Galvanic corrosion of the least nobel alloy can be a serious issue when two different alloys are coupled together or touching. The chart above gives the voltage difference when dissimilar metals are in a seawater environment. If the alloys are different you can always expect SOME galvanic corrosion, but it's not usually a concern unless the voltage difference between the two metals is 0.2v or more.
For example, manganese bronze fittings could be fastened with silicon bronze bolts in a sea water environment and experience little to no galvanic corrosion because the voltage difference is so very small. However, mild steel or stainless steel screws or bolts would cause an adverse reaction and lead to significant galvanic corrosion on that same manganese bronze fitting.
When two different metals or alloys are immersed in a corrosive solution or regularly connected by moisture, each will develop a corrosion potential. If the conditions for galvanic corrosion are present, the more noble metal will become the cathode and the more active metal will become the anode. A measurable current may flow between the anode and the cathode. If this occurs, the anode’s rate of corrosion in the service environment will be increased while the cathode’s corrosion rate will decrease. The increased corrosion of the anode is called “galvanic corrosion.”
Galvanic corrosion is sometimes used to extend the life of materials (i.e. zinc coatings on carbon steel and zinc anodes in water heaters), but, if it is not considered and the right conditions exist, it can lead to unexpected failures.
Requirements for Galvanic Corrosion:
In order for galvanic corrosion to occur, three elements are required.
If any of these elements is missing, galvanic corrosion cannot occur. If, for example, the direct contact between the two metals is prevented (plastic washer, paint film etc.) or if there is some other interruption in the conductive path, there cannot be galvanic corrosion and each metal will corrode at its normal rate in that service environment. Figure 1 shows examples of conditions that do not meet all requirements for galvanic corrosion.
Examples of bi-metallic combinations when galvanic corrosion cannot occur
When two different metals are coupled together in atmosphere or water, the likelihood of developing galvanic corrosion can be predicted using a “galvanic series.” In specialized applications, such as when dissimilar metals are embedded in concrete, corrosion data for that specific environment should be used.
Figure 2 shows the galvanic series measured in seawater for some common metals and alloys. When two metals are further apart in the list (e.g. a larger difference between the two numbers), the driving force for galvanic corrosion is increased. The most anodic (active) metals are at the top and most cathodic (noble) at the bottom. Both solid and hollow bars are shown for the stainless steels. The hollow bars represent actively corroding stainless steel, which has a different potential then passive (not corroding) stainless steel. In most applications, where dissimilar metals are combined, the passive (solid) bar should be used to determine the position of the stainless steel.
For example, if zinc (think galvanized steel) which is an active material and near the top of the list and stainless steel, a noble metal and near the bottom of the list were in direct contact and in the presence of an electrolyte (water), galvanic corrosion will occur if they are regularly exposed to an electrolyte.