Seacocks - DZR etc

[Plevier]

I've managed to find the following numbers on my seacock ball valves - DN25, DN20, PN30, MS58. There may be other numbers out of sight.
Am I right in thinking that the only marking relating to the metal is the MS58, and that MS58 is not DZR?
The ball valves are plated, the skin fittings and hose tails are not. In the heads, where all the skin fittings are in a cupboard below the basin, all the unplated items have a matt black finish. Example here https://picasaweb.google.com/113567...hkey=Gv1sRgCMWboZff7-O6eA#5799466420998233314 The ones for the galley and the engine intake are bright. Any ideas why?
Is the severity of dezincification related to salinity? My boat (Jeanneau) is 9 years old but the outer flanges of the skin fittings only show slight traces of going pink in patches. Example here https://picasaweb.google.com/113567...hkey=Gv1sRgCMWboZff7-O6eA#5799466898628279074 The boat started life at Levington well up the Orwell, which I guess would be low salinity, then moved to Tollesbury, I don't know how saline it is there, and I have it quite well up the river at Shoreham, again with lots of fresh water dilution. Will this slow DZ down?
They all feel rock solid when you heave on them. I don't know whether I need to be rushing to change everything or not.

 

[vyv_cox]

MS58 is a German standard for a leaded brass, so you are correct, it is not DZR. The skin fitting looks pretty good to me, no worse than mine which are 26 years old. I would not be worrying about them just yet if they seem strong enough.

Dezincification is a problem in domestic water supplies in soft water, e.g. where I live in North Wales. It does not happen in hard water. DZR was originally developed for the plumbing industry, which is why seacocks for a few inches of water pressure are rated for 30 bar or more. I don't think the salinity is a factor but I don't know for certain.

I cannot explain the blackening. Is it paint?

 

[Plevier]

Thanks Vyv. Very reassuring! Particularly as our water here is very hard off the chalk hills.
No the black is not paint. It's more like a surface chemical treatment but it is soft and scrapes off easily. I wondered if any sort of vapour could produce it, maybe from cleaning products kept in the locker or something like that?
I don't think "organic" gases could account for it - not used enough!