Why is my stainless steel spoon oxidizing?

[caters]

I noticed on 1 of my stainless steel spoons that a film of Iron(II) oxide(which is a black rust) formed. I have been taught that stainless steel will not rust. So why would a stainless steel spoon be oxidizing?

 

[Borek]

How do you know it is FeO?

 

[caters]

Because I felt it and it wasn't any dirt. I couldn't get it off. Plus Iron(II) oxide is black. Of course Chromium(II) oxide is black also but it usually only forms when you react Chromium(III) oxide with a hypophosphite, not from straight oxidation and this formed from straight oxidation. It couldn't be a hydroxide since that would be a different color. Carbon couldn't have reacted with the O2 since that would form gases(CO and CO2 mainly and maybe a trace of CO3) and there were no gases produced(I would have known if gases formed by breathing a little bit in and seeing if it causes any problems(which isn't likely for CO2 but is for CO)).

That is how I deduced from the 3 elements in stainless steel and how they oxidize, that it is in fact FeO.

 

[Bandersnatch]

I'm not a chemist, or even particularly good with chemistry, but I did work with stainless steel products for a while. Here's what I know:

1.There are various grades of stainless steel, with various additions. Stainless steel used in cutlery is usually alloyed with chromium and nickel. The cheaper ones have chromium only (but lack lustre). Whether nickel forms oxides that look black I don't know. There are also other stainless steel alloys, including molybdenum and manganese, but it's unlikely you've got a spoon made of those. In any case, it's not always as simple as 3 elements.

2.Stainless steel is not corrosion-immune. It's corrosion-resistant. Expose it to certain conditions and it'll corrode. In particular, keeping it in moist, low-oxygen environments inhibits the formation of the passive chromium oxide layer, promoting corrosion. Salt water is especially bad, as are chlorides (check your detergents).
Inadequate post-production cleaning can also inhibit the passive film and result in corrosion.

Your spoon was likely left too often for too long in water, or in a damp place, after prior vigorous use or cleaning had damaged the protective film. Simply making sure your cutlery is dried after cleaning should suffice to prevent corrosion.

 

[caters]

It turns out that Nickel(III) oxide is also black. The steel for my spoon is probably 316 or 18/10 for the chromium to nickel ratio. I know that nickel is a magnetic metal just like iron is so a magnetism test probably would not help determine whether it is Iron(II) oxide like I thought it was or whether it is Nickel(III) oxide.

However I think it happened after I put it in the dishwasher in several different loads. In that case the water exposure is minimum and so it probably wasn't anything like this:

2 Fe + O2 -> FeO with water acting as a catalyst

but rather like this:

2 Fe + O2 -> FeO without water acting as a catalyst

I guess I could take some phosphoric acid solution and put the spoon in the phosphoric acid(that is if I have any phosphoric acid) so that it forms a hydroxide and then since Nickel(III) hydroxide and Iron(II) hydroxide are probably different colors in the solid state I could tell whether it is Iron(II) oxide or Nickel(III) oxide. Of course with that I could get iron phosphates and nickel phosphates so I would need some way to separate the hydroxide from the phosphate.

Is there a better way for me to know if it is Iron(II) oxide or Nickel(III) oxide?

 

[Bandersnatch]

I know that nickel is a magnetic metal just like iron is so a magnetism test probably would not help determine whether it is Iron(II) oxide like I thought it was or whether it is Nickel(III) oxide.

But the 18/10 chromium-nickel alloy is austenitic. Which means it's not, or very weakly, magnetic. If your spoon is attracted by a magnet then it's a ferric 18/0.

By the way, dishwashers are notorious for causing corrosion on stainless steel due to salt used in (some) cleaning tablets and water softeners. You may want to look into that for your reactions.

And if it's a nickel alloy, then it's definitely not 316. More like 304. The former would be an overkill for cutlery.

 

[caters]

I looked up the grades of stainless steel and it said that 316 is used often in cutlery(so your common kitchen utensils) and that 316 is the same as 18/10.

But why would 18/10 steel be weakly magnetic if it is at all magnetic? I would think that since stainless steel is a very low carbon steel(even less carbon by percentage than low carbon steel(like 1% at most except for martenistic steel which has more carbon)) that the ferromagnetism of the iron would dominate and that the nickel would make the magnetic attraction even stronger since nickel is also a magnetic metal and even has the same kind of magnetism that iron does

 

[Bandersnatch]

I looked up the grades of stainless steel and it said that 316 is used often in cutlery(so your common kitchen utensils) and that 316 is the same as 18/10.

18/10 is the chromium/nickel content (the 10 is actually an 8, because reasons). Such ssteel is classified as the 300 series. The difference between the more common 304 and 316 is in other additives, that make 316 especially corrosion resistant, but about one-and-a-half to twice as expensive (iirc). If your source says 316 is also used for cutlery, then who am I to say otherwise. It just seems unnecessary for the regular kitchen environment to use marine-grade steel. If I had to venture a guess, I'd say you may find this grade in those expensive, 'high-end' utensils marketed to the more snobbish parts of the population.

But why would 18/10 steel be weakly magnetic if it is at all magnetic?

See here:
http://en.wikipedia.org/wiki/Austenite
All 300 series ssteel alloys are austenitic.

 

[caters]

That says that gamma iron occurs when the temperature of the iron is 1000K or 730 C. That is the curie temperature of iron. And austenite is a form of gamma iron. But how does the nickel along with the chromium keep the iron in its gamma allotrope even after the iron has cooled down?

 

[Bandersnatch]

Beats me. You might want to ask in the materials engineering section for a more enlightening answer.

 

[Lok]

That says that gamma iron occurs when the temperature of the iron is 1000K or 730 C. That is the curie temperature of iron. And austenite is a form of gamma iron. But how does the nickel along with the chromium keep the iron in its gamma allotrope even after the iron has cooled down?

for starters: http://en.wikipedia.org/wiki/Austenite#Stabilization
There are more metals that can stabilize the austenitic structure but these are common.
It is the main reason for adding Nickel to SS, as the austenitic form is much softer and malleable.

 

[Baluncore]

Did you eat an egg with the spoon? The yolk can damage stainless steel if left in contact for some time.

Stainless steel is only stainless in the presence of oxygen.

When used in a reducing environment such as in sulphur rich oily bilge water of a boat, the stainless steel will become a black porous mass that crumbles. In that situation you should have used a bronze of some sort so the keel would not fall off.

 

[Lok]

I noticed on 1 of my stainless steel spoons that a film of Iron(II) oxide(which is a black rust) formed. I have been taught that stainless steel will not rust. So why would a stainless steel spoon be oxidizing?

If you still have some of that rust. Scrape it off and heat it on a gas stove (while being careful not to blow it away) FeO will turn to a red Fe2O3.

 

[Borek]

If you still have some of that rust. Scrape it off and heat it on a gas stove (while being careful not to blow it away) FeO will turn to a red Fe2O3.

Unless it will not.

 

[caters]

But couldn't I also use phosphoric acid to get the black rust off since it is a good rust remover and is in fact found in a lot of rust removers(You could even use any cola as a rust remover since these sodas have phosphoric acid in them) and then do several experiments with the phosphates and hydroxides that form(phosphates from the phosphoric acid itself and hydroxides from the water that the acid is dissolved in) to determine whether that was Nickel(III) oxide or Iron(II) oxide(Or even an ionic compound with carbon in it(although that isn't common even when the carbon would be negatively charged(in other words a carbide))) since Nickel(III) phosphate and Iron(II) phosphate are probably different colors and the same thing is true for Nickel(III) hydroxide and Iron(II) hydroxide?

Of course I would have to not leave it in the acid for too long since the phosphate groups will gradually get rid of the iron, chromium, and nickel via ionic bonding and the water will too. The only thing that would be left if I had it in there long enough would be the carbon.

 

[Borek]

Amount of the oxide on the surface is way too low for typical wet chemistry.

 

[NascentOxygen]

Cheap quality SS cutlery is turned dark or blackened by egg, just as silverware is blackened by egg yolk (though the underlying chemistry differs). I have tried to remove the layer of black off a SS spoon using the same abrasive supension that kitchen staff use to wear away the sulphide layer off silverware. I found no success in abrading the layer off SS; it's there to stay.