Galvanic corrosion is an electrochemical process that occurs when two metals are in contact with one another in the presence of an electrolytic solution (e.g. salt water). The two metals and the electrolyte behave like a battery cell and the more electropositive metal (the cathode) is corroded. The table below shows the anodic index of common metals. Of the two metals, the one with the more negative anodic index value will corrode. By examining the table we can observe that connecting a sensor with a stainless steel or aluminum housing to a copper pipe, in the presence of an electrolyte, will cause corrosion of the sensor housing. The effect will be particularly severe for an aluminum housing because of its more negative anodic index. Common methods for preventing Galvanic corrosion include placing a non-conductive material between the two metals to prevent them from touching or adding a sacrificial metal with a more negative anodic index, which will corrode instead of the sensor housing. Specific method to prevent Galvanic corrosion of sensors include:
- Connecting the sensor to the metal pipe through a non-conductive bushing (e.g. made of a high-performance plastic) which electrically insulates the sensor from the pipe. The follow illustration shows a pressure sensor connected to a brass tee through a nylon bushing.
- If a plastic bushing is not suitable, a sacrificial bushing of more negative anodic index can be used. For example, if the sensor housing is made of stainless steel, an aluminum bushing can be used. If the sensor housing is made of aluminum, a zinc bushing can be used.
- Low temperature copper or nickel electroplating of the wetted sensor surfaces can be carried out to protect the sensor. However, this process is more costly than the other methods and may not be suitable for all sensors. For example, plating of a pressure sensor diaphragm may slightly alter the sensors sensitivity).
|High nickel-copper alloys||-0.15|
|Chromium plated steel||-0.60|
|Tin plated steel||-0.60|
|2000 series wrought aluminum||-0.75|
|plain carbon steel||-0.85|
|Wrought aluminum (except for 2000 series)||-0.90|
|Cast aluminum (non-silicon)||-0.95|