MEC281 MATERIALS SCIENCE

CHAPTER 5 CORROSION & PREVENTION METHODS

Rasdi bin DeramanFakulti Kejuruteraan Mekanikal

UiTM Pulau Pinang

CORROSION

Example: the tarnishing of silver, iron rusting, dissolution of metal in acid solution

What is the corrosion process? Corrosion- Deterioration / destructive of a metal or its alloys by oxidation. Metals will corrode if they come in contact with; moist atmosphere, air, water, salt solutions, acids, or alkalines.

Disadvantage: economic terms higher expense to spent for corrosion prevention and maintenance/ replacement

Advantage: etching, dry cell batteries

ELECTROCHEMICAL CONSIDERATION

• Corrosion process is normally electrochemical process.

• Involve chemical reaction.• ē transfer from one chemical species to another.

ELECTROCHEMICAL CONSIDERATION

• 3 important process in corrosion– Electrolyte (any solution that contains ion)– Anode (oxidation)– Cathode (reduction)

Anode

• Oxidation process occurs• Oxidation –metal atom lose/give up ē

– M M n+ + n ē– i.e ; Fe Fe 2+ + 2 ē

• Negative electrode

Cathode

– Reduction reaction occurs– Gains of ē (ē from metal atom that is oxidized)– The reaction at cathode

• M n+ + ē M n-1

• Or M n+ + nē M • i.e Mg 2+ + 2ē Mg

– Positive electrode

The Standard EMF series

• Measured cell voltage

• Represent only diff. in electrical potential from reference cell

• Reference cell : using standard hydrogen electrode

Standard EMF series

• two half-cell reactions for Fe and Cu Fe Fe2+ + 2ē half-cell reactions (oxidation)Cu2+ + 2ē Cu half-cell reactions (reduction) Fe + Cu2+ Fe2+ + Cu Overall reaction

Overall cell potential ΔV = V1

+ V2

= 0.340 + 0.440 = 0.780 V

Electrode potential • Not all metallic materials oxidize to form ions

with the same degree of ease

+

• Example:• One half of an electrochemical cell consists of a

pure nickel electrode in a solution of ions; the other is the cadmium electrode immersed in the solution. Write the spontaneous overall reaction and calculate the voltage that is generated.

• Solution:

Galvanic series- represents the relative reactivity of a number of metals and commercial alloys in seawater

TEST 2 MEC 281

• 4 APRIL 2009• 2.00 – 4.00 PM• CHAPTERS: 3, 4, 5 & 6

Corrosion rate

• The rate of material removal as a consequence of the chemical action

• the weight loss can be converted to corrosion rate in units – mm/yr– mpy (mils per year)

• Corrosion current is used to generate a corrosion rate

Q =(nFW)/M ………………..(1)But Q = It

(nFW)/M = It ………….……(2)

W= (ItM)/nF ………….…....(3)

But W is mass = (density, ρ ) x (volume)where Volume = (area, A) x (thickness, H)

Thus, W = ρAH and substitute in eq (3)

ρAH = (ItM)/nF ………….…(4)H/t =(IM)/(nFρA) ……….....(5)

where H/t is corrosion rate, CR in unit cm/S

Current density, Icorr = current flow/ Area, A/cm2Icorr = I/A

From eq. (5), Corrosion rate, H/t = {(Icorr)(M)}/(nFρ) ……….....(6)where this rate is in unit cm/s.

The normal practice, unit of Corrosion rate, CR are in mpy and mm/yr.

CR = { (128,704.7 x Icorr x M)/ n ρ } unit mpy

CR = { (3269 x Icorr x M)/ n ρ } unit mm/y

n = no. electrons involved in half cell reactionsIcorr = ampere/cm square

M =atomic weight in grams/mol

ρ = Density in grams/cm cubic

6 Factors Influence Corrosion Rate

Factor Condition Corrosion Rate

Concentration of solution High

Stress intensity High

Temperature High

Oxygen High

Flow rate of liquid High

Surface contact area High

Experiment 1: Specimens Steel and Copper immersed in an agar-agar electrolyte. Steel act as anodic site and Copper act as cathodic site.

Experiment 2:Place the straight nail and the bent nail in one of the petri dishes. Be sure the nails do not touch each other. Pour the lukewarm agar-agar over the nails until they are covered.

FORMS OF CORROSION

General (Uniform) Corrosion

This is the most common form of corrosion. In fact, all metals are attacked by uniform corrosion, but to different grades. It proceeds by a chemical or electrochemical reaction over a large surface area, effectively thinning and eventually consuming the entire thickness of the metal.

Electrochemical reactions occurs at more or less, the same rate over the entire surface.

Failure caused by uniform attack can be easily and accurately predicted. Timely visual examination, if possible, of selected points at the thinnest parts of a component or a structure is often sufficient, or some simple tests, e.g. immersing a sample in the solution involved, will provide accurate corrosion data for the prediction of the service life of a component.

General (Uniform) Corrosion (Cont…)

Galvanic corrosion

Occurs when 2 metals having different compositions are electrically coupled and exposed to an electrolyte

The more electro-positive = cathode, The least electropositive = anode

Mechanism of galvanic corrosion

Three conditions must be present:• Electrochemically dissimilar metals must be

present; • These metals must be in electrical contact; and • The metals must be exposed to an electrolyte.

• When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.

• It is this potential difference that is the driving force for galvanic current flow

• area ratio of metal also influence the galvanic corrosion rate

• Normally occurs at joining i.e fastener, welded

• When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.

• It is this potential difference that is the driving force for galvanic current flow

• area ratio of metal also influence the galvanic corrosion rate

• Normally occurs at joining i.e fastener, welded

Aluminum helicopter Aluminum helicopter blade has corroded near blade has corroded near where it was in contact where it was in contact with a steel with a steel counterbalance. counterbalance.

Steel pipe has Steel pipe has corroded near where corroded near where it was in contact it was in contact with a gate valve with a gate valve Copper alloy. Copper alloy.

Stress Corrosion Cracking (SCC)• The SCC occurred because of the simultaneous

effects of tensile stress and corrosion. Stress may be internally or externally applied. Internal stresses are produced by non-uniform deformation during cold working, by unequal cooling from high temperatures, and by internal structural rearrangement involving volume changes. Stresses induced by press and shrink fits, and those in rivets and bolts are internal stresses.

Stress Corrosion Cracking (SCC)

Intergranular ...

Stress Corrosion Cracking (SCC)

Transgranular ...

Erosion-corrosion• Corrosion accelerated by impact by solid particles• These may remove metal, or they may just

remove oxide and allow metal to corrode more quickly

• Erosion corrosion is commonly found in piping, especially at bends, elbows and abrupt changes in pipe diameter- position where the fluid changes direction or flow suddenly becomes turbulent. Propellers, turbine blades, valves and pumps are also susceptible to this form of corrosion.

Pitting Corrosion• As the name suggests, pitting is an extremely localised

attack that results in pits, i.e., holes of various sizes. They are called holes because that the diameter of the attack is generally small and that the depth-to-diameter ratio is often greater than 1. They may sometimes cluster together to give a look of rough surfaces.

• Pitting is one of the most destructive forms of corrosion; it causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. Most pits develop and grow in the direction of gravity, starting from horizontal surfaces and growing downwards.

Pitting corrosion Caused by Chlorides (halides) Passive metals & alloys = susceptible

Mechanism …Mechanism …

Crevice corrosionCrevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in narrow crevices such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps, under porous deposits and in many other similar situations.

Example of Crevice corrosion

Revision:

Type of Corrosion Cause

Galvanic corrosionDissimilar metals are in electrical contact.

Stress corrosion cracking (SCC)

Stress + Susceptible materials + Corrosive environment

Erosion corrosionSuspended particle in fluid motion

Crevice corrosionNarrow crevices associated with a stagnant solution.

Introduction The easier technique to prevent corrosion occurred:i. Understand the theory of all types of corrosion.ii. The factors influenced corrosion rate. 

6 Factors Influence Corrosion Rate

Factor Condition Corrosion Rate

Concentration of solution High

Stress intensity High

Temperature High

Oxygen High

Fluid flow rate High

Surface contact area High

Corrosion prevention and protection• 5 general techniques to consider:

– Material selection and treatment– Environmental alteration– Design consideration– Coating – Cathodic protection

1. Material selection and treatment• Selection of material depends on the corrosion

environment:

For example; Application in fresh water

• Suitable metal: cast iron, steel, Al alloys, Cu alloys and some stainless steel.

Application for seawater environment• Titanium, brass, some bronzes, cooper-nickel

alloys, nickel-chromium-molybdenum alloys.

Suitable metal for atmospheric application• Al alloys, cu alloys, galvanized steel.

1. Material selection and treatment (Cont..)

• Heat treatment for relieve the residual stress induce in metal that formed into finished shapes by bending

• Another factors that will be consider– Mechanical properties– Material cost – Material fabrication– Availability of materials

a) Mechanical properties

– When operation condition become aggressive, material from combination 2/3 metal are needed

– Effect of temp. will influence the mechanical properties i.e application of plain carbon steel is limited to 316-343oC due to loss of strength and susceptibility to oxidation and corrosion. Highly alloyed Ni-Cr-Fe alloy are adequate to corrosion resistance at elevated temp. compare to 18Cr-12Ni. 18Cr-12Ni sustain oxidation resistance at temp. up to 816oC and strength reduce above 650oC.

b) Material cost

– Life cycle cost or cost effectiveness rather than initial cost.

c)Material fabrication– Material should be selected on the basis of

their maintainability as well as their fabrication ability.

– Need to consider the ease of modification or repaired after exposure to operating condition.

d)Availability of materials– Consideration for future availability for repairs

or replacement or alternative replacement materials.

2. Environmental alteration

• Decrease the velocity of corrosive fluid• Use lower temperature• Remove O2 from water solution by deaerated process• Reduce the concentration of corrosive ions in a solution• Adding inhibitors in system

– Scavengers type- react with and virtually eliminate a chemically active species in solution such as dissolved O2

– Absorption type- attach themselves to the corroding surface and interface with either oxidation/reduction reaction or form a very thin protective coating

3. Design consideration• Prevent the formation of galvanic cell by using

similar metal or alloys or using non metallic connector ( fitting, gasket) to electrically insulate the two dissimilar metal.

• Weld rather than rivets. Or choose rivets that are cathodic to the materials being joined.

• Make the anode area larger than the cathode area.

• Avoid sharp bends in piping system when flow occurs. Erosion corrosion will occur at areas in which the fluid direction changes sharply, for example Cavitations.

• Design components so that fluid system are closed to avoid continuously dissolve gas.

• Easy removal and replacement of parts that are expected to fail rapidly in service, must be design

• The design should allow for complete drainage and easy washing

4. Coating

• Physical barrier applied on surfaces in the form of films.

• Used to isolate anode and cathode region.

• Important condition:– coating must be nonreactive in the corrosive

environment.– Resist to mechanical damage that exposes

the bare metal to corrosive environment.

• Types of material for coating:– Metals (Metallic coating); Ceramic (Inorganic

coating) & Polymers (Organic coating)

• Metallic coating– Acts as sacrificial anodes instead of coated

metal.– i.e: steel are coated with Zinc-plated or tin-

plated.– Metal coating are plated by electro plating

process.– metal./ part to be plated is made as cathode.

The electrolyte is a solution of a salt of the metal to be plated and direct current is applied.

• Inorganic coating/ceramic coating– i.e reaction vessels can also be lined with

corrosion-resisitant teflon.– Stable, adherent, nonporous, noconducting

oxide layer.– i.e glass fused coating a steel.

• Organic coating– Polymeric material, pints, varnish.– Cheaper.

5. Cathodic protection

• Generally, Cathodic protection can be applied to all types of corrosion but involve highly coast.

• The principle of cathodic protection is connecting the metal (body) to be protected with an external anode or (to the electrical DC current) so that all areas of the metal surface become cathodic and therefore do not corrode.

• Two types of Cathodic protection:

i. Sacrificial anode

ii. Impressed current

Passivity? Caused by thin hydroxide layer forming on metal surface, protecting the metal from anodic dissolution However, oxide will itself corrode under certain conditions

a) Sacrificial anode• Employ reactive metals as auxiliary anodes

that are directly electrically connected to the metal to be protected.

• The sacrificial anode corrodes, supplies electron to the metal and thereby prevents an anode reaction at metal.

• The metals commonly used, as sacrificial anodes are aluminium, zinc and magnesium.

Eg. Aluminium, Magnesium & Zinc anode

Steel structure

• The current has to flow in a loop and therefore the surface of structure that can be protected is the submerged surface that are received protection current from anodes.

PROTECTED SURFACE

STEEL PIPE

UNPROTECTED SURFACE

• Structures that are commonly protected by cathodic protectionare the exterior surfaces of:– Pipelines– Ships’ hulls– Storage tank bases– Jetties and harbour structures– Steel sheet, tubular and foundation pilings– Offshore platforms, floating and sub sea

structures

• Cathodic protection is also used to protect the internal surfaces of:– Large diameter pipelines– Ship’s tanks (product and ballast)– Storage tanks (oil and water)– Water-circulating systems.

Criteria for sacrificial anodes• A corrosion potential that is sufficiently

negative for the specific application; in general, alloying additions are made to make the potential more negative than that of the unalloyed basis metal

• A high anode efficiency, which means that impurities that result in self corrosion must be absent or minimum.

• An ability to remain active and to corrode uniformly and not to become passive

Why should use ICCP?

•Steel structure that will be protected is large

•Protection current requirement is high

•Coating quality varies in a large range

•Resistivity of soil is high

•Source of electric current is available

•Field for ground bed are available

b) Impressed current Cathodic Protection

ICCP Method (Cont…)

• Obtained from a DC power supply connected between anode and the metal to be protected.

• A connection from a current source to make electron flow to metal, to cause the metal to be cathode.

• Anode is usually surrounded by high conductivity backfill material ( “groundbed”) such as gypsum or bentonite which improve the electric contact between anode and surrounding soil.

• Add: it will protected large area.

ICCP Method (Cont…)

•Negative terminal of powers source is connected to the structure to be protected.

•Positive terminal is joined to an inert anodic (i.e. graphite)

•The source of electrons is an impressed current from an external power source for an underground tank.

•The anode is usually surrounded by high conductivity backfill material such as coke breeze, gypsum or bentonite which improve electric contact between the anode and surrounding soil.

TQCORROSION &

PREVENTION METHODS