This is due to the need of replacing corroded metal tools and structures to preserve functionality and safety (Roberge; Lancashire).
The rusting process is a complex reaction involving various stages upon the contribution of different compounds and production of derivatives. Rusting is an electrochemical process that occurs in the presence of water or moisture and source of electrolyte. This process cannot proceed to any considerable extent if any of the said requirements is not present. When the metal is in the air, there should be more than 50% relative humidity while above 80% relative humidity results to severe rusting of bare metal (Lancashire 3).
Above are the conditions that are conducive to corrosion of metals but the reason behind why metals or any other substance corrode in the first place need to be answered. Corrosion is an opportunity for metals to deteriorate. Metals like most substances need to undergo this process to be able to combine electrochemically with other substances in order to form new compounds. Many environmental conditions provide this opportunity and places with high moisture or relative humidity is one of these conditions, called rusting, although corrosion occurs when metal come into contact with various chemicals such as acids, bases, ammonia gas and other vapors and substances (Roberge).
A model (Figure 1) illustrates the chemical process that occurs during rusting. The first stage of the process is called anodic reaction. During this stage, the metal dissolves by generating electrons as shown in the upper portion of the model. The second stage is called cathodic reaction, during which the electrons that were produced in the first stage are then consumed. These two stages of the rusting process can occur adjacently or far apart (Roberge).
Figure 1. A schematic representation of the rusting process showing the generation and consumption of electrons (from Corrosion Doctors).
There are six chemical reactions that occur during the rusting process. First is the oxidation of the iron (Fe) which has come into contact with moisture or water droplet. The second chemical reaction involves the formation of water from the absorption by dissolved oxygen of the electrons produced from the first reaction. The third chemical reaction includes the generation of hydrogen gas through the consumption of electrons by hydrogen ions. The fourth chemical reaction is the production of the insoluble iron or rust through the reaction of hyrdroxide ions with ferric ions. Another reaction involves the production of rust from the combination of hydrogen ions and oxygen with iron ions. And another chemical process includes the formation of rust through the formation of iron hydroxides with the interaction between hydroxide irons with iron ions (Tarr).
The first chemical reaction is illustrated by the following process (Fig. 2). This chemical reaction occurs when water moisture comes into contact with a metal surface. The solid iron or Fe(s) oxidizes in the presence of water to produce aqueous iron or Fe2+(aq), producing two free electrons (Baldragon Academy).
Figure 2. Oxidation of iron.
The second reaction is the formation of water through consumption of electrons by hydrogen ions and dissolved oxygen as shown by Figure 3. Four electrons (e) are absorbed by four hydrogen