The first post you probably didn't expect

The first post! So much pressure to say just the right thing to the highly discerning internet public. What to say? I suppose it doesn't really matter since all who read this will be hanging on my every jot and tittle. After all, this is a BLOG!

I have decided what I, the 1st and/or 2nd ‘J’ in this little family, will blog about: Inter-granular carbide precipitation in 300 series stainless steel.

First some background: 300 series stainless steel is a fancy way of referring to steel that is less likely to corrode than, say, 1010 steel. Hmmm... how about some more background: steel is a mixture of the atomic elements Iron and Carbon. It is a very useful and strong material. Why? When molten steel solidifies all of the various atoms inside of it arrange themselves in a crystal structure; a structure that resembles individual wooden building blocks stacked in every direction, up, left, right, towards you and away from you. Let's consider each one of those wooden blocks to be an atom of iron and then let's wedge a penny in between each corner of each block. The penny is the atom of carbon. Now if you imagined that you glued the blocks together along the 8 faces and also glued the pennies at each block corner and, there you have it: steel. In your mind, try to push on the blocks. They are pretty strong, aren't they? Why? Well, obviously because you glued them, right? Well that's not all. That penny at each corner of each wooden block makes each row of blocks very difficult to slide out of place, even though the penny is much smaller than the wooden block. That is the same way steel gets its strength. Those little carbon atoms sit in between the larger iron atoms, making the whole structure very difficult to slide or break.

Okay, now back to stainless steel. Stainless steel isn't really stainless; it will corrode just like any other metal if the right conditions exist. In fact, this brings us back to inter-granular carbide precipitation in 300 series stainless steel. You see, stainless steel doesn't corrode easily for the same reason that cheese won't mold... IF, its packaged in an airtight plastic bag. Of course, we aren't literally placing some steel in a plastic bag and calling it stainless, but stainless steel does have its own inherent plastic bag. This protective sheet around stainless steel comes from another atom in the periodic table of elements: chromium. You see, chromium combines with the oxygen in the air and forms a mixture of these two elements called chromium oxide. This chromium oxide coating that now exists all around our stainless steel acts like a plastic bag around cheese, keeping all the bad stuff out.

Now a certain kind of stainless steel with a specific recipe of elements, including chromium, is called 300 series by those lovely engineers called Metallurgists. This type of steel has a problem. If you heat it up to a very high temperature, say 2000 degrees F (that's pretty hot, maybe 4 times hotter than the hottest setting on your oven) some of those dastardly carbon atoms decide that they want to get up and get out of there. Yep, they just go ahead and leave. Well sure, what would you do if you were a carbon atom, stuck in between a bunch of big sweaty iron atoms? It stinks, you have a sensitive nose, and you’re pretty sure the iron atom next to you hasn’t showered in a few millennia. That being said, you are very small and very agile. So you get up and move yourself away from the iron atoms and where do you go? Well, you notice that there are gaps in between the colonies of iron atoms, which of course makes sense because iron atoms, like people tend to form cliques and only hang out with the people they are most alike. So you move on over to the gaps in between the cliques and get a short reprieve from the smell. Of course a new problem arises, though, there's not enough room for all the carbon atoms to find sanctuary in the inter-iron gaps. Unfortunately, some of the carbon atoms just plain have to stay where they are, their noses burning in the sweaty stench.

Now, you’ll notice I used the term inter-iron gaps. Sound familiar? How about inter-granular? Yep, same thing. In fact, what’s the next word? Carbide. Those are the first two words in our topic: Inter-granular carbide precipitation in 300 series stainless steel. Wait a minute, I just realized that we now know just about every word in the above sentence except ‘precipitation’. Precipitation is a fancy way of describing the phenomenon of small objects coming out of larger objects. For example: precipitation. Like the rain kind. Isn’t rain just water coming out of clouds? The water always existed in the clouds it just took special circumstances to make it come out as rain.

So to add to our metaphor a bit, those small carbon atoms are trying to squeeze through the crowd of iron atoms and find sanctuary in the gaps between their groups. Our little carbide friends are not loners, however. Remember that chromium stuff that combines with the oxygen and creates a plastic baggy effect over the stainless steel? Well, in the atomic world, carbon and chromium are the best of friends. If circumstances permit they would very much like to be together almost like they are in love (ahhhhhhh). So off they go, hand in hand, to where they find reprieve from their smelly neighbors, and that is the precipitate: chromium carbide.

So to review, we now have a mass migration of carbon and chromium from where these two originally started among the iron. So why does all of this make stainless steel more susceptible to corrosion? Well now there isn’t as much chromium to combine with the oxygen. This leaves gaps in the plastic baglike protection of the stainless steel. Nature is not afraid to take advantage of something if she will benefit greatly, and, poof, you have rust on stainless steel.

Tada! Now you know what inter-granular carbide precipitation in 300 series stainless steel is and how it comes about.

You are welcome.