We all know that STEEL WILL RUST, especially when chlorine-treated water is around – chlorine is very corrosive. The reason stainless steel WON’T RUST is because of its chromium (Cr) content – nickel (Ni) content just makes it stronger. Cr, however, when being exposed to dry oxygen in the air, will form a thin and extremely compact oxide film (Cr2O3) at the surface which prevents anything underneath from corroding further. One can say that it forms a protective skin. That is the trick of stainless steel!
That is why you hear 18/10 is mentioned beside the 18 gauge when you are introduced to a particular brand of sinks. 18/10 simply means that the steel used to fabricate sinks contains 18% Cr and 10% Ni while 18 gauge indicates the thickness of the sheet that is used to fabricate the sink. 18/10 steel is also referred to by professionals as T304 steel which is, in an easier-to-understand term, surgical grade.
So because of its Cr content, T304 steel won’t corrode, period. In fact, for this reason, most sink manufactures and suppliers can offer limited life time warranty on their stainless steel products!
But why do people sometime find stains on it? Some stains look just like rust, especially on a new sink? Even Kindred Canada website has this sort of question on their FAQ page. There is also this hot topic entitled “How to Remove a Stain from a Stainless Sink” on this Finishing.com website that mentions Franke (Kindred’s top of the line) sinks having stains caused by various reasons. A person posted on this builder’s website mentioned a brand new Kohler sink (model K-3145) had stains left by liquid soap in just about a week or two!
Now we need to discuss the real possibility that will make SS sink rust on surface. Note that we are NOT talking about the rust stains left by a cast iron pan that are discussed on this page. We are NOT talking about stains that were caused by various hush chemicals as mentioned on this page either. We are talking a real chance that the sink surface that is made of the T304 steel will rust. Read on…
Like we mentioned above, one obvious possibility is the chlorine in the tap water. In North America, almost all cities choose to treat the water with chlorine by killing parasites, bacteria, algae, viruses and fungi. We know chlorine is highly oxidative and that is why it is so effective in killing biological contaminants. It is so corrosive that it can penetrate the thin Cr2O3 protecting film that has not yet finished forming. The result? Chlorine can react with iron to form iron chloride (FeCl3) which is brown in color. This can happen, especially when chlorine concentration is high and when you did not wipe your new SS sink dry each time for the first few weeks.
Secondly, the surface of any new sink is very chemically ACTIVE. It takes some time, usually a few weeks, before a relatively thick oxide protecting film can be formed. Before this happens, many paste-like stuffs can stick onto the new sink surface and become relatively hard to remove when you leave it there dry. Like we mentioned above, even liquid soap was reported to have left marks on a brand-new Kohler sink. Generally speaking, anything that is chemically basic (as opposed to acidic) tends to stick onto the new sink surface. That is why we can find that we can easily remove stains on new sinks with diluted vinegar. Baking soda will make it worse as it is basic in nature.
The reason that the new sink surface is active is because of the polishing processes involved. You see, most sinks have a finish that requires a polishing process which involves using an abrasive material in steps of grits, just like in the case of granite polishing, e.g. #30, #60, #120, #220… up to #600. The brushed finish is less polished comparing to the standard satin finish. Because this stainless steel surface is polished, some steel particles are left partly attached to the surface.
Note that the post-polishing rinsing process will only remove those dislodged and loose particles, by not those that are still part of a roughened surface. Due to their extremely small sizes, these tiny partly attached particles prevent the sink surface from forming a compact protective Cr oxide film. Moreover, these particles are more chemically active than the steel substrate. For example, they can react with acetone – which is used by most granite countertop installers to clean leftovers of the silicone glue that is used to fasten the sink underneath countertops (a less active solvent such as alcohol is better than acetone). So this roughed surface is more likely to react with the oxidizing component in the water such as chlorine.
How to reduce the chance for this to take place?
The best way is to clean your sink thoroughly and wipe it DRY. Without water, no rusting whatsoever can happen any more on most metallic surfaces. So leaving a cup (even made of porcelain or glass) overnight in the sink when the sink is wet is not a good idea because the cup bottom will trap some water underneath and increases the chance of its reaction with those tiny iron particles. So KEEPING IT DRY is the most important and easiest care you can implement for your stainless steel sink, especially if it is a new sink.
This is why the stain marks usually occur to new sinks when these partly attached steel particles are both fresh and more in quantities; with time, they will eventually be worn away. This is why it is a good idea to wipe your sink along the polished veins to remove these particles effectively.
However, there is another chance you can get rust on your sink surface… by leaving in it a knife, spoon or a set of forks that have already lost its protective chrome coating. In this case, the copper or other less reactive metals underneath the chrome coating plus iron will form many microscopic cells, like batteries, in the presence of tab water. This is called Galvanic corrosion and will make those steel particles even more reactive. So do not leave those objects wet in your stainless steel sink for overnight. You can submerse them UNDER the water using a closed strainer. This slows down the Galvanic corrosion. Do you know that corrosion of a steel boat occurs more near the water surface than under the water? More oxygen content in shallow water speeds up the corrosion process.
Note that this surface discoloration problem will eventually go away by itself in about two our three months. If you wipe your sink dry after each use, this thing will not happen at all. After a few months have passed, this will not happen even when you forget to wipe the sink dry. The reason is, most of the particles are gone and a smoother and more compact oxide film has been fully formed to protect the surface.
Lastly, as other sources point out, do not leave in sink any objects that will corrode by itself, especially when there is water around. Remember our above advice to take care of your stainless steel sink? KEEP IT DRY.
In our own control tests, we left a dinner knife (did not have any visible wear on it) and a iron nail (already rusted) in a stainless steel double sink overnight. We added a bit of tap water to make both wet. The next day we came back to find that the dinner knife did not leave any stain while the rust from the nail got into the veins of the polished lines! The rust simply got stuck in there. We could not even remove it by rubbing with a soft pad followed by rinsing with water. We had to use vinegar to remove it 🙁 Remember, this was not the rust from the sink – this was the rust from the rusting nail!
Note: if you think vinegar is too strong for the stains in your situation, a safer way is to use Windex which says on the label that it works for cleaning ceramic and stainless steel surfaces.
So we hope this article will help you take a good care of your stainless steel sink. Cheers!
This article is written by Aubrey Jun Zhang, who obtained a Ph.D. degree in Electrochemistry (a branch of Physical Chemistry) at the University of Calgary in 1994. His Ph.D. dissertation was entitled “Oxide Film Studies at Pd and Pd-containing Amorphous Alloy Electrodes.”