MIL-HDBK-1003/7
tubesheet material, aluminum bronze is significantly less susceptible to galvanic
corrosion than Muntz metal. The cost difference between aluminum bronze and silicon
bronze is slight. The actual material cost of silicon bronze is slightly lower but the
added thickness or supports required for high pressure designs negates any material
savings. Muntz metal is not the best material since it does not have the strength
required to ensure adequate tube-to-tubesheet integrity. However, as with freshwater,
Muntz metal tubesheets are often used with copper-nickel tubes.
New condenser tubesheet materials are under consideration as a result
of the ever increasing use of the new austenitic and ferritic stainless steel condenser
tube materials. Stainless steels such as Type 316L and other proprietary alloys are
similar to Type 304 stainless steel but with the addition of molybdenum that offers
increased resistance to general corrosion, pitting, and crevice corrosion attack.
Galvanic corrosion between the new "super" stainless steel condenser tubes and these
tubesheet materials is minimized because their similar compositions places them
relatively close on the galvanic series chart. The 316L and similar tubesheet materials
are also slightly cathodic to the "super" stainless steel tube alloys. This is
desirable since whatever corrosion takes place, if any, will occur on the thicker
tubesheet instead of the thinner walled tubes.
Polluted Water Service. Aluminum bronze is the preferred tubesheet material
7.3.4.3
for titanium tubes. For extremely polluted water, a titanium tubesheet (or titanium
cladded tubesheet) should be considered. This arrangement would prevent any potential
galvanic corrosion of the aluminum bronze as well as eliminate any problem with
corrosion due to the sulfides. A properly designed cathodic protection system should
protect the aluminum bronze tubesheet.
Recommended tubesheet materials for use with tubes made of the new
austenitic and ferritic stainless steels are the same as described under brackish water.
7.3.5
Water Boxes
7.3.5.1
Freshwater Service. Use carbon steel ASTM A285 Grade C or ASTM A283 Grade C
with copper alloy or stainless steel tubes.
Brackish and Seawater Service. Water box materials include carbon steel,
7.3.5.2
stainless steel, and 90-10 copper-nickel. In brackish water, there is no advantage to
using stainless steel over carbon steel since stainless steel is more expensive and is
also susceptible to corrosion. A feasible alternative is 90-10 copper-nickel but it is
significantly more expensive than carbon steel. Carbon steel is an acceptable choice
assuming that the interior of the water box is properly coated and that some form of
cathodic protection for the water box is provided.
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