Galvanic corrosion is an electrochemical process that occurs between two dissimilar metals, or between a metal and a conductive non-metallic material, when both are exposed to an electrically conductive media. In the case of a packing gland, it occurs between a metal component and the carbon or graphite packing. Under these conditions, the material that is closest to the anodic end of the galvanic scale will be corroded in preference to the one that is closest to the cathodic end of the scale. (See Table 1.) As the distance between materials on the galvanic scale increases, a corresponding rise occurs in the rate and the extent of the corrosion.
In a valve or a pump using packing made of either graphite or carbon, a galvanic reaction may be initiated as soon as any electrically conductive fluid, such as water, is introduced. Since graphite is more cathodic than the metals that make up valves and pumps, it is the metal that may be subject to corrosive attack.
Liquid Phase Needed
Even though a valve or pump may be packed with a graphite or carbon packing, many cases exist in which the metal parts will not be subjected to galvanic corrosion. For example, an electrically conductive fluid in a liquid state must be present for the galvanic reaction to take place. The temperature of a superheated steam valve prevents the accumulation of any significant amount of water, thereby nullifying the possibility of galvanic corrosion.
Another example of when galvanic corrosion protection may not be necessary is when the equipment is constructed of austenitic stainless steels (e.g., 300 series, 630, etc.). These stainless steels are much more resistant to galvanic attack.
On the other hand, the martensitic stainless steels (e.g., 400 series) are highly susceptible to galvanic attack. If a valve or pump is constructed of martensitic stainless steel and if it will be exposed to an electrically conductive fluid for any period of time, then consideration should be given to incorporating a galvanic corrosion inhibitor system into the carbon or graphite packing sets used to seal it.