Dissimilar pipes

Figure 2.5. Gasket insertion between pipe llangcs for sealing purposes and to minimize galvanic corrosion between dissimilar piping metals.

Thus, if dissimilar pipes are butt-welded with the electrolyte flowing through them, the most severe corrosion will occur adjacent to the weld on the active metal. The current of the galvanic cell takes the path of least resistance and this affects corrosion in that current does not readily flow around corners. In soft water, the critical distance between copper and iron may be 5 mm in seawater it may be several decimeters. The critical distance is greater the larger the potential difference between anode and cathode. Then, the geometry of the circuit affects galvanic corrosion and this is observed in the case of stray current corrosion.7 (Baboian)5... 

Ensure effective separation between piping sections of dissimilar metals. Examples of this are illustrated in Figure 2.5. As shown, dielectric nonabsorbent gaskets of adequate thickness can be inserted between dissimilar... 

Piping should not be directly attached to dissimilar metal structures via conductive materials. 

In situations in which piping protrudes partitions or bulkheads of dissimilar metals, proper precautions should be taken against galvanic corrosion. Possible solutions include the use of dielectric gaskets or sleeves and the use of plastic adhesive tapes. Examples are illustrated in Figure 2.6. 

In buried pipeline installations, avoid contact of piping with structures of dissimilar metals. Also, where possible, specify uniform quality, grade and surface conditions. Various quality sections should not be welded together in buried installations. 

Tinning of copper piping or components is a good approach toward minimizing galvanic action between dissimilar metals. 

Where the pipe run contains dissimilar ends (closed-open or open-closed), prohibited pipe lengths are ... 

Gilbert, P. T., Considerations arising from the use of dissimilar metals in seawater piping systems , 5th International Congress on Marine Corrosion and Fouling, Barcelona (1980)... 

Differences in soil potential. Since pipe-lines pass through zones of aerated and unaerated soil and possibilities for electrolytic-cell formation are great, the observed currents may have resulted from soil dissimilarities. 

Dissimilar-metal corrosion can occur even if the two metals are not initially in direct contact. For example, in homes where copper tubing is used for plumbing, there is always a small amount of dissolved Cu2+ in the water. When this water encounters steel piping or a chrome-plated bathroom sink drain, the more-noble copper will plate out on the other metal, producing a new metals-in-contact corrosion cell. In the case of chrome bathroom sink fittings, this leads to the formation of Cr3+ salts which precipitate as greenish stains. 

The corrosivity of soils also depends upon the oxidation-reduction potential as classified by Booth et al.15 The classification scheme of the corrosivity of soils is given in Table 4.4b. Macrogalvanic cells are formed in underground pipelines due to foreign structure the combination of new and old pipe dissimilar metals (stainless steel and carbon steel) differential aeration dissimilar soils and stray currents. All these lead to localized corrosion of underground pipelines. 

The cause of corrosion in the domestic hot water system in the building is attributed to the improper use of dissimilar metal pipes and associated components. The temperature difference in the heat exchanger and heat circulation locations makes the corrosion more severe. An area effect is also a contributor to the corrosion. The remedial measures should be aimed at reducing the existing galvanic cell, to minimize the temperature and area effects. 

When two dissimilar metals are used in the construction of equipment containing a conducting fluid in contact with both metals, an electric potential may be set up between the two metals. The resulting galvanic action can cause one of the metals to dissolve into the conducting fluid and deposit on the other metal. As an example, if a piece of copper equipment containing a solution of sodium chloride in water is connected to an iron pipe, electrolysis can occur between... 

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