What is galvanic corrosion?
Galvanic corrosion is also known as bimetallic corrosion. Many devices are made of a combination of various metals, such as aluminum and copper, iron and zinc, copper and iron, and so on. Under the electrolyte water film, a corrosion macro cell is formed, which will accelerate the corrosion of the negative potential metal. Factors affecting galvanic corrosion include the environment, the conductivity of the medium, and the area ratio of cathode and anode. Galvanic corrosion also occurs in humid atmospheres, the greater the humidity or the more salty the atmosphere (such as near the sea), the faster the galvanic corrosion. A galvanic couple composed of a large cathode and a small anode. The greater the anode corrosion current density, the more serious the corrosion.
Galvanic corrosion depends primarily on the potential difference between dissimilar metals. The potential here refers to the actual potential of the two metals in the electrolyte solution (corrosion cut-off), that is, the corrosion potential of the metal in the solution.
The galvanic sequence of common metals in flowing seawater is as follows:
Magnesium, zinc, aluminum, low carbon steel, Cr13/Cr17 stainless steel (active), brass, bronze, Cr17 stainless steel (passive), Cr18Ni9, rust steel (passive), silver, titanium, platinum and above Arranged in order of inactivity. The ones in the front are called base metals, and the ones in the back are precious metals. Active metals are always prone to corrode first.
Methods of Preventing Galvanic Corrosion
① Try to avoid dissimilar metals with great potential difference as conductive
②In order to avoid the unfavorable area ratio of large cathode and small anode, the parts with small area should use metal with relatively positive corrosion potential
③ When dissimilar metals with large potential difference are assembled together, an insulating sheet is generally added in the middle, and the gasket is fastened without moisture absorption. The selection avoids crevice corrosion
④ When designing, choose an anode part that is easy to replace. Or thicken it to prolong its life
⑤When possible, add corrosion inhibitor or paint to reduce the corrosion of the medium, or add a third piece of anode metal for cathodic protection, etc.
