how long is long enough to ascertain whether corrosion will occur...
What holds true for corrosion in other electronic components is no different for the Sparks
battery! Which doesn't take a science experiment or study to figure out what the outcome will be.
The only way to tell with 100% certainty that saltwater is not going to eventually damage your Sparks
battery is to take the
battery pack apart and inspect the PCB board, switch, components, and contacts for any corrosion.
Saltwater corrosion usually sets in quickly within a matter of hours to where you should be able to see components inside start to turn green or rust. If you think that you can just keep testing out the
battery pack without inspecting it internally, your just rolling the dice and taking a gamble. Over time environmental conditions such as condensation and humidity can eventually aid even the smallest traces of salt and start corrosion inside without you ever seeing any adverse effects until over time it finally has a catastrophic failure from it.
I don't recommend you opening the
battery unless you have experience handling or building Lipo's
batteries yourself. If by chance you puncture or damage a cell, you will have poisonous smoke and possibly a fire on your hands that you possibly won't be able to extinguish.
The critical thing that must be done almost immediately with any electronics that has been exposed to saltwater is that the power source needs to be disconnected from it, and it needs to be either rinsed with fresh water or cleaned off with a good electronics parts cleaner so that all the salt is completely removed from it. After this the electronics needs to be completely dried before it can be tested or used again.
If you have failed to remove any trace of the salt, it can come back to haunt you later on. Soaking the electronics in rice sometimes works as only a short term solution, however I have repaired many DJI drones that have been in either saltwater or swimming pools chlorinated water and soaked in rice to dry out afterwards. And the critical mistake the owners usually have made that lead to the aircrafts eventual demise even after having it flying and operating again, is them having not cleaned or completely rinsed out the saltwater or chlorine before putting their drones in the rice.
I don't use or even tell others to use the rice method works, I'd personally rather take everything apart and properly clean and dry it. If there is a part I know that has saltwater in it, and I can't take it apart to clean and dry it, it gets trashed and replaced.
Conclusion: Replace the
battery, or just keep it for use only on the ground.