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?
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
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.
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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
I guess my explanation of pitting corrosion was too brief. You might
look at the Key to Metals article on corrosion of aluminum at:
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
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.
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
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.