GALVANIC CORROSION G002
If you put two dissimilar metals or alloys in a common electrolyte and connect them with a voltmeter the voltmeter will show an electric current flowing between the two. This is how the battery in your automobile works.
When the current flows, material will be removed from one of the metals or alloys (the ANODE ) and dissolve into the electrolyte. The other metal (the CATHODE) will be protected.
Now let’s take a look at the Galvanic Series chart. The further apart the materials are located on this chart the more likely that the one on the ANODIC end will corrode if they are both immersed in a fluid that is considered to be an electrolyte. Salt water (water with a lot of chlorides) is one of the best.
A ship has lots of bronze fittings and a steel hull. Note that steel is located seven lines from the ANODIC end, and bronze is listed at twenty seven rows from the same end. Sea water is a perfect electrolyte so the bronze fittings would immediately attack the steel hull unless something could be done to either protect the steel or give the bronze something else to attack. The classic way to solve this problem is to attach sacrificial zinc pieces to the hull and let the bronze go after them. Again, looking at the chart, you will note that zinc is found on line three from the top of the chart. In other words the zinc is further away from the bronze than the iron is, so the galvanic action takes place between the zinc and the bronze rather than between the steel and the bronze. Zinc paint is used for the same reason in many applications.
Nickel base tungsten carbide contains active nickel. When this face material is used in a dual seal it is common to circulate water or antifreeze, containing water, between the seals (as mentioned in the beginning, water can be an excellent electrolyte because of the addition of chlorine and fluorine). You will note that active nickel is located twenty one rows from the top of the chart. Passivated 316 stainless steel is positioned nine rows from the bottom. This means that the stainless steel can attack the nickel in the tungsten carbide causing it to corrode.
The rate at which corrosion takes place is determined by :
- The distance separating the metals on the galvanic series chart
- The temperature and concentration of the electrolyte. The higher the temperature, the faster it happens. Any stray electrical currents in the electrolyte will increase the corrosion also.
- The relative size of the metal pieces. A large cross section piece will not be affected as much as a smaller one.
Many metal seal components are isolated from each other by the use of rubber O-rings or similar materials and designs. Shaft movement that causes fretting of the 316 stainless steel rubs off the passivated layer and exposes the active stainless to the electrolyte until the metal part becomes passivated once more. This is one of the reasons we see corrosion under O-rings, Teflon®, and similar materials.
Look under “corrosion-stainless steel” for a list of the common corrosion problems we experience with stainless steel seal components and concentrated cell or crevice corrosion where I discuss specific corrosion beneath rubber parts.