Why Can’t You Use Stainless Steel and Aluminum Together
The combination of aluminum and stainless steel causes galvanic corrosion. In order to understand why you shouldn’t use stainless steel and aluminum together, we first need to understand how galvanic corrosion works. Galvanic corrosion is the transfer of electrons from one material (anode) to another (cathode). In addition to knowing what galvanic corrosion is, we also need to understand the technical terms that go along with it.
Here are all of the technical terms we will be using during this post:
- Anode – material that is positively charged, electrons leave this material
- Cathode – material that is negatively charged, electrons enter this material
- Electrolyte – liquid that aids in the process of electron transfer
- Corrosion/corrode – Destroy or weaken metal gradually
How It Works
Galvanic corrosion occurs when two materials (an anode and a cathode) come into contact with each other and an electrolyte. Electrolytes can be environmental factors such as humidity or rainwater. When these factors come into play, electron transfer will begin to occur. Depending on the level of resistance in an electrolyte, this transfer can happen much faster. This is why salt water, an electrolyte with a very low resistance, is a common factor when considering what product to use. Due to this, it is incredibly important to consider what material you are going to use in an environment. When working with a marine, salt water environment, you even need to consider the type of stainless steel you are using.
There are multiple kinds of rust that can occur during the oxidization process. To find out more about them please read this blog post about Three types of rust that frequently occur.
For the rest of our post, instead of referring to anode and cathode, we will be using the example of aluminum (anode) and stainless steel (cathode). When aluminum and stainless steel are used in an assembly together, the electrons from the aluminum will begin to transfer into the stainless steel. This results in the aluminum weakening. This weakened aluminum causes it to deteriorate at a much faster rate. This can lead to an extended life of the stainless steel. Note: Aluminum, if left on its own with the electrolyte, will still lose its electrons eventually, but having stainless steel present will significantly speed up this process.
The galvanic corrosion practice is actually commonly used in plating to create a sacrificial layer on top of another material. Zinc plated steel and black oxide are commonly used examples.
Each and every assembly is situational. As metal relies on its environmental factors to corrode, and there may be places where you can use some metals together without seeing these effects. If the environment is very dry, sheltered from weather and dirt then you, may try using metals together. However, in most situations the environment is not temperature and humidity controlled, rust will occur. Due to this, Albany County Fasteners recommends never using aluminum and stainless steel together. We also recommend using metals exclusively for maximum life. Stainless with stainless, aluminum with aluminum, brass with brass. Mixing metals can affect the strength of the application, the lifespan of the fasteners, the corrosion of the materials, etc.
The other situation in which these materials can be used together with little impact on rust prevention is if the cathode area is very small when compared to the anode area. For example, if the base material is a large sheet of aluminum, then using very small stainless steel screws will not dramatically decrease the life. Conversely, if you use aluminum to attach a large sheet of stainless steel, the aluminum life will be dramatically shortened.
Albany County Fasteners recommends the use of neoprene EPDM or bonding washers in between stainless fasteners and aluminum materials, the neoprene forms a barrier in between the metals, preventing corrosion.
Environmental Factors To Determine
Many factors need to be considered when choosing the correct material for your installation.
||Why It Matters
|Duration of electrolyte contact
||The longer an electrolyte is in contact with aluminum and stainless steel, the more likely there is to be a transfer of electrons.
||The lower the electrolyte resistance the easier it is for electron transfer to occur. Ex: salt water has a very low electrolyte resistance.
||Water that sits and takes a very long time to dissipate can lead to extended exposure to electrolytes.
||Dirt (especially not in direct sunlight) can absorb an electrolyte and hold it for very long periods of time. This can result in increased exposure to the assembly if it is not kept clean.
||Both are environmental factors that lead to increased water in the air. If the environment is prone to these factors, the exposure to electrolytes is considered to be extended
||Crevices provide a catch for moisture (electrolyte) which can end up holding it against the materials for an extended period of time.
If you decide that you need to use two different materials together, we recommend using an anode as the base material and making sure that it is significantly larger than the cathodes. Cathodes can also be called noble metals or metals that have a high resistance to oxidation (rust). We have compiled a list of noble metals below:
From Anode To Cathode
To mitigate the effects of galvanic corrosion even further, it is recommended to use materials that are less likely to cause electron transfer when exposed to each other and an electrolyte. The following list is a list of materials. *Note: the closer the two metals on this list, the less likely they will be to suffer from the negative effects of galvanic corrosion.
- Magnesium Alloys
- Aluminum Alloys
- Mild and Carbon Steel, Cast Iron
- Chromium Steel (With Less Than Or Equal To 6% Chromium)
- Active Stainless Steels (302, 310, 316, 410, 430)
- Aluminum Bronze
- Lead-Tin Solder
- Active Nickel
- Active Inconel
- Manganese Bronze
- Silicon Bronze
- Copper-Nickel Alloys
- Silver Solder
- Passive Nickel
- Passive Inconel
- Passive Stainless Steel (302, 310, 316, 410, 430)
How Can I Stop Galvanic Corrosion?
There are a few steps you can take if you MUST use these materials together.
- Add an insulator between the two materials so they no longer connect. Without that connection, the transfer of electrons cannot occur. Well Nuts are a commonly used fastener to help separate materials that can suffer from galvanic corrosion.
- Use materials with the same potential. Metals with the same corrosion resistance are typically ok to use together.
- If you are in a situation where only one of the materials will come into contact with an electrolyte then transfer of electrons will not occur.
- If there is a coating on the cathode it can prevent the transfer through increased resistance.
- Consider your environment before installing. Choose materials that will work for your environment.
- Coat or paint your assembly (completely) so that the electrolyte cannot make contact with the materials
- Use neoprene EPDM or bonding washers as a barrier in between the metals.
If you’re curious about the types of materials we offer and more about them, check out our Materials Reference Guide.