Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: thedoc on 19/10/2015 21:50:01
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Roger Wilkinson asked the Naked Scientists:
Hi. I have a question relating to our bath tub. Its made of enamelled steel. The enamel is getting quite thin through wear and what looks like some kind of erosion as there is a line where the water has regularly been filled to. The problem that we are experiencing is that for the last few months when sitting in the bath for a few minutes we are getting skin burns that leave us red and sore for a few days. Its almost a though there is some kind of electrolysis going on between the bath and our skin. We do not use strong chemicals to clean the bath. The bath taps are not on the bath but in the wall, The bath is electrically grounded to earth via earth bonding. and the waste pipe is plastic.
We do live in a chlorinated area.
Any ideas would be most welcome.
What do you think?
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Most interesting. Whereabouts are the burns? Contact with the bath, around the water surface, under the water level or above it?
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Might the "burns" be that your skin is coming into contact with the cracked area and getting "pinched" or scratched by the sharp enamel margin at the crack edge?
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Its almost a though there is some kind of electrolysis going on between the bath and our skin ... The bath taps are not on the bath but in the wall, The bath is electrically grounded to earth via earth bonding. and the waste pipe is plastic ...
So the metal-bath is not earthed via the waste-pipe or the taps.
If there is an earth-wire attached to the bath that could be a source of electricity if there is a wiring-fault.
I'd have an electrician check that the earth wiring is actually earthed,
and isn't carrying a current from a faulty electrical device in the home.
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Some people are very sensitive to metals like nickel, which can cause mild to severe skin irritation by contact. Nickel-plated jewelry, or zippers, or buttons, etc. can cause dermatitis ranging from an itchy red rash, all the way up to weeping open sores (depending on the extent of the contact, and the degree of sensitivity.)
Perhaps the metal part of the tub under the peeling enamel is interacting poorly with the skin...?
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Thanks everyone for your sugestions.
thedoc: the burns are on areas of the skin that are in direct contact with the bath and allways below the waterline.
chris: I like this idea. I will check out the earth loop impedence Ze and continuity of the bonding cable from the bath to the incomming earth, i will also check to see if there is any significant earth leakage from any of the equipment on the circuits. we have a 30ma RCD on the incomming supply so i doubt if there can be much but i guess a few millamps return to the bath could be enough to cause a reaction.
Thanks again everyone.
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Sorry. everyone i got all your names and sugestions mixed up.
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Did you have any joy with your investigations?
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Not much luck with the investigations im afraid. Turns out that the bath is bonded to the water pipes. The connection is good from the bath to the incomming earth (0.15 Ohms) and the Earth loop at board is 0.34 Ohms. There is no voltage between bath and Neutral / Earth. I also clamped the earth for leakage with most things in the house turned on. = (1.23ma)
So a bit stumped really. I think we will just go with a new bath anyway as it looks a bit tired. But it would have been nice just know what wa causing it.
Thanks
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Chorine is reactive with iron and steel. If this reaction forms ferric chloride, the iron will be looking for an extra electron to form ferrous chloride. This would be favored with traces of metallic iron and extra chlorine.
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Chorine is reactive with iron and steel. If this reaction forms ferric chloride, the iron will be looking for an extra electron to form ferrous chloride. This would be favored with traces of metallic iron and extra chlorine.
No, I don't think ferric chloride would be acting as an oxidizer under these conditions. If there is any trace of chlorine around (as invoked to form the FeCl3) it would react to oxidize any ferrous ions back up to ferric (and would probably oxidize anything that could reduce ferric ions to ferrous ions anyway). Also, assuming that the water is open to air, any dissolved ferrous salts would oxidize up to ferric ions by the action of oxygen.
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The burns are below the waterline, where the oxygen concentration is lower. Hot water makes the oxygen concentration even lower. Like you said, the chlorine will renew the ferrus chloride back to ferric chloride after each time it bites, until oxygen is able to diffuse under the water and form iron oxide rendering it neutral.
One thing I would add is connected to a diet rich in garlic. Garlic can cause sulfide to appear in one'ss sweat on the skin. Iron sulfide would be one reaction product, if we had ferric chloride forming in the bath.
This idea came from an article I remember reading about a person who made clay pots and noticed black spots forming on the finished pots. This turned out to be due to sulfide on their hands due to eating a diet rich in garlic; sweat. Clay has iron in it.
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How about the enamel having lower thermal-conductivity (https://en.wikipedia.org/wiki/Thermal_conductivity) than the metal of the bath, as an explanation ?.
The water & enamel & metal-bath are all at about the same temperature , but where the enamel is worn-thin the underlying metal is conducting the heat to your nether-regions more effectively than via normal-thickness enamel.
[ cf. skin can be damaged when touching cold-metal , (ice "burn") , but not by touching (say) wood or plastic which is at the same temperature as the cold-metal ].
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A neat idea but touching bare metal below 50 deg C won't produce a burn, and most baths are around 35 - 40 degrees.
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Chorine is reactive with iron and steel. If this reaction forms ferric chloride, the iron will be looking for an extra electron to form ferrous chloride. This would be favored with traces of metallic iron and extra chlorine.
You may be on to something with the chlorine, but I think the erosion of the enamel is a symptom rather than a compounding factor.
If the chlorine concentration were somehow too high, or if there were some other corrosive material in the water, that could be responsible for irritating the skin and damaging the tub?
Seasurf, there are paper test-strips that you can use to test the pH and oxidative content (chlorine or ozone or peroxide, or anything that oxidizes iodide to iodine) Perhaps you should draw a bath and test the water to make sure nothing is out of whack.
Are you on a municipal water system?
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... touching bare metal below 50 deg C won't produce a burn ...
According to chef Heston Blumenthal, 55oC can cook a sirloin steak medium-rare , although it will take five hours ... http://www.theguardian.com/lifeandstyle/2011/dec/14/slow-roasted-rib-of-beef-with-bone-marrow-sauce-recipe
[ If the burning sensation still occurs in tepid/cold baths that would exclude my explanation in "Reply#12" above ].
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Only if the steak is dead. Live animals have circulating blood that transfers heat away from a small hot spot. And saunas run at 80 to 90 deg C with very few casualties because the rate of heat transfer from still air or wood is very low. But nobody would tolerate getting into a bath much above 45 deg C, well below the safety level for bare metal.