Neeves
Well-Known Member
I think some of this might be misleading:
There is no suggestion anywhere I have seen that G80 or G100 is a global panacea. For example it would be wasted on many vessel for whom weight and space is not an issue.
G70 chain, or the steel, or the cross section of the link is homogenous. The structure of the steel is consistent through the link. There is no 'compression' layer on the outside. To have a steel of a different physical structure would result in stresses between the layers. The whole basis of the temper is to produce a crystal structure to impart strength, not to produce an outer layer different to the bulk of the steel. Tempered steel, including chain, is produced in different thicknesses (and strengths) and to have an outer layer of one strength and an inner softer layer is simply not feasible. The strength of the steel is the same through the steel. You can see this in broken samples - there is a homogeneous break.
However all galvanising is an alloying process, or that is what happens, and the galvanised layer is actually composed of alloy layers of different compositions. The inner layer has the highest Fe content (of an FeZn alloy) and subsequent outer layers have less and less Fe - the outer layer is pure, or nearly pure zinc. The inner layer - the one that forms next to the base Fe, or steel, is very thin, a few microns and the exterior layers are less and less hard, though commonly harder than the base steel.
In use the outside layer of Zinc is soft and abrades easily but it does also serve to protect the under layers, because it is malleable. For structural steels this is advantageous - because it does not suffer abrasion but for anchor chain the outer layer is transitory and soon wears off.
The life of the galvanising is commonly a function of 2 factors, how well it is adhered )or how well it is alloyed) and how thick it is. Common standards of thickness are 70 microns - which is effectively based on engineering applications, structural steel work - where there is no abrasion. Our chain is made to the same standard, if you are lucky maybe a bit higher than 70 microns (by chance rather than design) and sometimes lower, I've seen 60 micron specs. But life of our chain is a function of the thickness of the galvanising. You can produce a thicker galvanising layer, but zinc costs money and we are small part of the market. If the layer is too thick - it can flake, and life of the coating then reduces. If there is inadequate preparation the alloying layers are not well formed and flaking can also occur.
Maggi had a poor reputation for galvanising - there were complaints of their G70 chain (of which Noelex has direct experience) and of their G40 chain (which have been reported on YBW). It does not appear to have anything to do with steel quality - but process. I noticed on samples I had a propensity to flaking - indicative of poor preparation - the chain was insufficiently clean before galvanising. Gal poorly adhered will break off on the seabed, and even in the gypsy.
These same complaints are not evident, or not so consistently, for other chain makers, American nor Chinese whether G70 or lower grades.
But returning to the FeZn layer that forms the interface between steel and gal. This is a very hard layer, and very thin. It has no impact at all on strength of the chain. But when it is breached - there is only steel underneath - and it will corrode.
Really as soon as you see rust developing on your chain you should then prepare to regalvanise as you are looking at raw steel and further use will abrade the steel and reduce strength.
However High Tensile steel is factorially harder than G30 steel - and weight for weight - the HT steel will last longer (though both will corrode at a similar rate). How fast 6mm chain in use abrades compared with 10mm chain is an unknown - you can guess but there is no basis - so your guesses are, simply, guesses. Because the HT chain, in this case, is lighter, and harder (and will be on the seabed for a shorter time - the steel (and the galvanising) will last longer. Or because the chain has a lower surface area it will abrade more quickly. To complicate - the galvanising on HDG is softer than that proposed for HT chain.
Instead of guessing - I'll conduct some tests.
There is another wear mechanism, for gal - and that is chemical. Some seabeds, primarily mud, are anaerobic and acidic. The chain can be chemically eroded. If you chain comes up black - its probably been in a sulphur rich environment. Hardness does not come into it - thickness of coating is important.
Dashew has been using G70 chain since the 70s or 80s - he has never suggested that galvanising life of the chain he uses, ACCO, now Peerless, has been an issue. Other people have followed his lead and also use Peerless G70 chain. No-one has ever highlit galvanising life as being an issue, specifically for G70. The users of G70 without exception have moved down a size, to a small chain - if there were serious issues with the life of small chain - it would be known, be the subject of internet discussion and Dashew would be defining his use of the product on the expedition vessels he sells.
. I hope people considering which chain to use find these type of discussions helpful. The number of people that have used G7 anchor chain is very limited at the moment so there will naturally be some disagreement concerning the relative merits of this choice. As long as the posts are polite and respectful (as yours always are, Thinwater) I think there is merit in threads such as this. If nothing else, it may convince more people to try G7 chain.
