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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

Bharat Khatri
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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 '19 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 '19 at 02:27
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    What is the tank itself made of? – DJohnM May 15 '19 at 05:23
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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 '19 at 14:28

2 Answers2

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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.

manassehkatz-Moving 2 Codidact
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Ecnerwal
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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: https://continentalsteel.com/stainless-steel/grades/ – Lee Sam May 14 '19 at 16:24
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    Not remotely "a local thing" - it's an ASME grade - the low-carbon version: https://www.sandmeyersteel.com/images/316-316l-317l-spec-sheet.pdf – Ecnerwal May 14 '19 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 '19 at 17:15
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    Most "316" or "304" sold today is in fact the "L" - low carbon version. It is just a factor of how much the corrosion resistance is lost due to welding. When stainless is heated, any carbon in it will want to join with the chromium to form carbides, which reduces the amount of chromium left for corrosion protection. "L" versions restrict that, but most companies just produce the "L" version alone because almost no one wants the non-L versions. – IronEagle May 14 '19 at 18:23
  • @LeeSam As a layman, I can't see how the link explains why the OP should stay away from the 400, 500 and 600 series, apart from the excellent polishability of cutlery grade steel. – Pavel May 15 '19 at 06:37
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    @Pavel [Here:](https://www.metalsupermarkets.com/7-things-consider-choosing-stainless-steel-grade/) "**Austenitic** stainless steels generally provide the most corrosion resistance because of their high amounts of chromium. This makes grade 304 an excellent choice when corrosion resistance is important. Grade 316 is similar to grade 304, but it has molybdenum as part of its chemical makeup, further increasing its corrosion resistance. [Ferritic and martensitic stainless steels ...] have less nickel and sometimes less chromium [...] which can result in a loss of corrosion resistance." – Peter - Reinstate Monica May 15 '19 at 08:10
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    @Pavel Low corrosion resistance or high cost. Series 300 seems to be common construction grade. – Lee Sam May 15 '19 at 09:20
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    What hazards are possible when cleaning the tank? – akostadinov May 15 '19 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 '19 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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