The alkaline batteries uses zinc at negative pole. Zinc has a very high energy density, it is relatively cheap, and is non-toxic.
Zinc is also an amphoterous metal: it means that it is corroded both from acid and alkaline solutions. Originally the zinc was used in carbon-zinc batteries, which uses a neutral electrolyte, so the fact it is amphoterous wasn't a problem.
With the invention of alkaline batteries, zinc was mixed with small quantities of mercury, to form an "amalgama" which is an alloy at all effects, even if it is created without any heat. This "amalgama" is not reacting with the potassium hydroxyde, which is the electrolyte present in alkaline batteries.
Following the 1990 mercury ban, manufacturers had to find a substitute for the very toxic mercury. Initially they were adding lead and zinc oxide to the zinc; to slow down the production of hydrogen gas.
The production of hydrogen was not relevant with mercury amalgama; it become important when mercury had to be removed from alkaline batteries.
In the first years of 1990 it was found that replacing the lead (which was less toxic than mercury, but toxic nonetheless) with small and equal quantities of bismuth and indium to the powdered zinc reduced the formation of hydrogen even more.
From then, the formulation of alkaline batteries has not changed much.
The problem lie in the fact that the quantity of hydrogen that is formed when the alkaline battery is idle is purely theoretical; many unaccounted factors contributes to the increase of the quantity of hydrogen production, first of all the purity of the chemical components used. This degradation "by design" of alkaline batteries often goes out-of-hand, and the battery leaks.
Reality is - that alkaline batteries are reliable only up to two years from the date of manufacture. For the known marketing reasons, the manufacturers extends the shelf life of the batteries artificially, with the results we all have seen... I had uncounted alkaline batteries, over the years, leaking in their packaging, still unused.
Equipment manufacturers suggest to replace batteries every year for a reason...
Regards
Anthony