Anodes & Corrosion Title

























Anodes Galvanic Or Dissimilar Metal Corrosion

A factor, which accelerates the corrosion process, may be differences in potential of dissimilar metals coupled together and immersed in an electrolyte. Many factors control the severity of the corrosion resulting from such dissimilar metal coupling: 

1. The relative differences in position (potential) in the galvanic series, with reference to a standard electrode. The greater the difference, the greater is the driving force of the reaction. 

2. The relative area relationship between anode and cathode areas. Since the amount of current flow and, therefore, total metal loss is determined by the potential difference and resistance of the circuits, a small anodic area will corrode more rapidly. It will be penetrated at a treated rate than a larger anodic area. 

3. Polarization of either the cathodic or anodic area may reduce the potential difference, and thus reduce the rate of attack of the anode. 

It is not wise to couple a small exposed area of a less noble metal with a large area of a more noble metal, in media where the less noble material may tend to corrode by itself. If such couples cannot be avoided, but one of the dissimilar metals can be painted or coated with a non-metallic coating, the cathodic material should be so coated, rather than the anodic one. If the two materials can be insulated from each other, e.g. by the use of an insulated joint in piping systems, the galvanic couple can be avoided. 

Where this is not possible, a waster heavy wall nipple section of the less noble material can be used and readily replaced when it fails. In piping systems handling natural waters, galvanic corrosion is not likely to extend more than three to five pipe diameters down the ID of the less noble pipe material. In metal components exposed to the atmosphere, galvanic effects are likely to be confined to the area immediately adjacent to the joint.