, and should be avoided. Using fastener systems of alu minum to aluminum, stainless to s t a i n l e s s , s t a i n l e s s t o f i b e r reinforced plastics (FRP), and stain less to wood limits the useful ness of these combinations and requires special precautions for satisfactory s er v ice. Pac k i ng t he s e fa stener joints with most greases is not ef fective, since moisture wicks up be tween the grease and metal surface, thus increasing crevice at tack. There are few spec ially com pounded greases that adhere to and displace water from t he metal lic The marine corrosion resistrance of copper-, iron-, nickel-, aluminum-, and titanium- based alloy fasteners are reviewed. Coated-steel systems are also in cluded. Several new alloys are characterized as candidate materials. Galvanic compatilbility is identified as the most critical consideration for marine fasteners. Guidelines are presented to assist designers in selection of appropriate marine fasteners materials.

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