(3) insulate the two metals from each other, making sure contact 

 is not restored in service by grounding or corrosion products bridging 

 the insulator; 



(4) use coatings: the anodic material must be completely covered 

 to prevent rapid attack at holidays in the coating; sometimes it is 

 also beneficial to coat the cathodic material to reduce current 

 density; and 



(5) place a more anodic third metal in contact with the other two 

 so that this third metal provides sacrificial protection. 



2. Joining, Cutting and Repairing Metals . 



a. Rivets and Bolts . Riveting, at one time, was the primary means of 

 joining metals together. Today the importance of riveting in construction 

 has lessened because of the developments of welding and high strength bolting. 

 The American Society of Mechanical Engineers (ASME) Boiler and Pressure 

 Vessel Code no longer lists riveting as an acceptable method for pressure 

 vessel fabrication, although repairs can be made to riveted vessels in 

 accordance with the code requirements that were used for the vessel con- 

 struction. Riveted joints have one important advantage over bolted joints. 

 Properly set, rivets do not loosen. In spite of its lack of favor in con- 

 struction, riveting is an important joining method in manufacturing. 



Rivets are made from bar stock by hot or cold forming the head. Round 

 button heads are most common but flattened and countersunk types are also 

 produced. For structural steel fabrication, steel rivets should be specified 

 to ASTM Standard Standard A502, Steel Structural Rivets. This standard lists 

 three grades, all of which are intended to be hot driven. Grade 1 is a 

 carbon steel rivet for general purpose, usually used for joining steel 

 conforming to ASTM Standard A36. Grade 2 is a carbon-maganese steel rivet 

 used for joining high strength carbon and high strength low alloy structural 

 steels. Grade 3 is about the same strength as grade 2 rivet steel, but 

 because copper and chromium are required in the steel composition, grade 3 

 rivets have enhanced atmospheric corrosion resistance approximately four 

 times that of carbon steel without copper. Grade 3 rivets correspond to 

 steels conforming to ASTM Standard 588, High Strength Low-alloy Structural 

 Steel with 340 megapascals (50 0.00 pounds per square inch) Minimum Yield 

 Point to 10.2 centimeters (4 inches) Thick. Steel rivets are also listed in 

 ASTM Standard A31, Boiler Rivet Steel and Rivets, for repair of riveted 

 boilers and pressure vessels, and in ASTM A131, Structural Steel for Ships. 

 Rivets meeting the requirements of ASTM A31 or ASTM A131 are not suitable for 

 structural construction unless these rivets have also met requirements of 

 ASTM A502. One important difference in the requirements between these 

 standards is that ASTM A5Q2 requires hardness tests, whereas the other 

 standards specify tensile tests on the rivet steel. 



Holes for rivets may be punched or drilled. If punched, it is recom- 

 mended that the holes also he reamed to remove distorted metal, particularly 

 if the structure may be subjected to vibration. For steel construction, the 

 holes are usually 1.6 millimeters (1/16 inch] in diameter larger than the 

 nominal diameter of the undriven rivet. Flame cutting of holes is not 

 recommended because of the micro structural changes that occur in steel. 



213 



