Constraints on materials

TemperatureGalvanic compatibilityAtomic oxygenMoisture absorption/desorptionFluid compatibilityThermal cyclingChemical (corrosion)

Constraints on materials, cont.. Temperature

The range of temperatures experienced will play a large part in the materials selection. Extremes are illustrated by the examples of cryogenic tanks and thermal protection systems for re-entry applications. Temperatures below room temperature generally cause an increase in strength properties, however the ductility decreases. Ductility and strength may increase or decrease at temperatures above room temperature. This change depends on many factors, such as temperature and time of exposure.

Materials shall be compatible with the thermal environment to which they are exposed.

Passage through transition temperatures (e.g., brittle-ductile transitions or glass transition temperatures including the effects of moisture or other phase transitions) shall be taken into account.

Constraints on materials cont..Galvanic compatibility If two or more dissimilar materials are in direct electrical contact in a

corrosive solution or atmosphere galvanic corrosion might occur. The less resistant material becomes the anode and the more resistant the cathode. The cathodic material corrodes very little or not at all, while the corrosion of the anodic material is greatly enhanced.

Atomic oxygen Materials are exposed to a flux of atomic oxygen. The flux level varies

with altitude, velocity vector and solar activity. The fluence levels vary with the duration of exposure.

Moisture absorption/desorption The properties of materials are susceptible to changes induced by the

take-up of moisture. Moisture absorption occurs during production and application of components.

Constraints on materials cont.. Fluid compatibility

In some occasions materials are in contact with liquid oxygen, gaseous oxygen or other reactive fluids or could come into contact with such a fluid during an emergency situation.

Thermal cycling Thermal cycling can induce thermal stresses and due to the difference in coefficient of

thermal expansion for example between fibres and matrix for composites and between

base metal and coating micro-cracks can form which could jeopardise long-term

properties.

Materials subject to thermal cycling shall be assessed to ensure their capability to

withstand the induced thermal stresses and shall be tested.

Chemical (corrosion) The chemical environment to which a material is subjected in its life span may cause

changes in the material properties. Corrosion is the reaction of the engineering material

with its environment with a consequent deterioration in properties of the material.

Corrosion will include the reaction of metals, glasses, ionic solids, polymeric solids and

composites with environments that embrace liquid metal, gases, non-aqueous

electrolytes and other non-aqueous solutions, coating systems and adhesion systems.

Environmental Degradation of Materials Corrosion))

Materials are “attacked” by their operating environment.We will focus on the degradation of metals. This is called

corrosion.

In metals, corrosion is produced by the loss of actual material, which leaves the piece as an ion in solution, and is carried away by an electrolyte.

Rust is a symptom of this problem in steel, but there can be corrosion without it.

What is Corrosion??

Electrochemical reaction involving an anode and a cathode.Deterioration of a material because of reaction with the

environment.

Combines many elements of engineering and impacts ALL engineering disciplines: Chemical Engineering, Mechanical Engineering, Material Engineering, Electrical Engineering and Civil Engineering.

What is Corrosion??

Corrosion involves the interaction (reaction) between a metal or alloy and its environment. Corrosion is affected by the properties of both the metal or alloy and the environment. The environmental variables include:

pH (acidity) Oxidizing power (potential) Temperature (heat transfer) Velocity (fluid flow) Concentration (solution constituents)

What is Corrosion??

Combination of the material and it’s environment - Examples:

No Problem: Lead in Water Aluminum in atmosphere Nickel in hydraulic fluid

BAD: Steel in marine environment Cu in Ammonia Stainless Steel(SS) in chloride (Sea water) Lead in wine

Please note the presence of stainless steel:

Yes, under certain circumstances, stainless becomes active.

Factors: (These are bad for any metal!)

1. Low aeration in water

2. Low velocity water

3. Presence of Cl-. Chlorine is one of the worst offenders in promoting corrosion.

UNIVERSALITY OF CORROSION

Not only metals, but non-metals like plastics, rubber, ceramics are also subject to environmental degradation

Even living tissues in the human body are prone to environmental damage by free radicals-Oxidative stress- leading to degenerative diseases like cancer and diabetes.

CORROSION DAMAGE Disfiguration or loss of appearance Loss of material Maintenance cost Extractive metallurgy in reverse- Loss of precious minerals,

power, water and man-power Loss in reliability & safety Plant shutdown, contamination of product etc

The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal.  Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. 

COST OF CORROSION

Direct & Indirect losses:

Direct loss: Material cost, maintenance cost, over-design, use of costly material

Indirect losses: Plant shutdown & loss of production, contamination of products, loss of valuable products due to leakage etc, liability in accidents

WHY DO METALS CORRODE?

Any spontaneous reaction in the universe is associated with a lowering in the free energy of the system. i.e. a negative free energy change

All metals except the noble metals have free energies greater than their compounds. So they tend to become their compounds through the process of corrosion.

Fundamental Components

Corrosion can be defined as the deterioration of material by reaction to its environment.

Corrosion occurs because of the natural tendency for most metals to return to their natural state; e.g., iron in the presence of moist air will revert to its natural state, iron oxide.

4 required components in an electrochemical corrosion cell:

1) An anode; 2) A cathode; 3) A conducting environment for ionic movement (electrolyte); 4) An electrical connection between the anode and cathode for the flow of electron current.

If any of the above components is missing or disabled, the electrochemical corrosion process will be stopped.

Example – The Daniell Cell

This example illustrates some of the basics of corrosion.

On the surface of the Zn bar we have the following

eZnZn 22

On the surface of the Cu bar we have the following

CueCu 22

Note the current path. The salt bridge provides for ion exchange.

Driving force

A driving force is necessary for electrons to flow between the anodes and the cathodes.

The driving force is the difference in potential between the anodic and cathodic sites.

This difference exists because each oxidation or reduction reaction has associated with it a potential determined by the tendency for the reaction to take place spontaneously. The potential is a measure of this tendency.

Summary:

What’s needed for Corrosion:

1. An anode. This is where the damage occurs. Oxidation takes place.

2. A cathode. Here’s where the reduction reaction takes place.

3. An electrolyte. (Almost any moisture will do.)

4. A current path between the cathode and anode.

ELECTROCHEMICAL NATURE

All metallic corrosion are electrochemical reactions i.e. metal is converted to its compound with a transfer of electrons

The overall reaction may be split into oxidation (anodic) and reduction (cathodic) partial reactions

Next slide shows the electrochemical reactions in the corrosion of Zn in hydrochloric acid

ELECTROCHEMICAL REACTIONS IN CORROSION

DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,

222 HZnClHClZn -------------------- -(1) Written in ionic form as,

22 222 HClZnClHZn ----------------------(2)

The net reaction being,

222 HZnHZn ------------------------- (3)

Equation (3) is the summation of two partial reactions,

eZnZn 2*2 -----------------------------------------(4) and 222 HeH ------------------------------------------(5)

Equation (4) is the oxidation / anodic reaction and Equation (5) is the reduction / cathodic reaction

REDOX reactions

Here is a typical reduction reaction involving hydrogen ions in solution. Note that the H gains electrons.

eFeFe 22

Here is an oxidation reaction. Fe is the symbol for iron. Note that metal looses electrons.

222 HeH

These Reactions want to occur in Pairs

We are assuming that the Fe is surrounded by a weak acid in which H+ ions are abundant.

This acid is called an electrolyte. It provides a home for the dissolve Fe+2 ion.

Note that there has to be an internal movement of electrons through the Fe.

General Reactions

Anode: (Metal basically dissolves in the electrolyte.)

Cathode: (This is a very common reaction!)

OHeOOH 442 22

enMM n

Surfaces near high O2 concentration are cathodic!

PASSIVATION

Many metals like Cr, Ti, Al, Ni and Fe exhibit a reduction in their corrosion rate above certain critical potential. Formation of a protective, thin oxide film.

Passivation is the reason for the excellent corrosion resistance of Al and S.S.

Galvanic Series •Ranking the reactivity of metals/alloys in seawater

PlatinumGold

GraphiteTitanium

Silver316 Stainless Steel (passive)

Nickel (passive)Copper

Nickel (active)Tin

Lead316 Stainless Steel (active)

Iron/SteelAluminum Alloys

CadmiumZinc

Magnesium

mor

e an

odic

(ac

tive

)

mor

e ca

thod

ic

(in

ert

)

Relationship between the rate of corrosion, corrosivity of an environment and corrosion resistance of a material.

Corrosion Samples in

Different Applications

Underground corrosion

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs, by which time considerable damage may be done.

Electronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections.  Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits.  These resistors form part of a radar installation.

Corrosion influenced by flow-1

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped.  The high

velocities in the pump accentuated the corrosion damage.

Corrosion influenced by flow – 2

This is a bend in a copper pipe-work cooling system.  Water flowed around the bend and then became turbulent at a roughly cut edge. 

Safety of aircraft

The lower edge of this aircraft skin panel has suffered corrosion. Any failure of a structural component of an aircraft can lead to

the most serious results.

Influence of corrosion on value

A very slight amount of corrosion may not interfere with the usefulness of an article, but can affect its commercial value.  At the points where these scissors were held into their plastic case some surface corrosion has occurred which would mean that the shop would have to sell them at a reduced price.

Motor vehicle corrosion and safety

The safety problems associated with corrosion of motor vehicles is illustrated by the holes around the filler pipe of this petrol tank.  The danger of petrol leakage is obvious.  Mud and dirt thrown up from the road can retain salt and water for prolonged periods, forming a corrosive “poultice”.

Corrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel.  This ship has suffered severe damage in the areas which are most buffeted by waves, where the protective coating of paint

has been largely removed by mechanical action.

Aluminium Corrosion

The current trend for aluminium vehicles is not without problems. This aluminium alloy chassis member shows very advanced corrosion due to contact with road salt from gritting operations or use in coastal / beach regions.

Damage due to pressure of expanding rust

The iron reinforcing rods in this garden fence post have been set too close to the surface of the concrete.  A small amount of corrosion leads to bulky rust formation which exerts a pressure and causes the concrete to crack.  For structural engineering applications all reinforcing metal should be covered by 50 to 75 mm of concrete.

“Corrosion” of plastics

Not only metals suffer “corrosion” effects.  This dish is made of glass fibre reinforced PVC.  Due to internal stresses and an aggressive environment it has suffered “environmental stress cracking”.

Galvanic corrosion

This rainwater guttering is made of aluminium and would normally resist corrosion well.  Someone tied a copper aerial wire around it, and the localised bimetallic cell led to a “knife-cut” effect.

Galvanic corrosion

The tubing, shown here was part of an aircraft’s hydraulic system.  The material is an aluminium alloy and to prevent bimetallic galvanic corrosion due to contact with the copper alloy retaining nut this was cadmium plated.  The plating was not applied to an adequate thickness and pitting corrosion resulted.

Galvanic corrosion

This polished Aluminium rim was left over Christmas with road salt and mud on the rim. Galvanic corrosion has started between the chromium plated brass spoke nipple and the aluminium rim. 

Galvanic corrosion

Galvanic corrosion can be even worse underneath the tyre in bicycles used all winter. Here the corrosion is so advanced it has penetrated the rim thickness.

Significance of Corrosion on Infrastructure

Engineer finds corrosion in collapsed bridge at North Carolina speedway (2000)

Corrosion & Catastrophic Failure

A Concrete bridge failure