Never being one to accept things I’m told on face value, I’ve often thought about what in the blacksmith’s world is accepted wisdom on erosion of mild steel. For those of you who aren’t into metallurgy here is a short explanation.
You might have noticed old ironwork form anywhere between 100 to 500 years in your area which is rusted but not eroded. The reason for this is twofold. Firstly, it is most likely wrought iron which, while being made of the same ore as the mild steel we mostly use today, goes through an initially different smelting process. This in short means it has a totally different molecular structure to mild steel. The structure of mild steel is crystalline, while wrought iron is fibrous. Adding to this, wrought iron will have inclusions or impurities caused by the smelting process. This fibrous structure, according to conventional wisdom on this subject, lends itself to a greater degree of burnishing through the forging process which gives it a surface that will be more resistant to moisture-penetration. The burnished surface is what we smiths call millscale, it has a dark gray or black colour which is where blacksmiths get their name. Millscale is densely packed iron and carbon along with other trace elements.
Conventional thought on this is that when a rust bloom forms on wrought iron that has been forged correctly to form millscale, it makes it almost impervious to water penetration and, therefore, more resilient to further oxidisation. Oxidisation of iron is what causes erosion. Conventional thought also says that the same is not true for mild steel that has been forged due to its crystalline structure. Because it does not burnish down in the same way as wrought iron, it will not give the same levels of water penetration prevention.
I have thought about this a great deal. Both materials looked the same once forged. To me they both seem to burnish to the same degree. But looks can be deceiving and, as I don’t have a handy electron microscope to check out the molecular differences to surface density, I went looking for studies into the defences in erosion rates of forged mild steel and forged wrought iron. Surprisingly, I could not find any such research. Now, I’ve only got Google at my disposal so that does not mean that such a research does not exist but as I couldn’t find one I decided to create my own limited experiment. I forged a simple BBQ out of mild steel, using only traditional joinery and took care to forge all of the material used, creating an even millscale throughout the BBQ. The BBQ has been left out in the elements ever since, seven years now and used often. It has moved to three different houses with us, not to mention trips to the beach and around the northwest Mayo.
As you can see, it has formed a rust bloom but there is zero erosion. Seven years is not a conclusive time period and this is not a controlled experiment but it does lead me to question conventional wisdom on this matter. I’ve see fabricated mild steel structures show signs of erosion in that timeframe, so why not this structure? Could conventional thought on this subject be misleading? As yet I feel it is too early and too limited a study to give a conclusive answer. However, it does call into question what we believe to be the truth about the material we use as blacksmiths and the way we treat and finish it. It is my theory that mild steel, when correctly forged using traditional joinery and not welded with modern welders, performs very well compared with wrought iron. I’d love to hear your thoughts on this subject and if you can point me in the direction of any studies or others experiments in this same field, I’d be glad.