It seems that heat and the moisture in a barbeque turns stainless steel to junk. Last year I paid nearly $40.00 for one of those flavour grids, made out of SS, that replace the ceramic coals on a grill. Today I found a rusty , flaking piece of what looked like mild steel. What gives.??
It's probably made of the same stainless that my rusty stainless steel rustproof shower shelf is made of.
I have no scientific explanation for it, but it's perfectly normal. I'd venture a guess that the stainless, having been heated above a red heat for lengthy periods of time, is oxidized such that the protective value of the stainless alloys is lost. What we need is a chemist or metallurgist to explain the reaction for us.
Harold
he probably had chromed steel, not stainless. I have a stainless grill and there is not a single trace of rust.
i
There are several different grades of stainless. Use a magnet on it. If the magnet sticks then it will probably rust.
From:
Lane
The amount of moisture to which you grill is exposed plays a big role. I had a stainless burner burn out completely while living in Utah, where it's not known for high humidity. Takes a few years, but it happens. Likely a response to products of combustion------acid rain?
I agree, it's possible the component in question could be plated, however.
Not all stainless alloys lend themselves to being heated repeatedly. In fact, hardly any of them do. If you get them hot enough long enough, they all break down if exposed to oxygen, which they are.
Harold
Found this on the sci.engr.metallurgy NG via a Google search.
------------------ "Chrome migration occurs when you overheat stainless steel or locally heat it at high temperatures for extended periods of time. Again, if the chrome in the surface layer falls below 12% you will see corrosion occuring with time."
------------------ Although he didn't say what temperature is "high temperatures", he war responding to rust at the HAZ location of a weld. Lane
Why -
We all know stainless rusts. Some like 316 doesn't because of the special alloy. Some are rust resistant, not rust proof.
Even cooking pan stainless has problems with some acids - Tomato for example.
Some are lab grade safe, some are air safe, some are water safe, some are salt water safe.
I have SS rusting in the shop, several kinds, and two kinds that are not.
Consider the Barbie - Very hot - Chemical reactions are elevated under heat.
Consider the hood of the Barbie traps moisture that gets in when it rains. Chemistry starts.
Martin
salt water safe.
The harsh reality is that all 300 series stainless has the potential to rusts to some degree unless it has been passivated. One of the certified MIL processes is to soak the material in a heated solution of nitric acid, distilled water and potassium (di)chromate for a given period of time, which removes the free iron. It is the free iron that rusts. The rusting, unless accelerated by heat erosion, tends to stop on the surface. The treatment apparently has no effect on finished diameters. High precision parts are subjected to the treatment without making any allowances.
My experience in the aero-space and defense industry taught me that (machined) stainless is almost always passivated to prevent surface rusting, even when the components may not be subjected to a nasty environment.
It's not uncommon to see surface rusting on old stock stainless sheet or bar that has been exposed to the elements. Often it's where the stock has been in contact with ferrous materials, likely leaving an excess on the surface from the contact.
Harold
Pretty much any stock book worth its weight discusses the typically used (300 series) stainless and the intergranular corrosion that occurs between the temperatures of 800 and 1500 degrees F. It doesn't take extreme temperatures to affect the material.
Certain alloys have been formulated to overcome the problem, but are highly unlikely to be used in utilitarian objects. Two that come to mind are 321 and 347.
Harold
Is this passivating process something that could easily be done in a home shop?
- Michael
Use Google to search on Sci.engr.joining.welding. Citric acid seems to work for passivating and is not too dangerous.
Dan
It is, but you'd likely have a hard time buying the nitric acid and the potassium dichromate. Chemicals are quite difficult to buy now, and even more difficult to dispose of. It was simple when I did it way back in the mid 70's. It was by rude awakening that I realized that what I was doing was passivating when I processed some components for an aerial surveillance camera developing unit many years ago (I still have some of the extra components as a memento of the job).
Needless to say, this was a defense job, which normally would have specified a MIL spec with reference to passivation. Instead, the print had specific requirements for the parts to be processed as I had mentioned in my previous post. I dutifully did exactly as the print specified, giving no thought to what I was doing. It was well after the fact that I realized that I had passivated the parts, nothing more. My biggest regret was not having made an exact record of the components and process. I could have used in on many occasions.
For me, it was no chore to do the process, because at that time I was refining gold as a hobby, so I had a fume hood and all the necessary chemicals at hand to do the job. Handling nitric acid can be challenging. It is quite hard on flesh.
Harold
Which brings up the question. What does Nitric acid really do to SS.
What I have seen on some Railroad rail stock was rust nicely on the outside. A carbide cutting tool running red hot and throwing fire and light with blue chips tend to ?? burn the carbon off the surface area ?? and make it much less liable to rust. Is the Nitric pasavator(sp) doing something to the carbon or what does Fe use as a catalyst to rust ?
Martin
potential to
nitric acid,
rusting,
allowances.
surface rusting,
environment.
Well, from my Elements of Materials Science Book (first time I've looked at it in years!), chromium combines with oxygen and forms an adsorbed ionic layer to isolate the surface:
Cr + 2O2 + 2e- = (CrO4) --
Hope that clarifies things : )
surface.
Quoting further from the same source:
"A chrominum-bearing steel is quite resistant to corrosion under an oxidizing condition; however, in the absence of oxygen, the reaction
Cr = Cr++ + 2e-
is free to proceed. Hence we find that those steels which are passive in the presence of oxygen or oxidizing acids such as HNO3 or H2SO4 become active in the presence of HCl, HF, or other oxygen-free acids. A steel is therefore placed within a galvanic series of alloys at one position or another depending on the oxidation level of the electrolyte"
Good call about the "green rot" of many 300 series stainless steels when held at those middle temperatures. Although we don't often see problems from this in our end (continuous flame roasters and cookers), it is a potential problem. As far as discoloration mentioned by some, people have to remember that is isn't called "stain-free" steel, it's "stain-LESS" steel.
T321 would be the best option in a stainless but would be quite expensive. We also often see 80-20 and 80-20CB specified for high temperature stuff (up to about 2000F) but again, it gets pretty spendy to get that exotic for a BBQ
Just on a side note, we have recently been doing a lot of commercial flame roasters in T314 stainless. The gas flames directly hit the stainless material and they seem to hold up well over time.
Koz
That sounds logical - floating the Chrome to the surface. The surface does look to bright to be steel.
Any voids in the 'reflow' area tainted with rust now.
Thanks, Martin - that also tells us a little of the alloy.
Interesting. I just checked an old Jorgensen stock book and it doesn't show it. Is it a new alloy, or just uncommon? How closely does it compare to 316?
I would have assumed 316L would have been a fair choice for your application, assuming intergranular corrosion wasn't a problem.
I can't help but think that humidity plays a huge role in how heated stainless holds up. I used some scrap bars of 303 S to support a mercury retort that was heated to red heat when used (for good reason) and found that aside from discoloration, the 303 bars held up perfectly. In fact, the retort itself was made of 304 stainless and never failed. They were used over a period of several years, although not daily. They were not subjected to any stress, and likely had lost a considerable amount of tensile strength due to intergranular corrosion, but it wasn't an issue in my application. By contrast, the burner I lost in an old barbeque corroded out totally in just a few years. The corrosion was around the small port holes around the H shaped burner, creating very hot burning areas some places, and a complete lack of flame in others. The barbeque, which resided out of doors, was likely damp or wet quite often. By contrast, the retort setup was kept perfectly dry.
Harold
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