Replacing lead weights with US nickel coins

[pwoolf]

Why not just make weights from Monel?

Monel would be awesome, but hard to source!

Nickels have almost as good of performance and are available inexpensively at any US bank.

 

[lexvil]

Why not just make weights from monel

Monel would be awesome, but hard to source!

Nickels have almost as good of performance and are available inexpensively at any US bank.

$3 per pound scrap price, not sure of manufacture ready price. The advantage is solid product takes much less space for a given negative buoyancy although less flexible but the shape of a nickel doesn’t pack to tightly.

 

[pwoolf]

Why not just make weights from monel

$3 per pound scrap price, not sure of manufacture ready price. The advantage is solid product takes much less space for a given negative buoyancy although less flexible but the shape of a nickel doesn’t pack to tightly.

I've used reclaimed scrap 316 stainless bar for dive weights and that works--just depends if you have a good source (picture of one attached). Unfortunately I don't have easy sources of monel scrap where I live.

But fortunately, cupronickel coins are present in many countries and no machining required!

 

[Johnoly]

....I've been using US 5 cent nickels instead of lead as a soft dive weight.

On behalf of us spearo's who have been ROBBED underwater, at 5 cents all I can say is....

The TAX MAN is going to get you too !!

 

[BoundForElsewhere]

Very interesting, I immediately tried looking up my local currency but your calculator doesn’t include Canada.

you'll need a lot of loonies. but then, Canada has plenty of loonies. not as much as the US but y'all are catching up quickly.

 

[runsongas]

@pwoolf that figure of 0.56 mm/yr is for distilled/de-ionized water that will leach ions from commercial lead and break it down faster. the next sentence is that the corrosion is reduced when higher chloride ion concentrations are present (aka salt water). From those rates, usage during dives isn't a concern as long as you don't lose the weights.

 

[Umuntu]

I've even made a handy calculator to estimate the number of coins (US and global) and their approximate value in USD for any weight you want.

I didn’t see the UK on your list.

Is the 5pence coin not made from cupronickel ?

I think your idea is brilliant ! On future dive trips I will take a couple of these empty bags with me, visit a local bank for coins, and then at the end of the dive trip use the coins to give the dive guides an impressive looking tip

 

[VikingDives]

Very interesting, I immediately tried looking up my local currency but your calculator doesn’t include Canada.

As the 51st state, you have ample access to nickels (sorry I couldn't help myself).

 

[Tanks A Lot]

[...]
If we take it at these ranges from Table 2 (0.002 to 0.013 mm/year) then they are around that of 316 stainless steel, which doesn't seem correct to me.

EDIT: Looking further I do find data on lead corrosion, interestingly it comes from the nuclear reactor industry.

https://inis.iaea.org/collection/NCLCollectionStore/_Public/31/060/31060544.pdf?r=1

They list a corrosion rate of 0.001 mm/year for "neutral carbonated ground water" but for salt water or distilled water they find 0.56 mm/year.

As @runsongas pointed out already, this is not quite applicable in the diving industry.

When we think of metals corroding, the intuitive assumption is that they will corrode faster in saltwater than freshwater. But this assumption is incorrect for lead.

The study you quoted mentions that nitrates and acetates are the main culprits for lead corrosion. Both are present in ocean saltwater, but in such tiny amounts compared to sodium or chloride, that I would nearly assume their effect to be negligible. The study further goes on to mention high corrosion rates of lead in low chloride solutions, but fails to give the exact number. The number at which this has been proven is very low indeed at 0.0005M¹. It further mentions that this corrosion rate decreases with the increase of chloride. And the chloride concentration of ocean saltwater at 0.5M to 1.9M is significantly higher.

These salts form additional passivation layers on the lead. More specifically, PbSO₄ and PbCl₂ are formed. This is in addition to the earlier mentioned Pb₅O(OH)(CO₃)₂ and PbO₂ layers. Of these layers, the PbCl₂ is probably the most durable one, and chloride is by far the most abundant salt in seawater. What happens is that the base alloy is protected by a layer of PbCl₂, and secondary passivation will be achieved by a layer of PbO₂ forming. You are certainly correct in saying that these layers are not as strong as, say, aluminum oxide layers, but I would argue that they are sufficiently strong to inhibit most further corrosion.

There is one study that looked at the corrosion rate of lead in seawater, which was done in 2013². They reach a corrosion rate of 0.75mg per dm² per day. We can do some back-of-the-hand calculations to see what this would actually mean. Let's assume we submerge a cube of 1Kg of lead in seawater.

Lead has a density of around 11.34g per cm³:

Calculating the volume of a 1Kg cube of lead:
Volume_cube = 1000g / 11.34g / 1cm³
Volume_cube ≈ 88.18cm³

Calculating the side-length of the 1Kg cube of lead:
Side-length_cube = ∛88.18cm³
Side-length_cube ≈ 4.48cm

Calculating the surface area of the 1Kg cube of lead:
Surface Area_cube = 6 x (4.48cm)²
Surface Area_cube ≈ 120.42cm²

Convert the cm² to dm²:
120.42cm² / 100 ≈ 1.2dm²

Calculate the daily corrosion rate of our 1Kg cube of lead, assuming a rate of 0.75mg per dm² per day:
Corrosion_day = (0.75mg / dm²) x 1.2dm²
Corrosion_day = 0.9mg

Calculate the days until the 1Kg cube of lead is completely corroded:
Days_{Until corroded} = 1000000mg / 0.9mg
Days_{Until corroded} = 1111111

Our block of lead would last us 1111111 days or just over 3000 years. I must admit that assuming the block to be a cube is incorrect, but I can't be bothered to calculate exact surface areas with the slits for the weight belts, or assume shots of lead. These numbers would definitely be different and shorten the lifetime of the lead. But on the other hand, no dive weight is going to spend 24h per day in saltwater either. I hope that the numbers make it clear that lead is, for all intents and purposes, impervious to saltwater, at least as far as the dive industry is concerned.

Again, I want to reiterate that I'm by no means advocating for the usage of lead; I think your goal is admirable. But I also think that an honest representation of the physics and chemistry is important, as people might attack your whole premise on some flawed assumptions.

Lead is horrendous because of its toxic properties, but unfortunately, its corrosion characteristics are excellent. This, coupled with its high specific gravity and low price, is the reason it was chosen in the dive industry. If you think about the historical context, there are only a handful of metals that are of interest as diving weights at all:

Metal	Gravity	Price per Kg ($)
Platinum	21.45	<30000$
Gold	19.32	<80000$
Tungsten	19.25	≈30$
Lead	11.34	≈2$
Silver	10.49	<900$
Copper	8.96	≈9$
Nickel	8.9	≈15$
Cobalt	8.9	≈25$
Iron	7.87	≈0.1$
Tin	7.31	≈28$
Chromium	7.19	≈8$
Zinc	7.14	≈3$

Platinum, Gold, and Silver can all be discarded right away because of their price. Iron, Zinc, Tin, and Chromium are not suitable for saltwater. This only leaves Tungsten, Lead, Copper, Nickel, and Cobalt as suitable materials. And lead seemed to be the natural choice historically, as it was dirt cheap, has a very high gravity, and excellent corrosion characteristics. Tungsten would be a suitable contender, but it is very hard to melt down, having the highest melting point of all known elements at 3422 °C. Lead, on the other hand, is molten down easily and formed into shape.

This leaves Copper and Nickel as the next in line. They are more expensive and harder to form into shape, but I believe the dive industry as a whole should move away from lead and use these elements, or better said, suitable alloys of these, for weights. They are more expensive, harder to work with, and have a not inconsiderably lower density. But I believe these tradeoffs are worth it. Especially dive professionals who handle weights daily may be exposed to a not insignificant risk of lead exposure. Unfortunately, there are no scientific studies done on the subject, as the number of people that are at risk is so minimal to begin with.

There is no need to panic and rush to replacements or see lead as a toxic killer that will end one's life if you touch it often enough. However, it has serious toxic properties, which may harm the person handling it in the long run and it can certainly harm the environment.

¹C.J. Semino *, A.L. Burkart, M.E. Garcia, R. Cassibba - Lead & Carbon Steel Galvanic Corrosion Evaluation (1996)
²A. A. Abdul Azim, Venice K. Gouda, Laila A. Shalaby & Safa E. Afifi - Corrosion Of Lead In Salt Solutions (2013)

 

[MarkA]

I didn’t see the UK on your list.

Is the 5pence coin not made from cupronickel ?

Nickel plated steel since 2012, at 3.25g each.