design considerations for aluminum hull structures - Ship Structure ...

-39-such a connection is sommhat higher than a conventional mechanicallyfastened joint, but life-cycle cost consideration may favor bimetallicstrip comections ifstificiently large quantitiesoftiaterialare involved.The exfoliation problem was discussed previously, and is well documentedin numerous reports. At this time, it can be ~oncluded that thisproblem has been solved by the introduction of new tempers for the highmagnesiumalloys which exhibited the problem. The survey conducted inconjunction with this study found no significant evidence of exfoliationin commercial alumirmn hulls. All of the known hulls which have experiencedexfolia-bion were of g4S6-H321 alloy, while W cmme~cia~ hnl~shave generally been constructed of S083, S086 or alloys with lowermagnesium content. Crew boats which were fabricated with ~086-H32 platingand were subjected to a relatively rugged operating environment and highpanel stress levels have reportedly stood up very well, with no apparentevidence of exfoliation. This would lead to the tentative conclusion that5086-H32 alloy is superior tO 5456-H321 alloywi~h rega~d to exf’olia~ionresistance, which is consistent with the higher magnesiun content of thelatter alloy. Fortunately, the foregoing discussion is now of academicinterest only, since the recent introduction of the HI16 and HI17 tempersfor both alloys has apparently solved the exfoliation problem.Obher areas Tnwhich corrosion has occurred as a result of improperisolation of aluminum from other metals include the following:(a)(b)(c)(d)Black iron piping systems in ballast tanks connected toaluminum bulkhead spools via rubber lined stainless steelsleeves. If the rubber lining is not p~operly installed,severe corrosion can be expected on the aluminum sleeves.Adequate cathodic protection is required within the ballast tankto protect the hull structure.Steel deck machine~ must be well isolated from aluminum foundations,using butyl rubber, neoprene gaskets, plastic chocking,or equal. Painting both surfaces with red lead is notrecommended.Miscellaneous minor details are often developed without coneiderabionof isolation requirements. Minor piping pystems, connectionsof hose racks ard other miscellaneous outfit, etc., must considerthese requirements, since minor details often create problems withcritical structures.Corrosion and pitting has been observed in way of overboati discharges,indicating the need for local inserts. Other instancesof localized pittiing”havegenerally been rest~icted to areas inwhich known galvanic couples have existed,ABRASION AND IKFACTSeveral instances have been noted in which steel mooring cables andanchor chains have caused moderate to severe abrasion of aluminum hullstructures. Minor abrasion has also resulted from contact with wharvesand pilings. This appears to be an inherent desi,gnproblem with aluminumhulls which must be overcome by the proper location of mooring gear andground ‘tackle,as well as pro-visionof expendable chafing strips whe?erequired; Abrasion of cargo decks has been a concern o,ncrew boats, wherehea~ equipment and pipe is being handled. This is generally solved byfastening wooden protection strips to the aluminum deck.