Copper pipe for refrigerators is 99.9% pure with a dash of Phosphorous. By specification it is de-oxidised, Nitrogen free and fully degreased (provided it's still properly packaged). You could say it's posh copper!

When supplied in a roll it's Soft, but straight pipe could be either Hard or Half-hard. The pipe is either work hardened or softened by heating to get the desired property. In use the soft form is easier to bend but weaker – about ⅓rd as strong as fully hard.

One concern may be the thickness of the pipe wall – I believe refrigerator tube is only good for about half the pressure taken by brake pipe. But as brake pipe is rated about 3000psi, perhaps not a problem on a small engine. (Warning – I'm not a boiler expert!). Did the seller provide a specification?

Dave

Brake pipe is 10% Nickel

But the OP refers to copper

q

For really small tube i buy the little handy packs that some model shops stock . Have a look at emma’s spare room machine shop on youtube , if you send her a message maybe she can point you in the right direction as she makes her own boilers and is based in the Brisbane area . For larger stuff like what you want for a boiler sometimes making friends with a local plumber or gasfitter can be an advantage , they usually collect offcuts to send to the scrapyerd for some pocket money.

What M.E suppliers have you tried ?