I don't suppose you have any way of figuring out what the alloy actually is, do you? Contacting the manufacturer, perhaps?
Alloyed brass with high-zinc content (to give that bright gold color), is also often alloyed with antimony, tin, or other additives, especially when it's for liquid applications, to prevent the leaching of zinc.
That being said, regular use, without letting water sit in it for long periods will render your worries moot. Most of the corrosion effects are due to acidic water, either stagnant or slow-moving, or neutral ph water with a high salt content. Neither of which you should be using for your plants unless you occasionally make an acidic tannin or coffee solution, and again, as long as you don't let it sit, there's no reason to worry about it.