"Acid" water and Joints don't mix well (dangerous cocktail, joints and acids) Just kidding .....
So that's my concern: I was told that the injection of rainwater into the washing machine was to call SAV DARTY much earlier than expected ....
By definition, rainwater is very weakly mineralized, ultimately it is only distilled water "contaminated" by substances present in the atmosphere. Main responsible for a slightly acidic pH: carbon dioxide (CO2) -> carbonic acid. The relative purity of rainwater means that minute amounts of dissolved CO2 are sufficient to vary the pH, so minute amounts of the structure of a concrete tank also dissolve to neutralize this water. At least, on the scale of a human life ...
In a washing machine, the use of detergents makes the water extremely basic, and consequently eliminates the possibly "corrosive" character that could present a weakly mineralized water. In addition, the absence of hardness allows substantial savings in products and eliminates the risk of scaling. Rainwater is soft water without softener, and therefore without rejection of regenerating salts. Personally, I see only advantages.