Cappy
Would be Luthier
- Messages
- 334
Dont worry about the mill scale.
Weld through it, and paint over. Mill scale is a good rust preventer in itself.
Gonna rack the wee brain here a bit so excuse any complete rubbish I write.
Thinking back to the days of college studying for my Mates ticket one of the subjects we covered was GSK (General Ship Knowledge) and part of it concerned shipbuilding, and shipbuilding practice. A great emphasis was placed on the preparation of the steel plate (still is as far as I know) with regard to the removal of the mill scale, either by sand blasting, pickling or flame cleaning, prior to use. It all has to do with the difference positions in the periodic table between mill scale and steel which creates the anode/cathode situation and makes an electrical circuit when wetted. This is how the process of corrosion starts and in salt water (being a better electrolyte) it happens even quicker.
The same can also happen between two steel plates, and can even occur on the same plate due to maufacturing irregularities in the rolling process, and hence slightly different electrical characteristics.
Perhaps goes some way to explaining why the welds always go rusty first
It's a tricky business and no mistake, as I also think that mill scale has a slightly different coefficient of expansion from the steel which is why you see it flake off as you heat it. It is supposed to be waterproof by itself but one small break in it and gets pushed off the steel surface as the underlying steel starts to rust.