I'd be cautious with prolonged exposure to bleach on stainless steel - strong bleach solutions are corrosive on stainless, IMHO.
Thought I would provide more complete information on the chemistry of stainless steel as it relates to homebrewers. Aiden's post on not using Chlorine Bleach with stainless steel kegs was not Wrong, but it can be used safely if the parameters are understood. Here are the parameters.
Corrosion of 300 series Stainless Steel
The 300 series austenitic Stainless Steels commonly used for kegs and cooking equipment are very acid resistant. The steel receives its protection from a passive oxide layer that forms on the surface when exposed to air (oxygen). Because acids are oxidizing, acids promote this protective layer. The oxide layer is primarily composed of Chromium oxides with Nickel oxide contributing.
Q: Why doesn't this work for aluminum?
A: The aluminum oxides are porous to do not prevent exposure of the base metal to the corrosive media. Anodizing is an electro-chemical process that builds up the surface oxide, making it thicker and non-porous. Calphilon Cookware is anodized aluminum.
Okay, back to Stainless. The corrosion resistance depends on the chromium. Austenitic stainless is a super saturated solution of chromium and nickel in iron. It is actually a very high temperature phase (arrangement) that has been frozen to preserve the distribution of elements. Austenitic stainless does not like heat. It performs well up to 600F, but higher temperatures cause atom diffusion which causes the properties to change. Compare it to Ice Cream. If you let Ice Cream get soft, then stick it back in the freezer, it will stay much the same. But if you let it melt completely and stick that back in the freezer, you get a frozen puddle which is no longer what you consider Ice Cream to be.
Welding is a local melting/freezing process whereby atom diffusion can take place if it is hot enough, long enough. There are time/temperature curves that describe this, but don't lend themselves to ascii. Suffice to say that for Type 304 stainless, five minutes at 600F or above will cause chromium diffusion that will later cause cracking in service. Exposure to the temperatures that cause diffusion is termed as being Sensitized. Type 304L stainless, L meaning less carbon, is more weldable and can spend
almost 20 hours above 600F before becoming Sensitized. Most kegs are made from 304L to facilitate the welded construction. What this means to brewers that use high BTU propane burners is that care must be taken to not overheat the kegs while boiling or this sensitization will occur over time. Diffusion is cumulative.
Okay, back to corrosion. The corrosion inhibitor is the passive oxide layer that protects the surface. All corrosion is basically galvanic. The electrochemical difference between a metal and a non-metal or two metals in an electrolyte causes electrons to flow and ions to be created. These ions combine with oxygen or other elements to create corrosion products.
Now lets say we already have an electrolytic solution containing ions, bleach water. These chlorides are caustic or alkaline and cause the protective oxide layer to deteriorate. If a stainless steel container is completely full of this electrolyte, every surface is at the same electrical potential and nothing happens. But what if there were a deep scratch in the wall, or a rubber gasket against the steel creating a crevice? Well, these areas can become electrically different from the surrounding area and a galvanic cell can be created. Inside the crevice, on a microscopic
scale, the chlorine ions can combine with the oxygen, both in the water and on the steel surface, to form chlorite ions, thus depleting that local area of oxygen. If the bleach water is still, no circulation, then that crevice becomes a tiny highly active site relative to the more passive stainless steel around it and corrodes. This is known as Crevice Corrosion. The same thing can happen at the surface if the keg is only half full. In this case, the steel above the waterline is in air and the passive oxide layer is stable. Beneath the surface, the oxide layer is at a
different potential and less stable due to the chloride ions. Now the crevice is represented by the waterline. Stable area above, less stable but very large area below, crevice corrosion occurs at the waterline. Usually this type of corrosion will manifest as pitting or pinholes. The mechanism described is accelerated by localization so a pit is most often the result.
A third way that chlorides can cause corrosion of stainless is by concentration. This mode is very similar to the crevice mode described above. By allowing chlorinated water to evaporate and dry on a steel surface, those chlorides become concentrated and change the electrical potential of the surface at that site. The next time the surface is wetted, corrosion will immediately take place, creating a shallow pit. The next time the keg is allowed to dry, that pit will probably be one of the last sites to evaporate, causing chloride concentration again. At some point in the cycle life of the keg, that site will become deep enough for crevice
corrosion to take over and the pit will corrode through.
Given the above information, knowing our enemy as it were, we can develop usage practices to ensure that our precious stainless is not attacked and pitted by the use of bleach as a sanitizer.
1. Do not allow the stainless steel vessel to sit for extended periods of time (hours, days) filled with chlorine water.
2. Fill the vessel completely so that all surfaces are at the same potential.
3. Swirl the water once in a while to eliminate local concentration/ de-oxidation.
4. After the 20-30 minute soak for sanitizing, rinse the keg with preboiled water to prevent evaporation concentration and either dry it completely with a towel (if storing) or fill it with beer.
These simple practices will preclude chlorine induced corrosion.
John Palmer - Metallurgist for International Space Station Alpha McDonnell Douglas Aerospace, Huntington Beach, CA
Cheers,
TL