Re: which acid corodes steel fastest

Dylan, you have asked a very difficult question -- perhaps an impossible one! Let me explain why.

There are about a dozen strong mineral acids that are very corrosive and attack metals. There are three that are commonly used and readily available: hydrochloric acid, sulfuric acid, and nitric acid.

But how quickly an acid will attack a metal depends on very many things. It depends especially on how smooth or rough the metal surface is (fine grains dissolve faster than larger grains, faster than rough surfaces, faster than smooth surfaces). It also depends on just how cold or warm it is. Warmer acids usually dissolve things faster. Also these acids are mixtures of another substance with water. Usually the less water and the more of the other substance there is, the faster the reaction. But this is not the case if the amount of water becomes too small a fraction.

These are problems that can be solved -- we simply have to make sure there is a fair competition when we are designing our experiments. Same temperature, same type of surface, and some set concentration of the acid (i.e. proportions between water and the other substance).

Here is where we run into our first real difficulty: suppose we say that our acid has to be a 50/50 mix of the acid substance and water. Do we mean that half of the weight has to be the acid substance, or do we mean that half of the molecules have to be of the acid substance? A mixture of 100 gram of hydrogen chloride and 100 gram of water is 50% by weight, but only 33% of the molecules are hydrogen chloride. A mixture of 100 gram of hydrogen sulfate with 100 gram of water has only 14% of hydrogen sulfate molecules. So if you make the comparison fair in one way, it is not fair in another!

The second, and most important difficulty is this: steel is not a single pure substance. If you had asked about copper, or tin, or lead, that would have been much easier, because these are pure simple substances. But steel is a mixture of iron, and carbon, and iron carbide, and varying amounts of other metals, and even small amounts of non-metals like silicon and phosphorus.

You should know that there is stainless steel, that is designed to resist corrosion, and that has a fair amount of nickel in it. It is quite different to the steel that is used in tools and locks, which is very hard and strong -- that often contains metals like manganese, vanadium, or tungsten. Some steel is magnetic, and some is not.

Some of these types of steel are attacked by acids quite quickly, and others are hardly attacked at all. Some will resist attack by one of the strong acids, but be badly corroded by another.