ABSTRACT (THE EFFECT OF L-METHIONINE ON CORROSION INHIBITION OF MILD STEEL IN 2M HCl – USING WEIGHT LOSS METHOD)

The inhibitive action of L- Methionine against the corrosion of mild steel in 2M HCl was investigated using weight loss method at 303K, 313K, 323K and 333K. Results obtained showed that L-Methionine is a good inhibitor for mild steel in 2M HCl solution with maximum inhibition efficiency of 93.08% in 5.0 x 10-4M at 323K. The result also reveals that increase in temperature increases corrosion rate and decreases efficiency; while increase in concentration of the inhibitor decreases corrosion rate and decreases efficiency

                                                           CHAPTER ONE

INTRODUCTION

The corrosion of metals at the solid/liquid interface by corrodents such as acid and base is a major problem encountered in numerous industrial processes (Kadhum et al., 2014). Millions of dollars are lost yearly because of corrosion; much of this loss is due to corrosion of iron and steel although many other metals may corrode as well as the problem with iron. Due to this harmful effect, corrosion is an undesirable phenomenon that ought to be prevented (Buchwershaija, 2009). Mild steel is one of the major construction materials which is extensively used in chemical and allied industries for the handling of acids, alkalis, salts and other solutions.

  1. Definition of Corrosion

Corrosion is the deterioration or loss of material and its critical properties due to chemical, electrochemical and other reactions of the exposed material surface with the environment (Fontana, 1986). Corrosion is a natural process that reduces the binding energy in metals with the end result involving a metal being oxidized as the bulk metal loses one or more electrons (Chinwko et al; 2014). Corrosion of metal differs from that of other materials in that it involves charge transfer (ions and electrons) and in most cases conducting solutions, that is electrolytes (Khamael and Mabrouk, 2009). The environment that corrodes a metal could be air, water, acid, base, soil and others.

  1. Chemistry of Corrosion

Corrosion of metals consists of the formation of hydrated oxide, Fe(OH)3, Fe(OH) or in Fe2O3.H2O. It is an electrochemical process which requires the presence of water, oxygen and an electrolyte. In the absence of anyone of these, corrosion does not occur to a significant extent.

            Corrosion is an electrochemical process. One part on a metal surface behaves as anode and undergoes oxidation and the other part act as cathode and undergoes reduction; while the liquid acts as an electrolyte. In acidic medium, the corrosion of a metal leads to the evolution of hydrogen gas at the cathode.

At anode: M                       Mn+ ne- (oxidation)……………………………… (1.1)

At cathode: 2H+ + 2e H2 (reduction)……………………………………….(1.2)

In alkaline medium

Anode: Fe   Fe2+ + 2e (oxidation)……………………………………………(1.3)

Cathode: O2 + 2H2O + 4e 4OH

Fe2+ + 2OH Fe (OH)2 (reduction)…………………………………………… (1.4)

When oxygen is in excess, Fe (OH)2 is oxidized to Fe(OH)3 and decomposes to hydrated ferric oxide according to equation (1.5):

4Fe (OH)2 + O2        4Fe(OH)3 + H2O…………………………………… (1.5)

The loose porous rust of Fe(OH)3 can also slowly transform into a crystalline form as Fe2O3.H20, the familiar reddish brown solid called rust. Since this process involves hydrogen ions or hydroxide ions, they would be affected by changes in PH

  1. Classification of corrosion

Corrosion of metals is classified according to nature of corrosion reaction. One method divides corrosion into low-temperature and high temperature corrosion. Another separates corrosion into direct combination (or oxidation) and electro-chemical corrosion.

The preferred classification here is:

  1. Dry or chemical corrosion

This occurs in absence of a liquid phase or above the dew point of the environment. Vapors and gases are usually the corrodents. Dry corrosion is most often associated with high temperatures like oxidation of metals.

Fe + O2 Fe2O3, Fe3O4, FeO

Dry chlorine is practically non-corrosive to ordinary steel, but moist chlorine dissolved in water, is extremely corrosive and attacks most of common metals and alloys. The reverse is titanium.

                  Dry-forms iron sulfide

H2O

                   Wet-forms oxy-sulfides (H2S + H2O) + hydrogen embrittlement

  1. Wet or electrochemical corrosion

This occurs when a liquid is present. This usually involves aqueous solution or electrolytes and accounts for the greatest occurrence of corrosion. Most of the knowncorrosion forms belong to this type of corrosion (Khamael and Mabrouk, 2009).

  1. Types of corrosion

There are several types of corrosion. These include; general attack corrosion, localized corrosion, galvanic corrosion, environmental cracking flow assisted corrosion (FAC), inter-granular corrosion, De-alloying, fretting corrosion.