Pique the Geek 20100711: Essential Materials. Zinc (Updated)

(10 pm. – promoted by ek hornbeck)

Most of us have heard of zinc in passing comments.  Some TeeVee commercials tout the virtues of it in the diet.  Actually, it is an essential trace element, and part of some coenzyme systems, and so is essential for health.  Interestingly, it is more important for men than for women because the prostate gland needs lots of zinc to produce semen and to maintain health.

That does not mean that women do not need zinc, just that they do not lose lots of it in seminal fluid.  The requirements for zinc in the metabolism is the same betwixt the sexes, but, as females lose iron during menstruation, men lose it during ejaculation.  Please follow, and I promise not to be so graphic for a while.

Zinc is usually thought of as a heavy metal, but it is really not.  With an atomic number of only 30, it is much lighter than many metals commonly encountered.  Iron has an atomic number of 26, and hardly every is thought of as a heavy metal.  On the other hand, lead, silver, copper, gold, and may others are much heavier than zinc.

The uses of zinc, other than coenzymes (vitamins in our bodies) are multiple.  Most folks contact zinc every day and do not realize it.  It is EVERYWHERE!  It is difficult to say what product is the most rich in it, but for Americans the one cent piece may be the most common.  Please allow me to give a brief history of what is called a US “penny”.

Where to start?  First, the United States NEVER minted a coin called a “penny”.  That is exclusively British, but by common usage our “cent” came to be called a “penny”, since they both were small denomination coins made mostly of copper.  Until the late 1850s our cents were quite large (as big, almost as a half dollar), and minted from pure copper.

Pure copper has a disadvantage as a coin metal:  it is soft and has extremely poor wearing properties.  Around 1857, experiments were conducted to alloy copper with nickel to make cent coins, and they were used for some years.  Since nickel was expensive, zinc was substituted, with a little tin, and the bronze cent was borne.  This bronze allow was continued until 1943 (when war efforts required copper to be put into munitions, and the first clad US coin was struck, zinc plated steel).  In 1944 spent shell casings were used, 95% copper and 5% zinc, our cents contained no tin.

In 1947 we went back to the 95% copper, 3% tin, and 2% zinc coin metal for cents until the close of 1962.  The cost of tin became high then, so after 1962, all US cents were brass, not bronze.  (By definition, a binary alloy of copper and zinc is brass, a binary alloy of copper and tin is bronze, but a ternary alloy of the three is also bronze).  Until 1982, US coins were the 95/5 alloy, but the cost of copper became too high then.

Thus came the copper plated zinc cent.  We have had them since 1982 (that was a transition year, so some of both compositions were struck).  Since then, all of our cents are zinc, with a very thin electroplated pure copper shell so that they would look like the old ones.  But they corrode easily when the copper plating is damaged.  They are also much lighter than a copper cent, the former having a mass of only 2.5 grams, and the latter having one of 3.11 grams.

Current US cents are toxic because of the zinc content, especially if the copper coating is flawed.  Small children and dogs are at particular risk.  In the old days, the brass or bronze cents would usually just pass through the system with little harm, but the zinc ones dissolve and cause problems.  It is a good idea to keep coins off of the floor if you have little children or dogs.

There are lots of industrial uses for zinc.  One of the most important is using it as an anticorrosion material for mild steel.  I guess that I should explain mild steel first.

Iron with little carbon in it, usually with quite a bit of silicon and phosphorous, called wrought iron, is pretty corrosion resistant.  However, it is extremely expensive to produce, because it takes lots of manual labor to pound out impurities from it.  During the 1800s to the early 1900s it was the best.  However, when the Bessemer process was developed to produce steel, wrought iron became a dinosaur.  I would challenge anyone reading this to say with certainty that he or she had ever touched wrought iron.  By the way, “wrought iron” fences are actually cast iron ones, so they do not count.  The only wrought iron in them was the cables to connect them.

After Bessemer steel was used, it was found that it rusted very quickly in comparison with wrought iron.  Even the better, modern furnace steels corrode faster than wrought iron.  This is because of the intimate play of chemistry and physics between iron and carbon.  The excess of carbon makes steel, well, steel, but it makes it subject to corrosion.  This is where zinc comes into play.

Wrought iron is sort of hard to find, since it is only produced in small amounts.  However, it has many proprieties that make it be best of materials.  Mild steel, on the other hand, is produced in huge amounts, and it is a useful material.  But is rusts really fast.  If anyone wants to know the mechanism, please ask in a comment.

This is where zinc coating come to importance.  Mild steel will rust at a glance, but when coated with zinc, becomes extremely stable, even in thin sheets.  Most sheet steel is zinc coated, and instead of lasting five years, can last 25 years.  That is because of the battery that zinc and iron produce.  Zinc is more reactive than iron, so when zinc plated steel is exposed to water, the zinc corrodes before the iron.  This makes zinc a sacrificial coating, and a good and cheap one.  Zinc also has the advantage of forming a compact, tough oxide that tends to protect both the zinc and the steel under it.  Finally, it actually alloys with the iron at the surface of the sheet, so a chemical bond forms and holds the zinc onto the steel, rather than just being painted onto the metal.  Over half of the zinc produced goes into protective coatings for steel.

The second largest use of zinc is for diecasting.  Zinc alloys have excellent casting properties, and are strong enough for many uses.  Such alloys are called pot metal, diecast metal, and some other names.  Whilst they preserve the outline of the mold extremely well, they are prone to pitting under damp conditions and so are almost always coated, either with paint or powder coat, or with metal plate.  Diecast materials are often chromium plated for automotive use, but plastics have replaced a lot of diecast in recent years because of weight.

Brassmaking comes in third for zinc use.  Brass, as mentioned above, is an alloy of copper and zinc (other metals may also be added for specific purposes) and has excellent corrosion resistive properties.  Many plumbing fixtures are made of brass, either plated or not.  Brass is in many respects better than copper for such uses, since it is harder, tougher, and machines better.  It is also cheaper since zinc is cheaper than copper for a long shot.  However, it is not as corrosion resistant, so it is not used for pipes, just fittings.  A large amount of brass is used in decorative work, since it takes a high polish.  It tarnishes readily, however, so has to have a protective lacquer to keep its sheen.

A significant role of zinc is still for electrical production in batteries.  (Actually, we are talking about zinc-carbon cells, a battery being two or more cells used together.  Thus, a AAA cell is not a battery, but a 9V one is indeed a battery.)  Volta built the first battery out of zinc and copper discs, separated by paper wetted with brine.  Old fashioned dry cells are zinc and carbon, as are the alkaline ones, the difference being the solution that conducts the current, acidic in the old fashioned ones, alkaline in the modern ones.  (By the way, if you keep your unused dry cells in the freezer they will last just about forever until you use them.  The same goes for conventional photographic film).

Zinc is an important trace mineral, being part of many enzyme systems.  The RDA (recommended daily allowance) is around 15 milligrams (less for females, more for males), but there is some evidence that this may be somewhat low.  In addition to my food intake, I personally take 65 mg of zinc supplements.  Until more data are available, I think that much more than that is not safe.  There are fairly good data that zinc boosts the immune system, but that too much suppresses it, so I am taking the safe route.

In excess, zinc interferes with both the iron and the copper metabolism.  Certain denture adhesive manufacturers are now being sued because of high zinc levels in some of their products being linked to copper deficiency.  Also of note is the anecdotal evidence of zinc being of value in prevention of the common cold.  My personal feeling is that it is not specific for the cold, but rather its general immune system boost is responsible.  I do not recommend any zinc product to be swabbed in the nose.  Whilst the cold prevention effect is iffy at best, it is definitely established that zinc preparations can have a direct toxic effect on the neurons responsible for the sense of smell.  FDA forced a recall of the Zycam swabs because of this very issue.

UPDATE:  In the rush to finish up tonight (I was busy today) I forgot to mention the link between zinc and the decrease in prostate cancer in men.  Whilst the data are far from clear, it seems that there is a link between adequate zinc intake and prostate health.  I am not sure what the mechanism is, but have a hypothesis.  There exists a highly toxic and known to be carcinogenic metallic element called cadmium which has properties very similar to zinc.  My postulate is that if a man is deficient in zinc, the prostate (which concentrates zinc) may concentrate cadmium instead.  This is pretty much conjecture on my part, but I seem to recall come studies in men who were given zinc supplements and the cadmium levels in their seminal fluid peaked, then dropped off dramatically.  This is thought to be due to zinc taking its proper place in the prostate, displacing the toxic cadmium.  I must say that this is far from proven.

Well, there was a little history and discussion of zinc, an extremely important material.  And you have done it again!  You just wasted many einsteins of photons reading this tinny piece.  And even though Ann Coulter becomes non sarcastic when she reads me say it, I always learn much more than I could possibly hope to teach by writing these posts, so keep those comments, questions, corrections, tips, and recs coming.  Remember, no science or technology issue is off topic here.

Warmest regards,

Doc

Crossposted at Docudharma.com and at Dailykos.com