Chapter 5: Corrosion & Non-ferrous Metal

CHAPTER 5 CORROSION & NON FERROUS

METAL

• Why does corrosion occur?

• What metals are most likely to corrode?

• How do temperature and environment affect corrosion rate?

• How do we suppress corrosion?

CORROSION AND DEGRADATION

Corrosion:--the destructive electrochemical attack of a material.--Al Capone's ship, Sapona,off the coast of Bimini.

THE COST OF CORROSION

INTRODUCTION

INTRODUCTION

Corrosion?

Why Do Metals Corrode?

Corrosion can lead to failures in plant infrastructure and machines which are usually costly to repair, costly in terms of lost or contaminated product, in terms of environmental damage, and possibly costly in terms of human safety.

Why corrosion should be avoid?

Corrosion: the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.

Corrosion

Categories of Corrosion

1) Dry corrosion

2) Wet / Electrochemical corrosion

Dry corrosion

Dry corrosion or oxidation occurs when oxygen in the air reacts with metal without the presence of a liquid.

Dry corrosion is surely the most visible of all corrosion processes, e.g. rusty bridges, flag poles, buildings and outdoor monuments.

Dry corrosion

Typically, dry corrosion is not as detrimental as wet corrosion, but it is very sensitive to temperature. If you hold a piece of clean iron in a flame, you will soon see the formation of an oxide layer!

The differences in the rate of dry corrosion vary from metal to metal as a result of the mechanisms involved.

Dry corrosion

In dry corrosion the oxygen has to be able to make contact with the metal surface.

Initially this is not a problem, but as soon as corrosion starts to occur the oxide layer, that forms on the metal surface, will limit the amount of oxygen that can further react with the metal.

Dry corrosion

Uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area. General thinning takes place until failure.

If surface corrosion is permitted to continue, the surface may become rough and surface corrosion can lead to more serious types of corrosion.

Uniform Corrosion

The formation of an oxide layer on the surface of a metal will, in some instances, lead to a reduction in the rate of corrosion.

When a metal oxidises and forms an outer layer, this layer can remain on the surface of the metal and limit further corrosion by inhibiting the ability of oxygen or other corrodents to reach the metal surface. This is known as passivation.

Formation of an Oxide Layer

Passivation of a metal surface through the formation of an oxide layer is found in many common metals and alloys.

Aluminium naturally forms a protective oxide layer (or scale) which slows down further oxidation and corrosion.

Stainless steel has chromium added to it, which forms a very protective oxide layer that prevents further corrosion.

Passivation

Not all oxide layers that form on metals are protective.

If the oxide does not form a continuous layer on the surface of the metal, it will not be able to reduce the amount of oxygen reaching the metal surface.

Cont.

Also known galvanic corrosion.

It will occur if an “electrochemical cell” is produced.

An electrochemical cell consists of an Anode, a Cathode, a Connection, and an Electrolyte.

Wet / Electrochemical corrosion

The anode is the metal that corrodes. It undergoes oxidation and therefore loses electrons.

The cathode can be a metal or any other conducting material. It undergoes reduction and therefore gains electrons. The reaction that occurs at the cathode is not necessarily related to the material that it is made from.


The connection is necessary for the electrons to travel between the anode and cathode and can be either physical direct contact or some form of wire.

An electrolyte must also be present to allow for migration of ions between the cathode and anode and participate in the formation of corrosion products.


Wet corrosion therefore involves an oxidation reaction at the anode and a reduction reaction at the cathode.

In the oxidation reaction metals give up electrons to become positively charged ions.

Reaction at anode:


The electrons generated from the metal are transferred to another material. This is the reduction reaction and occurs at the cathode.

Reaction at cathode:


For the electrons that are formed at the anode to move to the cathode there must be a path that they can follow. This usually means a physical contact between the anode and the cathode.

There must also be a way for the ions produced to come together so they can react to form the corrosion products. This usually is provided by a liquid (water, electrolyte) or moist conductor.

The Electrochemical Cell

The essential components are:

1) Anode

2) Cathode

3) Connection

4) Electrolyte

All of these components need to be present for corrosion to occur. If you remove even one of them then there will be no corrosion. This is the theory behind corrosion prevention.


If you have two metals in contact, with an electrolyte present, how do you determine which metal will corrode?

Eg. Zn and Cu


A metal higher in the series has a higher corrosion resistance than one below it in the series.

From the table we can see that gold (Au) has the highest corrosion resistance, and that if we were to join a steel pipe (Fe) with a copper fitting (Cu) then it is the steel that would corrode because it is below copper in the series.

Galvanic corrosion can cause unwanted accelerated corrosion when it is not considered during design or construction, however it can also be used to advantage.

When considering which metal will corrode we can look at the galvanic series. Metals closer to one another generally do not have a strong effect on each other, but the further apart two metals are, the stronger the corroding effect on the one higher in the list.

Anodes and cathodes arise in many ways. As well as connection between two different metals, a plain metal surface can have anodic and cathodic areas. For example:

- Grain boundaries can be anodic with respect to grain interiors.

- Cold worked regions are anodic to regions not cold worked.

Cont.

1) Varying stress

2) Varying oxygen concentration

3) Crevice corrosion

4) Intergranular corrosion / Grain interface

Forms of corrosion

Occurs in similar metals with varying stress concentration area.

Anode: high stress

Cathode: low stress

Varying stress corrosion

Stress corrosion is another form of corrosion that is important to many fields including civil structures.

Stress-corrosion occurs when a material exists in a relatively inert environment but corrodes due to an applied stress. The stress may be externally applied or residual.

This form of corrosion is particularly dangerous because it may not occur under a particular set of conditions until there is an applied stress. The corrosion is not clearly visible prior to fracture and can result in catastrophic failure.


Many alloys can experience stress corrosion, and the applied stress may also be due to a residual stress in the material. An example of a residual stress could be a stress remaining in a material after forming, or a stress due to welding.

Stress corrosion cracking will usually cause the material to fail in a brittle manner, which can have grave consequences as there is usually little or no warning before the failure occurs.


Stress corrosion is a form of galvanic corrosion, where stressed areas of the material are anodic to the unstressed areas of the material.

Practically the best way to control stress corrosion cracking is to limit or reduce the stresses a material is under while it is in a corrosive atmosphere.



Varying oxygen concentration

Is localized corrosion that has the apperearance of cavities (pits) and occurs on freely exposed surfaces.

Cathode: large passive area surrounding the pits.

Anode: area of the pits (since the pit is a cavity, it is difficult for the solution to leave the pit.)

Pitting

Pitting

Is localized corrosion that occurs at mating or closely fitting surfaces where easy access to the bulk of corrosive environment is hindered.

May occur at mating surfaces, assemblies of metals and nonmetals and deposits on the surfaces of metals.

Crevice corrosion

Crevice corrosion occurs when two components are joined close together to form a crevice. Corrosion occurs as the crevice accumulates water.

If the crevice is small enough a differential oxygen concentration in the water can form. When this happens the base of the crevice becomes anodic to the upper region.

Crevice corrosion often occurs under bolts and rivet heads as well as in shielded areas and under dirt or sand deposits.

Crevice corrosion

Crevice corrosion

Crevice corrosion

Is a microstructural corrosion that occurs along the grain boundaries of austenitic and ferritic stainless steels and chromium containing nickel alloys.

The heat of fabricating or processing operation can cause the chromium and carbon in stainless steels and chromium containing nickel alloys to combine forming, forming chromium carbide.

The grain boundaries become depleted in chromium and susceptible to corrosion in specific environments.

Intergranular corrosion


Intergranular corrosion occurs when the grain boundaries in a metal form an anode and the interior of the grain acts as a cathode. In serious cases this can lead to the grains falling apart.

This type of corrosion is a particular problem in stainless steels, however it can also occur in other metals.


In stainless steels the problem occurs after the metal is heated to between 425°C and 870°C. During the heating, the chromium in the stainless steel reacts with carbon in the steel and forms particles of chromium carbide at the grain boundaries. The regions near the grain boundaries become depleted in chromium.


This means that the regions around the grain boundaries are no longer protected by the chromium passivation, and therefore corrode intergranularly.


a) Cathode and anode protection

b) Material selection

c) Coatings (metals, organic and non

organic)

d) Design

Corrosion Prevention Techniques

Introduced by Humphrey Davey (1824).

Cathodic Protection is an electrochemical means of corrosion control in which the oxidation reaction in a galvanic cell is concentrated at the anode and suppresses corrosion of the cathode in the same cell.

1) Cathodic Protection

Figure shows a simple cathodic protection system. The steel pipeline is cathodically protected by its connection to a sacrificial magnesium anode buried in the same soil electrolyte.

Cathodic Protection

Concept : supply more electrons to the system or suppresses metal dissolution.

Two methods : external power source and galvanic coupling.

Cathodic Protection

By impressing a direct current between an inert anode and the structure to be protected. Since electrons flow to the structure, it is protected from becoming the source of electrons (anode). In impressed current systems, the anode is buried and a low voltage DC current is impressed between the anode and the cathode.

Cathodic Protection

Negative terminal to object.

Positive terminal to carbon electrode.

Electrons will be supplied to the tank.

This will prevent corrosion.

Cathodic Protection a) By impression electric current from an external power source

By coupling a given structure (say Fe) with a more active metal such as zinc or magnesium. This produces a galvanic cell in which the active metal works as an anode and provides a flux of electrons to the structure, which then becomes the cathode. The cathode is protected and the anode progressively gets destroyed, and is hence, called a sacrificial anode.

Cathodic Protection

Concept : sacrificial anode

Example:

Magnesium anode is connected to steel to prevent corrosion.

Cathodic Protection a) By appropriate galvanic coupling

Sacrificial anode continuously “consumed” by corrosion and needs replacement

Example:

Zinc: used broadly,e.g. galvanized zinc coating is a common distributed sacrificial anode for steel and magnesium: used for underground pipeline protection, i.e. in soil and other low conductivity environments.

Sacrificial anode method

Cathodic Protection

Predict the general corrosion rate and

susceptibility to localized attack of candidate

materials.

The scope of selection is not limited to metals.

2) Materials Selection

Choose material that is suitable with the environment. For example stainless steel is sensitive in nitric acid environment.

Choose metals closer to one another according to galvanic series.

Choose material suitable with the application.

Material Selection

Protective coatings are a simple way to reduce corrosion, by limiting the exposure of the metal to a corrosive environment.

Paint is a very common protective coating, but tar, pitch, bitumen and plastics are also used.

3) Protective Coatings

An important consideration for protective coatings is to ensure the coating is well adhered to the metal, and that it remains intact or is regularly repaired/recoated.

A further form of protective coating is to plate a coat of another metal onto the surface of the metal you wish to protect. One type of this coating is known as galvanising, where zinc is plated onto iron or steel. In the case of galvanising, the zinc acts as an anode and corrodes preferentially to the iron or steel.

Protective Coatings

Coating

Metal coating:

Example: Plating, ‘cladding’, ‘hot dipping’ dan ‘flame spraying’.

Zinc, nickel, tin, cadmium, gold, copper and platinum are metals used for plating.

Organic and non organic:

Coating applied to metals to protect them from environment.

Example of organic coating: paint, lacquers, varnish, tar.

Example of non organic coating: enamel, ceramic

Coating

The final, and most efficient way to prevent corrosion is to design your component or process to eliminate or reduce the possibility of corrosion. Some of the things to avoid are:

Design principles that are used to avoid corrosion include prevention of crevices by using continuous welds, avoiding locations where corrosives can concentrate.

Joins which require fasteners (e.g. bolts) usually create crevices. Welding is a better alternative, however, the weld shape should be monitored to ensure no crevices are present.

Avoid varying stress area such as sharp corner.

Avoid using metals further apart with one another in galvanic series.

4) Design

You can alloy metals to change their corrosion behaviour. Stainless steel is a very good example of this. Chromium is added to the steel to enhance its corrosion resistant properties. This works because the chromium in the metal forms a passivating layer, similar to that produced by aluminium.

You can also add small amounts of another metal to an alloy, which can shift its position in the galvanic series making it more cathodic than it was previously.

5) Alloying

NON FERROUS METAL

ENGINEERING MATERIAL

Nonferrous metals and their alloys do not contain iron as a principle ingredient.

These are used in industry because of the following characteristics:

a)Ease of fabrication

b)Resistance to corrosion

c)Good electrical and thermal conductivity

d)Light Weight

DEFINITION

1)Aluminum

2)Copper

3)Zinc

TYPES OF NON FERROUS METAL

This metal is highly used as it is good conductor of electricity. It is soft, malleable and ductile material with a reddish brown appearance. Its specific gravity is 8.9 g/cm3 and melting point is at 1083 °C.

It is largely used in making electrical cables and wires for electrical machinery and appliances. Copper alloys are categorized into two categories: copper zinc alloys and copper tin alloys. Brass is usually used for costume jewellery whereas bronze is used as bushings.

The most widely used copper zinc alloy is brass. It has a greater strength than that of copper but has low thermal and electrical conductivity. Brass is used for tube manufacturing, plumbing fittings and musical instruments.

The alloys of copper and tin are termed as bronzes. The useful range of compositions is 75-95% copper and 5-25% tin. This alloy is comparatively hard, resists surface wear and can be shaped into wires, rods and sheets easily. Bronze is used for making gears, air pumps, bushings and condenser bolts.

Copper (CU)

It is a white metal produced by electrical process from alumina oxide which is prepared from clay material called bauxite. It is a lightweight material having a specific gravity of 2.7 g/cm3 and a melting point of 660°C. In its pure state, aluminium is weak and soft but becomes hard and rigid once mixed with other materials (aluminium alloys). It has good electrical conductivity and good resistance to corrosion. Due to its light weight, it is used heavily in the aerospace industry. The commonly used aluminium alloys duralumin and Y-alloy.

Application: • Beverage can • Aluminium foil • Window cladding • Roof frame • Stairs • Bottle cap

Aluminium (Al)

Zinc is a bluish white metal which is in pure state. It has a specific gravity of 7.1 g/cm3 and its melting point is 420 °C. It is not very malleable and ductile at room temperature. It offers high resistance to atmospheric corrosion. It is used for covering steel sheets to form galvanized iron due to its high resistance to atmospheric corrosion. Zinc has its alloys too. The usual alloying elements for zinc are aluminium and copper. The aluminium improves mechanical properties and also reduces the tendency of zinc to dissolve iron. Copper increases the tensile strength, hardness and ductility. The alloys from zinc are used for washing machines, oil burners, refrigerators, radios and television sets.

Zinc (Zn)

Engineering

Chapter 5: Corrosion & Non-ferrous Metal