Heat exchanger galvanic protection

Galvanic corrosion can be controlled by the use of sacrificial anodes. This is a common method of controlling corrosion in heat exchangers with Admiralty tube bundles and carbon steel tube sheets and channel heads. The anodes are bolted direcdy to the steel and protect a limited area around the anode. Proper placement of sacrificial anodes is a precise science. 

Carbon steel heat exchangers, cast iron water boxes, screens, pump components, service water system piping, standpipes, fire protection systems, galvanized steel, engine components, and virtually all non-stainless ferrous components are subject to significant corrosion in oxygenated water. 

Galvanic or impressed current anodes are used to protect these components. The anode material is determined by the electrolyte zinc and aluminum for seawater, magnesium for freshwater circuits. Platinized titanium is used for the anode material in impressed current protection. Potential-regulating systems working independently of each other should be used for the inlet and outlet feeds of heat exchangers on account of the different temperature behavior. The protection current densities depend on the material and the medium. 

The L-foot fm covers the tube more or less completely to protect the base tube against corrosive attack, but still leaves a potential corrosive site at the base of the fin adjacent to the preceding fin. The double L-foot is intended to provide complete coverage of the tube, where corrosion would otherwise be a problem. Where corrosion is troublesome, soldered or galvanized tubes may offer a solution. The dimensions of fmned tubes are results of past experience in the design of air cooled heat exchangers. Tube diameters range from about 1.905 cm (0.75 in.) to 5.08 cm (2.0 in,). 

Copper-base alloys will corrode in aerated conditions. It is, therefore, sometimes appropriate to consider cathodic protection. It becomes particularly relevant when the flow rates are high or when the design of an item causes the copper to be an anode in a galvanic cell (e.g. a copper alloy tube plate in a titanium-tubed heat exchanger). Corrosion can be controlled by polarisation to approximately — 0-6V (vs. CU/CUSO4) and may be achieved using soft iron sacrificial anodes. 

Swedish iron is sometimes used as galvanic wastage plates in heat exchangers, particularly for marine applications. This is possibly based on tradition, since it cannot be the most economical method in the light of current cathodic-protection practice. The material is not currently used as an impressed-current anode. 

Galvanic anode systems are generally used in well-coated electrically isolated structures, offshore structures, ship hulls, hot-spot pipeline protection, heat exchanger water boxes and other environments of resistivity below 10000 Q cm. 

When two dissimilar metals are immersed in an electrolyte they usually develop different potentials in accordance with the theory already presented. If the metals are in contact the potential difference provides the driving force for corrosion. Severe corrosion often occurs as a result of the contact between two metals. In shell and tube heat exchangers where the tubes are fabricated from a corrosion resistant alloy, and the shell is made from mild steel for instance to reduce the capital cost, corrosion is very likely unless adequate protection is made. The less resistant of the two metals is caused to corrode, or to corrode more rapidly, while the resistant metal or alloy corrodes much less or may be even completely protected. The basis for galvanic corrosion is illustrated on Fig. 10.6. Metal A has a lower electrode potential than metal B. Ions migrate in the conducting solution while electrons flow across the junction of the two metals, as a result metal A is corroded at C. 

Research has also been focused on the need for inhibitors that will function adequately in recirculated systems made up of a variety of metals (and perh s nonmetals as well). Typical systems of this type include jacket water cooler circuits, which may include cast iron parts, copper-bound gaskets, lead-tin or silver solder joints, and brass or aluminum heat exchanger tubes or both. The inhibitive system must provide protection for all the different metals and not stimulate galvanic aaion between them. Films that may be deposited must be sufficiently thin so heat transfer is not seriously reduced. 

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