# Why do metals have to be separated to prevent corrosion? **URL:** \ **Category:** Factual Questions\ **Created:** [January 10, 2009, 4:35pm UTC](https://boards.straightdope.com/t/why-do-metals-have-to-be-separated-to-prevent-corrosion/480678 "2009-01-10T16:35:20Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [January 10, 2009, 4:35pm UTC](https://boards.straightdope.com/t/why-do-metals-have-to-be-separated-to-prevent-corrosion/480678/1 "2009-01-10T16:35:20Z")
My chemistry is shamefully shallow here and I need some brushing up. We’re having some roofing done and there are some places that could have aluminum and copper contacting each other. Roofing contractors are telling us that they will need to be separated by some type of membrane to reduce corrosion. Is this what they’re talking about - the two metals, with water in between, form a weak wet cell, and that can speed up decay of the metals, i.e. corrosion, as they interact? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [January 10, 2009, 4:52pm UTC](https://boards.straightdope.com/t/why-do-metals-have-to-be-separated-to-prevent-corrosion/480678/2 "2009-01-10T16:52:23Z")
[Galvanic Corrosion](http://en.wikipedia.org/wiki/Galvanic_corrosion) ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [January 10, 2009, 5:11pm UTC](https://boards.straightdope.com/t/why-do-metals-have-to-be-separated-to-prevent-corrosion/480678/3 "2009-01-10T17:11:58Z")
> [@CC](#): > > My chemistry is shamefully shallow here and I need some brushing up. We’re having some roofing done and there are some places that could have aluminum and copper contacting each other. Roofing contractors are telling us that they will need to be separated by some type of membrane to reduce corrosion. Is this what they’re talking about - the two metals, with water in between, form a weak wet cell, and that can speed up decay of the metals, i.e. corrosion, as they interact? Yes. That’s what they’re talking about. The aluminium will corrode faster, and the copper will corrode slower than either would alone, if they’re in electric contact. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [January 10, 2009, 5:47pm UTC](https://boards.straightdope.com/t/why-do-metals-have-to-be-separated-to-prevent-corrosion/480678/4 "2009-01-10T17:47:47Z")
> [@CC](#): > > My chemistry is shamefully shallow here and I need some brushing up. We’re having some roofing done and there are some places that could have aluminum and copper contacting each other. Roofing contractors are telling us that they will need to be separated by some type of membrane to reduce corrosion. Is this what they’re talking about - the two metals, with water in between, form a weak wet cell, and that can speed up decay of the metals, i.e. corrosion, as they interact? With metals they will corrode when exposed to oxgyen (air) reducing the metal to an oxide (rust). When two different metals are in contact a electrilictic reaction can occure at a rapid rate. Most common is connecting iron pipe to copper pipe. In this case an dielectric union must be used. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [January 10, 2009, 5:53pm UTC](https://boards.straightdope.com/t/why-do-metals-have-to-be-separated-to-prevent-corrosion/480678/5 "2009-01-10T17:53:16Z")
> [@Snnipe\_70E](#): > > …electrilictic… I had to squint real hard at that for a minute before I realized that you meant “electrolytic.”