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Top Myths About Stainless Steel

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Stuart21
User Rank
Silver
Stainless invention
Stuart21   10/16/2014 9:40:38 AM
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As an inventor I also have a good appreciation of the inventions of others. (IMHO)

One of my favorite inventions is the magnetic soap holder. It's great to find your soap dry every morning, not sitting in a puddle of soap mud, & rapidly disappearing.

Very happy to find a local source of magnetic soapholders when I lived in Taiwan, so I bought a few and stuck them through my kitchen & bathroom and in the factory. Didn't even get through the first cake of soap, though, before the keepers began to rust. Turns out its just a bit of sheet steel, with the proverbial 'sniff o' chrome' to made it look good till at least it gets out of the shop.

I contacted the company in East Taipei to express my displeasure - they replied that they can't use stainless as it is not magnetic. I sent them a picture of a magnetic knife rack, holding some pretty solid stainless knives to show them that stainless can be magnetic.

I contacted them again some years later and asked them if they had stainless keepers yet. They said yes, so I went and ordered a heap, some for my workers who also liked the item, and had asked me to get them one or two.

Tried the new ones when I received them, rusted just as quickly as the previous ones! Again expressed my displeasure to co, they replied that this was stainless steel 'according to their defintion'. 

Maybe will have to make my own keepers - laser cut some suitable sheet, & spin it. When I get time!

I am not knocking Taiwan manufacture - I spent a lot of time there & saw the quality of products go from not very good (some opinion polls even rated Made in China products better than Taiwans) to a very high standard, for at least the last decade or so.

I think the mag. soap holder co is just the last holdout to the 'old way'.

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?

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