Why use Zinc?

Wet corrosion

The process called wet corrosion is similar to the process in a simple battery. The essential features of a primary cell are two electrodes (an anode, which corrodes when current is taken from the cell and a cathode) and an electrolyte solution.

If two dissimilar metals are placed in contact in the presence of a conducting fluid, then the metals become the electrodes and the conducting fluid becomes the electrolyte. Small electric currents are then generated, in the process, the anode corrodes.

Where two dissimilar metals are concerned, that which becomes the cathode and that which becomes the anode is determined by their relative positions in the Galvanic Series. The one towards the lower end becomes the anode and the one towards the upper end becomes the cathode.

The Metals in Galvanic Series

Cathodic (Most Noble)

• Silver
• Monel Metal
• Copper-Nickle Alloys
• Bronzes
• Copper
• Brasses
• Nickel
• Tin
• Lead
• Cast Iron
• Steel
• Cadmium
• Zinc
• Aluminium
• Magnesium

Anodic (Least Noble)

For example Zinc is lower in the table than steel and is therefore anodic to steel, and in the presence of an electrolyte will corrode in preference to the steel. The fact that Zinc is anodic to steel is an important factor in its value as a protective coating, giving it an additional ‘sacrificial’ role to its role as a barrier.

Corrosion rate of zinc

The corrosion rate of zinc is directly proportional to the coating thickness and is only 10-20% that of steel.

Zinc corrodes much more slowly than steel in most natural environments. The formation of protective films on zinc during exposure are mainly responsible for its excellent resistance to atmospheric corrosion.

It has been reported that, on average, the rate of attack on zinc is only about one twenty fifth of that on steel in most atmospheres and various waters, but this ratio can vary considerably according to the environment and the duration of exposure. Zinc usually corrodes fairly rapidly during the early stages of exposure, but the corrosion rate slows down quickly with the formation of the protective films. Once such a film covers the zinc surface, the corrosion continues at a reduced rate in which the corrosion is normally directly proportional to the duration of exposure.

Application

Zinc can be applied by a range of different processes which makes it adaptable to most types and shapes of component.

Mechanical damage

On occasions mechanical damage may cause breaks in the Sherardized coating. Should this happen, the zinc will corrode in preference to the exposed steel thus protecting it in Sherardizing’s secondary but important role as a sacrificial protective coating. A further protective feature occurs as a result of this sacrificial action since comparatively bulky corrosion products accumulate in the damaged area and tend to inhibit further electrochemical action there.