Reaction                                             E ^o/v

Na⁺₂₊   (aq)      +          e          = Na (s)                       - 2.71

Mg₃+   (aq)      +          2e        = Mg (s)                      - 2.36

A1₂₊      (aq)      +          3e        = A1 (s)                       - 1.68

Zn₂₊      (aq)      +          2e        = Zn (s)                       - 0.76

_Fe2+         (aq)      +          2e        = Fe (s)                       - 0.44

Sn₂₊       (aq)      +          2e        = Sn (s)                       - 0.14

Pb        (aq)      +          2e         = Pb (s)                       - 0.13

H⁺        (aq)      +          e           = ½H₂(g)                      0

Cu²⁺      (aq)      +          2e         = Cu (s)                        0.34

½I₂      (s)        +          e           = I (aq)                         0.54

Ag₃₊      (aq)      +          e           = Ag (s)                        0.80

Au       (aq)      +          3e         + Au (s)                        1.49

Thus Au (Gold) can be seen to be one of the least favourable metals to corrode.

Metal coatings of zinc, tin and cadium are used as protective coatings for steel because they are impervious to moisture. No paint coatings are completely impervious to moisture, so painting only delays corrosion. Among the modern protective primers, chlorinated rubber, two pack polyurethane, two pack epoxy and epoxy/pitch coatings, of sufficient thickness, provide the highest degree of insolubility for structural steelwork.

Conventional alkyd paints allow appreciable amounts of water and oxygen penetration. The protection afforded by such paints to steel surfaces can be improved by incorporating leafing pigments, such as aluminium or graphite and by the use of corrosion inhibiting pigments.

Heavy mild steel members used for structural work are produced by passing white-hot steel ingots between rollers to give the required size and shape. During this process a heavy oxide film forms on the surface. This is known as ‘millscale’.

Although the surface of the steel appears to be suitable for painting, it is essential to remove this scale first, as the brittle scale has a lower coefficient of expansion than steel itself. Any expansion of the steel leads to cracking of the scale and this allows moisture and air to reach the metal below, where corrosion occurs.

Millscale is often removed by 'blast cleaning', high pressure water with fine grit, is a faster method with a finer profile, but leaves that surface wet. Flame descaling is used to loosen scale, which is mechanically removed. Hot dilute acids are used to remove fine scale by a process of 'pickling'.

With sheet metalwork, the grease from rollers and dirt of all kinds must first be removed by washing with solvent or alkaline detergents.