Question

Illustrate the process of steel corrosion. What are the protection methods? illustrate the process and the reason why those protection methods work.

Answer 1

The corrosion of structural steel is an electrochemical process that requires the simultaneous presence of moisture and oxygen. Essentially, the iron in the steel is oxidised to produce rust, which occupies approximately six times the volume of the original material. The rate at which the corrosion process progresses depends on a number of factors, but principally the 'micro-climate' immediately surrounding the structure.The corrosion of steel can be considered as an electrochemical process that occurs in stages. Initial attack occurs at anodic areas on the surface, where ferrous ions go into solution. Electrons are released from the anode and move through the metallic structure to the adjacent cathodic sites on the surface, where they combine with oxygen and water to form hydroxyl ions. These react with the ferrous ions from the anode to produce ferrous hydroxide, which itself is further oxidised in air to produce hydrated ferric oxide (i.e. red rust.) The sum of these reactions can be represented by the following equation:

Fe + 3O₂ + 2H₂O = 2Fe₂O₃H₂O
(Steel) + (Oxygen) + (Water) = Hydrated ferric oxide (Rust)

However, after a period of time, polarisation effects such as the growth of corrosion products on the surface cause the corrosion process to be stifled. New, reactive anodic sites may be formed thereby allowing further corrosion. In this case, over long periods, the loss of metal is reasonably uniform over the surface, and this is usually described as 'general corrosion'. A schematic representation of the corrosion mechanism is shown (above right).

The corrosion process requires the simultaneous presence of water and oxygen. In the absence of either, corrosion does not occur.

• There are three methods that may be used to protect steel from corrosion.

1. Passive Barrier Protection

Passive barrier protection works by coating the steel with a protective coating system that forms a tight barrier to prevent exposure to oxygen, water and salt (ions). The lower the permeability of the coating system to water, the better the protection provided. Two-pack epoxy coatings and chlorinated rubbers applied at sufficiently high film builds offer the most successful corrosion protection through passive barrier protection.

2. Active Protection

Active corrosion protection occurs when a primer containing a reactive chemical compound is applied directly to the steel. The reactive compound disrupts the normal formation of anodes on the surface of the steel in some way. For example, inorganic zinc inhibitive pigments, such as zinc phosphate, offer active anti-corrosive protection to the steel substrate (Zinc phosphate (Zn3(PO4)2) is only slightly soluble in water). It hydrolyses in water to produce zinc ions (Zn2+) and phosphate ions (PO43-). The phosphate ions act as anodic inhibitors by phosphating the steel and rendering it passive. The zinc ions act as cathodic inhibitors.

3. Sacrificial Protection (Cathodic Protection or Galvanic Protection)

The above-mentioned reaction between dissimilar metals can be used to protect steel against corrosion. The most widely used metal for the protection of steel is zinc. Zinc metal in direct contact with the steel substrate offers protection through the preferential oxidation of zinc metal. Zinc is a great choice in protecting steel, as not only does it corrode in preference to the steel, the rate of corrosion is generally slower. This rate, however, is accelerated in the presence of ions such as chlorides in coastal locations.