Looks similar to what we were seeing, after long term exposure to the acidic fixation agent. When we had over spray, it would accumulate on the peripheral parts, causing serious deterioration after awhile... Our rudimentary solution was to wipe down the machine, several times a day....
Not speaking from actual machine experience on this ( I neither own or operate a kornit), but I have done some work in a metal workshop and If I'm not mistaken the picture of the board looks like corroded aluminum. Aluminum corrodes forming small craters where the water and acid deposits, acompained by a white halo where the hole is. knowing that the pretreatment is done on the board and is indeed very wet I would bet is corrosion from the pretreat and water ( have you checked the PH for the pretreat once you have added the water, it might be acidic ( not much needed) enough to corrode aluminum, some metals do not behave well in permanent contact with moisture). I could be mistaken but when I saw the picture that popped in to my head.
Anyways it has to be the pretrat for sure because it is the only thing apart from the cotton that comes in to contact with the board, that or you have some strange critters eating away your machine( not likely)
An Easy fix would be to protect the metal with some sort of vinyl or adhesive plastic, afterwards check with kornit about the PH for the pretreat, and you can check also the ph for yours, that would easily point you in to the right direction.
Yes, the acidic nature of the pretreatment will cause corrosion if left standing on the pallet surface. Justin's recommendation for wiping off the pallet may be necessary, but I would first try cleaning it only once daily with a mild solvent cleaner (such as Simple Green) and see if the problem stops.
Another precautionary step is to check your chemistry for the proper measurements. The Fixation 201B needs to be mixed at a 1:3 ratio, or one part fixation, 3 parts tap water (do not use bottled or distilled water). Whether or not you get the 5 liter or one gallon containers of fixation, the dilution ratio is still the same for both.
Correct me if I'm wrong, but.... distilled (or purified) water is better.... no?
There are a lot of shops out there with well water or "city water" that has either untreated (high iron content in northeastern US)) or has high degree of chemical treatment (city water with biocides or chlorene).
I was told (with other mfgr. PT fluid concentrates NOT Kornit) that because of the variations of additives by water softeners or city water additives, that it wasn't a good idea and it "might" adversely affect the reaction between the PT and the ink....
And.... I would coat, or cover the platen with a silicone or teflon based coating. PT fluids will even pit stainless steel if allowed to remain. Wiping the residue off daily will prevent it from oxidizing the surface.
Tap water is the required water to use when mixing the pretreatment. Do not use distilled water. The minerals in tap water actually suspend the chemistry much better that distilled or purified water.
Board getting holes 
