TOPIC 5CORROSION AND NON-

FERROUS METALPREPARED BY:

AZMARINI BINTI AHMAD NAZRIPENSYARAH POLITEKNIK UNGKU OMAR

CORROSION

• Is a process of formation of the compound of pure metal by the chemical reaction between metallic surface and its environment.

• It is an oxidation process.• It causes loss of metal.• Example: formation of rust on the surface of iron, formation

of green film on the surface of copper.• The responsible factors for the corrosion of the metal are:a) the metal itself;b) The environment chemicals;c) Temperature;d) Design of the metals• Corrosion is often classified as wet and dry corrosion.

Dry Corrosion•Dry corrosion occurs in the absence of a liquid phase or above dew point of the environment.•It is direct reaction of atmospheric gases like oxygen, halogens, oxides of sulphur, oxides of nitrogen, hydrogen sulphide and fumes of chemicals with metal.•Oxygen is mainly responsible for the corrosion of most metallic substances when compared to other gases and chemicals.•There are three main types of dry corrosion:a) oxidation corrosion (reaction with oxygen)b) corrosion by other gasesc) liquid metal corrosion

Difference between dry and wet corrosion

Dry corrosion Wet corrosion Corrosion occurs in the

absence of moisture. It involves direct attack of

chemicals on the metal surface.

The process is slow. Corrosion products are

produced at the site of corrosions.

The process of corrosion is uniform

Corrosion occurs in presence of conducting medium.

It involves formation of electrochemical cells.

It is a rapid process Corrosion occurs at anode

but rust is deposited at cathode.

It depends on the size of the anodic part of metal.

Electrochemical Corrosion

a)Oxidation – Reduction Reaction•Eg : a piece of zinc metal placed in a beaker of dilute hydrolic acid.•Result: the zinc dissolves or corrodes in the acid, and zinc cloride and hydrogen gas are produced as indicated by the chemical reaction:

Zn + 2HCl ZnCl2 + H2

•This reaction can be written in a simplified ionic form, omitting the cloride ions, as

Zn + 2H+ Zn2+ + H2

•This equation consists of two half reactions:one for oxidation of the zincone for reduction of the hydrogen ions to form hydrogen gas

• These half reactions can be written as:Oxidation: Zn Zn2+ + 2e-

Reduction : 2H+ + 2e- H2

• Oxidation reaction: metals form ions that go into aqueous solution is called the anodic reaction. In the anodic reaction electrons are produced that remain in the metal, and the metal atoms form cations .

• Reduction reaction: a metal or nonmetal is reduced in valence charge is cathodic reaction. In the cathode reaction there is a consumption of electrons.

• Electrochemical corrosion reactions involve oxidation reactions that produce electrons and reduction reactions that consume them.

• Both oxidation and reduction reactions must occur at the same time and same overall rate to prevent a buildup of electric charge in the metal.

b) Galvanic Cells with Electrolytes• A galvanic cell can be constructed with two dissimilar metal

electrodes each immersed in a solution of their own ions.• Eg: Zinc electrode immersed in a 1 M solution of Zn2+ ions and

another of copper immersed in 1 M solution of Cu2+ with the solution at 25⁰C

• The two solutions are separated by a porous wall to prevent their mechanical mixing, and an external wire in series with a switch and a voltmeter connects on the two electrodes.

• When the switch is just closed, electrons flow from the zinc electrode through the external wire to the copper electrode and a voltage of -1.10V shows on the voltmeter

• Result: Zn electrode will be oxidized to Zn2+ ions, and Cu2+ ions will be reduced to Cu at the Cu electrode

• Oxidation : Zn Zn2+ + 2e- E⁰ = -0.763 VReduction : Cu2+ + 2e- Cu E⁰ = -0.337 VOverall Reaction : Zn + Cu2+ Zn2+ + Cu E⁰ = -1.100 V

Assignment 2 :

A galvanic cell consists of an electrode of zinc in a 1 M ZnSO4 solution and another of nickel in a 1 M NiSO4 solution. The two electrodes are separated by a porous wall so that mixing of the solutions is prevented. An external wire with a switch connects the two electrodes. When the switch is just closed:a)At which electrode does oxidation occur?b)Which electrode is the anode of the cell?c)Which electrode corrodes?d)What is the emf of this galvanic cell when the switch is just closed?

Phenomena in the Electrochemical Series•Eg : Rusting of iron•If a piece of iron is immersed in oxygenated water, ferrite hydroxite [Fe(OH)3] will form in its surface.•The oxidation reaction:

anodic reaction : Fe Fe2+ + 2e-

•The reduction reaction: cathodic reaction : O2 + 2H2O + 4e- 4OH-

• The overall reaction :2Fe + 2H2O + O2 2Fe2+ + 4OH- 2Fe(OH)2

Types of Corrosion:

a)Pitting Corrosion•Is a form of localized corrosive attack that produces holes or pits in a metal.•Pitting is often difficult to detect because small pits may be covered by corrosion products.•Pitting usually requires an initiation period, but once started, the pits grow at an ever-increasing rate.•Eg: The propagation process for a pit in an aerated saltwater environment.•The anodic reaction of the metal at the bottom of the pit.•The cathodic reaction takes place at the metal surface surrounding the pit.

b) Crevice reaction

• Can occur in crevices and under shielded surfaces where stagnant solutions can exist.

• It occurs under gaskets, rivets, and bolts, between valve disks and seats and in many other similar situations.

• Occurs in many alloy systems such as stainless steels and titanium, aluminum and copper alloys.

c) Intergranular Corrosion

• Is localized corrosion attack at and/or adjacent to the grain boundaries of an alloy.

• If a metal corrodes uniformly, the grain boundaries will only slightly more reactive than the matrix.

• However, under other conditions, the grain-boundary regions can be reactive, resulting in intergranular corrosion that causes loss of strength of the alloy and even disintegration at the grain boundaries.

d) Stress Corrosion

• Stress-corrosion cracking (SCC) of metals refers to cracking caused by the combined effects of tensile stress and a specific corrosion environment acting on the metal.

• During SCC the metal’s surface is usually attacked very little while highly localized cracks propagate through the metal section.

• The stresses that cause SCC can be residual or applied.• High residual stresses that cause SCC may result, for example,

from thermal stresses introduced by unequal cooling rates, poor mechanical design for stresses, phase transformations during heat treatment, cold working and welding.

Control Corrosion

a)Anode and Cathode Corrosioni.Anodic protection

•Based on the formation of protective passive films on metal and alloy surfaces by externally impressed anodic currents.•Controlled anodic currents by a device called a potentiostat can be applied to protect metals that passivate such as austenitic stainless steels, to make them passive and hence lower their corrosion rate in a corrosive environment.

In this method, an external d. c. power supply is used. The. negative terminal of power supply is connected to underground component and positive to an inert anode. e. g. graphite. Therefore, current passes to component and corrosion is suppressed

Usually, underground tanks and pipes are protected by impressed current method.

                                                                    

ii. Cathodic Protection

• Cathodic protection is also achieved by coupling a component to metal which is more anodic. This metal is called as sacrificial anode, e.g. Magnesium is anodic with respect to steel and corrodes when galvanically coupled. Magnesium gets consumed to protect the steel, hence it is called as sacrificial anode. These anodes are used in the following application;

• Protection of hull of ship. • Domestic and industrial water heater. • Elevated water storage tank • Underground pipes • Boilers

Sacrificial anode for cathode protection of (a) Underground pipe and (b) Domestic water heater

b) Material Selection• Metallic Materials i. For reducing or nonoxidizing conditions such as air-free acids and

aqueous solutions, nickel and copper alloys are often used.ii. For oxidizing condition, chromium-containing alloys are used.iii. For extremely powerful oxidizing conditions, titanium and its

alloys are commonly used.• Nonmetallic Materials – polymeric materials such as plastic and

rubbers are weaker, softer and in general less resistant to strong inorganic acids than steels. Ceramic materials are excellent corrosion and high-temperature resistance but have the disadvantages of being brittle with low tensile strengths.

c) Coatings

i. Metallic Coating• the metal will be protected with applied as thin coatings to

separate the corrosive environment from the metal.• Metal coatings are sometimes applied so that they can serve as

sacrificial anodes that can corrode instead of the underlying metal.

• Eg: the zinc coating on steel to make galvanized steel is anodic to the steel and corrodes sacrificially.

ii. Inorganic Coatings (Ceramic and Glass)• Coat steel with a ceramic coating to attain a smooth, durable

finish.• Steel is commonly coated with a porcelain coating that of a thin

layer of glass fused to the steel surface.

Continue……….

iii. Organic Coatings• Paints, varnishes and many other organic polymeric materials

are commonly used to protect metals from corrosive environments.

• These materials provide thin, tough and durable barriers to protect the substrate metal from corrosive environments.

d) Design

• The engineering designer must consider the materials along with the necessary mechanical, electrical, and thermal property requirements.

• All these considerations must be balanced with economic limitations.

• Some important general design rules:i. Allow for the penetration action of corrosion along with the

mechanical strength requirements when considering the thickness of the metal used.

ii. Weld rather than containers to reduce crevice corrosion.iii. Use galvanically similar metals for the whole structure.iv. Avoid sharp bends in piping system where flow occurs.v. Design tanks and other container for easy draining and cleaning.

NON-FERROUS METALS

• Non-ferrous naturally means a metal or alloy which contains little or no iron.•Common non-ferrous metals are aluminum, copper and magnesium.•Properties: as corrosion resistance, high thermal and electrical conductivity, low density and ease of fabrication.•Application: aluminum for cooking utensils and aircraft bodies, copper wire for electricity, copper tubing for residential water supply, zinc for galvanized sheet metal for car bodies, titanium for jet-engine turbine blades and for orthopedic implants and tantalum for rocket engines.

Aluminum

•Characteristic:a)High strength-to-weight ratiob)high thermal and electrical conductivityc)good corrosion resistanced)good manufacturing properties.

•Application:a)Containers and packagingb)Buildings and other types of constructionc)Transportation (aircraft and aerospace applications, buses, automobiles, railroad cars and marine craft)d)Electrical applications e)Consumers durables (Appliances, cooking utensils and furniture)f)Portable tools

CopperCopper are among the best conductors of electricity and heat, and they have good corrosion resistance. They can be processed easily by various forming, machining, casting and joining techniques.•Characteristic:a)High electrical and thermal conductivityb)good corrosion resistancec)good manufacturing properties.

•Application:a)Electrical and electronic componentsb)Springsc)Plumbingd)Heat exchangerse)Marine hardwaref)Consumer goods(such as cooking utensils, jewelry and other decorative objects.)

Zinc Zinc is bluish-white in color and is the metal fourth most utilized industrially, after iron, aluminium and copper.•Characteristic:a)Has relatively low melting temperature about 419Cb)Fair conductor of electricity

•Application:a)Zinc serves as an anode and protects the steel(cathode) from corrosive attack.b)Zinc is used for galvanizing steel against corrosionc)Alloying element; brass, for example is an alloy of copper and zinc.d)Zinc based alloy are used extensively in die casting for making such products as fuel pumps.