Passivation of Metals

The passivation of a metal as pertain to corrion refers to the formation of a protective layer of a creation product, which inhibits further reaction ion, the passivation of metals occurs in the presence of a particular conditions. Examples of metals that show passivation are stainless steal, nickel alloys.
These are two main conditions regarding the nature of the passive.

1. It is believed that the passive is always a diffusion barrier layer of reaction product (e.g. metal oxide or other compounds) that separated the metal from its condition and which slow doen the reaction rate.
2. It is believed that the passive metals are covered by chemistorbed films of the oxygen. Such a layer is supposed to displace the normally adsorbed water molecules and slow down the rate of anotic dissolution involving the hydration of metal ions, the two theories have in common that there is a protective film formed on the metal surface to create the passive state which results in increased corrosion resistance.

Processes of Corrosion

The processes of corrosion are:
Oxidation: An electrochemical   process   by which   an elements or species loose electrons and the consequence increase in its valence state.
Reduction: Is the process by which an element or species acquire one or more electrons. Thus reducing its valence with the metallic ores usually the oxides of metals reduce to their metals. The transformation of hydrogen is an example.
lonization: Is an electrochemical process that involves the dissociation of a compound into charged participles or ions, if there is no reductions reaction, there by will be no oxidation reaction.

Factors-Affecting Rate of Corrosion

The variable of the materials can have a deciding influence on ihc corrosion properties of the materials that are in the contact with it (Callister, 1997). These variables, which ulter mostly, determine the rate, extent and form of the corrosion process, influence the corrosion properly of materials: They are
Temperature: As a rule, increasing temperature increases corrosion rates. It is a result of the effect of temperature on the reaction kinetics themselves and the higher diffusion rate of many corrosion by-products at increased temperature. This depends on the variation of the dissolved oxygen contents and the predominant importance of cathodic diffusion control (Ijomah, 1991). In a closed system corrosion rate decreases with increasing temperature due to decrease in oxygen solubility.
Fluid Velocity: The higher the velocity, the higher the corrosion rate. At very low velocity the diffusion effects cause corrosion.
Suspended Solids: an increase in suspended solid level will accelerate corrosion rates. These solids include any inorganic or organic contaminants present in the water e.g. clay, sand, site or biomass.
pH: Corrosion rates always increase with decreasing pH (increasing acidity) it’s a direct conscience of increases in the concentration of an aggressive ion (H+) and increase in the solubility of most potentially corrosion products because it is an aggressive gas or oxidizing agent. As its concentration increases, corrosion rate increase until the diffusion to the surface reach maximum (Ekuma and Idenyi, 2006).

Corrosion Evaluation

The unit of corrosion rate is weight loss per unit area per time for most engineering application, the rate of material removal or change in thickness of a structure as a consequence of chemical action rather than weight loss in the most practicable measure of corrosion rate (Ijomah, 1991).
However, The corrosion penetration rate (CPR) or the rate of material removal as the consequence of the physicochemical interaction mostly expressed in mils/year or mmyear-1 is an important corrosion – monitoring index that gives reliable information to corrosion experts on the degree of corrosion progress in a given materials in services. The mathematical computation of CPR is based on the formular:
CPR =   KW

where, W is weight loss after exposure time t, R and A are density and exposed specimen area respectively and K is a constant whose magnitude depends on the system of unit used. For instance when K = 87.6, CPR is in mm year-1 and w, t, e and A are expressed in mg, h, gcm-3 respectively (Idenyi and Ekuma, 2006). For most application, a corrosion penetration rate of less than about 0.50mm year is acceptable (Ekuma and Idenyi, 2006).

Corrosion of Mild Steel 

Mild steel is one of the major construction materials, which is extensively used in chemical and allied industries for the handling of acid, alkali and salt solutions (Jaikumah et al., 2010). In the United Slates, for example, mild steel is used in 85% of all steel products. With such wide spread usage, the knowledge of the properties and corrosion behaviour of mild steel becomes imperative, especially for engineering and manufacturing firms, as well as students of metallurgy. Notably, all elements in steel, along with carbon, act as hardening agents. That is, they prevent dislocations from occuring inside the iron crystals, and stop the lattice layer from sliding past each other. This is what makes steel harder than ion. Thus, varying amounts of these hardening agents create different grades of steel. In this wise the ductilility, hardness and mild steel tensile strength are a function of the amount of carbon and other hardening agents present in the alloy.

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