Corrosion of Steel
The actual corrosion process that takes place on a piece of bare, mild steel is very complex due to factors such as variations in the composition/structure of the steel, presence of impurities due to the higher instance of recycled steel, uneven internal stress and/or exposure to a non-uniform environment.
It is very easy for microscopic areas of the exposed metal to become relatively anodic or cathodic. A large number of such areas can develop in a small section of the exposed metal. Further, it is highly possible that several different types of galvanic corrosion cells are present in the same small area of an actively corroding piece of steel.
As the corrosion process progresses, the electrolyte may change due to materials dissolving in or precipitating from the solution. Additionally, corrosion products might tend to build up on certain areas of the metal. These corrosion products do not occupy the same position in the galvanic series as the metallic component of their constituent element.
As time goes by, there may be a change in the location of relatively cathodic and anodic areas and previously uncorroded areas of the metal are attacked and corroded. As the below figure indicates, this eventually will result in the uniform corrosion of the area.
The rate at which metals corrode is controlled by factors such as the electrical potential and resistance between anodic and cathodic areas, pH of the electrolyte, temperature and humidity.
How Zinc Protects Steel from Corrosion
The reason for the extensive use of hot-dip galvanizing is the two-fold protective nature of the coating. As a barrier coating, it provides a tough, metallurgically bonded zinc coating that completely covers the steel surface and seals the steel from the corrosive action of the environment. Additionally, zinc’s sacrificial action protects the steel even where damage or minor discontinuity in the coating occurs.
Barrier protection is perhaps the oldest and most widely used method of corrosion protection. It acts by isolating the metal from the electrolyte in the environment. Two important properties of the barrier protection are adhesion to the base metal and abrasion resistance. Paint is one example of a barrier protection system.
Cathodic protection is an equally important method for preventing corrosion. Cathodic protection requires changing an element of the corrosion circuit by introducing a new corrosion element, thus ensuring that the base metal becomes the cathodic element of the circuit.
There are two major variations of the cathodic method of corrosion protection. The first is called “the impressed current method.” In this method, an external current source is used to impress a cathodic charge on all the iron or steel to be protected. While such systems generally do not use a great deal of electricity, they often are very expensive to install.
The other form of cathodic protection is called “the sacrificial anode method.” In this method, a metal or alloy that is anodic to the metal to be protected is placed in the circuit and becomes the anode. The protected metal becomes the cathode and does not corrode. The anode corrodes, thereby providing the desired sacrificial protection. In nearly all electrolytes encountered in everyday use, zinc is anodic to iron and steel. Thus, the galvanized coating provides cathodic corrosion protection as well as barrier protection.
For over four decades, AZZ has been protecting critical infrastructure from the destruction of metallic corrosion as North America’s leading provider of galvanizing for fabricated steel. In addition to hot-dip galvanizing, AZZ offers a wide range of high-quality metal finishing and coating services. Our comprehensive metal coating capabilities create enduring infrastructure for constructing a stronger, safer, and sustainable world.