structural aluminum mill finish corrosion resistance

[sunstructures]

Hello, I am designing an aluminum structure using 6061-T6 standard I and C shapes which will form a roof canopy, and remain exposed for architectural reasons. The project site is in Los Angeles, and is not located near the coast. However, it will be within a mile or less from 2 heavily traveled freeways (both interstate highways), with diesel truck traffic throughout the day. The site is also in an industrial area.
The Aluminum Association states that the natural mill finish of structural aluminum will develop a 2 to 4 ten-millionths of an inch thick impermeable oxide film that will protect it from corroding atmospheres.
I would like to avoid the additional expense of anodizing or painting the aluminum. Will the natural oxide surface film be adequate for this type of installation?
Thanks.

 

[swall]

How will the aluminum be attached? Steel bolts? Joined to any steel components?

 

[sunstructures]

The aluminum structure will be attached to a supporting steel structure. The steel structure is composed of tubular sections and finished with a Carbozinc coating instead of galvanizing. The connection from the aluminum structure to the steel supports will be made with stainless steel bolts. The aluminum roof canopy itself will be bolted together with stainless steel bolts. There may be some shop welding required for fabricating aluminum brackets and angles that join the aluminum sections together. My main question is whether the aluminum needs to be anodized for corrosion protection, or is the natural oxide sufficient?

 

[swall]

You will need to use galvanized bolts instead of stainless.Otherwise, I think the bare aluminum will be OK, given the evironment you have described.A primary constituent of air in industrial evironments and near highways is SO2. According to Uhlig (Corrosion and Corrsion Control), aluminum is satisfactory in these applications.But you need to use galvanized bolts to alleviate any concerns with galvanic corrosion when acid rain impinges on the structure.

 

[sunstructures]

Thanks. We commonly use hot-dip galvanized steel bolts in all our steel structures. In this case, working with aluminum, I assumed we should use the better material in the connectors, and figured on stainless steel. I understand the better match between Zn and Al on the galvanic potential scale, but when would it be a better choice to use the SS connectors?

 

[IFRs]

We use SS bolts in industrial environments. While in theory they are a higher galvanic potential, the overwelmingly large area of aluminum compared to SS protects the AL parts.

An excellent source book is "J. Randolph Kissell,Robert L. Ferry, Aluminum Structures: A Guide to Their Specifications and Design". Read a review at

http://www.aluminum.org/Template.cf...tManagement/ContentDisplay.cfm&ContentID=3662