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CORROSION PREVENTION IN METALS – I

Dr. Munther Issa Kandah

Chemical Engineering Department

Jordan University of Science & Technology

INTRODUCTION

CORROSION: is the destruction or deterioration of material because of reaction with its environment.

Corrosion touches all: inside and outside the home, on the road, on the sea, in the plant, and in aerospace vehicles.

Corrosion

Destruction/deterioration         chemically

chemical and mechanical

Surface               metal

nonmetal

Environment          Humid atmosphere

Industrial atmosphere

–         rust: cars

–         corrosion of underground pipes: Drinking water, oils or gas.

Localized                Loss in weight

Corrosion                                        Deterioration in properties

General                   Both

Metals                            Corrosion                      Non-metals

Include electrochemical reactions                        Does not include

(Transfer of electrons on the metal surface)           electrochemical

rxn

ELECTROCHEMICAL THEORY

The requirements for corrosion to occur are:

1.  Anodes and cathodes must be present                 to form a cell

2.  Direct current must flow

The anode is the area where corrosion occurs and where current leaves the metal and enters the solution.

Anodes and cathodes can form on a single piece of metal because of local differences in the metal or in the environment.

The metal at the anode dissolves and becomes an ion. It is oxidized and losses electrons.

Formation of ferrous (Fe++) ions in the corrosion of the iron

Hydrogen ions accept electrons at the cathode and form hydrogen gas.

Basic diagram showing requirements for corrosion of metal



The iron atom detaches itself and goes into the solution as ferrous ion.

The electrons are left on the metal and travel through it to the cathode area, and accepted at the cathode area.

Fe = Fe2+ + 2e-             (oxidation, anode)

2H+ + 2e- = H2             (reduction, cathode)

Fe + 2H+ = Fe2+ + H2     (overall reaction)

The quantity of current that passes through this cell is proportional to the amount of metal that corrodes.

Why do areas on the same piece of metal acts differently “some become anodes and others cathodes”?

Differences exist at the interface on the metal surface itself.

In hydrochloric, phosphoric, hydrofluoric, and water-soluble organic acids such as formic and acetic, the cathodic reaction is the evolution of hydrogen gas and the anodic reaction is the oxidation of a metal

M = Mn + ne-

The cathodic reactions are one or more of these:

Hydrogen evolution:                2H+  +  2e- = H2

Oxygen reduction (acid solution):

O2 + 4H+  +  4e- = 2H2O

Oxygen reduction (neutral or basic solution):

O2 + 2H2O + 4e- = 4OH-

Metal ion reduction:                M3+   +   e- = M2+

Metal deposition:           M+   +   e- = M

The emf series tells us the following:

1. A metal lower in the series will act as the anode and one higher will act as a cathode when these two metals are coupled or are in contact with each other.

– Sodium reacts violently with water.

– Platinum is not attacked by water or even strong nitric acid.

Copper sulfate will corrode iron.

2.  Metals above hydrogen are not attacked by acids:

Cu + H2SO4 = no reaction

This reaction does not proceed because it is endothermic or absorbs heat, in other words, energy must be supplied from outside to make it go.

In the presence of air or oxygen, however, copper corrodes by the following reaction:

2Cu + O2 + 2H2SO4 → 2CuSO4 + 2 H2O

This reaction is exothermic – it gives off heat.

Batteries

Attack on zinc by hydrochloric acid (air-free HCl)

– Zinc is oxidized to zinc ions and Hydrogen ions are reduced to hydrogen

– Zn =Zn2+ + 2e- Oxidation (anodic reaction)

– 2H+ + 2e- = H2         Reduction (cathodic reaction)

– Both reactions must occur simultaneously and at the same rate on the metal surface:

Rate of oxidation = Rate of reduction

Corrosion of zinc in aerated hydrochloric acid.

Two cathodic reactions are possible:

The evolution of hydrogen and the reduction of oxygen

On the surface of the zinc there are two electron-consuming (reduction) reactions.

Since the rate of oxidation must be equal to the rate of reduction, increasing the total reduction rate increases the rate of zinc solution.

Iron is immersed in water or seawater which is exposed to the atmosphere

Fe = Fe2+ + 2e-                              anodic reaction

Assume the water or the seawater is nearly neutral and contains oxygen

O2 + 2H2O + 4e- = 4OH-                 cathodic reaction

The Overall reaction:

2Fe + 2H2O +O2 = 2Fe2+ + 4OH- = 2Fe(OH)2

Ferrous hydroxide precipitates from solution. It is not stable in oxygenated solutions and is oxidized to ferric salt:

2Fe(OH)2 + H2O + 1/2O2 = 2Fe(OH)3      (Rust)

Passivity

Passivity in a metal refers to a relatively inactive state in which it (the metal) displays a more noble behavior than thermodynamic considerations predict. It is the reason a metal does not corrode when it should.

A normal metal is a metal which does not show passivity effect.

If chromium is added to iron to make a high chromium (over 12%) steel, this alloy should be less corrosion resistant than ordinary steel.

IF Chromium and the stainless steels become passive, or exhibit passivity, because of the formation of a protective film on the surface of the metal.

Aluminum quickly forms a surface coating of aluminum oxide, which stops further corrosion by many environments.

Lead is below hydrogen in the emf series, but it shows excellent resistance to sulfuric acid at concentrations below 90%. Lead sulfate quickly forms when lead is exposed to sulfuric acid and corrosion stops.

In strong sulfuric acid, lead corrodes because lead sulfate dissolves in this acid and the metal cannot become passive or passivity is destroyed.

Titanium forms a protective film of TiO2,  and high-silicon cast iron forms a protective silica (SiO2) coating.

It must be emphasized that passivity is a relative term.

A stainless steel may exhibit passive behavior in nitric acid and in water, but it would be attacked by strong hydrochloric acid.

In other words, a metal or alloy is passive or active only in relation to particular environments.

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