Metals themselves can also set up corrosion cells. When a pipe consists of only one type of metal, impurities in the pipe wall can develop into anodes and cathodes. Alternatively, when two dissimilar metals come into contact, galvanic corrosion will occur. Galvanic corrosion is often set up in the distribution system in meter installations and at service connections and couplings. The galvanic series, shown below, arranges metals according to their tendency to corrode. This series can be used to determine whether galvanic corrosion is likely to occur and how strong the corrosion reaction will be.
As you can see on the series, some metals (such as gold and silver) are very inactive and unlikely to corrode. Many of these metals have been traditionally used as jewelry because of their low tendency to corrode even when in the presence of salts (in sweat) and oils found on the human body. Although these inactive metals would make non-corrosive pipes, they are usually too expensive to use in the distribution system.
At the other end of the galvanic series are metals which are very active and have a high tendency to corrode. These metals can be used as sacrificial anodes, which we will discuss later. They should not be used for distribution system pipes.
Most of the metals used in piping - iron, steel, and copper - are found in the middle of the galvanic series. These metals have some tendency to corrode, with those higher on the galvanic series (such as iron and steel) tending more toward corrosion.
The distance on the galvanic series between two metals will also influence the likelihood of galvanic corrosion when the two metals are placed in conjunction with each other. For example, if aluminum is brought in contact with a steel pipe, the likelihood of corrosion is low since aluminum and steel are close together on the galvanic series. However, if a stainless steel fitting is used on an iron pipe, the likelihood of corrosion is much higher.
When galvanic corrosion occurs, the more active metals always become the anodes. This means that they are corroded, and in extreme cases can begin to leak. The less active metal becomes the cathode and is not damaged.
At the other end of the galvanic series are metals which are very active and have a high tendency to corrode. These metals can be used as sacrificial anodes, which we will discuss later. They should not be used for distribution system pipes.
Most of the metals used in piping - iron, steel, and copper - are found in the middle of the galvanic series. These metals have some tendency to corrode, with those higher on the galvanic series (such as iron and steel) tending more toward corrosion.
The distance on the galvanic series between two metals will also influence the likelihood of galvanic corrosion when the two metals are placed in conjunction with each other. For example, if aluminum is brought in contact with a steel pipe, the likelihood of corrosion is low since aluminum and steel are close together on the galvanic series. However, if a stainless steel fitting is used on an iron pipe, the likelihood of corrosion is much higher.
When galvanic corrosion occurs, the more active metals always become the anodes. This means that they are corroded, and in extreme cases can begin to leak. The less active metal becomes the cathode and is not damaged.
Source: http://water.me.vccs.edu/concepts/corrosiontypes.html
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