Experiment-10

Aim of the Experiment–Organic inhibitor:

Effect of organic inhibitor thiourea on mild steel in 1N HCl acid solution

Theory: – Inhibitor is a chemical substance when added in small concentration in a corrosive environmentretards the anodic or cathodic or both the electrochemical reactions, accordingly it is called anodic or cathodic or mixed inhibitor.

Types of inhibitors-

i) passivators

ii) hydrogen poisons

iii)scavengers

iv)organic inhibitors-

a) adsorption type inhibitors

b) vapour phase inhibitor

c) pickling inhibitor

Thiourea is an organic compound which acts as pickling inhibitors. Pickling is a process in which metal is dipped in acid bath to remove scale oxide or clean rust.

Here thiourea acts as a good inhibitor in pickling bath; they adsorb on metal surface and increase hydrogen overvoltage and therefore stops metal dissolution reactions preventing further corrosion.

Pickling inhibitors get adsorbed on the clean surface & increase the hydrogen overvoltage so that the cathodic reactionsare very much retarded and some

inhibitors also inhibit the anodic reactions as well. Thus pickling inhibitors are cathodic or mixed inhibitors.

Mechanism

When charge on metal is zero, metal always gets adsorbed by a layer of charge when metal is in an electrolyte.

-Inhibitors have to remove other ions that has already adsorbed on the metal surface.

-Inhibitors are adsorbed on the metal surface as charge decreases, so that the action of inhibitor gets easier.

The inhibitor forms a diffusion barrier layer over the surface of the metal by adsorption, there by trying to separate the metallic surface from the corrosive environment. Then they block either anodic sites or cathodic sites or both. In this way they retard the corrosion rate.

Observations:-

Results:— By studying graph we seethat for 1 N HCl solution the corrosion rate is higher thanwhenthio-urea inhibitor is added.

Precautions:— 1. Care is taken to ensure that the electrodes do not touch the glass-walls as it might lead to erroneous results.

2. Care is taken to avoid any parallax error.

3. The surface of the specimen should be polished smooth without any surface undulations or pits prior to experiment.