[Anastasia]

Hello

I am interested in which material is the best for making soap boiler (cauldron)? I need it for cooking traditional alkaline soap.

Thanks in advance

[Enthalpy]

Welcome, Anastasia!

If you only want the cauldron to resist corrosion, what you need is a "compatibility list" that mentions the considered materials and the corrosive compound, at the desired temperature. Your search engine can find many such lists.

If the corrosive compound is potassium hydroxide and the mixture is hot, maybe metals as simple as pure copper might suffice? Since soap existed before stainless steel and more exotic materials, this wouldn't be surprising. Some alloys may be cheaper but copper cauldrons exist ready to buy.

In case the boiler has more functions than a passive container, I can't tell.

[Anastasia]

Thank you very much for your replay

The concentrated solution of potassium hydroxide (KOH) was used for soft and liquid soaps.

I am talking only about traditional hard/bar soap that was made using sodom hydroxide (NaOH) and sea salt concentrated solution. Temperature maximum of 120°. Copper cauldron can be used in this case too?

Traditionally, stone or copper was used for the making of relevant cauldrons, later - iron or cast iron, today - stainless steel, best considered AISI 430.

I am interested in if there is a better modern material.

As for the copper cauldron, kindly tell me which company is making it?
I did not find such a company.

[Enthalpy]

If copper, iron and stainless steel can be used, it suggests that the cauldron has no other function than holding the liquid. Easier choice.
KOH and NaOH have very similar corrosive properties.

Iron and cast iron will corrode slowly under these conditions and pollute your soap. I see no advantage to them.

AISI 430 is just the cheapest among stainless steel, but AISI 304L resists corrosion better, is more common and therefore can be as cheap. I expect stainless steel to last longer than copper.
AISI 304L is one US standard name, widely understood. Other countries use different names for nearly equivalent alloys.

What is the size of the cauldron, and how many will you let manufacture or buy? In small amounts you'd probably buy from a catalogue, then it would be AISI 304L or maybe copper.

Have you already found a "materials compatibility list" that mentions hot NaOH?

[Anastasia]

Thanks

There are only two variants from metals: pure copper or stainless steel with high content of nickel.

[Enthalpy]

"There are only two variants": you mean, available in the desired shape and size?

AISI 430 is a ferritic stainless steel, containing no intentional nickel. The more common 304L contains nickel to make it austenitic. It resists corrosion better, but depending on the country, up to 1/3 of all women may suffer contact allergy with nickel and its alloys. In metallurgy, its 8% Ni wouldn't be considered much. Other austenitic alloys can contain much more, especially for the chemical industry.

Have you already found a compatibility list?

[Anastasia]

I know the compatibility list.

I am allergic to nickel and that's why I said AISI 430 in the beginning.

I probably will choose high-purity copper.

[Enthalpy]

Copper will tarnish a bit at heat. Not enough to punch through, but expect traces of copper compounds in the soap. As well, pure copper is extremely soft.

Allergy to nickel: the cheap stainless knives and forks that are ferromagnetic are made of AISI 430. If you can use these, you can have a cauldron of the same material.

[Anastasia]

Thank you very much for your reply

In my case pure copper does not create a problem, but it is not modern material and it is hard to care as well.

Stainless steel is modern.

I just mentioned AISI 430 from the very beginning because it does not contain nickel and is quite resistant to aggressive environment.

But the Italian firms interesting for me offer me the AISI 304.

However, I am more interested in AISI 304L proposed by you.

Let's see where to will develop processes