re 26) why is zinc coating important, I use paint.

Zn offers protection from corrosion just like your paint does. Apart from this there is a hidden advantage. Here is the theory behind use of Zn coated Mild Steel.

1. Using good conductive material as a enclosure privides shield against electric fields, plane waves and high frequency magnetic field.

3. Using magnetic material provides shield against low frequency magnetic field.

4.Steel has poor conductive Relative conductivity 0.10 but has high permiability relative permiability 1000. Thus it act as a good shield against low frequency magnetic field but due to its poor conductivity is not so good at shielding against high frequency magnetic fields and electric fields.

5. Zinc has got very good conductivity Relative conductivity 0.34 which is next to Aluminium [ 0.40 ]. Thus if your steel is coated with Zn its good conducting property makes it a good shield agains high frequency magnetic fields and electric fields.

6. Whereas your paint has not got the above advantage due to almost zero conductivity of paint.

Thanks & Regards,
Prahlad Purohit