Galvanic reactions (galvanic corrosion) can present a problem for numerous industries. Fortunately, there are ways to handle it, if not avoid it. Here, we are going to explain galvanic reactions and what causes them. We’ll also look briefly at heat exchanger plugging (we recommend TNS plugs). You may have questions about the plugs used regarding grade/material and more.
Let’s start by taking a look at galvanic corrosion. When two different alloys or metals are regularly connected by moisture or immersed in a corrosive solution, the potential for corrosion will be developed by each. The role of the cathode will be played by the more noble metal if conditions for corrosion are present. The part of the anode will be played by the more active metal.
Between the cathode and the anode, a measurable current may flow. The rate of corrosion for the anode will increase if this occurs in the service environment. A decrease will result in the corrosion rate of the cathode.
Using Galvanic Corrosion
To extend the life of materials (i.e., zinc anodes and zinc coatings on carbon steel in water heaters) galvanic corrosion is sometimes used. Unfortunately, unexpected failures could result if the conditions aren’t exactly right. Three elements are required for galvanic corrosion to occur. These are as follows:
- A conductive electrolyte solution must connect the two metals (water would be an example). A conductive path is created by the electrolyte solution. This could occur when there is regular fog exposure, rain, condensation, immersion, or other sources of moisture during which the two metals are dampened and connected.
- Direct electrical, metal-to-metal contact.
- Two metals with two corrosion potentials that are different.
Relative Surface Area
For heat exchanger plugging, galvanic reactions are a concern when dissimilar metals are used. In theory, due to the ionic interplay between the two materials, this can cause advanced corrosion in the lesser material (in the presence of an electrode). The proportional surface area of the more noble material, however, can have a lot to do with galvanic reactions. The less noble material will have a more drastic reaction.
In addition to the above-stated requirements, an important factor is the relative surface area of each metal that is exposed (not mass). If the anode is very small and the area of the cathode very large, it’s likely that the produced current will be extremely high and encourage faster anode corrosion.
Here’s an example: Let’s say you have a stainless steel window frame. Carbon steel screws have been used to attach it. More likely than not, at an accelerated rate, the screws will corrode. If, however, stainless steel screws were used to attach a window frame made from carbon steel, galvanic corrosion would be extremely little, if any.
In areas where moisture accumulation is likely and will remain for extended periods, dissimilar metal combinations should be avoided. Dissimilar metals can be used together, however, in well-drained exterior applications if favorable service ratios exist. Electrically insulating them from one another is the best solution. For instance, if you weld together stainless steel and painted carbon steel, paint the welded joint. Regularly used with metals are stainless steel fasteners coated with inert washers (such as neoprene).
In heat exchanger plugging, whenever possible, it’s best to match tube material to plug material. This isn’t always possible, however. So, if you must choose a different material for plugging, follow this rule of thumb: In a less noble tube material application, use a more noble plug material. This is due to the relative surface area effect negating concerns of galvanic reaction.
If you have any questions regarding anything you’ve read above, we will be happy to answer them for you. Feel free to contact Torq N’ Seal Plugs at 201-641-2130!