Dissimilar Metal Contact Causes Accelerated Corrosion

Here is a classic example of something you don't want to do. This is a picture of a stainless steel bolt & nut going through and coming into direct contact with carbon steel. This is a handrail section of an elevated water tank balcony. The S.S. bolt was installed on the handrail to connect a chain that crosses the balcony handrail opening.

The carbon steel is more readily corrodible (less noble) than the stainless steel (i.e.-carbon steel rusts faster than stainless steel). And when you put the two metals in direct contact with one another, the less noble metal becomes the anode and therefore corrodes (rusts) at a much more assertive rate. This can obviously detrimentally affect the integrity of the structure if nothing is done to rectify this incongruence. It probably won't affect the integrity of the entire tank, but it can considerably affect the localized area of the handrail.

This is a phenomenon that is explained in most NACE (National Association of Corrosion Engineers) or SSPC (Society for Protective Coatings) courses. This occurence is explained by what is referred to as the "Galvanic Series." The galvanic series ranks metals in accordance to their respective rates of corrosion (or their "Nobility"). If a metal is more noble, then it has a greater resistance to corrosion; and vice versa. Subsequently, if you put two metals in contact with one another that have a considerable difference in their respective nobility, the less noble metal will always corrode an an accelerated rate. And that is what is exemplified in this picture.

This is a common occurrence with water tanks. And considering the myriad of things water tanks get attached to them (cell phone antennas, satellite dishes, cables, wires, telemetry, 911 transmitter devices, etc...) it is important to have a professional inspection or corrosion consultant look at and inside your tanks on a regular basis. Protect that asset and "Long Live the Tank."