This is a very typical example of stagnant zine corrosion caused by not using an anti corrosive. It's got nothing to do with silver.
Taking the top off and cleaning the block with some non abrasive metal Polish will make it look a lot better but there is most likely some degree of permanent damage to the finish.
The good news is, it's just the finish. The function of the block is unaffected. Using corrosion inhibitor will prevent the damage from worsening too much and there is no risk to other parts of the loop. Corrosion is not contagious, and the metal plating that disappeared from the block simply dissolves back into solution the same way it was when it was put on the block in the first place. Filters can't filter dissolved metals.
Always use corrosion inhibitor.
Many of us here in the Australian forums have had good results with liquid Utopia over many years. I think mainly because we have never had a lot of options to buy coolant locally and it costs a ton to import a bottle. The sysprep is garbage though, don't bother with it. It's just a detergent.
Last edited by Ashcroft; 02-01-2019 at 10:01 PM.