Today I gave my first chemistry lesson of the year to the first year undergraduates, now this is a basic course in chemistry which might make some of the more experienced chemists think something along the lines of ‘basic chemistry, it can not be that that important’. But I have to say that the fact that it is basic chemistry makes this statement very wrong.
While the basic course might be on a low level, we need to make sure that we give the students a strong foundation on which to build the “house of chemistry”. It is true that the wise man builds his house upon the rock while the foolish man builds his house upon the sand.
What the students need is a strong and robust rock which is able to take the weight of the things which they later learn. In common with house building it is very hard to repair the foundations after the house has been built. For those of you who do not know the process of foundation repair is known as underpinning, it is a slow and expensive process. What has to happen in underpinning is that a small hole is dug next to the building, and then the foundations at that spot are repaired. Then the hole is refilled and a new hole dug nearby to enable the next part of the building to be repaired. This is a long and slow process, any attempts to speed up the process by digging a big hole around the building can make the whole building fall down !
In the same way if a poor job is done in the first year chemistry class then the ‘house of chemistry’ will need a very expensive, difficult and irksome repair later. We need to avoid teaching anything which needs to be removed later, also we need to make sure that the students will be ready at the end of the course to move onto the next thing.
Today one of the students demonstrated that he is thinking about the chemistry in a deeper way, he asked how long a sacrificial anode would last. The is a open ended but good question, the problem is that without doing a series of experiments it is not possible to give an answer which is based on anything more than a wild stab in the dark.
The example he was asking about was a zinc coated steel water tank which is in contact with copper pipes. The tank contains a magnesium anode which is hanging on the end of a wire which is connected to the tank.
What I think is needed to answer this industrially important question would be a series of experiments to determine the corrosion potential and corrosion current associated with the zinc surface. Also the cathodic reaction on the surface of the copper pipes would need to be understood as would the anodic reaction on the surface of the magnesium block.
An easy way to deal with the problem would be to add the magnesium anode and then measure the current flowing through the wire from the tank using a zero resistance ammeter. This is an electronic device which uses op amps to measure the current. Based on the current, the weight of the block and the atomic mass of magnesium an estimate can be made of how long the block will last before a new block is needed.
This simple approach might be easy to use, but it has a series of disadvantages. It is possible to over protect some alloys, for example steel which is exposed to an excessive degree of cathodic protection will becomes brittle because of the formation of hydrogen. While a metal surface which is too far from the protective anode will be under protected and thus be subject to corrosion. I think that the student had wandered into a complex area which is addressed at length by Pierre the corrosion expert in his book.
I feel that Pierre has done such a good job of explaining this topic that I feel I am unable to do nearly as well as him, I think rather than wasting some electronic ink with my attempts to paraphrase Pierre that it would be better if you read his text book if you want to know about cathodic protection.
 Handbook of Corrosion Engineering,Pierre. R. Roberge, McGraw Hill, 2000,New York, ISBN number 0-07-076516-2.