When stainless steel and aluminum come into contact, galvanic corrosion can occur under specific environmental conditions. This reaction leads to the deterioration of one of the metals. While not an immediate process, it can compromise the integrity of structures over time if not properly addressed.
How Stainless Steel and Aluminum Interact
The interaction between stainless steel and aluminum primarily involves galvanic corrosion, an electrochemical process. This process requires three components: two dissimilar metals, electrical contact between them, and an electrolyte, a conductive liquid like moisture or saltwater. The metal with a more negative electrical potential becomes the anode and corrodes, while the other acts as the cathode. Aluminum typically serves as the anode, corroding while stainless steel functions as the cathode.
The rate of galvanic corrosion is influenced by several factors, including the presence and type of electrolyte. Saltwater, for instance, significantly accelerates the corrosion process due to its high conductivity. The ratio of the surface areas of the two metals also plays a significant role; a large cathode (stainless steel) relative to a small anode (aluminum) can lead to rapid corrosion of the aluminum. For example, small stainless steel fasteners with a large aluminum component increase this risk.
The specific grades of stainless steel and aluminum also affect their electrochemical potential difference. While stainless steel’s chromium content forms a protective oxide layer that enhances its corrosion resistance, this does not prevent galvanic corrosion when in contact with aluminum and an electrolyte. Aluminum, despite its own protective oxide layer, is less noble than stainless steel, making it susceptible to accelerated corrosion when coupled.
Strategies to Avoid Material Reactions
Preventing galvanic corrosion between stainless steel and aluminum involves interrupting one of the three necessary conditions. A common approach is to create a physical separation using non-conductive materials as barriers. Materials like rubber, plastic, nylon, or specialized tapes and gaskets can effectively prevent direct electrical contact. For instance, plastic or nylon washers and sleeves with stainless steel bolts in aluminum components can isolate the metals.
Applying protective coatings to one or both metals can mitigate the risk. Coatings such as paints, primers, powder coatings, or anodizing on aluminum act as dielectric barriers, blocking electrical current flow between the metals. It is often beneficial to coat both surfaces, and if only one can be coated, the more anodic metal (aluminum) should be prioritized. These coatings prevent the metals from contacting the electrolyte, slowing or preventing corrosion.
Proper design includes ensuring adequate drainage to prevent moisture or electrolyte buildup where the metals meet. Sloped surfaces help water run off, reducing exposure time to an electrolyte. Selecting fasteners of the same material as the primary component is ideal to avoid galvanic issues. If dissimilar fasteners are necessary, choosing a fastener material cathodic to the main component can help, or using insulating washers.
Another strategy involves cathodic protection, where a third, more reactive metal (a sacrificial anode) is intentionally introduced to corrode instead of the aluminum. Metals like zinc or magnesium are often used as sacrificial anodes because they are more anodic than both aluminum and stainless steel, preferentially corroding and protecting the other two. This method is particularly useful in harsh environments like marine settings.