The doors, hatches, and manholes we sell are made out of three materials: carbon steel, aluminum, and stainless steel. Anyone who has spent any time around ships knows that rust is one of the biggest maintenance headaches. Hours are spent chipping paint, chipping off rust, patching areas, and repainting every exposed metal surface.
The problem only gets bigger if you start mixing metals. Galvanic corrosion is what happens when you put two different metals together in an electrolyte solution, and seawater is a perfect electrolyte solution. The salt dissolved in water forms an electrical circuit between the two metals, with the current flowing from the anode (the more electronegative metal) to the cathode (the more electropositive), which will cause the anode to corrode. The metals that are in common use have been ranked based on how electronegative (anodic) or electropositive (cathodic) they are. The further apart the metals are in this ranking, the more likely that the anode will corrode.
So, do you need to have an all steel or an all aluminum boat, or can you use aluminum and steel in the same boat? Yes, you can use them together if you take the proper precautions. Steel and aluminum are right next to each other on the rankings of electronegativity, so they don't have as large of a voltage difference between them when immersed in seawater. A good paint system will prevent this galvanic corrosion from occurring by isolating the two metals. Better yet, if you invest in a primer that has zinc incorporated into the primer, the zinc will act as a sacrificial metal (the zinc in the primer will oxidize before the item covered by the primer will oxidize), and provide increased protection from galvanic corrosion.
Our customers have successfully used doors and hatches with aluminum covers or door panels, and steel coamings. The aluminum door panels and covers are much easier to lift and operate, yet the steel coaming or frame allows you to weld it into the steel structures of the ship. As long as the metal is painted, you don't have to worry about accelerated corrosion.
If you are interested in reading more about galvanic corrosion, or would like to check out the research done for this post, check out the link to this article: http://www.npl.co.uk/upload/pdf/bimetallic_20071105114556.pdf
Also, here is a fun article about instances in history where we learned about galvanic corrosion the hard way: http://corrosion-doctors.org/Corrosion-History/Lessons.htm