Expert Galvanic Corrosion Prevention & Testing Services
Galvanic Corrosion Testing
Galvanic corrosion takes place when two dissimilar metal alloys are electrically connected and simultaneously exposed to the same corroding environment. When the dissimilar alloys are galvanically coupled, the less corrosion resistance alloy becomes the anode electrode which corrodes. The alloy with the more noble corrosion potential becomes the cathode electrode which experiences reduction reactions and is protected against corrosion attack. The corrosion at the anode electrode is enhanced the larger the magnitude of the difference in the corrosion potential between the two coupling metals in the corroding media. The attack is greatest near the junction of the dissimilar electrodes and becomes larger the larger the cathode surface area is.
To assess the severity of a corrosion attack induced by galvanically coupling two dissimilar alloys in a corroding environment, their corrosion potential and the galvanic corrosion current generated by the coupling is measured in such corroding environment. Assessment of galvanic corrosion can also be done by overlaying the polarization curves of each of the coupling alloys tested separately in the corroding environment. To minimize galvanic corrosion, the two coupling alloys must have nearly similar corrosion potentials in the working corroding environment. Another form to suppress galvanic corrosion is to insulate the coupling alloys with a protective coating layer.
As examples, here we provide a couple of galvanic corrosion cases. Based on the standard reduction potentials for iron (Fe), zinc (Zn), and aluminum (Al), Fe is the most noble and Al is the most active among them. Aluminum is then expected to generate a large galvanic current when coupled to Fe. In practice, Al forms a strong protective layer which makes it very resistant to corrosion and is expected to suppress galvanic current when coupled to Fe. The magnitude of the galvanic current from coupling two dissimilar alloys is generally determined by the difference between their respective corrosion potentials in a given corroding environment.
The attached graphs display the galvanic current generated from coupling iron to zinc and iron to aluminum in an aqueous solution of 0.1M H2SO4. The current generated by coupling iron to zinc is about 8.70 mA, and that generated from coupling iron to aluminum is about 20 µA, for the same cathode to anode area ratio; iron being the cathode in both cases. The relatively large galvanic current of the iron-zinc couple will cause zinc to be aggressively corroded while hydrogen evolution is expected to take place on the iron surface. In contrast, the small galvanic current of 20 µA measured from coupling iron to aluminum will induce minimum corrosion on aluminum.
The standard method to evaluate Galvanic corrosion tests in electrolytes is ASTM G71.
Galvanic current due to room temperature galvanic coupling of a zinc plate, 2 cm2 area, to a 500 µm thick iron plate, 1 cm2 area, in 0.1M H2SO4 solution. The generated galvanic current by the coupling is about 8.7 mA, a high enough current to induce significant corrosion to zinc.
Galvanic current due to room temperature galvanic coupling of an aluminum plate, 2 cm2 area, to a 500 µm thick iron plate, 1 cm2 area, in 0.1M H2SO4 solution. The generated current by the coupling is about 20 µA. This low galvanic current is not expected to induce significant corrosion to aluminum.