I learn best by doing. The next best thing for me is a word picture. With this in mind, I thought I would talk about corrosion in terms of historical events as a back drop to help imprint some corrosion concepts.
In AD 89 the Roman Legions sacked the town of Inchtuthil in Scotland. They broke and burned nearly everything prior to retreating elsewhere for another conquest. They noticed after burning the town that steel and iron nails were left. A lot of them. They did not want this valuable material to be left for the enemy to use to rebuild. After all the enemy could use these nails for swords and other items to wage war with the Romans in the future. In 1961, Professor Richmond of Oxford University found a pit in Scotland. The pit contained 76,840 small nails, 86,128 Medium, 25,088 large, and 1,344 extra-large nails. Seven tons in total. The pit was dug by the Romans to hide the nails from the Scotts. Seven tons of iron and steel was a lot to move in those days, and it was apparently decided to hide it in a pit rather than haul it elsewhere.
I can easily envision a talented blacksmith way back then stumbling onto some sort of recipe for an excellent grade of sword steel. Perhaps his son a soldier may have come back alive telling him of how well it performed in battle. Swords had to be hard enough to take and hold an edge, but this meant it would be brittle and might break in the heat of battle. So, it also it had to be soft enough to not shatter, but this meant it would not hold an edge very well. It was a balancing act of give and take. Perhaps the blacksmith happened to remember that small nails were harder than the large ones, and perhaps he remembered that 4 large nails and one small nail was just the ticket to bring his men home from battle alive. A blend of hard and softer nails seemed to him to work. He would weld forge them together and come up with an amalgam of the two. A nail alloy of sorts. Inevitably though, there would be pockets and bits of hard and soft nails in his final swords. After all he was not perfect with his old forge. This may or may not be the way they did things back then. I don’t think it too far-fetched. Plus, it suits my needs here.
Now let’s jump to Japan in about 1337 and we find Tamahagane steel, or “Jewel Steel”. It is the stuff that the famous Katana swords carried by the Samurai were made of. It is made (even today by some) of black sand (magnetite) and black carbon and probably some other discrete and very closely guarded materials. These materials were loaded into a kiln and baked for 7 days straight. At the end of the 7 days they would break into the clay kiln and what emerges is a kind of tree bark looking sort of material with varying degrees of steel and carbon steel. The master sword smith taps on them to listen for a ringing sound. He looks at them for physical appearance and hefts them in his hand and finally selects several of each to begin forging the sword. We now know that what they discovered way back then was that by mixing high carbon and lower carbon steels made for a sword that was literally 10 times stronger and sharper than other swords of the day. Again, we can see that perhaps not all of the carbon was mixed in to the other. This is process is more refined, but similar to the nails in the Roman analogy. Heck almost 1300 years had elapsed.
I can hear you saying great Jim, but what does this have to do with CP? Well if we look at a mill report from modern steel for X-70 pipeline steel, we will see that there is a portion or percentage of iron, carbon (sound familiar?) and a host of other things in the mix that allows the pipe steel to be both strong and ductile (bendable). After all pipe steel has to perform in some crazy environments and not yield to sometimes immense pressures and heat exerted on it both internally and sometimes externally as well. Pipe steel is manufactured today with amazing accuracy and precise proportions. However there are microscopic areas of steel, iron, carbon and whatever else is in the recipe. They of course all touch each other some here and some there. In the heating and cooling process of hardening and tempering, crystals are formed on a molecular scale. Just like Tamahagane and the Roman sword made from nails. Little parts make up the whole.
Hang in there I am getting close to what I want to help you understand. If I put a digital multi-meter on two differing lumps of steel, metal, or some alloy I will see a voltage reading. Albeit very small, it will show up. Gold, silver, copper, zinc, and magnesium when compared to each other with a multi-meter all have varying degrees of voltage differences called potentials. In fact, for CP purposes the different metals are referred to as more “noble” and less noble. And more active and less active. Gold is generally at the top of most lists as most noble and least active. And the least noble and most active is magnesium (in most common lists).
The varying list of metals in pipe steel on a minute scale is what causes corrosion. Inevitably in steel there are discrete portions of one metal touching its neighbor and there is an exchange of voltage (electron transfer) present at this location which causes corrosion. With cathodic protection we can level the playing field by introducing electricity in the form of anodes or induced current from a rectifier and overcome the differing potentials that lead to corrosion.
Of note: Atomic scientists have studied the almost 2000-year-old Inchuthil nails to estimate the corrosion effects on barrels of nuclear waste.
Jim Lee makes his home in Decatur TN, and works across the United States. Mr. Lee holds the following certifications:
