I doubt such things were ever made of iron in the modern era, the implication is that it's a product that's straight from the forge and still coated with millscale which is black in colour; in contemporary use it's likely just slang for uncoated steel, I've never seen a shackle supplied with millscale on, only galvanised, electrozinc plating or bare steel.
That cites high tensile fasteners holding up a WTG; of nugatory relevance to a mooring shackle.
Millscale would be the last coating wanted on a product intended for subsea use. It is renowned for promoting corrosion and is always blasted off before painting.
Im just around the corner in Porth Penrhyn
One broke loose last week that's why I'm paranoid now
I find that the swivel is usually the weak link. The pin seems to wear the ring such that it becomes very slack & the nut on the pin part rounds off & something breaks. Not sure what goes first -pin,ring or nut- as I only see the swivels that I recover. Not the ones that have finally broken
Apparently the mooring became all twisted up due to the owner not checking and that snapped the shackle,may of been poor quality,boats need periodic checking.
That's correct it was the boat next to mine with a bridle set up that was twisted to it's max for months but the owner just left it like that even when he was on the boat.
Very many years ago I sailed a regatta at Morecambe, a drying bay renowned for fast tidal flow. I got talking to a council employee responsible for channel markers, who pointed out a red buoy only just visible on the water surface. This buoy, chain and sinker was infamous for gradually disappearing beneath the sand. They had made attempts to recover them but were never able to dig deep enough. The buoy was removed regularly before it sank but the tackle was not recoverable.
I would have to take issue with your four week theory. For many years, although not the last two, we have been in the habit of leaving our boat anchored for periods of four weeks at a time. It has not removed all the galvanising anywhere on the chain. What does happen is that the galvanising is turned black, presumably because the anchorage is black peaty mud, which will be quite acidic. So much so that the last time the chain went to be regalvanised, an operative took a cursory glance, and thought it had been painted black, and grit blasted it before galvanising.
but the shackle wears where the two components, shackle and chain rub each other constantly adding fresh silica and are under constant tension - for my tests the gal is worn off with no, or minimal, tension - simply the links rubbing on the seabed. You might think the smaller chain would wear differently to the bigger chain - but they wear at roughly the same rate. Gal removal is a function of the coating thickness and quality of the galvanising (care taken in cleaning prior to galvanising) and the type of galvanising (some processes produce a harder gal coating the higher hardness results in better abrasion resistance. The ideal is a thicker coating of a harder galvanising.