The Equipment Design Process
375
World War II. Certainly cathodic protection of large areas is not economical
unless these areas are coated. The techniques of surface preparation, primer
formulation, anticorrosion paint systems, and removal of fouling organisms
from the hulls and other undersea surfaces are of prime importance.
Zinc anodes, as sacrificial anodes, have been used successfully for protecting sea chests of saltwater piping systems. When the sea chest is steel and the
valve is nonferrous, a waster sleeve of mild steel is recommended as additional protection. Mixed ferrous and nonferrous systems should be designed
to include a waster piece of extra-heavy galvanized steel on the nonferrous
section, connected in such a way as to afford easy removal.
For protection against corrosion in stagnant systems, such as bilge pump
strainers and valves where various combinations of steel, stainless steel, and
bronze occur, a zinc anode should be used. One rule of thumb is 1 ft 2 of zinc
anode for each 50 ft 2 of bare metal.
For cupronickel piping, corrosion and erosion-corrosion are minimized by
having maximum flow rate (limited to 15 f/s) and by reducing or eliminating
air. During idle periods (1 week or more in duration), the saltwater piping
system should either be drained or operated daily.
11.3.7.1 Protection of Condensers and Saltwater Exchangers
For condensers and exchangers, corrosion control measures involve design,
material selection, fabrication, and maintenance. Designs avoid erosion corrosion by paying attention to the configuration of waterboxes and injection
piping. Specifications generally require limiting the flow rate of either seawater or brine flow to 15 ft/s for both 70–30 and 90–10 cupronickel to minimize the effects of erosion-corrosion.
If at all possible, avoid operating a condenser in polluted water.
During shutdown there is a tendency for seawater to pocket, especially in
the sagging section of tubes. Such pocketing causes serious corrosion and
can be avoided by washing with freshwater and then drying out (using an
air lance to empty sagging tubes). If the condenser cannot be completely
dried out during shutdown, it should be filled completely with freshwater.
Stones, pieces of wood, shells, etc. lodging in condenser tubes cause pitting corrosion. These are removed periodically by the use of a water lance,
soft rubber plugs driven through by an air gun or by a rotating bristle brush.
Wire brushing and abrasive cleaning of condenser tubes are specifically prohibited because these remove all the protective coating.
11.3.7.2 Use of Aluminum in Marine Service
One of the first rules of using aluminum in seawater is to avoid machining the
as-rolled or as-extruded surface if possible. The machined surface corrodes
far worse than the unmachined surface. The second rule is to avoid using
steel wool or wire brushes, other than stainless steel, on aluminum. Sanding
375
World War II. Certainly cathodic protection of large areas is not economical
unless these areas are coated. The techniques of surface preparation, primer
formulation, anticorrosion paint systems, and removal of fouling organisms
from the hulls and other undersea surfaces are of prime importance.
Zinc anodes, as sacrificial anodes, have been used successfully for protecting sea chests of saltwater piping systems. When the sea chest is steel and the
valve is nonferrous, a waster sleeve of mild steel is recommended as additional protection. Mixed ferrous and nonferrous systems should be designed
to include a waster piece of extra-heavy galvanized steel on the nonferrous
section, connected in such a way as to afford easy removal.
For protection against corrosion in stagnant systems, such as bilge pump
strainers and valves where various combinations of steel, stainless steel, and
bronze occur, a zinc anode should be used. One rule of thumb is 1 ft 2 of zinc
anode for each 50 ft 2 of bare metal.
For cupronickel piping, corrosion and erosion-corrosion are minimized by
having maximum flow rate (limited to 15 f/s) and by reducing or eliminating
air. During idle periods (1 week or more in duration), the saltwater piping
system should either be drained or operated daily.
11.3.7.1 Protection of Condensers and Saltwater Exchangers
For condensers and exchangers, corrosion control measures involve design,
material selection, fabrication, and maintenance. Designs avoid erosion corrosion by paying attention to the configuration of waterboxes and injection
piping. Specifications generally require limiting the flow rate of either seawater or brine flow to 15 ft/s for both 70–30 and 90–10 cupronickel to minimize the effects of erosion-corrosion.
If at all possible, avoid operating a condenser in polluted water.
During shutdown there is a tendency for seawater to pocket, especially in
the sagging section of tubes. Such pocketing causes serious corrosion and
can be avoided by washing with freshwater and then drying out (using an
air lance to empty sagging tubes). If the condenser cannot be completely
dried out during shutdown, it should be filled completely with freshwater.
Stones, pieces of wood, shells, etc. lodging in condenser tubes cause pitting corrosion. These are removed periodically by the use of a water lance,
soft rubber plugs driven through by an air gun or by a rotating bristle brush.
Wire brushing and abrasive cleaning of condenser tubes are specifically prohibited because these remove all the protective coating.
11.3.7.2 Use of Aluminum in Marine Service
One of the first rules of using aluminum in seawater is to avoid machining the
as-rolled or as-extruded surface if possible. The machined surface corrodes
far worse than the unmachined surface. The second rule is to avoid using
steel wool or wire brushes, other than stainless steel, on aluminum. Sanding