What you are tasting is iron. I would out it down to the yeast and your process and not so much the equipment, unless your mash tan is really old and damaged.
Excessive iron to the point of creating a metallic taste can be caused by multiple factors.
The Yeast
Yeast and iron have an interesting relationship. Many different yeast strains are rich in iron, and yeast grows and ferments better when iron is present. Thanks to the presence of iron in most grains, including wheat, oats, and barley, yeast tends to ferment these grains just fine.
During the mashing process, when ground grains are boiled and literally “mashed in” to make a thick wort, the trace amounts of iron in the grains are released and they will serve the yeast well once it is added to the wort.
Yeast also carries iron that it does not typically release into the wort. If the levels of iron are too high in the wort, however, the iron locked into the yeast cells will “autolyse” or break out of the cell wall and dissolve into the beer.
Once the iron in the yeast and the iron from the grains are all dissolved into the wort, iron levels may get high enough that it is detectable to the palate.
Water
Water also naturally contains iron, but again, at normal levels, even with the iron from the grains, the iron levels should remain low enough to be both helpful for fermentation and harmless to humans.
It would still serve brewers to check the iron levels in their water, however, particularly if the source of the water is a well or especially hard water.
Hard water can bring the overall pH of the wort up as well, which can cause an increase in oxidation and release the iron from the yeast cell walls, once again driving up the overall iron levels to detectable and higher levels.
Thus, brewers should check both iron levels and the pH of the water used in brewing.
The Container
Finally, and most commonly, excess iron can come from the container in which the wort is mashed. Most brewers today will use the classic copper kettle that is coated with a protective film so as not to leach any minerals into the brew.
But inexpensive, old, or damaged metal containers can often leak iron into the wort and drive up iron levels across the board.
