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I need a ladder in my underground water tank at home, to enable easy access for cleaning and maintenance operations. The water from the underground tank is used for drinking, apart from other household chores.

What would be the most suitable metal for this submerged ladder in the underground water tank? I don't want any water contamination due to sustained contact between ladder metal and tank water, as the water is used for drinking. TDS of water supply is ~125 mg/liter.

EDIT - I would also want to know any metal treatment (paint, varnish, etc) to prevent water contamination due to sustained contact between ladder metal and tank water.

underground tank ladder

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    Is it possible to instead have the ladder lowered into the cistern, and stored outside the tank when not in use?
    – ratchet freak
    May 14, 2019 at 12:35
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    Leaving the ladder outside the tank is a potential safety issue. If someone falls in while the tank is only half-full it could be deep enough to drown in, while still not having a good escape route.
    – user3757614
    May 15, 2019 at 2:27
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    What is the tank itself made of?
    – DJohnM
    May 15, 2019 at 5:23
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    Not remotely an expert in this sort of thing but--is there any particular reason not to use aluminum?
    – Hearth
    May 15, 2019 at 14:28

2 Answers 2

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Stainless steel is the obvious choice. Which exact grade is probably more dependent on what you can easily find than what the "most perfect for the job" grade might be, but 316 or 316L would be high on the list and are quite commonly available.

Be very careful about ventilating the tank if entering it "for maintenance" - tanks can be deadly. If the water is clean going in and there's no route for contaminants to enter, the need for internal maintenance should be minimal.

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    I’ve never heard of 316L, it must be a local thing. I use a lot of 304, also very common. Stay away from the 400, 500, and 600 Series. Here’s a link that explains: continentalsteel.com/stainless-steel/grades
    – Lee Sam
    May 14, 2019 at 16:24
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    Not remotely "a local thing" - it's an ASME grade - the low-carbon version: sandmeyersteel.com/images/316-316l-317l-spec-sheet.pdf
    – Ecnerwal
    May 14, 2019 at 16:36
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    I live in a high corrosion area and never heard of that. Learned something already today.
    – Lee Sam
    May 14, 2019 at 17:15
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    What hazards are possible when cleaning the tank?
    – akostadinov
    May 15, 2019 at 13:49
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    Asphyxiation is the usual / major problem. Gases can collect in underground tanks, which are neither breathable nor noticeably objectionable. If you enter a tank without thoroughly ventilating it, you may pass out and die, as may anyone who enters the tank to (attempt to) rescue you.
    – Ecnerwal
    May 15, 2019 at 13:59
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To add a bit to the question from Pavel in the comments of Ecnerwals answer:

Stainless steel structures in swimming pools are known to be prone to SCC. The use of standard austenitic stainless steels like 304 and 316 is forbidden in this application. The best steels to use for this purpose are the high nickel austenitic steels such as the 6% Mo grades. However, in some cases, duplex steels such as 2205 (1.4462) and the superduplex grades can be considered.

Stress Corrosion Cracking (SCC) is a form of corrosion which occurs with a particular combination of factors:
Tensile stress
Corrosive environment
Sufficiently high temperature
Normally 50 deg C but can occur at lower temperatures around 25 deg C in specific environments, notably swimming pools.
Unfortunately, the standard austenitic steels like 304 (1.4301) and 316 (1.4401) are the most susceptible to SCC

source

Sure, swimming pools contain chlorine and usually have a higher temperature and stuff, but better safe than sorry.

Since OP asked for the most suitable steel, I would like to suggest 1.4462 Duplex stainless steel. It is non-corrosive steel with excellent properties.

Properties:

  • particularly high corrosion resistance,
  • very good mechanical properties,
  • suitable for use at temperatures from -100 °C to 250 °C,
  • medium good forgeable, good weldable,
  • polishable, magnetizable

Application:

  • construction industry
  • chemical and petrochemical industry
  • food industry
  • offshore
  • shipbuilding
  • machine engineering
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