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Zinc has bigger game than you do

February 14, 2012

 

Still struggling to pick out that one in a million Valentine’s Day gift? Worry no longer, because I have the perfect gift idea sure to make anyone melt. No – it’s not superhot plasma from your favorite star gone supernova (the Biebs?) – We’re talking zinc baby. Baby. Baby. Baby. Oh baby.

Zinc coatings on steel to be exact, through the process of “hot dip” galvanization.  The hot dip part is literal – pieces of steel are dunked in a bath of molten zinc metal and when they are removed a thin layer of zinc is atomically attached to the surface of the steel. The “galvanized” part belies the true function of the coating, considering this word is derived from a certain Italian who took liberties with dead frogs.

“Why bother?” Your significant other may ask.  “Here I had this nice piece of steel and now you’ve gone and hot dipped it. I think we’re at two different places in our relationship.” We’ve all been there.

The role of the zinc is to protect the iron in iron alloys (most importantly steel) from rusting. Like a proverbial knight in shining (preferably hot dipped) armor, zinc prevents the steel from rusting by selflessly doing all the rusting itself. As I describe it, this is starting to sound a lot less romantic and more like zinc has wandered into the friend-zone. Poor zinc.

If you read my last article, you know that natural corrosion resistance is rare among the elements, but ironically zinc protects steel by being less good at resisting the undesirable reactions with oxygen and water that cause the formation of rust on steel. A good analogy for this process is if both zinc and iron open lemonade stands on the same block and oxygen is the customer. Every time iron is trying to seal the deal zinc unleashes a humorous ad campaign and slashes their prices, so naturally oxygen buys lemonade from zinc instead. Now replace lemonade with electrons and you’ve got a half-decent understanding of how zinc does its job – as far as oxygen is concerned it came to this two bit lemonade town to buy lemonade from somebody (form an atomic bond), and zinc sure is looking pretty good right now. The only discrepancy with this analogy is that in the business world iron would have to resort to foul play and steal all of zinc’s lemons or deface their billboards or something. Probably shouldn’t write that on your next chemistry exam though. But don’t feel bad for zinc – the corrosion products of zinc with the environment (zinc oxide and zinc carbonate), actually prevent continued corrosion of the underlying zinc. This is called “passivation” which is the way that many other metals, like aluminum, avoid needing a hot dip.

So we’ve covered the “from the heart” part of a good V-day gift, but a box of galvanized nails might not send the right message to the apple of your eye. The good news is hot-dip galvanized steel looks really cool. The next time you’re passing a guard rail, street light pole or the like, you might notice a kind of patchwork series of flecks on the surface of the steel. No, that’s not art deco, those are the macro-sized crystalline grains of zinc busy oxidizing away. You can see some of these flecks inside the heart at the beginning of this article. In fact, all metals crystallize in this way, but because of how the zinc coating is applied the crystals of zinc that form on the surface of the steel are able to grow very large. If you were to take a high powered microscope and have a look at the surface of un-galvanized steel you would see a very similar grain structure but about 1000 times smaller. A good example is shown below from the Flikr photostream by the department of engineering at the University of Cambridge –  I admit I am a little jealous at all the cool science photos they have. Do yourself a favor and check them out.  The grains in this image are about 30-50 micrometers, which is about half the width of a human hair.

Normally the grain size in so called “engineering materials” like steel is intentionally kept very small because it improves strength. Zinc, whose only role is as a sacrificial coating, can flaunt its’ big ole’ grains. The only other example I can think of where crystal grains are visible like this is in polycrystalline silicon solar cells. The fact that all three of these materials have the same general appearance is not by chance, but more on that another time.

 

 

 

If love is sacrifice, then zinc has got it down. I will close with a poem that you can use with your gift:

Roses are red,

Rust is red too,

A coating of zinc,

says I – LOVE – YOU

 

And don’t I just.

 

– Gordon

 

Original image sources:

hot dip galvanized

grains of steel

 

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Gordon Waller
About Author

Gordon is pursuing his PhD in Materials Science and Engineering and conducts research on lithium batteries. He likes cooking, textbooks, internet memes and the great outdoors. His viewpoint on science is that everything is more interesting when you understand how it works, also being able to impress (read annoy) your friends with your techno-lingo is a major perk.

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.