- Published on Saturday, 12 April 2014 08:11
- Written by Don Bush
A: This is a good question, and the answer should begin with discussing exactly what is meant by galvanic corrosion.
When two different metals are electrically coupled in the presence of an electrolyte (i.e., an electrically conductive fluid such as salt water), an electrical voltage or potential is developed between the two metals. This voltage will drive electrical current and will cause accelerated corrosion of one of the metals. The metal that exhibits accelerated corrosion is called the anode while the other is the cathode. The cathode is said to be more “noble” than the anode.
For a given electrolyte, a galvanic series can be developed. This is essentially a listing of metals and alloys in order of their relative nobility—from highest to lowest—in that particular electrolyte. The most common galvanic series is that for seawater. A shortened, simplified version of this series is shown in Figure 1 as an example.
Figure 1. Galvanic series for seawater
Graphite Gold Silver Titanium
Alloy 20 (N08020, CN7M) 316 (S31600, CF8M)
Nickel-Copper (N04400, M35-1) Copper
410 SST (S41000, CA15)
Tantalum Chromium Nickel Copper
Gray Cast Iron
Carbon Steel (A105, A106 Grade B, WCC)
Lead Aluminum Cadmium Zinc
One noticeable factor here is that some of the metals and alloys don’t appear in this list in the order that one might expect. Some materials that are commonly used for resistance to seawater or salt-spray are lower on the list than materials not often used. An example would be zinc vs. carbon steel. Zinc has fairly good corrosion resistance in these environments, whereas carbon steel rusts very quickly. However, when electrically coupled, the zinc becomes the anode and corrodes, and the carbon steel becomes the cathode and does not corrode.
This has practical significance. For example, zinc coatings applied by hotdip galvanizing are often used to protect carbon and alloy steels from corrosion in marine environments. The zinc coating protects the underlying steel in two ways: 1) as a barrier coating by resisting corrosion, and 2) by “galvanic protection,” which protects the underlying steel even if there is a break in the zinc barrier.
Another characteristic of galvanic corrosion is that it exhibits an area effect. If the surface area of the cathode exposed to the electrolyte is increased or the surface area of the anode exposed to the electrolyte is decreased, the anode will corrode at a higher rate. If the exposed surface area of the cathode is much larger than the exposed surface area of the anode, the corrosion rate can be very high. Conversely, if the exposed surface area of the cathode is much smaller than the exposed surface area of the anode, the corrosion rate at the anode will be very low.
This area effect plays strongly into decisions regarding the use of dissimilar metals in corrosive situations. For example, carbon steel is clearly anodic to 300-series stainless steels in corrosive solutions. Because of this, one would expect that using these materials in combination would cause galvanic corrosion of the steel. However, many carbon steel valves are sold with 300-series stainless-steel trims, which don’t cause a problem. The reason is that the carbon steel valve thereby causing very little galvanic corrosion potential. This is also the reason graphite can be used for gaskets and other sealing components in valves, even though it is nobler than any metal.
If this combination is reversed, and a trim material is chosen that is less noble than the valve body, the potential for accelerated corrosion of the trim is increased. If the valve body and the piping are both nobler than the trim, the potential for accelerated corrosion of the trim can be very high.
There have been a number of instances where failure to consider the effects of galvanic corrosion have resulted in excessive corrosion.
- Excessive corrosion in 440C valve trim installed in a duplex stainless-steel body in a duplex stainless-steel piping system in a corrosive refinery application
- Excessive corrosion of CW2M valve bodies and trim installed in a titanium piping system flowing water containing chlorine dioxide in a paper mill
- Excessive corrosion of N05500 shaft and pins in a superaustenitic stainless-steel valve installed in a superduplex stainless-steel piping system in seawater service
- Excessive corrosion of S17400 shafts in butterfly valves with CF8M bodies installed in a 316 stainless-steel piping system (process fluid unknown)
- Excessive corrosion of carbon steel valve bodies installed in a 304 stainless-steel piping system flowing chloride-containing water
- Because of all this, when the process fluid is an electrolyte, it is best to ensure that the trim materials are at least as noble as the body, and that the body material is at least as noble as the piping.