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Top Myths About Stainless Steel

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Amclaussen
User Rank
Platinum
Re: cooking and eating with stainless steel
Amclaussen   9/1/2014 10:00:21 AM
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You are absolutely right, cookiejar!  Here in Mexico we use a lot of tomato (both the red and the green "jitomate" varieties)... those are quite acidic, thus any stainless (or any metal) spoon that touches the "salsa" destroys the true flavor of it, and curiously, it starts decomposing it so that it lasts a lot less, even refrigerated.   So we use wooden spoons or plastic. Amclaussen.

Dave Palmer
User Rank
Platinum
Re: PASSIVATION OF STAINLESS
Dave Palmer   8/29/2014 2:04:45 PM
@a.saji: The need for passivation (or pickling, electropolishing, or other surface treatment processes) depends not only on what you require, but also the condition of the parts prior to surface treatment.

In some cases, a stainless steel part may be passive without going through a passivation process.  In other cases, a stainless steel part might not be passive even though it's gone through a passivation process.

A good way to check for passivity in most stainless steel is to swab the surface with a solution of copper sulfate in sulfuric acid.  This solution is blue in color.  If it leaves a copper-colored deposit on the metal, the surface is not passive.

a.saji
User Rank
Silver
Re: PASSIVATION OF STAINLESS
a.saji   8/29/2014 6:49:07 AM
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@Dave: great explanation mate. Is your final conclusion is it all depends on what you require is it ? 

a2
User Rank
Gold
Re: PASSIVATION OF STAINLESS
a2   8/28/2014 11:13:05 PM
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@xxbruce: What does it do exactly ? I haven't heard the process name but has a positive feeling about it. 

a2
User Rank
Gold
Re: Stainless Steel and galling threads
a2   8/28/2014 11:10:37 PM
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@Mr.Wirtel: I don't think it's a possibility. 

Mr. Wirtel
User Rank
Gold
stainless steel in carpentry work.
Mr. Wirtel   8/28/2014 9:05:56 PM
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@Dave. Just another thought about stainless steel. I had a deck built from Treated Pine held together by galvanized screws. After years I noticed many of the screws were deteriorating so I decided to replace them with a much more expensive stainless steel screw. After $50 worth of screws and many hours crawling around on my hands and knees replacing screws, I found that they react negatively to the treated lumber much faster than the galvanized screws. Dave, you just explained why with the comment about sodium chloride. Treated lumber contains calcium chloride and I am guessing it attacks that layer of oxygen. Thanks

Mr. Wirtel
User Rank
Gold
Re: Stainless Steel and galling threads
Mr. Wirtel   8/28/2014 8:50:34 PM
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Is it possible for you to substitute a brass nut? You can find some extremely durable brass, but it is hard to machine.

Dave Palmer
User Rank
Platinum
Re: PASSIVATION OF STAINLESS
Dave Palmer   8/28/2014 8:28:53 PM
@xxbruce: Your memory is good.  Passivation of stainless steel is typically done in either nitiric or citric acid.  The purpose of the process is to remove any free iron that might be present on the surface.

Working or cutting a stainless steel part using carbon steel tools can leave a surface film of carbon steel behind.  If not removed, this surface film can rust -- after all, it's just carbon steel.  Passivation will remove this surface film.

Just because a part has been through a passivation process, though, doesn't mean it's actually passive.  In order for passivation to work, the surface needs to be clean of grease and oil.  Passivation baths won't remove this type of contaminant.  Therefore, prior to passivation, parts are typically cleaned with either an alkaline cleaner or a solvent degreaser.

Passivation also won't remove oxides (such as heat tint from welding processes) or particulate contamination.  For example, if you've made the mistake of blast-cleaning a stainless steel casting with carbon steel shot -- like I did once -- don't expect the passivation bath to remove the embedded shot particles.  For this kind of heavy-duty job, you need to use a much more aggressive process called "pickling." This process uses a mixture of hydrofluoric and nitric acids, pretty nasty stuff.

As Ken Herrick mentions below, electropolishing is another surface treatment process for stainless steel.  Ken's description of the electropolishing process is correct.  You can think of it as electroplating in reverse.  The surface layer of the material is electrochemically removed, leaving a bright, clean, and smooth surface.  Electropolishing is expensive, but very effective at increasing the corrosion resistance of stainless steel parts.

Passivation, pickling, and electropolishing are all ways of trying to obtain a passive surface on stainless steel.  Which process (or combination of processes) you need depends on what you're starting out with.

xxbruce
User Rank
Iron
PASSIVATION OF STAINLESS
xxbruce   8/28/2014 7:40:19 PM
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In the past I was a design engineer for a company manufacturing de-greasing equipment constructed of some family of stainless. There was a process in which they treated the surface with, if memory serves me correctly, nitric acid. I believe the process was called "PASSIVATION". Is anyone familiar with this process and what does it accomplish?

cookiejar
User Rank
Gold
cooking and eating with stainless steel
cookiejar   8/28/2014 7:34:36 PM
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Stainless steel cookware seems all the rage these days.  Few people realize that it is reactive and significantly affects the taste of food.
Shortly after we were married, we carefully followed my mother's recipe for a chicken vegetable broth which was simmered for 4 hours in our stainless steel stock pot.  To our dismay, the taste was very bland indeed, missing all the aromatic flavors of the vegetables and spices.  My bride figured it was a mistake in my mom's recipe.  My mom insisted the recipe was correct and after some thought gave us a cheap old porcelain coated stock pot to try. To our surprise, that solved the problem and the batch cooked in it was excellent.  We also found that the soup tasted much better using a China (porcelain) spoon rather than a stainless steel one.  The Chinese have obviously figured that out a long time ago.  Maybe that's also why they use chopsticks rather than metal cutlery - taste. 
So it is obvious that stainless steel is reactive when cooking in it.   Another demonstration of this property is to wipe your onion soaked hand with anything stainless and the odor will be gone - instantly.  Could the increase in salt consumption have something to do with neutralizing the taste killing properties of stainless steel?
When we cook in our kitchen these days we use porcelain covered ware which makes such a huge difference to the smell and taste of food.  My wife uses chopsticks which I've been unable to master so I often use a China spoon, in private of course - never with guests around. 

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