We’ve all seen images of old, rusted shipwrecks lying on a deserted beach. We know that the ocean, plus a metal object, plus a bit of time, equals a damaged reddish-brown, rusted object. Yet, many treasure hunters have found almost perfectly preserved gold coins on the ocean floor. So if metal rusts and gold is a metal, how can this be?
Gold does not rust in salt water. Some metals, like iron and copper, will experience a chemical reaction to the salt water very quickly. Gold, on the other hand, does not go through the same chemical reaction and does not get flaky, reddish-brown spots, or turn completely green like other metals when exposed to salt water.
This article will explore why metals rust, how salt water speeds up this process, and why gold is not likewise affected.
Table of Contents
Why Salt Water Makes Metal Rust
Image by Lucas Gruwez via Unsplash
A metal, like iron, is, by its nature, very reactive and has to be exposed to a combination of water and oxygen for it to corrode. Corrosion happens through the electrochemical process of oxidation.
The metal does not need to be submerged in water for this to happen; simply being exposed to moisture, even if it is just in the air, is enough.
How Rust Happens
Corrosion, and specifically rust, happens to iron (Fe) when it is placed in an electrolyte — a medium that enables the flow of electrons. The iron is an anode that will lose electrons during a chemical reaction.
These electrons will go to the cathode, another piece of metal that is not as reactive or a different spot on the iron itself.
Water (H2O) is a good electrolyte, so when water comes into contact with iron, it gains the oxygen (O2) in water and loses electrons creating iron oxide.
The final formula for rust by itself will be Fe2O3H2O. The exact color of the rust will be determined by water’s nature that then hydrates this new chemical compound (source).
Salt Water Speeds Up the Process
Salt water, water containing sodium chloride (NaCl), is a better electrolyte than water without salt. The balance of chloride and oxygen in salt water determines the exact rate of corrosion.
A chloride content of 3.5%, which is the average salinity of our seawater, creates the perfect environment because oxygen solubility is at its best, and the water is very conductive, i.e., carries the electron from the anode to the cathode very well (source).
Why Gold Does Not Rust
For a metal to rust, it must contain iron. Without iron, rust (2Fe2O3) cannot be created from the combination of metal, water, and oxygen. However, other metals, like copper, silver, and gold, can corrode, which is a different process to rust.
Gold Properties
Gold is chemically unreactive. This means that it doesn’t react with oxygen, one of the most reactive elements, as iron does. This also means that gold doesn’t tarnish over time, as most other metals do.
Copper, for example, doesn’t rust, but it does tarnish to become a green or blue color if not properly cared for.
The chemically unreactive property of gold, as well as its malleability and pliability, its low maintenance, and the fact that it is a great conductor, has made it a highly sought-after metal for many hundreds of years, and it was probably one of the metals used by humans to create highly sought after decorative items.
Today, it is also used in electronics, dentistry, and aerospace industries.
However, gold is so soft, malleable, and pliable that, in its purest form, it can easily be scratched. It must be combined with other metals, like copper, silver, or zinc, to create an alloy, and only then can it be formed into a usable object like jewelry or coins (source).
Gold Can Corrode
Although gold cannot rust and is unreactive, it is not part of noble elements’ family, meaning noble gasses. Noble gasses, like helium, are so unreactive that they are used in everyday life because of their non-reactivity. Gold, on the other hand, does react to certain other elements.
The only acid that can dissolve gold is aqua regia. It is used to purify gold and remove it from substrates. This acid is, of course, hazardous and should be handled with great care. Fortunately, it cannot be stored as the chemicals in the acid oxidizes very quickly, making the acid useless (source).
Gold can also react with chlorine gas to create auric chloride. This compound has no commercial use and is mainly used as a catalyst in organic synthesis.
The conditions in which gold reacts to either chlorine gas or aqua regia is not something that you will come across at random, and, as a result, your pure fine gold should not be reacting, rusting, corroding, or tarnishing under normal circumstances.
When Gold Items Tarnish, Corrode, or Rust
Any commercial gold item — be it bars, jewelry, coins — can potentially become tarnished, lose their luster, or show signs of rusting. This is because of the nature of those items and the fact that, even when items are said to be pure gold, they are usually still alloyed with other metals to improve the strength of the item.
There are precautions you can take to always ensure that you are buying high-quality items.
Bullion
Gold bullion is the term given to gold bars, coins, and ingots that can be bought as an investment. If you are buying gold bars, for example, always buy from a reputable seller, and, even then, make sure to check the serial number, weight, and purity.
Gold bars
All gold bars over a certain weight should have a serial number that identifies that bar. If the seller cannot or does not want to provide it, it is most likely either not pure gold or potentially illegal or smuggled gold.
The weight and purity of your bar will determine the price. An investment bar should not have a purity lower than 99.5%. At this purity, the likelihood of seeing any tarnish on the gold bar is little to none.
Coins
Coins can be a bit of a different story. The two most popular gold coins, the American Eagle and the Krugerrand, both contain a fair amount of other metals. The American Eagle contains 3% silver and 5.33% copper. The Krugerrand contains 8.33% copper. The added metal makes the coin more durable.
Although the gold content is less, the coin’s value can be proportionally higher than a gold bar due to its collectability.
On the other hand, coins like this can become tarnished if they are not stored correctly, and rust-like spots can appear as the other metals in or on the coin interact with the environment.
When a coin becomes tarnished, or spots start to appear, it can be tempting to clean the coin. This is not recommended. Cleaning a gold coin can lead to the unwittingly damaging and devaluation of the item, so be sure to check out the
If one really wants to clean a gold coin, it is advised to allow an expert to perform the job and that you ensure the coin’s storage conditions are subsequently improved.
Jewelry
Gold jewelry can lose its luster and become damaged quite easily. As with bullion, this is because the gold is mixed with other metals or the piece is only gold plated.
Mixing the gold is especially important when making jewelry because the piece must cope with everyday wear and tear.
In jewelry, the composition of gold not only makes the piece more durable, but it also changes the color of the finished product. A mix of gold and copper, for example, creates a reddish gold, while gold and aluminum create a blue-tinged gold.
The purity of gold jewelry is measured in carats — the higher the carat, the purer the piece.
The mixed, or alloyed, gold in jewelry will dull or tarnish over time as it comes into contact with our skin and moisture in the environment. Further exposure to chemicals like chlorine in swimming pools, perfume, and hairspray will also have an effect.
This is even more true for gold-plated jewelry as the plating is incredibly thin, and it can flake as it becomes more and more damaged.
In general, when you notice that metal has dulled or discolored, it can be washed with some water and non-phosphate washing liquid. Be sure to rinse the soap off completely, and then dry and polish the item afterward as a buildup of soap suds can cause it further dull.
Final Thoughts
Gold has been a valuable commodity for thousands of years, and not only is it a beautiful metal, but it is also not susceptible to rust in any type of water or moisture. Unlike iron, gold doesn’t react to oxygen; in fact, it reacts to hardly anything.
This, and various other properties, make it an appealing metal to use in a number of industries. However, gold is also very soft and has to be combined with other metals into an alloy for its most common uses — namely, bullion and jewelry.
The gold alloy in gold coins and jewelry can be corroded, tarnished, or dulled over time as the other metals react to the environment around it. The correct storage and careful cleaning of gold items can help to mitigate this.