Problem never ends. I forget to remove alkaline batteries from equipment before storing it and then months or years later try to use it and find it dead with corroded batteries inside. Most of the time, it's just a tedious clean-up job, but sometimes the device is trashed. I've also noticed that alkalines are much more likely to leak when left in equipment than when stored outside equipment.
I looked into why alkalines leak, and it's due to hydrogen gas generated inside and pushing out the electrolyte, which is mostly potassium hydroxide. It's the KOH that does the damage. Interestingly, hydrogen gas is not generated in the reactions that produce output current. Instead it's hydroxide attacking zinc in the battery. I tried to find the chemical reaction on the net, but best I could find is that it might involve formation of zincate ion. So the reaction is likely:
Zn + 2 OH- + 2 H2O = Zn(OH)4-2 + H2
The reaction is analogous to what happens when Al foil is put in lye solution. The reaction will proceed at all times and will be more rapid in a warmer environment. But why faster when the battery is in equipment rather than stored? I hypothesize that when the battery is in equipment, the zinc in the battery acts as a sacrificial anode for minor corrosion that normally occurs on metal parts in a device, including the battery contacts. Once some hydrogen forces out KOH solution, reactions on the contacts and other metal parts accelerate which makes the internal zinc reaction accelerate too, leading to even more expulsion of electrolyte.
Can the zinc/hydroxide reaction be prevented? Not really. If it's an alkaline battery, then by definition, it will have zinc in contact with hydroxide. There are two traditional solutions proposed: use a hydrogen getter (i.e. absorber or adsorber) or use a vent with a membrane that will allow hydrogen to pass, but not electrolyte. Both have problems. Hydrogen getters have used expensive materials, like palladium, but less expensive ones, like Suisorb, have been developed. Hydrogen getters may be self-defeating in this application though. They remove a reaction product (hydrogen) which may accelerate the reaction until the getter is saturated and useless. Hydrogen-permeable membranes apparently are used with varying success in alkalines. Problem is the battery can be in various orientations, and if the membrane is below the electrolyte, hydrogen will exert pressure to force electrolyte through the membrane directly or by rupturing it. Having multiple membranes in different orientations might work, but clogging could still be a problem eventually.
I've never seen a 9 volt alkaline leak. I'm sure it happens; I just haven't seen it yet. However, a 9 volt alkaline has six internal 1.5 volt cells surrounded by a secondary container. I assume leakage does occur from the 1.5 volt cells but rarely penetrates the secondary container. AA, AAA, C and D cells could be made with a secondary container, but some electrical capacity would have to be sacrificed to the dead space of the container.
Mike in California