Galvanic series
The galvanic series determines the electrochemical potential and nobility of metals and metal alloys.
Each alloy or metal has a distinctive corrosion potential. The more negative a metal or alloy is, the more likely it is to suffer galvanic corrosion.
When two metals are submerged in an electrolyte, while also electrically connected by a metallic conductor, the less noble will experience galvanic corrosion.
The less noble metals becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.
Galvanic cell
A galvanic cell arise when two metals with dissimilar compositions come into contact in the presence of an electrolyte. The rate of corrosion on the less noble metal is determined by the electrolyte, the difference in nobility and the relative areas of the anode and cathode exposed to the electrolyte.
Read more about the electrochemical cell and process of corrosion.
Galvanic series diagram
The below diagram lists metals and alloys potential in the order of reactivity in sea water vs. Zn and Ag/AgCl and in soil vs. Cu/CuSO4 reference electrodes. Top listed metals/alloys are the least active (most noble) and proceeds down to the most active (anodic) metal/alloy.
Galvanic series chart
The chart can be used to determine the likelihood of a galvanic reaction, and galvanic corrosion or bimetallic corrosion, between two different metals in a seawater environment. The closer a metal or an alloy is in the galvanic series, the less are the effects of galvanic corrosion compared to those metals far apart.
Read more about the process of corrosion.
The galvanic potential is specific for a specific electrolyte, temperature and flow rate. The diagram refers to flowing sea water and temperature in the range 10 to 30 ℃.
Corrosion rate
Corrosion rate can be defined as the speed at which any metal in a specific environment deteriorates.
Life prediction of steel in water is challenging for existing structures as well as new. Corrosion is a complex function of many factors such as salinity, dissolved oxygen, stray currents, pH and temperature and more, which makes it difficult to establish predicatable rates of degratiation. This explains why many corrosion rate charts exist, but mostly for unique locations and conditions.
As a rough indication, the average corrosion rate for carbon steel in seawater can be assumed to be in the area of 0.1 to 0.15 mm/year, which equals 100-150 mA/m². Localised corrosion rate however, are often several orders of magnitude higher.
Stainless steel can be either passive or active
Stainless steel may corrode either in active or passive state, dependent on the electrolyte conditions. In normal aerated water condition the passive metal has an oxide film that prevents further attack, while the same metal become active and exhibit a potential near 0.5 volts in low-velocity or poorly aerated water.
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