I need to measure humidity in a corrosive environment, high vapor concentration 60% formic acid. What sensor could I use? A DHT22 is suitable for those situations?
I don't think so. The DHT22 uses a substrate with a something that changes resistance when the humidity changes. If only the DHT22 is packaged in the wrong plastic bag, the chemicals of the plastic bag can already make the DHT22 very inaccurate.
Try a few capacitive humidity sensors, and see which one lasts the longest time.
You need a barrier between the Formic acid and the sensor that is impervious to the big molecules but lets the water through. CPVC sounds like a possibility, I recall it is permeable to humidity anyway. The sensor will see changes very slowly. And I recommend a capacitive humidity sensor also.