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?
Reply to
Scott
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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.
Reply to
Scott
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
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Reply to
Pittsburgh Pete
Look to the original specs! If the alloy contains copper, it will corrode easily in an unprotected state.
Michael Dahms
Reply to
Michael Dahms
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.
Reply to
Scott
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:
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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
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To navigate from page to page just click on the double arrow buttons > or click on an underlined topic. You'll find lacquering (painting) discussed.
Pittsburgh Pete
Reply to
Pittsburgh Pete
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.
Reply to
SDevlin989
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
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Pittsburgh Pete
Reply to
Pittsburgh Pete

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