METALS TREATMENTS for CORROSION RESISTANCE
Post 344 ⇒ by Gautam Shah →
Corrosion is oxidation of metals in reaction with an oxidant such as oxygen. Rusting is formation of iron oxides through gradual destruction of the material by chemical reaction with the environment. Corrosion or degradations as a process of wearing away of the mass also occurs in ceramics (efflorescence of bricks and terracotta), stones, glass and polymers. Corrosion can be local or more widespread affecting the entire surface. Corrosion is a diffusion process and occurs on the exposed surfaces. One of the main methods of preventing such destruction is to isolate the surface from its environment, including the atmosphere and contact with other passive materials.
Both, Ferrous and Non ferrous metals, have a tendency to corrode that is the atom lose electrons and become ions. This is called an anodic reaction, and for the corrosion process to proceed there must be a corresponding cathodic perceptor that adsorbs the electrons.
Reasons for corrosion could be:
Structural (joint system)
There are several ways prevent, reduce or terminate the corrosion.
☐ Metals get corroded by the environment. In most cases, to create an ideal environment for surrounding the metal will require a very large and extensive system, and possibly large quantity of energy to maintain it.
☐ Corrosion occurs due to the discharge of electrons. Metal electrons migrate due to electrical potential that builds up amongst different metals and between the metal and an electric conductive substance, of an assembly. If cathodic polarity is imposed by supplying an electric DC current to reverse the direction or by attaching sacrificial anodes such as aluminium or magnesium, whose atomic relationship with steel is such that a current is generated without external assistance (the anodes are earthed and have electrolytic continuity with the structure).
☐ Next course is to isolate the metal from corrosion conducive aspects of environment (such as moisture, water, acidic fumes, chlorides etc.). Coatings and plating provide the required isolation. The most appropriate way would be to place the isolating element as close to the metal surface, as possible, and if feasible even integrate it into the metal itself. Electroplating, metalizing etc., are some of the techniques that are on the verge of integration.
☐ Corrosion can also be controlled by constitutionally upgrading the metal, by one of the following techniques.
▫ Mechanical -stress induction and relief techniques at Normal, Hot and Cold temperatures,
▫ Chemical -additive and subtractive treatments
▫ Alloying – solid solution, precipitation, amalgamation.