held a sufficient time for solutioning to occur, then cooled rapidly to 

 produce a metastable, ordered, close-packed-hexagonal beta phase structure, 

 much like the transformation structure that is formed during the quenching 

 of high carbon steel from a temperature above the austenitizing temperature. 

 This structure is very hard but too brittle for most engineering purposes 

 and must be tempered by heating to an intermediate temperature, typically 

 595° to 650° Celsius, and holding for a sufficient time to reprecipitate 

 fine acicular alpha phase particles in the tempered beta phase structure. 

 Tempering stabilizes the structure and restores ductility and toughness. 



e. Galvanic Coupling . When two dissimilar metals are in electrical 

 contact with each other and immersed in an electrolyte, a potential is 

 established and electrical current may flow. This potential is related to 

 the relative tendency of each of the metals to go into solution. The more 

 active metal acts as the anode and corrodes at a faster rate than it would 

 by itself. The more noble (stable) metal acts as the cathode and is pro- 

 tected. This phenomenon is known as galvanic corrosion. The two dissimilar 

 metals electrically connected are called a galvanic couple. Table 28 presents 

 a galvanic series for flowing seawater at ambient temperature for several 

 metals and alloys. This galvanic series is based on practical measurements 

 of corrosion potentials at equilibrium in seawater. Galvanic corrosion is 

 most likely to occur if the two metals are widely separated in the series. 

 The rate of corrosion is dependent on current density. 



Table 28. Galvanic series in flowing seawater (2.4 to 

 4.0 m/s) at ambient temperature. 



Magnesium 



Zinc 



Aluminum alloys 



Calcium 



Carbon steel 



Cast iron 



Austenitic nickel cast iron 



Copper - nickel alloys 



Ferritic and mortensitic stainless steel (passive) 



Nickel copper alloys, 400, K-500 



Austenitic stainless steels (Passive) 



Alloy 20 



Ni - Cr - Mo alloy C 



Titanium 



Graphite 



Platinum 



If two dissimilar metals must be joined, several steps may be taken to 

 minimize galvanic corrosion: 



(1) Choose metals close together in the galvanic series to 

 reduce the potential; 



(2) avoid unfavorable area effects by keeping the cathodic area 

 small in relation to the anode, thereby reducing current density; 



212 



