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The thing that I'm still struggling with is that this corrosion rate suggests that lead has an extraordinarily low corrosion rate, at or below that of stainless. But that does not comport with my experience with uncoated lead weights and, say, my stainless steel regulator. The dive weight is bangled to all hell and has obvious corrosion marks, while the regulator looks like brand new.
Comparing lead to stainless steel is difficult I guess. You would first have to define which stainless steel we use for comparison. Is it austenitic, ferritic, martensitic, duplex, or so on. Each of these categories further has several subcategories of stainless steel. A common choice would be 316L stainless steel, which belongs to the austenitic group. And incidentally 316L seems to have fairly similar corrosion characteristics in seawater as lead does.
I wonder, do these corrosion rates only tell half the story of a just a lead object in water. What about mechanical action?
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This is completely true and touches upon an important point. It may be that the weight you perceive as corroded are merely severely deformed, showing white powdery lead oxide. Lead deforms so easily, you can even scratch it with your fingernails.
Given this picture, an abraded, manipulated weight like lead will be constantly deforming. This means that the passivation layer will be constantly cracking and flaking off when, say the weight rubs against a belt, sack, or wet suit. This case would be much worse for shot, as it will behave as a ball mill.
The scientific studies, also the ones I mentioned are useless in some very real sense for real world scenarios. Oftentimes they assume un-stirred, stagnant water, which is obviously not the case. Moving water would effect the formation of passivization layers and the resulting corrosion rates significantly. However, they also assume the lead is immersed for 24 hours, which is also clearly not the case.
Coins are better in that the underlying cupronickel is as hard as the hardest passive surface on lead, and the passive surface is even harder.
Stainless is even better as it is hard and its passive layer is very hard, so there is very little deformation, so the passive layer stays intact.
What do you think about this?
I think I would leave the argument of corrosion of lead aside. If you look at lead weights that get recovered after nearly a decade of immersion in saltwater on old diving hats or similar, they still look great. I remember seeing a picture of a lead anchor from the first century BC, that still looked decent. If I can dig that picture up I add it here later.
However, what I do would mention is that while lead doesn't really lose any weight over time to corrosion, it still forms oxides. And these oxides can flake off and find their way into a person or animal fairly easily via ingestion. They may not be enough to make a dent in the leads mass, they may be enough to become a problem in a living organism. Touch the lead with your hand, then eat a banana on the boat -> You likely ingested a little lead.
The whole situation is far more complex than what we can solve via back of the hand calculations. The truth will likely lie somewhere in between "Lead is completely impervious to saltwater" and "Lead corrodes rapidly". Lead will be fairly unaffected by the water, but mechanical action and abrasion will take a toll on it, if ever so slight.
I agree that the ocean faces a ton of challenges, many of which we have zero power to change. BUT, the good news is that replacing lead in our dive kit IS something we can do now. Coins made from marine-grade alloys like cupronickel are similarly priced to lead, last longer, and eliminate heavy metal exposure completely. Unlike other ocean challenges, this is one we can solve immediately at little to no extra cost.
This should be a more widespread opinion. Just because we can't change every problem at once, that doesn't mean that we can't start by changing the little things, especially when doing so is relatively easy.
[...]Lead exposure via SCUBA weights is a risk small enough to ignore. Just wash your hands after handling and before eating, and the risk is nil.
This is certainly true to some extend, but unfortunately it doesn't end there. People all too often cast their own lead weights and in poorer countries this is routinely done without any precautions whatsoever. Not using it in the first place would negate the possibility and stop the problem in its tracks.
With that said, next time I have 100 nickels, I'm curious to see if I could cast a cupronickel weight. Seemingly would need a steel mold, don't know if a standard aluminum one would take the temperature. Be an interesting project.
Unfortunately that won't work. Lead is so easy to work with, melting already at 330°C or so. Aluminium withstands 660°C but Cupronickel needs more than 1100°C to melt.
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Blanket hysteria of "OMG! Pb! Eeeek!" is over the top dramatics. If you work with a lot of it every day, yeah, it may be a concern. (I once worked at an indoor range and had a colleague that also smelted and cast lead bullets on a commercial basis. He had annual checks of lead concentration in his blood. Never crossed into a problematic concentration.) Slight, occasional contact? Just not so much of a problem.
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I think no one here is advocating for the immediate dismissal of lead, or even suggesting that it is an immediate danger and threat and the most pressing issue. However, it is undeniable that lead is rather devious poison for living organisms. It is a poison that accumulates in organisms and only really leads to complications once a certain threshold is passed. There was good reason to ban leaded fuels and thankfully the world has stopped using them completely, with Nigeria using its last stockpile in 2021.
Your friend likely had precautions in place, as it should be. But these precautions are not universally in place, as I mentioned above especially in poorer places. If a better or safer alternative exists, I would suggest using it. Just because something can be made safe, that doesn't mean its a smart thing to do, especially if alternatives exists.
You have a good point in pointing out that cupronickel itself may have environmental issues. I would go as far as saying that Copper and Nickel mining are horrendous for the environment as well, there is no doubt about that. But how these compare to lead and the mining of lead, I have honestly no idea.
@Mobulai
I sincerely wish that XenForo would support proper LaTeX, this would make writing math formulas so much nice and easier to read. It does not as far as I'm aware, but it does support certain tags, like:
[sub]XXXX[/sub] for subscript and [sup]XXXX[/sup] for superscript
