Galvanic corrosion between stainless steel 304 and aluminum alloy
When installing solar panels, you want to use the strongest fasteners possible to ensure that your project holds together in high winds and winter weather. Despite the dissimilar metals and risk of corrosion, stainless steel fasteners are the recommended fastener with aluminum parts in solar mounting industry.
Because stainless steel is stronger than aluminum, it is safe to use stainless steel screws as fasteners.
Stainless steel is an alloy of carbon steel that is, itself, resistant to corrosion. However, stainless steel is reactive with aluminum, and when a stainless steel screw is in contact with an aluminum base metal, the aluminum is likely to corrode.
Aluminum and stainless steel together also appears to be a bi-metallic corrosion risk, from the ‘nobility’ table.
With this combination the affect of relative surface area on corrosion is important.
A large area of ‘cathode’ relative to ‘anode’ will accelerate the anodic corrosion. Although aluminum is anodic to stainless steel, large relative surface areas of aluminum to stainless steel can be acceptable, dependent on local conditions.
Stainless steel fasteners in aluminum plates or sheets are normally considered safe, whereas aluminum rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion.
When the stainless steel will generate a large corrosion current which will be concentrated on a small area of sacrificial metal. The aluminum will corrode quickly, and so aluminum fasteners in stainless steel are not acceptable. However, a stainless screw in aluminum is frequently used although corrosion of the aluminum immediately around the stainless is quite possible.
An example of the safe use of stainless steel and aluminum together is where stainless steel fasteners and hold down bolts are used to secure aluminum roadway or bridge parapet guards.
Even with no insulation between the metals, there should be little risk of corrosion.
Below is a list of best practices for corrosion prevention:
- Use one material to fabricate electrically isolated systems or components where practical.
- Avoid the unfavorable area effect of a small anode and large cathode. Small parts or critical components such as fasteners should be the more noble metal.
- Insulate dissimilar metals wherever practical [for example, by using a gasket]. It is important to insulate completely if possible.
- Apply coatings with caution. Keep the coatings in good repair, particularly the one on the anodic member.
- Add inhibitors, if possible, to decrease the aggressiveness of the environment.
- Design for the use of readily replaceable anodic parts or make them thicker for longer life.
- Install a third metal that is anodic to both metals in the galvanic contact, sacrificial anode.