We have been noticing black specks that leave a greasy trail when smeared in our bathroom sinks and on dishes washed in the dishwasher in recent weeks. After some research, I find that the 2 main reasons for this are 1) a deteriorating black rubber hose to/from the water heater, and 2) a deteriorating magnesium anode rod in the water heater. I pried open a bit of insulation on the pipes to/from water heater and found that they are either copper or solid plastic pipes. That leaves reason (2). How do I determine if the anode rod in my water heater is magnesium (as opposed to aluminum) without calling a professional? Is it typically written somewhere on the heater? Our heater was replaced in the past 3 years. I'm attaching 2 pictures below showing parts of the top of the heater, one of which seems to be the head of the anode rod (I don't know which one).

This part is close to the center of the heater This part is close to the edge of the heater

Also, are there any other major causes for the black stuff that I should be looking out for? I find that decaying black rubber hoses is another reason, but the hoses to the sinks in our house look like the picture below, which does not seem to be the flexible black rubber hose that decays. Can anyone confirm?

enter image description here

Yet another possible reason I find is the decaying inner lining of the water heater expansion tank. Below is a pic of the label on the expansion tank, which I believe was replaced when the heater was replaced in Dec 2017. Does it looks like the kind that has an inner lining that decays quickly?

enter image description here


2 Answers 2


Water heater anodes usually last about 3 to 5 years or so depending on the chemistry and hardness of your water, so chances are, if your water heater is only 3 years old, that's not the problem. That being said, it wouldn't hurt to take a peek at the anode and make sure it's doing OK.

The top of the anode rod is typically visible on the top of your water heater. It will look kind of like a bolt head on the top of the heater. You should be able to loosen it with a wrench and extract the anode to inspect it and replace it. If you don't see one of those on the top of the heater, the anode might be integrated with the hot water outlet pipe. You can disconnect the pipe and extract the anode that way.

Anode location - https://www.plumbingsupply.com/understanding-water-heater-anode-rods.html

From what I know, magnesium anode rods have a "bump" in the center of the cover nut while aluminum/zinc rods have smooth-faced nuts. I don't know if this is a "universal thing" or it varies from manufacturer to manufacturer, but that has been my experience.


Aluminum/Zinc Anode Rod End


Magnesium Anode Rod End

Most water heaters have aluminum anodes installed by default from the factory, but it sounds like you may have a magnesium rod and high sulfates in your water. That combination can cause hydrogen sulfides to form as the anode reacts with the water and produces black, oily particles to settle inside the water tank and float through the plumbing in one degree or another.

You might be able to get rid of the black specks by draining or flushing out your water heater (there should be a drain tap a the bottom of the tank that you can hook a garden hose up to) and replacing your magnesium anode with an aluminum or aluminum/zinc anode rod.

  • Thanks for the detailed answer. I updated the question with pics attached. Seems to me that the first pic I attached is the anode rod, and based on your answer, it's likely aluminum, correct? Commented Feb 14, 2020 at 15:56
  • 1
    It's likely aluminum but I'm not sure if that yellow dot takes the place of the "bump" on the nut to indicate that it's magnesium, or if it's something else entirely. At this point, I'd try flushing out the water heater tank and check to see if there's black gunk settling in the bottom. That could tell you if the heater tank is the source, or if the black stuff is coming from somewhere else in the system. (continued..)
    – gnicko
    Commented Feb 14, 2020 at 17:20
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    The relief tank says it has a polypropylene liner, so I wouldn't expect that to go bad in three years. But then you say the black gunk is oily and not like bits of plastic, so it could be some kind of organic matter "growing" in there somewhere. Also, I'd take the anode out of the water heater, while you're there, and get a visual on what's going on with it. Look for excessive corrosion and maybe black gunk on the bottom of it.
    – gnicko
    Commented Feb 14, 2020 at 17:22
  • 1
    Another thought: The "hoses" that you're talking about are likely old braided stainless steel sheathed connecting hoses. It could be that your dishwasher is connected with one of those or the water heater is connected to the rigid plumbing with a flexible connector. Are you seeing the black bits everywhere or just specific locations in the house?
    – gnicko
    Commented Feb 14, 2020 at 17:29
  • 1
    2 of 3 is kind of odd. I'd expect all 3, and the kitchen and laundry sink, outdoor hose bibs, etc. Are all the connections to the affected sinks, bathtubs, dishwasher the white Pex/pvc tubing? Do you have anything that looks like this: image.made-in-china.com/2f0j00SBvaFKOGSNrh/… ...maybe connecting the water hearter to the plumbing system, or at the affected locations? Older versions of those flexible hoses sometimes had rubber/plastic lining that could deteriorate under certain conditions....
    – gnicko
    Commented Feb 14, 2020 at 21:48

A black oily substance in sinks can be caused by hard/caustic water eating away at rubber compounds (washers, gaskets, etc.) in the domestic water system. This could possibly be caused by an anode rod that has been eaten away resulting in higher concentrations of corrosives in the water. However, that's unlikey after only three years but it's not impossible. It's not too difficult to check. Just follow the directions in your user manual or look for instructions on-line. They are usually very accessible from the top of your water heater. But make sure to follow instructions on shutting down the water heater and turning off the water as well as depressurizing the tank.
A more likely scenario is very hard water eating away at those rubber fittings in your faucet and dishwasher. A water softener will probably solve that for you. In the short-term check the washers and O rings in your faucets. Replacing them might buy you some time.

  • Thanks! I updated the question with a pic of the rubber hoses under our sinks. They don't look like the kid that decays causing black specks, correct? Commented Feb 14, 2020 at 15:57
  • Not sure about the lining in the hose but the exterior isn't rubber.
    – HoneyDo
    Commented Feb 17, 2020 at 21:19

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