Corrosion

Engineering Chemistry (revised edition)ISBN: 978-81-265-4475-2

Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

Corrosion in Metals and Alloys

The destruction of metals or alloys by surrounding environment through chemical or electrochemical changes.

Causes of Corrosion

Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals. The most familiar example of corrosion is rusting of iron exposed to the atmospheric conditions.





Corrosion Cell



Types of Corrosion

Dry Corrosion (Direct Chemical Attack)

There are three main types of dry corrosion.

1.Oxidation corrosion (reaction with oxygen):

The overall reaction can be given by



Mechanism for oxidation corrosion

The oxide layer formed can be

1.Stable

2.Unstable

3.Volatile

4.PorousEngineering Chemistry (revised edition)

ISBN: 978-81-265-4475-2Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

2. Corrosion by other gases such as Cl2, SO2, H2S, NOx: In dry atmosphere, these gases react with metal and form corrosion products, which may be protective or non-protective. Dry Cl2 reacts with Ag and forms AgCl which is a protective layer, while SnCl4 is volatile. In petroleum industries at high temperatures, H2S attacks steel forming FeS scale which is porous and interferes with normal operations.

3. Liquid metal corrosion: In several industries, molten metal passes through metallic pipes and causes corrosion due to dissolution or internal penetration. For example, liquid metal mercury dissolves most metals by forming amalgams, thereby corroding them.



Wet corrosion (Electrochemical theory of corrosion)

Metal comes in contact with a conducting liquid or when two dissimilar metals are immersed dipped partly in a solution.



The reactions involved in the galvanic cell are as follows:

1.Anodic reaction: Metal undergoes oxidation (corrosion) with positive release of electrons:

2.Cathodic reactions: These reactions are dependent on the constituents of the corrosion medium.

•If the solution is aerated and almost neutral, oxygen is reduced in presence of H2O to OH− ions.

•If the solution is deaerated and almost neutral, H2 is liberated along with OH− ions.

•If the solution is deaerated and acidic, H+ ions are reduced to hydrogen gas



3. The metal ions formed at the anode combine with hydroxyl ions and form the corresponding metallic hydroxide Fe(OH)2, which further gets oxidized to hydrated ferric oxide [rust].

In the presence of limited oxygen, black rust is formed as follows:





Types of Electrochemical Corrosion

Differential Metal Corrosion (Galvanic Corrosion)

When two dissimilar metals are in contact in corrosive environment.



Other examples of differential metal corrosion (galvanic corrosion) include the following:

1.Buried iron pipeline connected to zinc bar.

2.Steel pipe connected to copper plumbing.

3.Steel propeller shaft in bronze bearing.

4.Zinc coating on mild steel.

5.Lead–tin solder around copper wires.

Preventive Measures

1.Placing a thin layer of an insulator between two metals or materials.

2.Selecting materials having very less potential difference.



Differential Aeration Corrosion (Concentration Cell Corrosion)

When metal is exposed to differential air or oxygen concentration.



The examples of differential aeration corrosion include:

1.Half-immersed iron plate in aqueous solution.

2.Steel pipe carrying any liquid exposed to atmosphere.

3.Ocean going ships.

4.Steel storage tanks.

Preventive Measures

1.Using metallic coating, electroless plating or chemical conversion methods.

2.Maintaining the material’s own protective fi lm.

3.Controlling the chemistry of fluids and using inhibitors.



Waterline Corrosion

Waterline corrosion: (a) Water storage tank and (b) ocean going ship.



Pitting corrosion

Localized attack on iron surface in chloride medium

Preventive Measures

1.Preparing surfaces with best possible finish. Mirror-finish resists pitting best.

2.Removing all contaminants, especially free-iron by passivation.

3.Designing and fabricating to avoid trapped and pooled liquids.



Other Types of Corrosion

1.Intergranular corrosion:



2. Soil corrosion: Underground pipes, cables, tank bottoms, etc., get corroded due to moisture, pH of soil, ionic species like chlorides and micro-organisms like bacteria. It is further enhanced by differential aeration of various parts of the soil.





Small ratio of anodic to cathodic region leading to high rate of corrosion

Large ratio of anodic to cathodic region leading to low rate of corrosion.



Corrosion Control Methods

1. Design and material selection: number of angles, corners, edges etc. should be minimized

2. Protective coatings: prevents surface corrosion



3. Cathodic protection: metal at cathodic region is unprotected

Impressed current method.

4. Anodic protection: impressed anodic corrosion provides anodic protection

5. Corrosion inhibitors: Examples of anodic inhibitors include chromate, tungstate, molybdate, etc.



Practical Applications of Corrosion Control Methods

Materials need protection when they are exposed to a variety of corrosive environments, such as atmosphere, aqueous solutions, soils, acids, bases, inorganic solvents, molten salts, liquid metals, etc. Some practical applications of corrosion protection methods are discussed as follows.

1.Protection of steel structures: Steel is used in modern industrial, residential and commercial buildings, bridges, dams, offshore platforms and nuclear containments etc. A two-fold approach is used for protection:

a)Design and detailing of structure

b)Surface treatment with coatings Engineering Chemistry (revised edition)ISBN: 978-81-265-4475-2


Practical Applications of Corrosion Control Methods

2.Protection of metallic structures situated in electrolytes: These metallic structures include pipelines, underground storage tanks, water storage tanks, ship hulls and interiors, lock gates and dams, water treatment facilities, well casings, bridge decks, and steel buttresses and the common electrolytes are water and soil.

Corrosion may result due to soil pH, O2 concentration, moisture content, ions etc.Protection methods – Cathodic protection, using copper based alloy, galvanized steel etc



3. Temporary protection of metallic parts of machines: During the time of manufacture, assembly, storage and use, bare metal surfaces of metallic components of the machines as well as the finished machine, need to be protected from corrosion.

The different types of surfaces protected include:

• Components of automobiles.

• Components of electrical appliances.

• Components of agricultural and industrial machinery.

• Tools and machine tools.

• Screws, rivets and bolts used as fasteners.

• Steel and coated steel strips, sections and tubes.

• Strips and sections of aluminum and other alloys.

• Construction equipment.Engineering Chemistry (revised edition)

ISBN: 978-81-265-4475-2Copyright©2014 Wiley India Pvt. Ltd. All Rights Reserved

The methods employed for this protection include use of:

1.Solvent based fluids, which deposit a coating when the solvent evaporates. 2.Grease-like materials to form a protective coating. Materials include petroleum jelly, mineral oil based greases, vegetable oils based greases and semi-fluid compounds based on viscous oils. 3.Protective oils.4.Resin and plastic coating that can be stripped-off when the protection is no longer required.5.Volatile corrosion inhibitors that continuously release vapors of anticorrosive compounds.6.Contact corrosion inhibitors that inhibit corrosion when in direct contact with the metal.7.Water-based emulsified protectives that are applied to the surface to be protected either by dipping, brushing or spraying and deposit a waxy film after evaporation of water.8.Desiccants such as silica gel are used as dehumidifiers to protect machine parts from moisture.



Permeability of Oxygen to Patch Forming Material

The strength of Portland cement, used as building material, is enhanced by additional reinforcement in form of steel rods, wires, bars or mesh. These are embedded into the fresh and unused concrete.

When the hydrated cement hardens, it forms a protective layer around steel reinforcement, thus protecting it from corrosion. Even when the cement cracks due to mechanical or environmental stress, considerable reinforcement is maintained.

This protective cement layer is, however, destroyed on exposure to aggressive environment by carbonation process and attack of chloride ions, particularly in sea water.



Permeability of Oxygen to Patch Forming Material (Contd...)

The repair and maintenence of concrete/ cement structure with suitable patch forming material is important method for corrosion resistance and protection. The composition consist of more than one type of cements along with additive like silica fumes, slag and fly ash, polymer additives, fine polymer fibers.

These offers alkaline medium, low shrinkage, low permeability and high strength.

Fiber reinfored polymers (FRP) are used for repairing corrosion damaged structure.

Low water to cement ration, result in reduction of permeability that slow down the ingress of water, O2, CO2, and Cl- ions.

Scaling of Ferrous Materials

An iron scale is heterogeneous in nature and is composed of the following iron compounds in varying ratios:

The iron scales are produced by deposition of oxidized iron compounds. The scale can form passive layer over the metal and protecting it from further corrosion. In water distribution system, scaling contributes to contamination of water both from dissolution of scales and scale detached from the surface.

Corrosion

31

Corrosion: Deterioration of metal by electrochemical process

e.g. Rust on iron, due to formation of hydrated ferric oxide i.e. Fe2O3.xH2O

Tarnish on silver

Green Patina on Cu

Corrosion causes enormous damage to building, bridges, ships and cars.

Fundamental Process in Corrosion

32

Conditions for rusting:Oxygen gas and water

Electrochemical reaction:Anode: Metal surfaceFe(s) → Fe2+ (aq) + 2e- E0

red = -0.44 VCathode: Another region of same metal surfaceO2( g) + 4 H+ + 4e- → 2H2O (E0

red = 1.23VOverall reaction: 2Fe(s) + O2 (g) + 4H+

(aq) → 2Fe2+ (aq) + 2H2O E0

cell = Ec – EA = 1.23 – (-0.44) = 1.67V

4Fe2+ (aq) +O2 (g) + (4+2x)H2O(l) → 2Fe2O3.xH2O(s) +8H+

Rust

Corrosion Prevention

33

Barrier Protection: Coating with paintAlloy: Stainless steel i.e. Alloy of Fe-Cr Sacrificial Protection: By covering Fe(s) with more active metal. Such as Zn and Sne.g. Fe surface covered with Zn metal

Zn → -Zn 2+(aq) + 2e- E0red = -0.76

Fe → Fe2+ (aq) + 2e- E0red = -0.44

The Process of covering Fe with Zn is known as Galvanization.

Sn → Sn2+ + 2e- E0red = -0.14Fe → Fe2+ (aq) + 2e- E0

red = -0.44

Corrosion Prevention

34

Electrical/Cathodic Protection: Fe is connected with more active metal Like Mg and Zn.

e.g. Under ground water pipesFe(s) → Fe2+ (aq) + 2e- E0

red = - 0.44 VZn (s) → -Zn2+(aq) + 2e- E0

red = - 0.76 VMg (s) → Mg 2+(aq) + 2e- E0

cell = - 2.37 V

Corrosion on other metals

Aluminum: Al2O3 layer will formedCopper: CuCO3

Silver: Ag2S

Documents

Corrosion