aluminum

I need to put a set of aluminum spiral stairs outide on a deck which is over fresh water. The manufacturer is telling me that I need to
paint it or it will "pit". However, I see many aluminum products including boat lifts and such which are unpainted and do not show any "pitting". I know there are different grades of aluminum, but will this be a problem to put these stairs outside without painting them or do I really need to paint them. Can someone help me to understand this a little better?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Scott wrote:

What grade of aluminium?
Michael Dahms
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The specs dont say the grade of the aluminum, it olny states it is a high strenth aluminum alloy. It is a very strong aluminum, but I dont know what the different grades are.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Scott wrote:

Look to the original specs! If the alloy contains copper, it will corrode easily in an unprotected state.
Michael Dahms
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@yahoo.com (Scott) wrote in message

Scott:
Normally aluminum is protected from corrosion by formation of a thin oxide film which is stable and prevents further corrosion. If the oxide film breaks down locally, then corrosion occurs via pitting. Contrast this with steel where corrosion more commonly occurs relatively uniformly by weight loss. Chloride ions (as found in sweat for example) are a common source for pitting.
Another potential problem with aluminum is that it is more active than other metals such as iron and copper. If you put your ladder in a moist outside environment and connect it to steel or copper, then the ladder will corrode and protect the adjacent steel or copper from corroding.
The worst situation for galvanic corrosion is having a small area of aluminum connected to a large area of copper. Normally this should not be a problem, since your ladder is a large object. However, you need to beware of a pathological form of attack called deposition corrosion. If you have other metals like copper, lead, nickel, and tin located "upstream" of the ladder, then it is possible for water passing over them to dissolve those metals. Then those ions can plate out on the surface of your aluminum and set up an galvanic cell with a large surface area of the other metal. The typical "gotcha" is a comination of a copper roof with runoff into aluminum gutters. Less than 0.1 ppm of dissolved copper can start pitting.
If you have other metals "upstream", then you better think about painting your ladder.
Pittsburgh Pete ------------------------------------------------------------------------------- DISCLAIMER
We don't believe what we write, and neither should you. Information furnished to you is for topical (external) use only. This information may not be worth any more than either a groundhog turd, or what you paid for it (nothing). The author may not even have been either sane or sober when he wrote it down. Don't worry, be happy.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@hotmail.com (Pittsburgh Pete) wrote in message

Pete, the aluminum staris will not be attached to any other metal, it is attached at the top and bottom to a wood surface. I think the problem is I dont know the alloy content of the alluminum. I didnt quite understand how you stated that chloride ions can cause some corrosion.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@yahoo.com (Scott) wrote in message

Scott:
I guess my explanation of pitting corrosion was too brief. You might look at the Key to Metals article on corrosion of aluminum at: http://www.key-to-metals.com/Article14.htm It talks about how the corrosion resistance varies depending on what alloy you have (as Michael Dahms asked). There is also a discussion of pitting corrosion in the Scandiavian online aluminum handbook "Alubook", starting at http://www.alu-info.dk/Html/alulib/modul/A00101.htm To navigate from page to page just click on the double arrow buttons << or >> or click on an underlined topic. You'll find lacquering (painting) discussed.
Pittsburgh Pete
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Caution!
Just because the material you are mounting against is not metal does not mean that you won't have problems. The fasteners are likely metal (probably steel). Since you do not know which aluminum alloy you are using the effects could be unpredictable.

The chloride ions will act to break down the passive surface layer on the metal. Once the layer is broken, the corrodent has access to the base metal and corrosion will occur.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@aol.com (SDevlin989) wrote in message

If steel fastners are used, then some galvanic corrosion is likely adjacent to them. This is also discussed in the Alubook site discussed in my previous post. See http://www.alu-info.dk/Html/alulib/modul/A00104.htm
Pittsburgh Pete
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.