Most stains on plumbing fixtures are of the dingy or yellow-brown variety. Just as common are the darker and more dramatic stains rust leaves behind. But have you ever had blue or green staining on fixtures? It happens! But why? And what can you do about it?
What Is It?
With your typical (non-blue/green) stain, the cause is often as simple as what's in the water: minerals, sediment and even treatment chemicals can stain on their own or through interaction with pipes and fittings, resulting in annoying (but usually harmless) stains on amenable surfaces.
Note: Most rust stains fall in the harmless category (being the result of high iron levels in the water supply, often from wells), but if your home has any galvanized steel pipes you should investigate further: it's possible the pipes are corroding, which will end in a leak.
Blue or green staining occurs exclusively with copper pipe and fittings (including brass, which is copper + zinc), and is caused by the corrosion and dissolution of the metal itself. Water quality and even improper electrical grounding hasten corrosion, adding excessive amounts of dissolved copper to the water coming out of fixtures. When the copper-laden water is allowed to sit, it stains. Unsurprisingly, the water may also have a metallic taste to it. Because this staining is the result of the pipe itself dissolving, it's far from harmless.
The science of plumbing... The blue/green color of a copper stain is the result of a chemical reaction that creates a copper hydroxide/carbonate. Just as iron turns orange when rusted (Iron oxide), corroded copper will be a blue/green; take a look at the Statue of Liberty or an old penny. In the absence of a driver like acidic water or electricity, a layer of corroded, oxidized copper will actually adhere to the copper's surface, creating the patina or "living finish" that makes copper pieces so popular. This patina also reduces further corrosion inside copper pipe.
How It Happens
The excessive corrosion leading to copper staining has two primary causes: acidic water or stray electrical current running through the plumbing system. The latter is rare, but possible when an older home's plumbing is incorrectly used as a ground for the home's electrical system - or when DC current from a mass transit power system finds its way there. Either case can result in an electrochemical process called electrolysis, which accelerates metal corrosion. It can be difficult to determine if electrolysis is behind copper leaks and staining, but because it is uncommon, you can hold off calling the electrician until other possibilities have been ruled out. However, if you experience other electrical issues or get a shock touching pipes, that should clearly be addressed immediately.
Pro Tip: Closely related to electrolysis is galvanic corrosion - the accelerated corrosion of one metal when two different metals are joined. This is especially common with copper-galvanized steel connections. Although copper is less reactive in this situation, it does still corrode (though at a slower rate than the steel), and potentially enough to cause staining. The galvanized steel section will begin to leak and eventually fail. The solution - as prescribed by most plumbing codes - is to install a dielectric union between the metals to eliminate direct contact.
Acidic water (< 7pH) is the most common cause of copper staining. The dissolution of copper (cuprosolvency) really kicks off at about 6.8 pH, and is further enhanced when the water supply is high in dissolved oxygen, carbon dioxide or other gases. As the acidic water flows through the copper supply pipes, small amounts of metal are eaten away and dissolved into the water. Since we don't usually dry off every water-using fixture after we're done with it, the water sits and eventually stains the right surface... and unfortunately, most fixture surfaces are vulnerable.
Other water chemistries - like excessive bicarbonates, chlorination combined with alkaline water (with a pH above 7), or overly "soft" water - can also be behind copper staining. This is especially so in new homes where the protective patina has not yet formed on pipe interiors; dormant and lesser-used homes will suffer from these kinds of aggressive water chemistries as well. Bad installation is another not-uncommon cause: insoluble flux (now largely prohibited by code) left on or in pipe will eat away at it, due to its acidic nature.
Stopping the Problem(s)
There are several possible causes behind copper staining, and the only way to figure out what's going on is to have the water professionally tested. It's entirely possible that your own staining problem has no one particular cause, making it even more important to have a comprehensive overview of your water quality. Water treatment professionals can recommend the best solutions for your particular situation.
In general, acidic water can be made less so with the installation of an acid neutralizer. These devices use minerals to raise the water's pH closer to neutral. For extremely acidic water (< 5.5 pH), water treatment professionals use a more advanced system to directly feed treatment chemicals into the supply.
Note: Acid neutralizers will make your water "harder", but to what extent depends on many factors. Should the newly-hard water start to affect fixtures or skin/hair, you might also need to invest in a water softener.
Removing the Stains
Correcting the copper corrosion at the source is obviously the most important thing to do, but it doesn't help with the stains that are already there. What you can safely use depends on the material of the fixture affected; our basic advice is to start with something "gentle" and work your way up to heavier-duty cleaners and abrasives only if called for. This can help to avoid unnecessary damage to finishes. The "homemade" cleaners below have been reported to work well for copper stains.
For reference, consult our guides for cleaning porcelain, stainless steel, and even copper itself.
White vinegar is rightly touted as an excellent cleaning tool, and with the addition of salt, may be able to get rid of the blue/green stains. Try dissolving a tablespoon of salt in a cup of warm vinegar, soaking a rag with the solution, and letting it sit on the stain. The stain may begin to lift immediately - if not, try lightly scrubbing with the rag, and repeat the process if necessary. Because vinegar is a relatively weak acid and the salt is dissolved, this should be safe for any fixture material.
Another relatively safe option for most materials is to use lemon juice (or vinegar) and cream of tartar to create a mildly acidic paste that may also lift copper stains with some light scrubbing. Going up a level, you can also try using an ammonia and water solution (following the manufacturer's dilution guidelines) along with a bit of dish soap. Baking soda can be used as a mild abrasive to help with stubborn stains. Scrub with a rag or non-abrasive sponge.
If these aren't doing the trick, some have seen success using toilet cleaners containing sodium bisulfate. Before using any cleaning solution - but particularly harsh chemical cleaners - we highly recommend a spot-check in an inconspicuous area of the fixture to make sure you aren't trading one blight for another.
