Question 45 QMED04 - Boiler Technician-Watertender

The Correct Answer is C **Why Option C is Correct:** Option C ("The plates should be electrically insulated from one another by use of non-conductive gaskets and non- conductive ferrule sleeves with the bolts.") is correct because bimetallic corrosion (also known as galvanic corrosion) occurs when two dissimilar metals are electrically coupled in the presence of an electrolyte (such as moisture or salt water). Aluminum and steel have different electrical potentials; steel is generally more noble (cathodic) than aluminum, which is more active (anodic). When they are in contact, the aluminum will preferentially corrode to protect the steel. To minimize this corrosion, the crucial step is to break the electrical circuit between the two metals. Using non-conductive materials—gaskets between the main plate surfaces and ferrule sleeves/washers around the bolts—effectively insulates the aluminum from the steel, preventing the flow of galvanic current and halting the corrosion process. **Why the Other Options are Incorrect:** * **A) The plates should be bolted together in such a way as to ensure good electrical contact between the plates.** This is incorrect because maximizing electrical contact directly maximizes the galvanic current flow, which dramatically *increases* the rate of bimetallic corrosion, leading to rapid degradation of the more active metal (aluminum). * **B) Steel and aluminum are so close together on the Noble series that no particular provision need be made for bolting steel and aluminum plates together.** This is incorrect. While the required protective measures depend on the environment, steel and aluminum are sufficiently far apart on the galvanic series to cause severe corrosion, especially in aggressive environments (like marine or industrial settings). Specific provisions, such as insulation, are almost always necessary. * **D) The plates should be bonded together electrically by joining the plates by a bonding strap in addition to bolting together.** This is incorrect for the same reason option A is incorrect. A bonding strap ensures excellent electrical continuity, which promotes and accelerates the flow of corrosive galvanic current between the dissimilar metals.