Plastic vs. Aluminium vs. Stainless vs. Steel

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Originally Posted by Blknight.aus

Depends on the grade and how you weld it, most stainless hardens with age and tends to crack around the welds(usually from flexing for whatever reason, brittle might not have been the right word to use.

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brittle is the correct word.....as it work hardens itself it becomes brittle.......

but how much flex can you have in a properly made tank.....?
if it has reinforcing internally by means of baffle plates.....there arent any forces acting on it.....
its either sitting in the vehicle or slung under it........if there is flex from the mounts.....then use some rubber bushes......


or...am i missing something here.....? :eek:

heat expansion for one.
flexing on the mounts especially if your hammering on corrigations
weight of the fuel.

might take 5-6 years but it does happen.

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Originally Posted by Brian Hjelm

Stainless steel is tough, not brittle. Grey iron is brittle. Ask any machinist.

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Sorry Brian, maleable steel is just that, soft and bendy. Stainless, mit nickel, is tough, hard, and will crack in bending long before steel. Soft grey cast, yes, that is brittle, can be broken just by dropping on the deck.

Agree with Dave on food grade poly for water, also a new fuel grade poly in grey, that farmers fit to the back of their 4x4's for fuel transfer, these are even set up to take pumps and what have you, come complete with mounts and all, or you could look into the expanding poly bladders used on trawlers, as longer range fuel tanks. These expand on filling and collapse as they are emptied, look like a big red pillow.

regards Shorty.

I will take issue with you on the application of the term hard to stainless steels. For a start the term "stainless steel" is a trade name, patented 1916 for cutlery, which has come into common use to describe corrosion resistant alloy steels containing high percentages of chromium. These steels that also contain high percentages of nickel are not hardenable by heat treatment, but can attain high tensile strength by cold working. These types are called chromium-nickel austenitic steels. Other types that contain little or no nickel are hardenable by heat treatment and are called martensitic chromium steels. There are also ferritic chromium steels with no nickel content that are not hardenable by heat treatment. Sae nos. (five digit) of hardenable (martensitic) grades commence with 5 as do the ferritic alloys without nickel. The non-hardenable grades commence with 3. The stuff can be the very devil to machine, often requiring a bit of experimentation with speeds and feeds and tool grades to achieve the required finish and rate of production. It is usually a fair bastard to tap and you can get a lot of tap wear and breakage. Cutting tool manufacturers usually describe these steels as "difficult" materials with "stringy" or "long chipping" characteristics. Grades used for machining components in automatic machines usually contain a fair bit of sulphur and/or selenium to make them free-machining. The SAE no. of free-machining grades has an F suffix or an Se suffix

Okay Brian, I stand corrected. Miss use of a trade name that has become generic. Also various other types, Staybrite, Rostfri, all trade name recipes. you can't disagree however over the tendency for these materials to crystalise along side the welded seams, which does tend to make that area brittle, as a lay term. Hell, all metals are succeptible to this due to the magnetic influences of electrical welding, to a greater or lesser degree.

Shorty.

Much of the problems you mention at welded joints in these steels are from incorrect welding procedures particularly pre and post, and/or choosing unsuitable grades of steel or filler rod. There are grades that are recommended for welding, 30304, 30305, 30321, 30347. The last two are recommended where heat treatment after welding is not feasible. These are all chromium/nickel alloys. Bloody hell, Shorty, you have made me drag out dusty text books to check my memory of this stuff. I have not been on the tools except for hobbies for years.

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Originally Posted by Vida

Still unsure what I want to use for my additional water and diesel tanks.

Standard diesel tanks are nowadays either made out of steel or plastic - so why would you use the more expensive stainless - is rust the only reason? Aluminium has about the same weight as plastic tanks - is aluminium an good alternative for diesel tanks or does that material not go well with diesel?
I understand that aluminum and plastic would need an additional protection plate to prevent damage.

The only reason you would use stainless is to prevent rust from the outside. See also comments on stainless below. Mild steel is easy to fabricate and cheap.


Water tanks are normally made out of plastic. Is Aluminium an alternative to store drinking water or does aluminium contaminate the water on the long run? What about if you put silver based water purifier in the water - does this affect the aluminium?

Likely to cause corrosion. The same problem can be caused by putting bore or other highly mineralised water in aluminium There are no other reasons not to use aluminium for water. Mild steel will rust but galvanised steel is suitable if the joints are done properly and will be cheaper than anything else.

My water tank will be mounted inside the vehicle, protected from stones/rocks.

Any views and comments are welcomed.

Cheers,
Daniel

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In general, mild steel is cheapest and easiest to fabricate, but unsuitable for drinking water as the interior will rust, and for fuel the exterior will rust unless protected by painting and the prevention of water and mud traps.

Plastic is suitable for both water and fuel, but is difficult to fabricate other than mass production, and is easily cut, worn or punctured - it needs protection if under the vehicle, and care with the mounts.

Stainless steel is suitable for both water and fuel, but most grades work harden with vibration, requiring great care in design and construction and often making them the heaviest option. It is also usually the most expensive.

Aluminium is suitable for both water and fuel (most heavy tucks have aluminium fuel tanks). Use as a water tank it needs care in the use of chemicals in the tank. relatively expensive.

Galvanised steel is suitable for water (traditional rainwater tanks use it) and can be made from galvanised sheet or from mild steel and galvanised. Do not use for diesel, as any water contamination will result in oxidation of the zinc and the oxidation products will damage your fuel system. If made from galvanised sheet it will probably be the cheapest option for water tanks.

If properly designed, all the tanks should be around the same weight although the steel tank may need to have added weight to allow for corrosion and the stainless tank to allow prevent work hardening and resulting cracking.

John

What about making the tank from mild steel (cheap and easy to work) and then sealing it with special POR15 tank sealant. It is impervious to diesel and even high octane fuels, so it should stand up to water.

http://www.por15.com/s.nl/it.A/id.1467/.f

I think you tip it in the tank (it is like very thick paint) and swirl it around then tip out the excess.

why not buy 20 litre bladders for water instead of installing a tank

you then dont have permanent space taken up in vehicle

you can easily empty them out after a trip and dont have problem of having to drain or constantly treat tank with water purification tablets

you split your water over several bladders to avoid problem of damage to tank causing water loss in trip

water bladders can be placed virtually anywhere in vehicle and that means different configurations of useable space for different types of trips being undertaken

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Originally Posted by Brian Hjelm

Much of the problems you mention at welded joints in these steels are from incorrect welding procedures particularly pre and post, and/or choosing unsuitable grades of steel or filler rod. There are grades that are recommended for welding, 30304, 30305, 30321, 30347. The last two are recommended where heat treatment after welding is not feasible. These are all chromium/nickel alloys. Bloody hell, Shorty, you have made me drag out dusty text books to check my memory of this stuff. I have not been on the tools except for hobbies for years.

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it can be "brittle" (as in prone to fracture) esp in sheet form in high vibration or high temperature variation applications espicially beside welds but also at any point of vibration, the more chrome, the worse it is.

if there is vibration or high temperature variations it will work harden and fatigue, some grades simply take longer to do it. there is no grade of stainless that is immune, normallised or not.

with stainless a lot of times thinner is better than thicker, but you need to get the harmonics right by placing supports and baffles in exactly the right places.

lamella separators and or spray dryers (eg soap powder plants) are a prime examples.

even stainless adgitators in syrup tanks will have blades crack at the hubs due to the low level harmonics, whether cast or welded.