Galvanic anodes tubes

Until recently membranes were not considered compatible with cathodic protection as the anode would have to be under the membrane to pass current, and the anode generates gases that must escape. Membranes and cathodic protection have been used in car parks on thin slabs with some success. Some of the newer probe anodes have venting tubes and so they can be used under membranes. Galvanic anodes can be used as they do not generate gasses electrochemical chloride removal (or realkalization) could be done before the replacement of the membrane. 

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. 

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. 

Another method of providing protective current is to use the heads or water boxes as sacrificial anodes. These water boxes, when made of heavy steel or of cast iron, provide galvanic protection to the tube ends and tube sheet as they corrode. 

Figure 19.1 A galvanic cell. The salt bridge (an inverted U tube) containing a KCI solution provides an electrically conducting medium between two solutions. The openings of the U tube are loosely plugged with cotton balls to prevent the KCI solutbn from flowing into the containers while allowing the anions and cations to move across. The lightbulb is lit as electrons flow externally from the Zn electrode (anode) to the Cu electrode (cathode).

Electrochemical techniques Cathodic protection has been found effective against anodic SCC. Lead-tin solders and nickel plating have been used to protect stainless steel tube ends against SCC by water. This technique must be carefully controlled to be effective. Austenitic stainless steel has been known to suffer chloride SCC while simultaneously causing galvanic corrosion of steel components. 

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