ASHRAE Journal - April 2011 - 52

forming actual field experiments. This is complex and designers prefer to use past experience for their determination. The range of current outputs for different sacrificial anode materials with standard dimensions is generally known or can be obtained from the anode manufacturers. The number of anodes can be determined from total estimated current requirement divided by current per anode. It is important to maintain electrical continuity of the anode during installation. Considering the feasibility for accommodating the number of anodes in the water heads, suitable anode material can be chosen to restrict the number of anodes. Selection of sacrificial anode material also depends upon the degree of protection required, cost of anode material and its rate of consumption by corrosion. Carefully balanced zinc alloy, which corrodes evenly at a steady rate, is preferred as sacrificial anodes in marine condensers. Magnesium anodes can also be used but because of their higher driving voltage they are quickly spent. Systematic location of the anodes is critical to their overall effectiveness. The location of anodes is generally decided using past experience, ease of maintenance and symmetry. The anodes must be regularly serviced and replaced when spent. In seawater cooled shell-and-tube condensers, anode rods are provided in the water heads, typically in two ways, as shown in Photos 1 and 2. Photo 1 shows the anode rods completely inside the water head. The advantages of such an arrangement are that the depth of water heads can be minimized to reduce the overall length of the condenser and leakages through the anode holding holes in water heads can be avoided. The length limitations are critical for naval duty as less space is available onboard. The leakages can be severe in some cases, such as with seawater-cooled condensers onboard a submarine, because seawater pressure is in the range of 20 to 30 bars (2000 to 3000 kPa). The major disadvantage is that you must open the condenser water heads to
52 ASHRAE Journal

know the status of the anode rod corrosion. In Photo 2, a provision is made to insert the anode rods in the water head from the outside. The advantage is that the operator can easily determine the corrosion status of the anode rod by removing only the anodes. The disadvantage is the depth of water heads are

In some cases, anodes are provided even after using the same construction material for tube sheets, tubes, water heads and seawater piping. The corrosion of anodes in these cases does not indicate any sacrifice, as galvanic corrosion of the base metal is not probable even if anodes are absent. Thinking that the anode rods are protecting condenser corrosion, the operator will continue to replace the consumed rods unnecessarily. Designers should avoid such redundant selections, which confuse operators. Seawater pipelines may be made of a variety of materials: titanium, copper, nickel, etc. When connecting pipelines of different materials, use flange joints with gaskets to electrically insulate the two materials. Electrical discontinuity must be maintained between the two pipes. Therefore, flange bolts must be rubber coated, plastic/rubber washers should be used and the pipe should be supported using clamps with internal rubber lining. The electrical discontinuity between metals prevents electrons from flowing and causing galvanic corrosion. Over a period of time, a salt bridge can form in a flanged joint, which allows galvanic corrosion. The pipe should be cleaned periodically from the inside to avoid salt deposition.

Pitting Corrosion
Pitting corrosion is a form of extremely localized corrosion that leads to the creation of small holes in the metal. This kind of corrosion is insidious. It causes little loss of material, showing a small effect on its surface, while it damages the metal deep inside. Corrosion often obscures the pits on the surface and makes pitting difficult to detect. All forms of pitting are caused by the same basic mechanism. During corrosion, the protective film may not form or local destruction of film may occur. This local void in the protective surface can set up a galvanic cell. In a local galvanic cell, lack of oxygen around a small area creates an anode. The area with excess oxygen becomes a cathode. The corrosion penetrates the mass of the metal, with limited diffusion of ions, further pronouncing the localized lack of oxygen.
April 2011

Photo 1 (top): Anode rods inside water head. Photo 2 (bottom): Provision to insert anode rods in water head from outside. increased, causing an increase in the overall length of the condenser and the chance of leakages through the anode holding holes in the water heads. A common maintenance problem with this arrangement occurs when operation and maintenance staff use these anodes as handholds and footholds. The staff must be trained and prohibited from doing so, as the anodes may break inside the waterhead.
ashrae.org

ASHRAE Journal - April 2011

Table of Contents for the Digital Edition of ASHRAE Journal - April 2011