This guide is not intended to be comprehensive; rather, it is intended to function as an overview and general guide. For specific application requirements, consult a metallurgist or metallurgical engineer.
What is Galvanic Corrosion?
When two dissimilar metals are in contact in the presence of an electrolyte (such as moisture or an aqueous solution), an electrochemical reaction occurs, resulting in galvanic corrosion. The metal with a higher anodic index will act as the anode and undergo corrosion, while the metals with a lower anodic index will act as the cathode and experience minimal corrosion.
The "anodic index" is a measure used to rank different metals or alloys based on their tendency to corrode when coupled with another metal in a galvanic cell. It provides an indication of the relative nobility or reactivity of metals in a galvanic corrosion scenario.
The anodic index is typically expressed as a numerical value or a ranking. Higher anodic index values indicate a greater tendency for a metal to corrode, while lower values indicate a higher resistance to corrosion.
What other factors should be considered when determining the probability of galvanic corrosion?
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Electrolyte Environment: The type, composition, and conductivity of the electrolyte in which the metals are immersed play a significant role in galvanic corrosion. Different electrolytes can have varying corrosive effects on specific metal combinations. (e.g.: Air versus water)
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Surface Area Ratio: The relative surface areas of the two dissimilar metals in contact affect the rate of galvanic corrosion. A larger surface area of the anodic metal compared to the cathodic metal typically leads to more accelerated corrosion.
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Electrical Conductivity: The conductivity of the electrolyte and the electrical path between the metals influence the efficiency of the galvanic cell. Higher conductivity and more direct electrical contact between the metals can increase the likelihood of galvanic corrosion.
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Temperature: Elevated temperatures can accelerate corrosion rates. Higher temperatures generally increase the activity of electrochemical reactions, leading to more rapid galvanic corrosion.
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Presence of Protective Coatings: Protective coatings, such as paint, plating, or passivation layers, can provide a barrier between the metals and the electrolyte, reducing the risk of galvanic corrosion. The presence, quality, and integrity of these coatings are important factors to consider.
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Mechanical Factors: Mechanical stresses, such as vibration, abrasion, or rubbing between the metals, can affect galvanic corrosion. These factors can disrupt protective coatings or increase the contact area, potentially promoting corrosion.