If it is flux it will rust!!!!!!Take it from experience.Try cleaning it off,if it is rust it will leave a groove or pit in the blade.If it didnt leave any pits then it is safe to say that it is flux,neutralize it with window cleaner with ammonia in it,then put some wet baking soda on it this will help to finish neutralizing it.Sometimes you can get a bubble trapped inside the joint and it can leach out for a long time if not neutralized....Hope this helps,Bruce
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