my pipe I made is aluminized steel. I don't think that theres reason to be concerned about the aluminum flaking off since it doesn't flake off exhaust pipes.
I have no idea what the size of the pipes are in metric, but when I built mine I used an exhasut pipe adapter. 3"OD - 2 1/2"OD then I bought an adapter where one side was 2 1/2 ID which slips over the 2 1/2 OD side. I cut the other end of the adapter off and welded the cut piece right over the small end of tha adapter effecticely increasing its outside diameter. Once that was done I cut the tube to length leaving room for about 1/4" gap between both the TB and the MAF.
Idealy you want a tube that is 3" OD on one end and 2 1/2 ID on the other end. The inside diameters of both ends of the tube are the exact size or slightly larger than the bores of the MAF and TB respectively.
I thought about using gromets to seal the tubes to the intake pipe. That would likely be fine since the tube is just a connection betweent the MAF and the TB and it is not subjected to high or low pressures. I decided to weld everything becasue it ensures a perfect seal and it is much stronger than a gromet. Becase if air leaks through a gromet or any place on the tube it will be air leaking in that has not been metered by the MAF.
