Corrosion is the wearing away or alteration of a metal by electrochemical reaction or by direct chemical attack. We will focus on electrochemical corrosion, also known as galvanic corrosion. Metals have a specific relative electrical potential. When dissimilar metals are in contact with each other in the presence of an electrolyte (moisture for example), a low level electric current flows from the metal having the high potential on the galvanic series (see chart) to the metal having the lower potential. This is known as galvanic action. The metal in the higher position on the chart corrodes at a faster rate. This occurs due to the conduction of electrons through the metals from anode (+) to cathode (-), and a conduction of ions through the electrolyte which forms the corrosion products. Water is a common electrolytic solution, with salt water being even more damaging due to the high concentrations of dissolved salts. It is important to know from which of the dissimilar metals the current will flow when selecting materials. (Hopefully you will find this article exciting and interesting. We in Marketing try to be selective in what we publish from our Engineering Department. For example, our engineers have produced a video on mildew, which we will NOT be distributing. Sorry!).
To minimize galvanic corrosion in a fastener assembly, try to use the same, or similar metals, especially if an electrolyte is present. If dissimilar metals are coupled together, those that are most widely separated on the chart are most likely to cause corrosion. When dissimilar metals are used in the presence of an electrolyte, separate them with a dielectric (insulating) barrier, such as a paint or coating. Avoid assemblies where the anodic material is small compared to the cathode. There is a higher current density when the current flows from the smaller area to the larger. The threaded fastener which is typically small should be lower on the galvanic chart than the parent material being fastened. In some applications, galvanic action can be used advantageously by coupling the part requiring protection to a more positive (less noble) metal which will sacrificially corrode, e.g. "zincs" on boat motors.
|Metal or Alloy|
|↓||Steel or Iron|
|↓||Chromium Iron (Active)|
|↓||Type 304-Stainless (Active)|
|↓||Type 316- Stainless (Active)|
Positive ( + ) to Negative ( - )
|Lead, Tin Solders|
|↓||Brasses, Copper, Bronze|
|↓||Type 304 Stainless (Passive)|
|↓||Type 316 Stainless (Passive)|
|CATHODE ( - )
The best way to preclude galvanic action from occurring is to eliminate the electrolyte conductor from the fastener assembly. Some additional helpful hints are:
Some coatings may require that the tapped holes be plugged prior to application.
The reality is that it is virtually impossible to completely eliminate corrosion under all conditions. Metals (like people) have an inherent ability to return to their natural state, and unlike most people, metals do so willingly. Corrosion is costly in its many forms. It must be considered at the initial design stages. Hopefully, this information will help the designer to understand the principles of galvanic corrosion, and to select components, coatings, sealants, etc. to minimize these effects to an acceptable level. For severe corrosion applications (immersion in colas, diet sodas, etc.) we suggest that metallurgists or corrosion experts be consulted.
If you have any questions, please contact us.