Vinegar is a commonly used household item that most people have in their pantries. It has many uses, from cooking to cleaning. One interesting reaction that vinegar can cause is turning metals black. If you’ve ever left a steel wool pad soaking in vinegar, you’ve probably noticed it turns dark and black after a while. But what causes this reaction? Let’s take a closer look at the science behind vinegar’s effect on metals.
Vinegar Contains Acetic Acid
The secret ingredient in vinegar that causes it to react with metals is acetic acid. Acetic acid is a weak organic acid that gives vinegar its sour taste and strong smell. Vinegar contains between 4-8% acetic acid by volume, with most table vinegars around 5-6%.
When acetic acid comes into contact with certain metals, it causes an oxidation reaction. Oxidation is when a chemical reaction adds oxygen to a material. In this case, the acetic acid oxidizes the surface of the metal, changing the metal’s chemical properties.
Acetic Acid Reacts with Iron in Steel
One of the most common metals that vinegar reacts with is iron. Iron is a major component of steel and other alloy metals. When acetic acid touches iron, the acetic acid oxidizes the iron (Fe) molecules to iron oxide (Fe2O3). This chemical reaction turns the metal surface black.
Iron oxide is better known by its common name, rust. As the acetic acid causes the iron to rust, it creates that black/dark gray color on the surface of the metal. This is the same process that causes iron and steel objects to rust when exposed to regular air and moisture over time. However, vinegar speeds this process up dramatically.
Other Metals that React with Vinegar
While steel is one of the most vinegar-reactive metals, other common metals will also oxidize to form black coatings when exposed to vinegar’s acetic acid. These include:
- Copper – Turns green before becoming black as the copper oxide layer thickens
- Bronze – Contains copper so also turns green then black
- Brass – Contains copper and zinc, creates a black/dark gray oxidation
- Pennies – Modern pennies are copper-plated zinc, turns green/blue from the copper
- Silver – Oxidation causes black silver sulfide coating
- Aluminum – Direct vinegar exposure creates a dark gray oxidized layer
So if you put any steel wool, copper pennies, silver jewelry, or other metal items in vinegar, expect them to become tarnished or dark in color. The length of time the blackening takes depends on the type of metal, its smoothness, and how concentrated the vinegar is.
Using Vinegar and Hydrogen Peroxide to Blacken Steel
While vinegar by itself will darken steel over time, there is a more effective DIY solution that speeds the process up. The combination of vinegar and hydrogen peroxide rapidly blackens steel objects. Follow these steps:
- Get a container such as a plastic bin or jar. It needs to be big enough to hold the steel objects.
- Fill the container with undiluted white vinegar.
- Add 1-2 squirts of 3% hydrogen peroxide solution to the vinegar.
- Place your steel wool, nails, knives, etc in the solution and wait at least 20-30 minutes. Longer times will create a darker black finish.
- Remove steel from the mixture and rinse with water. Dry completely. The black color may continue to develop as the iron oxidizes further when exposed to air.
The chemical reaction between the vinegar, peroxide, and steel is rapid. As the acetic acid and peroxide interact, they dramatically speed the oxidization effects on the iron in the steel. This quick home-solution lets you intentionally blacken steel objects for various effects.
Using Blackened Steel
Here are some ways people utilize blackened steel produced through vinegar treatments:
- Distressed decorative objects – Creates an antique forged metal look
- Arts and crafts – Blackens steel wire for sculpture, beads, jewelry
- Woodburning tips – Oxidized steel wool creates darker burn marks on wood
- Gun bluing – Used by gunsmiths to blacken gun barrels and components
- Knives – Can create blackened engraved knife blades
- Home decor – Blackens hardware like hinges, handles, hooks
The vinegar and peroxide solution works particularly well for staining and aging steel wool for craft purposes. The wool readily soaks up the mixture and transforms into a deep black color.
How to Stop Vinegar from Blackening Metals
While the oxidation reaction can be useful in some cases, other times you want to prevent your metal from tarnishing. Here are some ways to stop vinegar from reacting with metals:
- Rinse immediately – If vinegar spills onto a metal surface, quickly rinse it under water to dilute the acetic acid.
- Dry thoroughly – Ensure no vinegar remains on the surface where it can continue reacting.
- Use non-metal containers – Store vinegar in plastic or glass to prevent corrosion of metal lids/caps.
- Coat in oil – Covering steel in mineral oil or beeswax prevents oxidation.
- Seal with varnish – Applying polyurethane or lacquer seals out moisture and vinegar.
- Protect with plastic – Cover susceptible metals with plastic wrap or bags if storing in vinegar.
With prompt cleanup and proper storage, it’s possible to use vinegar for household cleaning and cooking without worrying about unwanted blackening of your stainless steel pots and utensils.
The Chemistry Behind Vinegar as a Rust Accelerant
Now that we’ve covered what metals vinegar blackens and how to use it or prevent it, let’s take a closer look at the chemical reactions involved. Understanding the chemistry provides deeper insight into why this common household ingredient can so effectively corrode metals.
Acetic Acid Structure
Acetic acid (CH3COOH) is classified as a weak organic acid. This means it only partially dissociates into ions when dissolved in water. The molecular structure of acetic acid consists of:
- Methyl group (CH3)
- Carbonyl group (C=O)
- Hydroxyl group (O-H)
The hydroxyl hydrogen easily dissociates as an H+ ion, contributing to the acidic abilities of acetic acid. However, acetic acid is not as strong of an acid as hydrochloric or sulfuric acid. But it is the combination of acetic acid molecules and H+ ions that allow vinegar to corrode metals.
Redox Reactions
The chemical process of vinegar oxidizing metals involves redox reactions. Redox stands for reduction (electron gain) and oxidation (electron loss). In vinegar’s reaction with iron, here is what happens:
- Iron atoms lose electrons (oxidation)
- Acetic acid gains electrons (reduction)
This electron transfer converts the metal into a positively charged ionic form. For iron, solid metal Fe transitions to aqueous Fe2+ ions. The ionic iron can then react with oxygen and water molecules in the vinegar to produce iron oxide hydrates like Fe(OH)2 and Fe(OH)3 which appear as black rust.
Meanwhile, the acetic acid acts as a reducing agent, gaining electrons to become acetate ions. So vinegar efficiently causes oxidation of metals through its chemical reducing power.
Acid Dissolution
In addition to redox reactions, acetic acid also reacts with some metals through an acid dissolution mechanism. Many base metals will dissolve in the presence of acids to form salts.
For example, acetic acid can convert copper metal to copper acetate:
Cu(s) + 2CH3COOH(aq) → Cu(CH3COO)2(aq) + H2(g)
This reaction causes copper and bronze objects to develop the characteristic green patina when exposed to vinegar. Over time, the oxidation builds up into a black coating.
Vinegar’s acidic level is not high enough to dissolve iron. But acid dissolution is an important secondary mechanism contributing to its ability to corrode metals.
Conclusion
Vinegar is an effective tarnish remover, but it can also intentionally blacken metals through oxidation. Its acetic acid rapidly reacts with iron, copper, silver and other metals by triggering redox reactions. Understanding the chemical processes involved provides insight into how best to avoid or enhance vinegar’s effects on metal surfaces.
So next time you see a rusty penny or some darkened steel wool in a bottle of household vinegar, you’ll know the science behind the reaction. Vinegar’s acidic power makes it a useful rust accelerator for intentionally distressing and aging projects with an antique forged metal look.