aluminium ladder inside water tank - please help

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aluminium ladder inside water tank - please help
in a under ground water tank, to go inside the tank generally we provide cast iron rungs. instead of c.i.rungs, can we use aluminium ladder??? what are the advntages & disadvantage of aluminium ladder inside the water tank. please help..

i have never spec'd an aluminum ladder for use under water.  for potable water tanks, i have always seen like materials for the ladder and the tank (ie, steel and steel), both of which are coated with epoxy based paint. i have also seen aluminum ladders for sewer pump stations...all of which had to be removed due to significant corrosion (this may be the result of chemical corrosion as oppossed to electro-chemical).
is this tank steel, and if so is it coated?  i would consult with the ladder manufacturer on this one.  given that aluminum is more anodic than steel, the aluminum runs a good risk of corroding.  there are several other factors to consider as well, like ph of the water, what other materials are present in the water (maybe a sacrificial anode?).  also don't forget the little things such as the bolts...steel in contact with aluminum under water.
given the different variables that could be encountered, it would be well worth it to play it safe and use like materials.  otherwise, i would consult a qualified corrosion or metallurgical engineer.
years ago when i worked for the navy i was sort of a medium strength expert on aluminum but it has been awhile.  i do re  
have you considered using stainless steel? if the water tank is concrete, then you may have problems with the aluminum in contact with it. you can use the individual rung type ladder with bent stainless bar.
i assume your underground tank is concrete.  generally concrete and aluminum will react when in contact with each other. iron or steel is preferred.
have you considered a frp (fiberglass) ladder?  fibergrate.com is a good place to start.  they offer different types of resins depending on the corrosive environment (it doesn't need to be corrosive to use frp).
hope this helps,
mike
high-purity aluminum piping can be used for di water.
however, all commercial aluminum alloys will corrode in water, especially if it contains chlorine or chloride, and corrosion in contact with steel or concrete has already been mentioned.
as a metallurgist in the aluminum finishing business, i will say that a powder coat of pe or similar over properly sealed hard anodized aluminum should last for many years. however, i would agree with the mjflorio that a fiberglass ladder is probably the most cost-effective, corrosion-resistant solution.