When assembling aluminum 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 screws are the recommended fastener for aluminum panels.
Galvanic corrosion occurs when an electrically conductive liquid, such as water, connects the anode -- aluminum -- with the cathode -- stainless steel. Because the anode, or aluminum, corrodes quickly in comparison to the stainless steel, using aluminum screws in stainless steel would quickly weaken the structure as the aluminum degrades. Exposure to saltwater increases the rate of corrosion, as salt is a better electrical conductor than plain water.
To prevent a corrosive electrochemical reaction between the stainless steel screws and aluminum, you can protect the connecting points from exposure to water with tape or paint, seal the metals at the points of contact or use rubber or plastic washers. Once you've removed the electrical conductor by preventing contact with water, the stainless steel and aluminum are relatively inert, even when in direct contact. Because stainless steel is stronger than aluminum, it is safe to use stainless steel screws as fasteners.