Why is a tin can that appears like galvanised steel rusting?

Question

I bought a canned pineapple and finished the content. The part of the can that is in contact with the liquid appears like galvanized steel (crystalline surface or spangles sort of appearance), while the part that are not looks pretty shiny like the outside of the can.

I then washed the can and added water to it, and heated the bottom of the can with a butane torch. After the water was heated up, I removed the torch and left it out for a while.

When I came back later, I emptied the water but noticed that the bottom rim of can had quite a bit of rusting. After inspecting it closely, I found that there are spots of (concentrated) rust on the cylindrical wall of the can as well. There are also light patches of rust on the inside bottom of the can. The seam on the cylindrical wall that is in contact with water is also badly rusted.

I thought galvanised steel only rusts when the layer of galvanisation wears out. So, is the tin made with galvanised steel?
If yes, why are there concentrated spots of rust on the wall and other rusting?
If not, why does the inside of the can look like galvanised steel?
And BTW, why does the emptied and washed can smell "metallish"?

Answer 1

First of all, any 'tin' can you are likely to find is actually made of steel. Tin is too expensive. Also, tin does not 'rust', although it oxidizes. Your rust is iron oxide. Galvanized steel is steel with a thin zinc coating, likely hot-dip galvanization. What you describe (crystalline surface or spangles sort of appearance) does sound like galvanization, although I am not aware of any steel cans that are galvanized.

I thought galvanised steel only rust when the layer of galvanisation wears out. This is true. If your can is indeed galvanized, then it has likely chipped off, possibly on the rims of the can where the lid it crimped or pressed together, or by cutting the can open. Strong acids will dissolve zinc Also interesting to note: sodium chloride (salt) will dissolve zinc over time by converting the galvanization to zinc chloride, but that's not applicable here.

So, is the tin made with galvanised steel? Well, that does sound like what you were describing, but galvanized steel is not fit for foodstuffs. Zinc is mildly toxic and ingesting it can make you ill. I like feetwet's suggestion; that universally the inside of metal food cans are lined, by actually an epoxy that is sprayed on. This is almost certainly to be the case. Perhaps this makes sense: if your can was indeed galvanized, then the rational by the manufacturer was that the lining will keep the zinc separated and the food safe. Sounds legit. It might be good to be aware that the type of epoxy being used here is the one that uses as its plasticizer Bisphenol A and Bisphenol S (BPA/BPS). Ill leave it up to you to decide if that should bother you or not.

If yes, why are there concentrated spots of rust on the wall and other rusting? Well, if there is rust, then we can deduce that water must have be coming in contact with the iron in the steel, so whatever was protecting the steel has some failure.

And BTW, why does the emptied and washed can smell "metallish"? While zinc does have a metallic smell to it, iron oxide's is much stronger. Since you visually identified rust, this is what you are smelling.

Answer 2

Most modern steel cans for food are given a hermetic plastic coating on the interior. Before that steel cans were tin-plated. (I believe all-tin cans haven't been used in over a century.)

Depending on how hot you got the can when testing it you could have melted or vaporized the plastic coating, exposing the unprotected steel. Also, if the can is dented then the coating can be compromised and the steel will rust where exposed.

Finally, when you open the can you are cutting through the hermetic coating, exposing the steel at least on the edges where you cut, which is why opened cans quickly develop a "metallish" rust smell.

Answer 3

Believe it or not, the causative agent is the Ascorbic acid (presence in pineapple juice), which is known to cause pit corrosion (described here) with stainless steel! Here is a source "Effect of ascorbic acid on the pitting resistance of 316L stainless steel in synthetic tap water", to quote:

"Above the 10^-4 M of A.A concentration, A.A generates soluble chelate rather than absorbs on the steel surface and it causes passive film deterioration and severe pitting corrosion."

And per the question: "why are there concentrated spots of rust on the wall", which supports not normal rusting, but pit corrosion, which I have experienced. Interestingly, I actually did a related thread on it in another forum when my cutlery was unexpectedly (and seemingly, unexplainedly) attacked. Others reported similar experiences, with my experience starting with mango juice exposure to stainless steel steak knives left unwashed in a sink.

Apparently, in the presence of moisture, oxygen, iron, Ascorbic acid (a source of H+ and a chelate) and tap water minerals (electrolyte), an electrochemical based pit corrosion reaction can proceed with time.

Answer 4

The tin is on the outside surface of a tin can . The inside may be zinc coated ( as apparently yours was ) ,tin coated ,or coated with an organic, eg. lacquer. These protective coatings have limits to life depending on the contents ( salts , pH , and the particular acid ) and any damage - scratches, dents.

Answer 5

Food cans are usually tin coated steel. They may or may not have an organic coating, and many fruit cans leave exposed tin to reduce browning of the fruit with age. The surface phenomena that you see that looks like galvanizing is the organic acids in the fruit attacking the tin, which forms tin oxide and has a grey modeled appearance. It was just as shiny on the inside as you see on the outside when the can was first filled.

The tin coating is only resistant to corrosion in the absence of oxygen; once you opened the can, the tin is able to oxidize and leave exposed steel, subsequently allowing rust to form.