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Old 05-19-2015, 09:43 AM   #1
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Screws and Aluminum and Rust

If one is forced to use a screw that will come in contract with the aluminum what is the best metal make up of the screw? Should the screw be coated? Thanks in advance.
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Old 05-19-2015, 10:13 AM   #2
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Always use stainless, especially if it is exposed to weather.
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Old 05-19-2015, 10:21 AM   #3
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I second that recommendation.

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Old 05-19-2015, 10:21 AM   #4
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Cadmium plated aircraft screws work well, should you choose to go that route.
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Old 05-19-2015, 10:26 AM   #5
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I replace with stainless on each project and where corrosion becomes an issue.
Depending upon application I sometimes use a nylon washer/insulator and/or corrosion-Ex to retard the galvanic reaction.
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Old 05-19-2015, 10:27 AM   #6
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The 18-8 stainless screw grade should be type 304 or 305 as they have a higher corrosion resistance due to a lower carbon content (.08).
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Old 05-19-2015, 11:31 AM   #7
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Galvanic Corrosion

Stainless steel fasteners have been covered and are the best option but one more is Loctite 222 for small fasteners. It prevents galvanic corrosion between the threads of dissimilar metals and prevents them from vibrating loose. Because it is low strength you will be able to back the screws out using a screwdriver. Do not use Loctite 262 unless you never want to remove the screw. 222 is pink in color and 262 is red. Hope you find something that works for you.
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Old 05-19-2015, 11:35 AM   #8
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Stainless and here is what you coat the threads with prior to use.
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Old 05-19-2015, 12:47 PM   #9
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As an old retired plant engineer, I agree with most of the comments esp using Loctite to prevent galvanic corrosion. There are other products available for that but my experience with Loctite in general is they know their business. They also have a very good customer service department who will talk with you on the phone.

Don't know about the Permatex with aluminum and suggest you read the label and/or go to their web site for more info before using. I would also suggest using aluminum screws where the application will allow the lower tensile strength. That will generally eliminate galvanic corrosion or at least minimize it depending on the alloy.
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Old 05-19-2015, 12:56 PM   #10
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I noted in a previous thread on this topic that I used SS on the stainless fittings on my sailboat Aluminum mast. Never had a problem. Some one noted that they were an Aerospace engineer and they use SS on airplane aluminum all the time.

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Old 05-19-2015, 01:12 PM   #11
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Screws and Aluminum and Rust

Ditto on the stainless steel recommendations. Galvanic corrosion is the big concern joining dissimilar metals. Rust is a by-product of ferrous metal corrosion, but using stainless will ease your concerns over discoloration.
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Old 05-19-2015, 02:10 PM   #12
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Aluminum and Stainless steel are pretty good at self insulating each other and should not have a huge problem with galvanic corrosion with each other. A hot dip galvanized washer as an anode is not a bad idea. Thin electroplated washers won't last long because the coating is not thick enough to last long. Belly skins have the most corrosion issues because of road salt and water leaks from above and soaked insulation.

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Old 05-19-2015, 09:52 PM   #13
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Good info. Thanks everyone.
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Old 05-20-2015, 10:15 AM   #14
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Quote:
Originally Posted by Jacob D View Post
As an old retired plant engineer, I agree with most of the comments esp using Loctite to prevent galvanic corrosion. There are other products available for that but my experience with Loctite in general is they know their business. They also have a very good customer service department who will talk with you on the phone.

Don't know about the Permatex with aluminum and suggest you read the label and/or go to their web site for more info before using. I would also suggest using aluminum screws where the application will allow the lower tensile strength. That will generally eliminate galvanic corrosion or at least minimize it depending on the alloy.
A follow up on Jacob's note of caution: I've used Permatex for years in the salt water marine environment of stainless screws/bolts and aluminum. You have to try real hard to get much worse than that.

Product Description
Size: 8 Ounce
Permatex Anti-Seize Lubricant is a highly refined blend of aluminum, copper and graphite lubricants, for use during assembly to prevent galling, corrosion and seizing, and to assure easier disassembly. Featuring salt, corrosion and moisture resistance, this lubricant is ideal for marine use. With a temperature range from -60 degrees Fahrenheit to 1600 degrees Fahrenheit, it will not evaporate or harden in extreme temperatures.
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Old 05-20-2015, 02:29 PM   #15
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Anti Seize is never a bad idea with heat and corrosion environments.

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Old 05-21-2015, 10:40 AM   #16
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If anti-seize is used it should be zinc based anti-seize with no copper content according to the builder of my aluminium jet boat. Loctite product number 39901 is zinc based and will minimize galvanic corrosion.

Also, I would not use zinc plated washers or any zinc plated fasteners because the zinc acts as a sacrificial anode and after a few years the underlying carbon steel will come into contact with the aluminium resulting in corrosion.
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Old 05-21-2015, 01:08 PM   #17
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The double dip galvanized steel washers are made for corrosive environments and that is why the coating is so thick. Stainless steel is fine as well but it might tend to corrode the aluminum since it is cathodic to aluminum.

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Old 05-23-2015, 08:21 PM   #18
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Quote:
Originally Posted by NWRVR View Post
If anti-seize is used it should be zinc based anti-seize with no copper content according to the builder of my aluminium jet boat. Loctite product number 39901 is zinc based and will minimize galvanic corrosion.

Also, I would not use zinc plated washers or any zinc plated fasteners because the zinc acts as a sacrificial anode and after a few years the underlying carbon steel will come into contact with the aluminium resulting in corrosion.

All these products minimize galvanic corrosion--that's what we're talking about here. I personally don't buy your jet boat builder's no copper story since original Permatex has been used on aluminum in the saltwater/aluminum boating industry, with no problems, long before Locktite ever produced an anti seize product. That said, if one is afraid of copper, use a copper free product. Heck, use the food safe stuff if you want, just use Stainless and some anti seize--done deal.
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Old 05-24-2015, 01:20 AM   #19
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Quote:
Originally Posted by Howard L. View Post
All these products minimize galvanic corrosion--that's what we're talking about here. I personally don't buy your jet boat builder's no copper story since original Permatex has been used on aluminum in the saltwater/aluminum boating industry, with no problems, long before Locktite ever produced an anti seize product. That said, if one is afraid of copper, use a copper free product. Heck, use the food safe stuff if you want, just use Stainless and some anti seize--done deal.
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My North River Commander 21 is nine years old without any signs of corrosion so I'll go with what the builder recommends. In the late 80s I owned a marine repair business and we drilled out many corroded bolts from out drives and out boards. Preventative maintenance has come a long ways since then so I'll go with what the guys recommend that build the high dollar aluminium toys today. All we can do is post recommendations for the original poster and from that, he/she can make a decision or seek more competent advice.
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Old 05-25-2015, 08:38 PM   #20
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Absolutely, to each his own. Just use something


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